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Sample records for low-latitude tec observations

  1. SWARM Observations of the Motion of Low-latitude Plasma Depletions Coordinated with Ground-based TEC Measurements

    NASA Astrophysics Data System (ADS)

    Valladares, C. E.; Pradipta, R.; Sheehan, R. E.; Coisson, P.; Knudsen, D. J.

    2015-12-01

    During the early phase of the SWARM mission, the distance between the trajectories of all three satellites of the constellation was tens of km and the temporal separation was of order one minute. This unique geometry allows us to conduct multiple and almost simultaneous in-situ measurements through the same low-latitude plasma depletion to investigate their spatial coherence and the motion of structures embedded within the equatorial plasma bubbles. We have used the number density measured with the Electric Field Instrument (EFI) on-board the three satellites of the SWARM constellation during December 2013 and January 2014 and concurrent TEC values obtained by ground-based GPS receivers to fully diagnose the bubble characteristics at multiple scale sizes. We have applied correlation and cross-spectra analysis to the density values measured by the EFI probes to derive the longitudinal variability of plasma density structures and their velocity. Our results indicate a very strong variability of the plasma bubbles in longitude. More specifically, it shows that structures with scale sizes corresponding to 100 and 10 seconds are not in phase. TEC values measures on the ground indicated that TEC plasma depletions moved with a velocity of order 100 m/s and have a westward tilt of order 10°. This presentation will show results for several specific days of SWARM observations during passes in the American sector.

  2. TEC Response to X-ray Solar Emissions Observed in the Equatorial and Low-latitude Brazilian Region

    NASA Astrophysics Data System (ADS)

    Becker-Guedes, F.; Nicoli Candido, C. M.; de Siqueira, P. M.; Paula, E. R.; Takahashi, H.; De Nardin, C. M.; Costa, J. E. R.

    2014-12-01

    Some spurious effects affecting radio communications happen when the X-ray solar flux in the interplanetary medium reaches values above a certain threshold. The magnitudes of these effects depend on the X-ray peak brightness and the duration, which drive the intensity of the ionosphere response when the associated electromagnetic wave hit the sunlit side of the Earth atmosphere. An important aspect defining the severity of damages to HF radio communications and LF navigation signals in a certain area is the local time when each event takes place. In order to improve the understanding of radio signal loss or degradation in the Brazilian sector due to solar X-ray emissions, we analyze total electron content (TEC) maps and curves at selected sites obtained by a GPS network formed by tents of dual-frequency receivers spread all over Brazilian territory. We observe ionospheric local changes during several X-ray events in the 0.1-0.8 nm range identified by GOES satellite. Considering the duration, peak brightness, and local time of the events, our goal is to understand the degree of changes suffered by the ionosphere after these solar X-ray emissions using GPS receivers, namely in the equatorial region and around the southern crest of the equatorial ionospheric anomaly.

  3. Characterization of ionospheric variability in TEC using EOF and wavelets over low-latitude GNSS stations

    NASA Astrophysics Data System (ADS)

    Dabbakuti, J. R. K. Kumar; Venkata Ratnam, D.

    2016-06-01

    Investigation of ionospheric variability is essential for improving the daily ionospheric modeling and forecasting services of Global Navigation Satellite System (GNSS) applications. As India is a low-latitude region, more care has to be taken here to characterize the ionosphere due to irregularities and Equatorial Ionization Anomaly (EIA) conditions. Therefore, an appropriate method is required to diagnose the ionospheric variations during geomagnetic, solar and other disturbances. In this paper, the temporal ionospheric time delay variations were studied based on the Empirical Orthogonal Function (EOF) analysis and wavelet transforms (WT).These analyses were carried out with Total Electron Content (TEC) datasets obtained from three GNSS stations located in low-latitude regions. EOF analysis was performed on the TEC datasets, which were decomposed into a time series of orthogonal eigen values (or base functions) and associated coefficients. EOF base functions and their associated coefficients signify the hourly time variations and the day of the year variations. The results reveal that the first few EOFs represented the majority of TEC variability pertaining to the physical processes of the ionosphere. The accuracy of the EOF model was validated by the evaluation of observational TEC data with International Reference Ionosphere (IRI) 2012 models. The EOF model coefficients for each GNSS station showed a strong correlation with the IRI models and also described the correlation between the impacts of the level of geomagnetic activity on the ionosphere. The correlation coefficients for the first three EOFs were more than 0.95. The phase relationship of ionospheric TEC anomalies, with respect to the geomagnetic indices (Dst), were analyzed by wavelet transforms.

  4. Receiver DCB estimation and GPS vTEC study at a low latitude station in the South Pacific

    NASA Astrophysics Data System (ADS)

    Prasad, Ramendra; Kumar, Sushil; Jayachandran, P. T.

    2016-11-01

    The statistical estimation of receiver differential code bias (DCB) of the GSV4004B receiver at a low latitude station, Suva (lat. 18.15°S, long. 178.45°E, Geomag. Lat. 21.07°S), Fiji, and the subsequent behaviour of vTEC, are presented. By means of least squares linear regression fitting technique, the receiver DCB was determined using the GPS vTEC data recorded during the year 2010, CODE TEC and IRI-2012 model for 2010. To substantiate the results, minimization of the standard deviation (SD) method was also used for GPS vTEC data. The overall monthly DCB was estimated to be in the range of 62.6 TECU. The vTEC after removing the resultant monthly DCB was consistent with other low latitude observations. The GPS vTEC 2010 data after eliminating the resultant DCB were lower in comparison to Faraday rotation vTEC measurements at Suva during 1984 primarily due to higher solar activity during 1984 as compared to 2010. Seasonally, vTEC was maximum during summer and minimum during winter. The winter showed least vTEC variability whereas equinox showed the largest daytime variability. The geomagnetic disturbances effect showed that both vTEC and its variability were higher on magnetically disturbed days as compared to quiet days with maximum variability in the daytime. Two geomagnetic storms of moderate strengths with main phases in the local daytime showed long duration (∼52 h) increase in vTEC by 33-67% which can be accounted by changes in E×B drifts due to prompt penetration of storm-time auroral electric field in the daytime and disturbance dynamo electric field in the nighttime to low latitudes.

  5. Features of TEC behaviour over the low-latitude North-American region during the period of medium solar activity

    NASA Astrophysics Data System (ADS)

    Sergeeva, M. A.; Maltseva, O. A.; Gonzalez-Esparza, J. A.; De la Luz, V.; Corona-Romero, P.

    2017-10-01

    The study is focused on Total Electron Content (TEC) behavior in low-latitude North-American region within the twelve-year period from 2005 to 2016. The obtained systematic (regular) TEC variations (diurnal, seasonal patterns) conform to the known tendencies described by climatological TEC models. TEC variations during disturbances have a number of distinctive features. The majority of positive TEC enhancements occur already with Dst <-30 nT. The intensity of TEC change does not depend on Dst-index minimum value. Almost all cases of day-time TEC disturbances are accompanied by night-time disturbances with the possibility of relative night-time deviations being larger that is confirmed by data of low-orbit satellites CHAMP and DMSP. The simultaneity of changes of critical frequencies of the ionosphere with changes of TEC during disturbances was observed at Ramey (Puerto Rico) and Eglin (U.S.A.) stations when ionosonde data was available. This proves that TEC reflects the character of ionospheric disturbances. Spatial non-synchronism of TEC change that was sometimes observed can be explained by local character of the area covered with positive TEC disturbance. It was shown the potential provided by local GNSS networks for the detailed analysis of TEC variations over the considered region and estimation of the ionosphere state in near-real time.

  6. A comparison of GPS-TEC with IRI-TEC at low latitudes in China in 2006

    NASA Astrophysics Data System (ADS)

    Wan, Qingtao; Ma, Guanyi; Li, Jinghua; Wang, Xiaolan; Fan, Jiangtao; Li, Qi; Lu, Weijun

    2017-07-01

    A comparison between GPS derived total electron content (TEC) and IRI-2012 is studied at low latitudes in China in 2006. 4 GPS receivers form a small scale GPS network, and the spatial coverage starts at (26.1°N, 119.3°E) and ends at (22.8°N, 108.3°E), the corresponding magnetic coordinate is from (19.6°N, 191.6°E) to (16.2°N, 180.4°E). The network covers a region at or near the northern crest of the equatorial anomaly with ∼11° and ∼3° in longitude and latitude, respectively. The GPS-TEC is computed with the grid-based method. The ABT-2009 and NeQuick are selected for the bottomside and topside profile model for IRI-2012. There are similar monthly and seasonal variation trends between GPS-TEC and IRI-TEC in 2006, while the peak value of GPS-TEC and IRI-TEC appear at different times. The IRI-TEC is almost larger than GPS-TEC at day time, while the difference is small at night time. The largest difference between GPS-TEC and IRI-TEC can reach 16.5 TECU, and the ratio of the largest difference and GPS-TEC is about 65%. The largest latitudinal gradient of GPS-TEC at 22-24°N was 5 TECU/deg, while that of IRI-TEC was 2.5 TECU/deg. These above results indicate that the IRI-2012 can reproduce the TEC variation trends at low latitudes in China, but the IRI-2012 overestimates the GPS-TEC at day time in this region.

  7. Impacts of CME on the TEC at middle and low latitudes during maximum of the 24th solar cycle

    NASA Astrophysics Data System (ADS)

    Migoya Orue, Yenca Olivia; Amory-Mazaudier, Christine; Radicella, Sandro; Nava, Bruno; Kashcheyev, Anton

    2015-04-01

    In this study we analyzed the impacts on the GNSS-derived Total Electron Content (TEC) of four selected CME hitting the Earth during the year 2013 at different stations of middle and low latitudes (Ebre, Rabat, Alexandria, San Fernando, M'barara, Matera and Dakar). In order to analyzed the seasonal behavior of TEC under these disturbed conditions in the mentioned stations we have selected four CME events occurred during the different seasons (January 19, March 17, July 9 and October 2) of year 2013, at a maximum of the sunspot cycle 24. At the beginning of each event there is an increase of TEC followed by a decrease. The first increase of TEC is a consequence of the Prompt Penetration of the Electric Field (PPEF). The depletion of the TEC is associated to the Disturbance Dynamo Electric Field (DDEF). In order to interpret the observations we analyzed the convection patterns at high latitudes given by the radar SUPERDARN. At low latitudes, we derived the ionospheric electric current disturbance Diono from ground magnetic variations. Diono is the sum of the DP2 (PPEF) and Ddyn (DDEF) electric current systems. Finally we found that the strength of the impact at middle and low latitudes depends on the time of the impact of the CME and the season.

  8. Performance evaluation of GNSS-TEC estimation techniques at the grid point in middle and low latitudes during different geomagnetic conditions

    NASA Astrophysics Data System (ADS)

    Abe, O. E.; Otero Villamide, X.; Paparini, C.; Radicella, S. M.; Nava, B.; Rodríguez-Bouza, M.

    2017-04-01

    Global Navigation Satellite Systems (GNSS) have become a powerful tool use in surveying and mapping, air and maritime navigation, ionospheric/space weather research and other applications. However, in some cases, its maximum efficiency could not be attained due to some uncorrelated errors associated with the system measurements, which is caused mainly by the dispersive nature of the ionosphere. Ionosphere has been represented using the total number of electrons along the signal path at a particular height known as Total Electron Content (TEC). However, there are many methods to estimate TEC but the outputs are not uniform, which could be due to the peculiarity in characterizing the biases inside the observables (measurements), and sometimes could be associated to the influence of mapping function. The errors in TEC estimation could lead to wrong conclusion and this could be more critical in case of safety-of-life application. This work investigated the performance of Ciraolo's and Gopi's GNSS-TEC calibration techniques, during 5 geomagnetic quiet and disturbed conditions in the month of October 2013, at the grid points located in low and middle latitudes. The data used are obtained from the GNSS ground-based receivers located at Borriana in Spain (40°N, 0°E; mid latitude) and Accra in Ghana (5.50°N, 0.20°E; low latitude). The results of the calibrated TEC are compared with the TEC obtained from European Geostationary Navigation Overlay System Processing Set (EGNOS PS) TEC algorithm, which is considered as a reference data. The TEC derived from Global Ionospheric Maps (GIM) through International GNSS service (IGS) was also examined at the same grid points. The results obtained in this work showed that Ciraolo's calibration technique (a calibration technique based on carrier-phase measurements only) estimates TEC better at middle latitude in comparison to Gopi's technique (a calibration technique based on code and carrier-phase measurements). At the same time

  9. Performance evaluation of GNSS-TEC estimation techniques at the grid point in middle and low latitudes during different geomagnetic conditions

    NASA Astrophysics Data System (ADS)

    Abe, O. E.; Otero Villamide, X.; Paparini, C.; Radicella, S. M.; Nava, B.; Rodríguez-Bouza, M.

    2016-11-01

    Global Navigation Satellite Systems (GNSS) have become a powerful tool use in surveying and mapping, air and maritime navigation, ionospheric/space weather research and other applications. However, in some cases, its maximum efficiency could not be attained due to some uncorrelated errors associated with the system measurements, which is caused mainly by the dispersive nature of the ionosphere. Ionosphere has been represented using the total number of electrons along the signal path at a particular height known as Total Electron Content (TEC). However, there are many methods to estimate TEC but the outputs are not uniform, which could be due to the peculiarity in characterizing the biases inside the observables (measurements), and sometimes could be associated to the influence of mapping function. The errors in TEC estimation could lead to wrong conclusion and this could be more critical in case of safety-of-life application. This work investigated the performance of Ciraolo's and Gopi's GNSS-TEC calibration techniques, during 5 geomagnetic quiet and disturbed conditions in the month of October 2013, at the grid points located in low and middle latitudes. The data used are obtained from the GNSS ground-based receivers located at Borriana in Spain (40° N, 0° E; mid latitude) and Accra in Ghana (5.50° N, -0.20° E; low latitude). The results of the calibrated TEC are compared with the TEC obtained from European Geostationary Navigation Overlay System Processing Set (EGNOS PS) TEC algorithm, which is considered as a reference data. The TEC derived from Global Ionospheric Maps (GIM) through International GNSS service (IGS) was also examined at the same grid points. The results obtained in this work showed that Ciraolo's calibration technique (a calibration technique based on carrier-phase measurements only) estimates TEC better at middle latitude in comparison to Gopi's technique (a calibration technique based on code and carrier-phase measurements). At the same

  10. Analysis of Ionospheric Scintillation spectral and TEC in the Chinese low latitude region

    NASA Astrophysics Data System (ADS)

    Li, Guozhu; Ning, Baiqi; Yuan, Hong

    GPS L-band scintillations and total electron content TEC were recorded at Sanya 18 33° N 109 52° E for the period July 2004 - July 2005 Automatic recorded raw digital scintillation data are analyzed to obtain the spectral characteristics of irregularities producing ionospheric scintillations and to estimate the correlation between amplitude scintillation and power spectral density Concurrent measurements of TEC were used to analyze ROTI defined as the standard deviation of the rate of change of TEC Results show that spectral slope and auto correlation interval correspond quite well with amplitude scintillation index S4 during the generation evolution and decay phase of scintillation activity which indicates the formation evolution and erosion of small-scale irregularities The statistical results of S4 indices and spectral slopes indicate that the spectral slopes increase with S4 indices for weak scintillation S4 0 3 but for moderate and strong scintillation spectral slopes tend to be in saturation It is also find that the large and small scale irregularities coexist when scintillation occurs In the analyzed dataset the ratio of ROTI S4 is found to vary between 0 3 and 8

  11. Analysis of ionospheric scintillation spectra and TEC in the Chinese low latitude region

    NASA Astrophysics Data System (ADS)

    Li, G.; Ning, B.; Yuan, H.

    2007-04-01

    GPS L-band scintillations and total electron content (TEC) were recorded at Sanya (18.33°N, 109.52°E) during the period July 2004-July 2005. Automatic recorded raw digital scintillation data are analyzed to obtain the spectral characteristics of irregularities producing ionospheric scintillations and to estimate the correlation between amplitude scintillation and power spectral density. Concurrent measurements of TEC are used to analyze ROTI, defined as the standard deviation of the rate of change of TECνll. The statistical results of S4 indices and power spectral indices indicate that the power spectral indices increase with S4 indices for weak scintillation (0.1 ≤ S4 < 0.3), but for moderate and strong scintillation, spectral indices tend to be saturated. In the analyzed data set, the ratio of ROTI/S4 is found to vary between 0.3 and 6, and the variation in estimated zonal drift velocities during geomagnetic quiet days (Kp < 3) shows that the motion of the irregularities is highly variable in the initial phase of irregularity development. After about 22:00 LT, the estimated drift velocities tend to follow the same pattern.

  12. Comparison of IRI model predictions with low latitude ionospheric observations

    NASA Astrophysics Data System (ADS)

    Bittencourt, J. A.; Chryssafidis, M.

    During a period of high solar activity (1979/1980), IRI-predicted electron density profiles were compared with measurements made at Fortaleza (Brazil), 2 degrees off the dip equator. A few discrepancies were found. They are attributed mainly to dynamical effects associated with low latitude E x B electromagnetic plasma drifts and thermospheric neutral winds that are not correctly reproduced in the CCIR numerical maps and in the IRI profile shapes as well. In particular, the dependence on the magnetic declination angle, which strongly affects the electrodynamical plasma motions at low latitudes, is not satisfactorily considered in the models.

  13. Modeling and analysis of GPS-TEC low latitude climatology during the 24th solar cycle using empirical orthogonal functions

    NASA Astrophysics Data System (ADS)

    Dabbakuti, J. R. K. Kumar; Venkata Ratnam, D.

    2017-10-01

    The Total Electron Content (TEC) is an essential component describing the temporal and spatial characteristics of the ionosphere. In this paper, an empirical orthogonal function (EOF) model is constructed by using ground based Global Navigational Satellite System (GNSS) TEC observation data at the Bangalore International GNSS Service (IGS) station (geographic - 13.02° N, 77.57° E; geomagnetic latitude 4.4° N) during an extended period (2009-2016) in the 24th solar cycle. EOF model can be decomposed into base functions and its corresponding coefficients. These decomposed modes well represented the influence of solar and geomagnetic activity towards TEC. The first three EOFs modes constitute about 98% of the total variance of the observed data sets. The Fourier Series Analysis (FSA) is carried out to characterize the solar-cycle, annual and semi-annual dependences by modulating the first three EOF coefficients with solar (F10.7) and geomagnetic (Ap and Dst) indices. The TEC model is validated during daytime and nighttime conditions as well as under different solar activity and geomagnetic conditions. A positive correlation (0.85) of averaged daily GPS-TEC with averaged daily F10.7 strongly supports those time-varying characteristics of the ionosphere features depends on the solar activity. Further, the validity and reliability of EOF model is verified by comparing with the GPS-TEC data, and standard global ionospheric models (International Reference Ionosphere, IRI2016 and Standard Plasmasphere-Ionosphere Model, SPIM). The performances of the standard ionospheric models are marked to be relatively better during High Solar Activity (HSA) periods as compared to the Low Solar Activity (LSA) periods.

  14. Study of the low latitude ionospheric turbulence observed by DEMETER

    NASA Astrophysics Data System (ADS)

    Li, F.; Lefeuvre, F.; Parrot, M.

    Following previous works from Molchanov et al 2002a 2002b 2004a 2004b and Hobara et al 2005 data bases dedicated to the systematic analysis of the power and spectral indices of the electric field have been elaborated Two data bases are considered one for the survey mode and the other for the burst mode For the survey mode estimations of the turbulence parameters are performed from the 8 first Fourier components of the averaged power spectra 0-150 Hz frequency band A single slope power law model f - alpha is assumed A quality factor allows to test that hypothesis For the burst mode the power spectra are derived from the waveforms One and two slope models are systematically tested Results are presented and the possibility to use these data bases for correlation with seismic activity is discussed Y Hobara F Lefeuvre M Parrot and O A Molchanov Low-latitude ionospheric turbulence observed by Aureol-3 satellite Annales Geophysicae 23 1259--1270 2005 Molchanov O A Hayakawa M Afonin V V Akentieva O A and Mareev E A Possible influence of seismicity by gravity waves on ionospheric equatorial anomaly from data of IK-24 satellite 1 Search for idea of seismo-ionosphere coupling Seismo Electromagnetics Lithosphere-Atmosphere-Ionosphere Coupling edited by Hayakawa M and Molchanov O A TERRAPUB Tokyo 275--285 2002a Molchanov O A Hayakawa M Afonin V V Akentieva O A Mareev E A and Trakhtengerts V Yu Possible influence of seismicity by gravity waves on ionospheric

  15. Characterisation of GPS-TEC in the African equatorial and low latitude region and the regional evaluation of the IRI model

    NASA Astrophysics Data System (ADS)

    Adebiyi, S. J.; Adimula, I. A.; Oladipo, O. A.

    2016-06-01

    With the increasing application of Global Navigation Satellite System (GNSS) products and services, knowledge of the Total Electron Content (TEC) variation is vital, particularly in historically under-sampled regions. The ionospheric induced-error, which is the largest and most variable error source of GNSS applications, is proportional to TEC along the satellite-receiver path. Simultaneous Global Positioning System (GPS) measurements from six African equatorial and low latitude stations in the southern hemisphere are used to investigate the latitudinal variation of TEC over the region during the year 2013, a year of moderate solar activity. The analysis reveals some detailed features of seasonal, month-to-month and solar activity dependence of TEC. The seasonal variation of TEC revealed that the daytime and the pre-midnight values of TEC for stations located close to the geographic equator is considerably higher in equinoxes and June solstice compared to stations farther from the equator, however, the difference is insignificant during the December solstice. The month-to-month variation of TEC shows semi-annual symmetry/asymmetry in TEC values for stations closer/farther from the equator. TEC sensitivity to solar activity shows significant seasonal and latitudinal characteristics. Generally, a relatively good correlation exists between TEC and F10.7 for stations around the Equatorial Ionization Anomaly (EIA) region compared to those found at stations close to the equator. Beyond the EIA region, the correlation coefficients drop in all seasons. TEC predicted by the three topside options of the International Reference Ionosphere (IRI) 2012 model [i.e. the NeQuick (NeQ), IRI-2001 Corrected (IRI-01 Corr) and the IRI-2001 (IRI-01) options] exhibits latitudinal and seasonal characteristics. The NeQ option performed better than the other two options at stations located within the equatorial region in most of the months and seasons. Outside the EIA region, the IRI-01 Corr

  16. Mesopheric Temparature Inversions Observed in Long-Term Lidar Measurements at Mid- and Low-Latitudes

    NASA Technical Reports Server (NTRS)

    Leblanc, T.; McDermid, I. S.; Keckhut, P.; Hauchecorne, A.

    1998-01-01

    Results of an investigation of mesospheric temperature inversion layers using long-term lidar measurements at mid- and low-latitudes are reported. In this paper, new results from different lidar observations of the invasion layers will be presented.

  17. Mesopheric Temparature Inversions Observed in Long-Term Lidar Measurements at Mid- and Low-Latitudes

    NASA Technical Reports Server (NTRS)

    Leblanc, T.; McDermid, I. S.; Keckhut, P.; Hauchecorne, A.

    1998-01-01

    Results of an investigation of mesospheric temperature inversion layers using long-term lidar measurements at mid- and low-latitudes are reported. In this paper, new results from different lidar observations of the invasion layers will be presented.

  18. An additional layer in the low-latitude ionosphere in Indian longitudes: Total electron content observations and modeling

    NASA Astrophysics Data System (ADS)

    Thampi, Smitha V.; Balan, N.; Ravindran, Sudha; Pant, Tarun Kumar; Devasia, C. V.; Sreelatha, P.; Sridharan, R.; Bailey, G. J.

    2007-06-01

    The paper presents the observations and modeling of an additional layer in the low-latitude ionosphere in Indian longitudes. The signatures of the additional layer are observed as ledges or humps between the equatorial ionization anomaly trough and crest (EIA) in the latitudinal profiles of total electron content (TEC), measured using a single ground-based beacon receiver located at Trivandrum (8.5°N, 77°E, dip 0.5°N) in India. The ground-based ionograms also show the presence of the so-called F3 layer for a short duration corresponding to these signatures, and the layer is found to drift upward to the topside ionosphere. The study provides first observational evidence that the so-called "humps" in the latitudinal variation of TEC are nothing but the upward propagating F3 layer. This conclusion is supported by theoretical modeling using the Sheffield University Plasmasphere Ionosphere Model. It is shown that upward ExB drift and strong equatorward neutral wind (perturbed by atmospheric waves) can produce the humps in the latitudinal variation of TEC through the reduction in the downward diffusion of ionization along geomagnetic field lines. The model results also show that the F3 layer drifts to the topside and forms topside ledges.

  19. Ionosonde observations of daytime spread F at low latitudes

    NASA Astrophysics Data System (ADS)

    Jiang, Chunhua; Yang, Guobin; Liu, Jing; Yokoyama, Tatsuhiro; Komolmis, Tharadol; Song, Huan; Lan, Ting; Zhou, Chen; Zhang, Yuannong; Zhao, Zhengyu

    2016-12-01

    Spread F on ionograms has been considered to be a phenomenon mainly occurred at nighttime. This study presented a case study of daytime spread F observed by the ionosonde installed at Puer (PUR; 22.7°N, 101.05°E; dip latitude 12.9°N), where daytime spread F that lasted for more than 2 h (about 08:30 LT 10:45 LT) was observed on 14 November 2015. To investigate the possible mechanism, ionograms recorded at PUR and Chiang Mai (18.76°N, 98.93°E; dip latitude 9.04°N) were used in this study. We found that traveling ionospheric disturbances were observed before the occurrence of daytime spread F. Meanwhile, the movement of the peak height of the ionosphere was downward. We suggested that downward vertical neutral winds excited by traveling atmospheric disturbances/atmospheric gravity waves might play a significant role in forming daytime spread F over PUR during geomagnetic storms.

  20. Characteristics of low-latitude TEC during solar cycles 23 and 24 using global ionospheric maps (GIMs) over Indian sector

    NASA Astrophysics Data System (ADS)

    Dashora, N.; Suresh, Sunanda

    2015-06-01

    The characteristics of quiet time equatorial and low-latitude total electron content over the Indian sector using global ionospheric map (GIM) data (1998-2014) are obtained filtering out the solar flare and storm effects. The results are examined and interpreted in the context of large number of previous studies. The newly found features from this study are as follows. Marked difference in nature of equinoctial asymmetry is noted between solar cycles 23 and 24. Long absence of winter anomaly both during low and high solar activity (HSA) in LL (low-latitude) regions is found. Climatology of the diurnal cycle is provided in four categories using new criteria for demarcation of solar activity levels. Highest correlation (~77%) between GIM ionospheric electron content (IEC) and PI (solar EUV proxy index) is noted over equator in contrast to previous studies. The minimum positive contribution of PI in variation of IEC requires minimum of 2 years of data, and if more than 7-8 years of data are used, it saturates. Root-mean-square width of PI can be used to define the HSA. Strong QBO (quasi-biennial oscillations) in IEC is noted in tune with the one in PI over both LL locations but QBO remains surprisingly subdued over equator. The semiannual oscillations in GIM-IEC are found to be stronger at all locations during high solar activity and weaker between 2005 and 2011, whereas the annual oscillations are found to be substantially stronger only during HSA-23 and weakest over southern LL location throughout 17 years.

  1. TEC Observation at Equatorial Anomaly in China

    NASA Astrophysics Data System (ADS)

    Shen, H.; Ma, G.; Chen, Y.; Huang, W.; Zhang, D.; Wu, Y.; Luo, R.; Xiao, F.; Qin, J.

    2006-05-01

    Since May 2005, a GPS receiver chain had been established at equatorial anomaly region in south China. The chain is composed of 4 GPS receivers located at Fuzhou, Xiamen, Guangzhou and Nanning whose geomagnetic latitudes are 14.4°N, 13.2°N, 11.8°N, 11.4°N, respectively. The GPS observation is set at 1 Hz sampling rate, which enabled GPS-TEC data with a high time resolution. The four sites are just in northern equatorial anomaly, so they have favorable condition for the observation to study the TEC variations and ionospheric scintillation. With the data we have obtained, TEC enhancement with different scales and TEC depletions have been recognized. This paper presents the preliminary results of TEC observation with the chain, with emphasis on spatial scale of the TEC related phenomena.

  2. East Asian observations of low-latitude aurora during the Carrington magnetic storm

    NASA Astrophysics Data System (ADS)

    Hayakawa, Hisashi; Iwahashi, Kiyomi; Tamazawa, Harufumi; Isobe, Hiroaki; Kataoka, Ryuho; Ebihara, Yusuke; Miyahara, Hiroko; Kawamura, Akito Davis; Shibata, Kazunari

    2016-12-01

    A magnetic storm around 1859 September 2, caused by a so-called Carrington flare, was the most intense in the history of modern scientific observations, and hence is considered to be a benchmark event concerning space weather. The magnetic storm caused worldwide observations of auroras, even at very low latitudes, such as Hawaii, Panama, or Santiago. Available magnetic-field measurements at Bombay, India, showed two peaks: the main was the Carrington event, which occurred in day time in East Asia; a second storm after the Carrington event occurred at night in East Asia. In this paper, we present results from surveys of aurora records in East Asia, which provide new information concerning the aurora activity of this important event. We found some new East Asian records of low-latitude aurora observations caused by a storm which occurred after the Carrington event. The size of the aurora belt of the second peak of the Carrington magnetic storm was even wider than that of usual low-latitude aurora events.

  3. A multi-technique study of the 29-31 October 2003 geomagnetic storm effect on low latitude ionosphere over Indian region with magnetometer, ionosonde, and GPS observations

    NASA Astrophysics Data System (ADS)

    Panda, Sampad Kumar; Gedam, Shirish S.; Rajaram, Girija; Sripathi, S.; Pant, Tarun Kumar; Das, Rupesh M.

    2014-12-01

    The present study demonstrates the ionospheric response to the extreme geomagnetic storms during 29-31 Oct 2003 (the Halloween storm), in the low latitude anomaly Indian region, based on multi-instrument measurements namely magnetometer, ionosonde, and GPS observations. Unlike earlier reports, we have suitably chosen the best quiet days (CQ-Days) amongst 10 international quiet days (Q-Days), on the basis of equatorial electrojet strength and pattern, that drives the distribution of plasma over the low latitude. We stress that arbitrarily selecting the Control/Q-Days may lead to erroneous interpretations and will not yield a clearer understanding of the equatorial electrodynamics. Our analysis confirms the anomalous increase in TEC at all stations on 29 Oct 2003 and suppressed TEC across anomaly crest latitudes during 30-31 Oct 2003. The sharp transition in h'F is noticed during the progressive period of the storm, though it was relatively lower at the equator. The respective foF2 remained subordinate at Trivandrum. However, we did not notice such foF2 changes at Delhi. Observations at various latitudes confirm the maximum positive deviation of TEC at mid-latitude POL2 (140 %), followed by the low latitude Jodhpur (108 %), and the rest of the stations showing relatively lower enhancements with deviations ranging between 60-90 %. However, at Ahmedabad, the lowest divergence from the mean CQ-Days, attribute the typical quiet day formation of crest at this latitude. Although the results are well agreeing with earlier reports, miniature differences is noticeable due to our way of choosing the best reference days in the analysis.

  4. A comparison of TEC predicted by IRI-2012 with that measured at three different stations in low latitude Indian region for the years (2010-2012)

    NASA Astrophysics Data System (ADS)

    Karia, S. P.; Patel, N. C.; Pathak, K. N.

    2017-07-01

    The present study reports the comparison of GPS measured Total Electron Content (TEC) with that predicted by the latest IRI-2012 model at three different stations located within the Equatorial Ionization Anomaly region (EIA) in the Indian sector. The data used for the study are retrieved from three different stations, namely, Surat (geographic latitude 21.16°N, geographic longitude 72.78°E; geomagnetic latitude 12.90°N), Hyderabad (geographic latitude 17.25°N, geographic longitude 78.30°E; geomagnetic latitude 8.65°N) and Bangalore (geographic latitude 13.02°N, geographic longitude 77.57°E; geomagnetic latitude 4.58°N). The period of comparison is three years for rising solar activity from 2010 to 2012. Here it is to note that both Hyderabad and Bangalore are IGS station with the station code (HYDE and IISC respectively). The results for the comparison of seasonal variation shows a good agreement between the measured and modeled TEC for all seasons with deviation of (±15 TECU) for all three years at Surat and Bangalore and with a deviation of (±25 TECU) at Hyderabad. Both topside options NeQuick and IRI01-Corr derived nearly equal TEC at all three stations. It is observed that the GPS TEC data shows the EIA crest at (23.5°N) where as the IRI TEC predicts the EIA crest at (19.7°N) on an average for all the years 2010-2012.

  5. On the NH3 absorption depression observable at Northern low latitudes of Jupiter

    NASA Astrophysics Data System (ADS)

    Tejfel, Victor G.; Vdovichenko, Vladimir D.; Lysenko, Peter G.; Karimov, Alibek M.; Kirienko, Galina A.; Bondarenko, Natalya N.; Kharitonova, Galina

    2016-10-01

    From February to April of 2016, we carried out a special series of spectrophotometric observations of Jupiter to study the current behavior of the ammonia absorption at the low latitudes of the Northern hemisphere, where in 2004 we have found a well-defined depression of the 787 nm NH3 absorption band intensity (V.Tejfel et al., Bull.AAS, 2005, Vol. 37, p.682). In subsequent years, an existence of this depression was annually confirmed by spectral observations, although we were noticing its variable character. During observations of 2016 we obtained more than 2,500 CCD-spectrograms, including the spectra of the central meridian, the GRS, and 12 scans of Jovian disk on different dates (70 zonal spectra in each scan). The 787 nm NH3 absorption band was extracted with using of ratios of the Jovian spectra to the Saturn's disk spectrum that was taken as a reference. The depression of absorption in this band begins almost from the equator, and its maximum occurs at the planetographic latitude of 100N then the absorption increases again approaching to the latitude of 200N. The equivalent bandwidths corresponding to these latitudes are equal to 18.7 ± 1.4 A, 14.4 ± 1.0 A and 17.8 ± 0.8A. The 645 nm NH3 absorption band also shows depletion at the low latitudes of the Northern hemisphere, but it is less pronounced. At the temperate latitudes of the Northern hemisphere this band's absorption is systematically lower than the Southern Hemisphere's ones. We will continue research in this direction, especially because recently a significant depletion of gaseous NH3 has also been found with using of the VLA with high resolution (I. de Pater et al., Science, 2016, Vol. 352, Issue 6290, p.1290-1294) at the low latitudes of the Northern hemisphere in the region of the NEB.

  6. Direct observational evidence for disturbance dynamo on the daytime low-latitude ionosphere: A case study based on the 28 June 2013 space weather event

    NASA Astrophysics Data System (ADS)

    Thampi, Smitha V.; Shreedevi, P. R.; Choudhary, R. K.; Pant, Tarun Kumar; Chakrabarty, D.; Sunda, S.; Mukherjee, S.; Bhardwaj, Anil

    2016-10-01

    A case of the westward disturbance dynamo (DD) electric field, influencing the daytime equatorial and low-latitude ionosphere, during a geomagnetic storm that occurred on 28-29 June 2013 is presented. The GPS total electron content (TEC) observations from a network of stations in the Indian equatorial, low and middle latitude regions along with the radio beacon TEC, ionosonde, and magnetic field observations are used to study the storm time behavior of the ionosphere. Negative ionospheric storm effects were seen over the low and middle latitudes during the storm time due to the presence of a westward DD electric field. Observations show that the suppression of the equatorial ionization anomaly (EIA) from the morning hours itself on 29 June 2013 took place due to the prevailing westward DD electric field, providing evidence for the model calculations by Balan et al. (2013). Simulations using the GITM model also agree well with our results. The present study gains importance as the direct observational evidences for disturbance dynamo effects on the daytime low-latitude ionosphere and the EIA are sparse, as it has been difficult to delineate it from the compositional disturbances.

  7. Observation of energy spectrum of electron albedo in low latitude region at Hyderabad, India

    NASA Technical Reports Server (NTRS)

    Verma, S. D.; Bhatnagar, S. P.

    1985-01-01

    The preliminary results are presented of the measurement of the energy spectrum of low energy (5-24 MeV) albedo electrons, moving upward as well as downwards, at about 37 km (-4 mb) altitude, over Hyderabad, India, in low latitude region. The flux and energy spectrum was observed by a bi-directional, multidetector charged particle telescope which was flown in a high altitude balloon on 8th December 1984. Results based on a quick look data acquisition and analysis system are presented here.

  8. Simultaneous observation of whistlers and emissions during a geomagnetically quiet period at low latitude

    NASA Astrophysics Data System (ADS)

    Singh, Ashutosh K.; Singh, K. K.; Singh, A. K.; Lalmani

    2011-02-01

    A unique night-time natural electromagnetic disturbances in the VLF/ELF range received during a magnetically quite period at a low latitude Indian ground station, Jammu (geomag. lat. 19°26' N, L=1.17) has been reported. During the routine observation of VLF waves at Jammu, whistlers and different types of VLF/ELF emissions such as whistlers of varying dispersion confined to a small band limited frequency range, hisslers, pulsing hiss, discrete chorus emissions of rising and falling tones with multiple bands, oscillating tone discrete emission, whistler-triggered hook and discrete chorus risers emissions, etc. have been observed simultaneously during the quiet period on a single night. Such type of unique simultaneous observations has never been reported from any of the low latitude ground stations and this is the first observation of its kind. The results are discussed in the light of recorded features of whistlers and emissions. Generation and propagation mechanism are discussed briefly. Plasma parameters are further derived from the dispersion analysis of nighttime whistlers and emissions recorded simultaneously during magnetically quiet periods.

  9. Low-latitude total electron content enhancement at low geomagnetic activity observed over Japan

    NASA Astrophysics Data System (ADS)

    Kutiev, Ivan; Otsuka, Yuichi; Saito, Akinori; Tsugawa, Takuya

    2007-07-01

    Numerous total electron content (TEC) values derived from GPS signals are averaged within 1.5° × 1.5° cells in a 1-hour time frame, and the relative deviations of these average values from corresponding monthly medians are used to produce latitude-time plots over Japan. The paper analyzes the appearance and development of enhancements of TEC of equatorial origin (ETEs), occurring outside initial and main phases of geomagnetic storms. ETE structures appear mainly as single-crest structures in the evening hours local time, with TEC peak around 1900 LT. TEC usually decreases with latitude, and the structures disappear below 40°N. In some cases the TEC peak is found above the plot boundary of 24°N, as depletions toward the equator are also observed. The observed enhanced structures are linked to the well-known evening prereversal enhancement of ion drift in the equatorial F region. Double-crest ETEs are also observed, with the second peak occurring in early morning hours. Most of the ETE events appear in periods of low geomagnetic activity, 1-3 days after the main phase of the storms. In some cases the time of rising of ETE structures coincides with the increase of interplanetary electric field (IEF), a fact showing the importance of directly penetrating electric field in formation of ETEs. Often, ETEs appear repeatedly in 2 or 3 consecutive days. It is supposed that planetary atmospheric waves are responsible for this phenomenon. Most of the observed features of ETEs can be explained by the published results of simulations of the coupled thermosphere-ionosphere-plasmasphere (CTIP) model and the thermosphere/ionosphere/electrodynamic general circulation model (TIEGCM). It is suggested that ETE structures are produced mainly by a disturbance winds dynamo electric field, built up after the main phase of the storms. Some ETE events, appearing at the end of a prolonged period of low geomagnetic activity, can be linked to directly penetrating IEF in equatorial

  10. Chandra X-ray Observations of Jovian Low-latitude Emissions: Morphological, Temporal, and Spectral Characteristics

    NASA Technical Reports Server (NTRS)

    Bhardwaj, Anil; Elsner, Ronald F.; Gladstone, G. Randall; Cravens, Thomas E.; Waiate J. Hunter, Jr.; Branduardi-Raymont, Graziella; Ford, Peter

    2004-01-01

    Chandra observed X-rays from Jupiter during 24-26 February 2003 for about 40 hours with the ACIS-S and HRC-I instruments. The analysis of Jovian low-latitude "disk" Xray emissions are presented and compared with the high-latitude "auroral" emissions. We report the first Chandra ACIS-S measured X-ray spectrum (0.3-2 keV) of Jupiter's low-latitude disk The disk X-ray emission is harder and extends to higher energies than the auroral spectrum. The temporal variation in the Jovian disk X-rays is on an average consistent with those in the solar X-rays observed by GOES, and TIMED/SSE. Contrary to the auroral X-rays, the disk emissions are uniformly distributed over Jupiter; no indication of longitudinal dependence or correlation with surface magneh field strength is visible. Also, unlike the approx. 40 +/- 20 min periodic oscillations seen in the auroral X-ray emissions, the disk emissions do not show any periodic oscillations. The disk spectrum seems to be consistent with resonant and fluorescent scattering of solar X-rays by the Jovian upper atmosphere. Jupiter's disk is found to be about 50% dimmer in soft X-rays in February 2003 compared that in December 2000, which is consistent with the decrease in solar activity. No evidence of lightning-induced X-rays is seen in the Chandra X-ray data. The Jovian disk spectra observed with Chandra-ACIS is stronger than that observed with XMM-Newton two months later during April 28-29, 2003. The XMM-Newton Xray image of Jupiter shows evidence of limb darkening on the anti-sunward side as seen from Earth, as well as an asymmetry with respect to the subsolar point: suggesting a solar driven process.

  11. The Carrington Event and observation of aurorae at very low latitudes

    NASA Astrophysics Data System (ADS)

    Moreno Cárdenas, F.; Cristancho Sánchez, S.; Vargas Domínguez, S.

    2017-07-01

    The Carrington Event was a spectacular blaze of light observed on the solar surface on September 1, 1859, produced by intense activity occurring in the Sun and having remarkable consequences on Earth, e.g. extraordinary aurorae reported during the dawn on September 2th. The supreme solar-terrestrial event is the most energetic of which we have records and the associated geomagnetic storm produced a major auroral oval that expanded towards the equator of the planet. In this work we show, based on historical evidence, that the associated aurorae displayed in Montería, Colombia, at latitude 8° 45' N. We propose that the location of the Earth's geomagnetic north pole, the lowest in at least five centuries, added to the very energetic solar event, allowed the aurora to reach such low latitudes.

  12. Ionospheric TEC observations from TOPEX satellite

    SciTech Connect

    Vladimer, J.A.; Ewell, V.R.; Lee, M.C.; Doherty, P.H.; Decker, D.T.; Anderson, D.N.; Klobuchar, J.A.

    1996-12-31

    Variability of Total Electron Content (TEC) in the equatorial anomaly region of the ionosphere can be studied extensively using the results of measurements taken by the NASA/CNES satellite, TOPEX/Poseidon. The NASA radar altimeter (NRA) is the first space-borne dual-frequency altimeter capable of accurately measuring vertical ionospheric TEC below 1,340 km. TOPEX TEC observations have already been used to support results from an ionospheric measurement campaign that was conducted in equatorial anomaly regions of South America by Phillips Laboratory in Spring, 1994. The best agreement in TEC values is seen during intervals of longitudinal proximity of the satellites` paths. The TOPEX over-ocean data can be used as a supplement to land based measurements in applications to ionospheric research at low and middle latitudes. This study focuses on comparisons between TOPEX vertical TEC data and GPS equivalent vertical TEC measurements taken near the East and West coastal regions of South America. Also the Phillips Laboratory Global Parameterized Ionospheric Model (PIM) is utilized in an effort to estimate slant to vertical conversion errors.

  13. Observations of Baroclinic Coastally Trapped Waves Around Islands at Low Latitudes

    NASA Astrophysics Data System (ADS)

    Schramek, T. A.; Terrill, E. J.; Colin, P.

    2016-02-01

    Low latitude island environments present a unique region to investigate the nature of coastal trapped waves. Phase speed and modal structure of trapped waves are governed by the local stratification and topography around the islands. Observations of temperature and water level were collected for a 12 month period in 2013 around the island nation of Palau located in the tropical West Pacific. Palau encompasses a large barrier reef with sleep offshore slopes and topographic variability in the along shore direction encompassing multiple headlands. During the observational campaign Typhoon Haiyan passed through the island chain providing a large broad band forcing to the oceanographic environment via sustained winds of 155 knots. The data suggest the presence of island-trapped internal waves which propagated around the perimeter of the barrier reef at a phase speed of 4m/s. Isotherm heave greater than 25m was seen at each of the observation locations. Frequencies of the observed waves were near to super inertial. Recent work on coastal trapped waves has pushed the theory and observations past sub-inertial frequencies into the super-inertial range. These observations help provide insight to the nature of propagation of near and super-inertial trapped waves in the island environment.

  14. Performance evaluation of ionospheric time delay forecasting models using GPS observations at a low-latitude station

    NASA Astrophysics Data System (ADS)

    Sivavaraprasad, G.; Venkata Ratnam, D.

    2017-07-01

    Ionospheric delay is one of the major atmospheric effects on the performance of satellite-based radio navigation systems. It limits the accuracy and availability of Global Positioning System (GPS) measurements, related to critical societal and safety applications. The temporal and spatial gradients of ionospheric total electron content (TEC) are driven by several unknown priori geophysical conditions and solar-terrestrial phenomena. Thereby, the prediction of ionospheric delay is challenging especially over Indian sub-continent. Therefore, an appropriate short/long-term ionospheric delay forecasting model is necessary. Hence, the intent of this paper is to forecast ionospheric delays by considering day to day, monthly and seasonal ionospheric TEC variations. GPS-TEC data (January 2013-December 2013) is extracted from a multi frequency GPS receiver established at K L University, Vaddeswaram, Guntur station (geographic: 16.37°N, 80.37°E; geomagnetic: 7.44°N, 153.75°E), India. An evaluation, in terms of forecasting capabilities, of three ionospheric time delay models - an Auto Regressive Moving Average (ARMA) model, Auto Regressive Integrated Moving Average (ARIMA) model, and a Holt-Winter's model is presented. The performances of these models are evaluated through error measurement analysis during both geomagnetic quiet and disturbed days. It is found that, ARMA model is effectively forecasting the ionospheric delay with an accuracy of 82-94%, which is 10% more superior to ARIMA and Holt-Winter's models. Moreover, the modeled VTEC derived from International Reference Ionosphere, IRI (IRI-2012) model and new global TEC model, Neustrelitz TEC Model (NTCM-GL) have compared with forecasted VTEC values of ARMA, ARIMA and Holt-Winter's models during geomagnetic quiet days. The forecast results are indicating that ARMA model would be useful to set up an early warning system for ionospheric disturbances at low latitude regions.

  15. Observations of Low-Latitude Plasma Density Enhancements and their Associated Plasma Drifts

    NASA Technical Reports Server (NTRS)

    Klenzing, J. H.; Rowland, D. E.; Pfaff, R. F.; Le, G.; Freudenreich, H.; Haaser, R. A.; Burrell, A. G.; Stoneback, R. A.; Coley, W. R.; Heelis, R. A.

    2011-01-01

    Plasma density structures are frequently encountered in the nighttime low-latitude ionosphere by probes on the Communication/Navigation Outage Forecasting System (C/NOFS) satellite. Of particular interest to us here are plasma density enhancements, which are typically observed +/- 15 deg away from the magnetic equator. The low inclination of the C/NOFS satellite offers an unprecedented opportunity to examine these structures and their associated electric fields and plasma velocities, including their field-aligned components, along an east-west trajectory. Among other observations, the data reveal a clear asymmetry in the velocity structure within and around these density enhancements. Previous observations have shown that the peak change in drift velocity associated with a density enhancement occurs simultaneously both perpendicular and parallel to the magnetic field, while the 1results in this paper show that the peak change in parallel fl ow typically occurs 25-100 km to the east of the peak perpendicular ow. We discuss this and other aspects of the observations in relation to the characteristics of the plasma depletions formed near the magnetic equator detected by the same probes on the C/NOFS satellite and to previous observations and theories.

  16. Equatorial Anomaly TEC Observation in China

    NASA Astrophysics Data System (ADS)

    Ma, G.; Zhang, D. H.; Chen, Y.; Wu, Y.; Luo, R.; Xiao, F.; Shen, H.; Huang, W.; Tan, J.

    2005-12-01

    Since May 2005, a GPS receiver chain had been established at equatorial anomaly region in China. The chain is composed of 4 GPS receivers located at Fuzhou Xiamen Guangzhou and Nanning. Shown in the following are geographic and geomagnetic coordinates of the 4 stations. 1,Fuzhou: (26.1N, 119.3E--14.4N, 188.4E); 2,Xiamen: (24.5N, 118.1E--13.2N, 187.4E); 3,Guangzhou: (23.1N, 113.2E--11.8N, 182.8E); 4,Nanning : (22.8N, 108.3E-- 11.4N, 178.2E). The aim of the observation is to study the TEC and ionospheric scintillation at equatorial anomaly in China area. This paper presents a preliminary result of TEC observed with the chain, including geomagnetic quiet times and TEC response to geomagnetic storms.

  17. Lower thermospheric-enhanced sodium layers observed at low latitude and possible formation: Case studies

    NASA Astrophysics Data System (ADS)

    Xue, X. H.; Dou, X. K.; Lei, J.; Chen, J. S.; Ding, Z. H.; Li, T.; Gao, Q.; Tang, W. W.; Cheng, X. W.; Wei, K.

    2013-05-01

    We report two lower thermospheric-enhanced sodium layer (TeSL) cases observed at a low-latitude station, Lijiang, China (26.7°N, 100.0°E), on 10 March and 10 April 2012, respectively. The TeSLs in the two cases were located at altitudes near 122 and 112 km, respectively. In addition, strong sporadic sodium layers (SSLs) near 100 km accompanied the TeSL observed on 10 March 2012. Both the TeSLs and SSLs exhibited tidal-induced downward motion. The adjacent ground-based and space-borne ionospheric radio observations showed strong Es layers before the appearance of the TeSLs, suggesting an " Es- TeSLs (SSLs)" chain formed through the tidal wind shear mechanism. Assuming that the vertical tidal wavelengths remain unchanged, it is found that in different regions caused by the tidal wind shear, different TeSLs evolution processes are expected: (1) in a tidal-convergence region, a TeSL/SSL with a downward propagation phase is enhanced due to a rapid decrease in the Na+lifetime at the lower altitude; (2) in an ion convergence-divergence interface region, a TeSL/SSL will still follow the tidal downward phase progression, but sodium density does not exhibit evident enhancement; and (3) when a TeSL/SSL enters into a tidal wind-divergence zone, the layer density tends to decrease.

  18. Recent Plasma Observations Related to Dayside Magnetic Merging and the Low-latitude Boundary Layer

    NASA Technical Reports Server (NTRS)

    Chandler, Michael O.; Avanov, Levon A.; Craven, Paul D.; Mozer, Forrest S.; Moore, Thomas E.

    2007-01-01

    We have begun an investigation of the nature of the low-latitude boundary layer in the mid-altitude cusp region using data from the Polar spacecraft. This region has been routinely sampled for about three months each year for the periods 1999-2001 and 2004-2006. The low-to-mid-energy ion instruments frequently observed dense, magnetosheath-like plasma deep (in terms of distance from the magnetopause and in invariant latitude) in the magnetosphere. One such case, taken during a period of northward interplanetary magnetic field (IMF), shows magnetosheath ions within the magnetosphere with velocity distributions resulting from two separate merging sites along the same field lines. Cold ionospheric ions were also observed counterstreaming along the field lines, evidence that these field lines were closed. These results are consistent with the hypothesis that double merging can produce closed field lines populated by solar wind plasma. Through the use of individual cases such as this and statistical studies of a broader database we seek to understand the morphology of the LLBL as it projects from the sub-solar region into the cusp. We will present preliminary results of our ongoing study.

  19. Variability of Schumann resonance parameters observed at low latitude stations in China

    NASA Astrophysics Data System (ADS)

    Ouyang, X.-Y.; Xiao, Z.; Hao, Y.-Q.; Zhang, D.-H.

    2015-10-01

    This paper presents a comprehensive analysis of the Schumann resonance (SR) parameters observed at low latitude stations in China for the first time. Variations of SR peak frequency and intensity on different timescales (from minutes to years) are analyzed in detail. Diurnal and seasonal variations are shown and the source-observer distance is calculated to confirm the contributions of lightning activity. Differences in the profiles of SR intensity between the NS and EW components are due to the effects of the source-observer distance and the relative position of the observer to the sources. Diurnal frequency variations are more complicated and cannot be directly linked with the three thunderstorm centers. Seasonal variations are clear for intensity but not for frequency. The differences in the diurnal and seasonal variations between the SR intensity and frequency show that the greatest contributor to SR intensity is global lightning activity, while the SR frequency is not affected solely by lightning, as certain other factors involving ionosphere properties may play non-negligible roles. We also emphasize that our observations do not show a distinct day-night change in the SR parameters, and that the SR intensity does not show abrupt changes across terminators. This observation is consistent with previous simulations. Finally, the response of the SR to a solar flare is discussed. The flare leads to a sudden increase of about 0.2 Hz relative to the 2σ level of the SR frequencies in the first three modes, which is in agreement with other works in the literature. This frequency enhancement is explained using theoretical calculations.

  20. Observations of polarization electric fields and plasma drifts associated with low-latitude TIDs

    NASA Astrophysics Data System (ADS)

    Huang, C.

    2015-12-01

    Ionospheric plasma structures and TIDs are often observed in the conjugate hemispheres. It is widely believed that the polarization electric field is responsible for the conjugacy. However, observed conjugate disturbances so far are mostly in plasma density and TEC, and there is almost no report of observations of polarization electric fields associated with TIDs. We present the C/NOFS measurements of TID-like disturbances in the plasma density and drift velocity. The plasma drift perpendicular to the geomagnetic field is manifestation of the polarization electric field. Plasma flow parallel to the geomagnetic field is highly correlated with the polarization electric field and is consistent with the prediction of TID theories, confirming that the polarization electric field is caused by TIDs. Surprisingly, the polarization electric field is observed not only at night but also during daytime, implying that the polarization electric field is not shorted out by the dayside E layers. The satellite data are compared with the Jicamarca radar data when C/NOFS is 20 degrees away from the radar but nearly at the same magnetic latitude (along the same field lines). This may be the first observation of polarization electric field associated with TIDs and verifies that the polarization electric field can be indeed transmitted thousands of kilometers along the geomagnetic field lines. The observations provide direct evidence for justifying the mechanism for the generation of similar ionospheric disturbances in the conjugate hemispheres.

  1. Magnetically Conjugate Observations of the Low Latitude Ionosphere in Western South America

    NASA Astrophysics Data System (ADS)

    Hickey, D. A.; Martinis, C. R.; Baumgardner, J. L.; Milla, M. A.; Mendillo, M.; Meriwether, J. W.

    2015-12-01

    An all-sky imager (ASI) installed at Villa de Leyva, Colombia (5.6° N, 73.5° W, 16.3° mag lat) in October 2014 is used in conjunction with another ASI near the magnetically conjugate point at El Leoncito in Argentina (31.8° S, 69.3° W, -19.6° mag lat) to study irregularities and perturbations in the ionosphere. A third ASI in Jicamarca, Peru (11.95° S, 76.87° W, 0.1° mag lat) provides context for the structures generated near the magnetic equator on the west coast of South America. The region sampled by these instruments covers from ~40° S to ~15° N and from ~ 80° W to ~65° W . The Jicamarca Radio Observatory has radar systems and other instruments that measure the upper atmosphere which, combined with the ASIs, allow us to uniquely study equatorial and low latitude processes. The ASIs are able to detect airglow depletions at 630 nm associated with equatorial spread F (ESF) that can also observed with coherent radar scatter measurements at Jicamarca. Simultaneous conjugate observations of ESF are compared to see how the large-scale structures behave at these locations. The ASIs are also used to look for a signature of the midnight temperature maximum (MTM) that is seen as an increase in brightness propagating poleward. Radar and Fabry-Perot interferometer data is used to measure this increase in temperature and combining them with the ASI data we will be able to probe the extent of MTM effects and investigate how they vary with latitude in both hemispheres.

  2. SuperDARN HOP radars observation of ionospheric convection associated with low-latitude aurora observed at Hokkaido, Japan

    NASA Astrophysics Data System (ADS)

    Nishitani, N.; Hori, T.; Kataoka, R.; Ebihara, Y.; Shiokawa, K.; Otsuka, Y.; Suzuki, H.; Yoshikawa, A.

    2016-12-01

    The SuperDARN HOkkaido Pair of (HOP) radars, consisting of the Hokkaido East (2006-) and West (2014-) radars, are the SuperDARN radars located at the lowest geomagnetic latitude (36.5 degrees), and have been continuously measuring ionospheric convection at high to subauroral and middle latitudes with high temporal resolutions (<= 1 to 2 mins). These radars enable us to study the two-dimensional evolution of ionospheric convection ever 1 or 2 minutes. In this paper we study two low latitude aurora events observed in Hokkaido, Japan from 15 to 19 UT on March 17, 2015 and from 1900 to 2030 UT on December 20, 2015, identified using optical instruments such as all-sky CCD camera, wide field of view digital camera and meridian scanning photometer. Both events occurred during the main phase of the relatively large geomagnetic storms with minimum Dst of -223 nT and -170 nT respectively. The ionospheric convection at mid-latitude regions associated with the low-latitude auroral emission is characterized by (1) transient equatorward flows up to about 500 m/s in the initial phase of the emission (the geomagnetic field data at Paratunka, Far East Russia show corresponding negative excursions), and (2) sheared flow structure consisting of westward flow (about 500 m/s) equatorward of eastward flow (1000 m/s), with the equatorward boundary of auroral emission embedded in the westward flow region which expanded up to below 50 deg geomagnetic latitude. These observations imply that the electric field / convection distribution plays important roles in continuously generating the low latitude auroral emission. In particular the observation of the equatorward flow (dawn-dusk electric field) up to as low as about 50 deg geomagnetic latitude is the direct evidence for the presence of electric field to drive ring current particles into the plasmaspheric regions.

  3. Electron density in the intermediate heights for low latitude stations: observations and models

    NASA Astrophysics Data System (ADS)

    Mosert, M.; Radicella, S. M.; Adeniyi, J. O.; Ezquer, R. G.; Jadur, C.

    The electron density (NF1) and height (hF1) of the F1 inflection point measured at three low latitude ionosonde stations were compared with the parameters of the N170 point (electron density at 170 km) and with those predicted by the IRI model. The validity of the empirical equation proposed by Radicella and Mosert to predict the height hF1 was checked. Daytime electron density profiles from Ibadan, Ouagadougou and Tucumán covering different seasonal and solar activity conditions were used in the study. The results indicate that the two points are close together most of the time and that the Radicella-Mosert formula descrbies the data better than the current IRI model.

  4. Ionospheric studies using a low-latitude ionospheric model (LION-model) and ground-based ionosonde observations.

    NASA Astrophysics Data System (ADS)

    Pillat, V. G.; Bittencourt, J. A.; Fagundes, P. R.

    Ionospheric observations made with ionosondes of the type CADI at S a o Jos e dos Campos 23 2 o S 45 9 o W dip latitude 17 6 o S and at Palmas 10 2 S 48 2 W dip latitude 5 7 S Brazil under conditions of high and low solar activity are presented and compared with ionospheric results obtained from a realistic fully time-dependent Low-Latitude Ionosphere Model denominated LION model which simulates the dynamic behavior of the low-latitude ionosphere In the LION model the time evolution and spatial distribution of the ionospheric particle densities and velocities are computed by numerically solving the time-dependent coupled nonlinear system of continuity and momentum equations for the ions O O 2 NO N 2 and N taking into account photoionization of the atmospheric species by the solar extreme ultraviolet radiation chemical and ionic production and loss reactions and plasma transport processes including the ionospheric effects of thermospheric neutral winds plasma diffusion and electromagnetic E x B plasma drift The Earth s magnetic field is represented by a tilted centered magnetic dipole This set of coupled nonlinear equations is solved along a given magnetic field line in a frame of reference moving vertically in the magnetic meridian plane with the electromagnetic plasma drift velocity The model results reproduce adequately the main characteristics and dynamic behavior of the low-latitude ionosphere under quiet

  5. Temporal Development of Dayside TEC Variations during the October 30, 2003 Superstorm: Matching Modeling to Observations

    NASA Astrophysics Data System (ADS)

    Verkhoglyadova, Olga P.; Tsurutani, Bruce T.; Mannucci, Anthony J.

    Strong electron density enhancements are formed in the dayside low-latitude ionosphere by promptly penetrating electric fields (PPEFs) during intense interplanetary electric field (IEF)/superstorm events. Although some of the basic physical ideas of this newly recognized phenomenon are qualitatively understood and have been implemented in various existing ionospheric (and plasmaspheric) models, these models have consistently underestimated the satellite and ground total electron content (TEC) measurements by a wide margin. As one example, no model has been able to duplicate the TEC increase of the October 30, 2003 IEF/superstorm event to within a factor of 2. We present an effort first to determine the ionospheric electric field from magnetic measurements of the Equatorial Electrojet and then to simulate TEC enhancements for this superfountain event based on a modified SAMI2 model, called SAMI2*. We find our results in a reasonable quantitative agreement with the CHAMP measurements. It is noted that dynamic dynamo (storm-time) winds are not necessary to replicate the CHAMP TEC observations, only the PPEFs and the resultant superfountain effects.

  6. Temporal Development of Dayside TEC Variations during the October 30, 2003 Superstorm: Matching Modeling to Observations

    NASA Astrophysics Data System (ADS)

    Verkhoglyadova, Olga P.; Tsurutani, Bruce T.; Mannucci, Anthony J.

    Strong electron density enhancements are formed in the dayside low-latitude ionosphere by promptly penetrating electric fields (PPEFs) during intense interplanetary electric field (IEF)/superstorm events. Although some of the basic physical ideas of this newly recognized phenomenon are qualitatively understood and have been implemented in various existing ionospheric (and plasmaspheric) models, these models have consistently underestimated the satellite and ground total electron content (TEC) measurements by a wide margin. As one example, no model has been able to duplicate the TEC increase of the October 30, 2003 IEF/superstorm event to within a factor of 2. We present an effort first to determine the ionospheric electric field from magnetic measurements of the Equatorial Electrojet and then to simulate TEC enhancements for this superfountain event based on a modified SAMI2 model, called SAMI2*. We find our results in a reasonable quantitative agreement with the CHAMP measurements. It is noted that dynamic dynamo (stormtime) winds are not necessary to replicate the CHAMP TEC observations, only the PPEFs and the resultant superfountain effects.

  7. Geomagnetically conjugate observation of plasma bubbles and thermospheric neutral winds at low latitudes

    NASA Astrophysics Data System (ADS)

    Fukushima, D.; Shiokawa, K.; Otsuka, Y.; Nishioka, M.; Kubota, M.; Tsugawa, T.; Nagatsuma, T.; Komonjinda, S.; Yatini, C. Y.

    2015-03-01

    This is the first paper that reports simultaneous observations of zonal drift of plasma bubbles and the thermospheric neutral winds at geomagnetically conjugate points in both hemispheres. The plasma bubbles were observed in the 630 nm nighttime airglow images taken by using highly sensitive all-sky airglow imagers at Kototabang, Indonesia (geomagnetic latitude (MLAT): 10.0°S), and Chiang Mai, Thailand (MLAT: 8.9°N), which are nearly geomagnetically conjugate stations, for 7 h from 13 to 20 UT (from 20 to 03 LT) on 5 April 2011. The bubbles continuously propagated eastward with velocities of 100-125 m/s. The 630 nm images at Chiang Mai and those mapped to the conjugate point of Kototabang fit very well, which indicates that the observed plasma bubbles were geomagnetically connected. The eastward thermospheric neutral winds measured by two Fabry-Perot interferometers were 70-130 m/s at Kototabang and 50-90 m/s at Chiang Mai. We compared the observed plasma bubble drift velocity with the velocity calculated from the observed neutral winds and the model conductivity, to investigate the F region dynamo contribution to the bubble drift velocity. The estimated drift velocities were 60-90% of the observed velocities of the plasma bubbles, suggesting that most of the plasma bubble velocity can be explained by the F region dynamo effect.

  8. Observations of and Influences on Low-Latitude Vertical Plasma Drifts

    NASA Astrophysics Data System (ADS)

    Miller, E. S.; Chartier, A.; Paxton, L. J.

    2016-12-01

    Many workers have suggested that the morphology (position and relative intensities) of the crests of the equatorial ionization anomaliesis related to the time history of the equatorial vertical drift. In this work, we compare observations of the vertical drift using an HF radiosignals of opportunity in the Central Pacific with UV 135.6-nm observations of the equatorial anomalies from the DMSP/SSUSI andTIMED/GUVI instruments. Furthermore, we explore the role of E region density in modulating the vertical plasma drift using a passive HFsounding experiment in the Caribbean. Coupling between nighttime medium-scale traveling ionospheric disturbances (MSTIDs) and sporadic-Elayers has been suggested as a growth-rate-increasing process. While we observe sporadic-E in the local hemisphere coincident to increases in thealtitude of the F-region altitude, we also observe uplifts without sporadic-E in the local hemisphere. Apart from the trivial explanation that sporadic-E is occurring in the conjugate hemisphere, another possible explanation is that the E region may enhance the vertical drift, but is not required to produce enhanced vertical drifts. These studies represent fruitful areas of future intersection between ground-based observations and ICON and GOLD science.

  9. Observations of a Unique Type of ULF Waves by Low-Latitude Space Technology Five Mission

    NASA Technical Reports Server (NTRS)

    Le, G.; Chi, P.; Strangeway, R. J.; Slavin, J. A.

    2011-01-01

    We report a unique type of ULF waves observed by low-altitude Space Technology 5 (ST-5) constellation mission. ST-5 is a three micro-satellite constellation deployed into a 300 x 4500 km, dawn-dusk, and sun synchronous polar orbit with 105.6deg inclination angle. Due to the Earth s rotation and the dipole tilt effect, the spacecraft s dawn-dusk orbit track can reach as low as subauroral latitudes during the course of a day. Whenever the spacecraft traverse across the dayside closed field line region at subauroral latitudes, they frequently observe strong transverse oscillations at 30-200 mHz, or in the Pc 2-3 frequency range. These Pc 2-3 waves appear as wave packets with durations in the order of 5-10 minutes. As the maximum separations of the ST-5 spacecraft are in the order of 10 minutes, the three ST-5 satellites often observe very similar wave packets, implying these wave oscillations occur in a localized region. The coordinated ground-based magnetic observations at the spacecraft footprints, however, do not see waves in the Pc 2-3 band; instead, the waves appear to be the common Pc 4-5 waves associated with field line resonances. We suggest that these unique Pc 2-3 waves seen by ST-5 are in fact the Doppler-shifted Pc 4-5 waves as a result of rapid traverse of the spacecraft across the resonant field lines azimuthally at low altitudes. The observations with the unique spacecraft dawn-disk orbits at proper altitudes and magnetic latitudes reveal the azimuthal characteristics of field-aligned resonances.

  10. Observations of a Unique Type of ULF Waves by Low-Latitude Space Technology 5 Mission

    NASA Technical Reports Server (NTRS)

    Le, G.; Chi, P.; Strangeway, R. J.; Slavin, J. A.

    2010-01-01

    We report a unique type of ULF waves observed by low-altitude Space Technology 5 (ST-5) constellation mission. ST-5 is a three micro-satellite constellation deployed into a 300 x 4500 km, dawn-dusk, and sun synchronous polar orbit with 105.6 degree inclination angle. Due to the Earth's rotation and the dipole tilt effect, the spacecraft's dawn-dusk orbit track can reach as low as subauroral latitudes during the course of a day. Whenever the spacecraft traverse across the dayside closed field line region at sub auroral latitudes, they frequently observe strong transverse oscillations at 30-200 mHz, or in the Pc 2-3 frequency range. These Pc 2-3 waves appear as wave packets with durations in the order of 5-10 minutes. As the maximum separations of the ST-5 spacecraft are in the order of 10 minutes, the three ST-5 satellites often observe very similar wave packets, implying these wave oscillations occur in a localized region. The coordinated ground-based magnetic observations at the spacecraft footprints, however, do not see waves in the Pc 2-3 band; instead, the waves appear to be the common Pc 4-5 waves associated with field line resonances. We suggest that these unique Pc 2-3 waves seen by ST-5 are in fact the Doppler-shifted Pc 4-5 waves as a result of rapid traverse of the spacecraft across the resonant field lines azimuthally at low altitudes. The observations with the unique spacecraft dawn-disk orbits at proper altitudes and magnetic latitudes reveal the azimuthal characteristics of field-aligned resonances.

  11. Observations of a Unique Type of ULF Waves by Low-Latitude Space Technology 5 Mission

    NASA Technical Reports Server (NTRS)

    Le, Guan; Chi, P.; Strangeway, R. J.; Slavin, J. A.

    2011-01-01

    We report a unique type of ULF waves observed by low-altitude Space Technology 5 (ST-5) constellation mission. ST-5 is a three micro-satellite constellation deployed into a 300 x 4500 km, dawn-dusk, and sun synchronous polar orbit with 105.6 inclination angle. Due to the Earth's rotation and the dipole tilt effect, the spacecraft's dawn-dusk orbit track can reach as low as sub auroral latitudes during the course of a day. Whenever the spacecraft traverse across the dayside closed field line region at sub auroral latitudes, they frequently observe strong transverse oscillations at 30-200 mHz, or in the Pc 2-3 frequency range. These Pc 2-3 waves appear as wave packets with durations in the order of 5-10 minutes. As the maximum separations of the ST-5 spacecraft are in the order of 10 minutes, the three ST -5 satellites often observe very similar wave packets, implying these wave oscillations occur in a localized region. The coordinated ground-based magnetic observations at the spacecraft footprints, however, do not see waves in the Pc 2-3 band; instead, the waves appear to be the common Pc 4-5 waves associated with field line resonances. We suggest that these unique Pc 2-3 waves seen by ST-5 are in fact the Doppler-shifted Pc 4-5 waves as a result of rapid traverse of the spacecraft across the resonant field lines azimuthally at low altitudes. The observations with the unique spacecraft dawn-disk orbits at proper altitudes and magnetic latitudes reveal the azimuthal characteristics of field-aligned resonances.

  12. Observations of a Unique Type of ULF Waves by Low-Latitude Space Technology 5 Satellites

    NASA Technical Reports Server (NTRS)

    Le, G.; Chi, P. J.; Strangeway, R. J.; Slavin, J. A.

    2011-01-01

    We report a unique type of ULF waves observed by low-altitude Space Technology 5 (ST-5) constellation mission. ST-5 is a three micro-satellite constellation deployed into a 300 x 4500 km, dawn-dusk, and sun synchronous polar orbit with 105.6deg inclination angle. Due to the Earth s rotation and the dipole tilt effect, the spacecraft s dawn-dusk orbit track can reach as low as subauroral latitudes during the course of a day. Whenever the spacecraft traverse across the dayside closed field line region at subauroral latitudes, they frequently observe strong transverse oscillations at 30-200 mHz, or in the Pc 2-3 frequency range. These Pc 2-3 waves appear as wave packets with durations in the order of 5-10 minutes. As the maximum separations of the ST-5 spacecraft are in the order of 10 minutes, the three ST-5 satellites often observe very similar wave packets, implying these wave oscillations occur in a localized region. The coordinated ground-based magnetic observations at the spacecraft footprints, however, do not see waves in the Pc 2-3 band; instead, the waves appear to be the common Pc 4-5 waves associated with field line resonances. We suggest that this unique Pc 2-3 waves seen by ST-5 are in fact the Doppler-shifted Pc 4-5 waves as a result of rapid traverse of the spacecraft across the resonant field lines azimuthally at low altitudes. The observations with the unique spacecraft dawn-disk orbits at proper altitudes and magnetic latitudes reveal the azimuthal characteristics of field-aligned resonances.

  13. Response of equatorial and low latitude ionosphere to 2015 St. Patrick's Day super geomagnetic storm: Results from a chain of ground based observations over Indian region

    NASA Astrophysics Data System (ADS)

    Samireddipalle, Sripathi; Singh, Ram; Sreekumar, Sreeba; Suneel Kumar, Buduru

    2016-07-01

    In this paper, we present unique results of equatorial and low latitude ionosphere response to one of the major geomagnetic storms of the current solar cycle that occurred during 17-18 March 2015 where Dst reached its minimum of -228 nT. Here we utilized data from magnetometers, chain of ionosondes located at Tirunelveli (8.73°N, 77.70°E; geom: 0.320N), Hyderabad (17.360N, 78.470E; geom: 8.760N) and Allahabad (25.45°N, 81.85°E; geom: 16.50N) along with multi station GPS receivers over Indian sector. The observations showed a remarkable increase of h'F to as high as ~560 km over Tirunelveli (magnetic equator) with vertical drift of ~70 m/s at 13:30 UT due to direct penetration of storm time eastward electric fields which exactly coincided with the local time of Pre-Reversal Enhancement (PRE) and caused intense ESF irregularities in ionosondes and scintillations in GPS receivers at wide latitudes. Plasma irregularities are so intense that their signatures are seen in Allahabad/Lucknow. Stormtime thermospheric meridional winds as estimated using two ionosondes suggest the equatorward surge of gravity waves with period of ~2 hrs. Suppression of anomaly crest on the subsequent day of the storm suggests the complex role of disturbance dynamo electric fields and disturbance wind effects. Our results also show an interesting feature of Traveling Ionospheric Disturbances (TIDs) possibly associated with disturbance meridional wind surge during recovery phase. In addition, noteworthy observations are nighttime westward zonal drifts and PRE related TEC enhancements at anomaly crests during main phase and CEJ signatures during recovery phase.

  14. Equinoctial asymmetry in low latitude ionosphere as observed by SROSS-C2 satellite

    NASA Astrophysics Data System (ADS)

    Bardhan, Ananna; Aggarwal, Malini; Sharma, D. K.; Rai, J.

    2014-09-01

    The ionospheric plasma parameters (electron, ion temperatures and ion composition-Te, Ti, O+ and H+) measured by SROSS-C2 satellite at an average altitude of ~500 km has been investigated to study the behaviour of the ionosphere in equinoxes during half a solar cycle (year 1995-2000, F10.7 ~70-195). The region under study spans over 5-35°N geog. latitude and 65-95°E geog. longitude in the Indian sector. We found an equinoctial asymmetry in the diurnal behaviour of Te, Ti, O+ and H+ varying with increase in solar activity. The strength of equinoctial asymmetry in Te and Ti is strong during early morning and daytime and strength decreases with increase in solar activity whereas during night time no asymmetry/weaker is observed in low/high solar activity respectively. During the day time, a very strong equinoctial asymmetry in O+ is observed during solar minimum which diminishes with increase in solar activity. The similar diurnal behaviour of H+ as that of O+ is observed during low solar activity but no clear equinoctial asymmetry is observed during solar maximum, as H+ being highly dynamic. The transition height (O+/H+) is the lowest in early morning during solar minimum, which increases during local day-time. The rate of increase in transition height is different in both the equinoxes (higher in vernal than autumn) with respect to dependence on the solar activity, during daytime. Hence equinoctial asymmetry is stronger during solar minimum period than maximum, with higher/lower transition height in vernal during daytime/nighttime respectively.

  15. Very high resolution observations of waves in the OH airglow at low latitudes.

    NASA Astrophysics Data System (ADS)

    Franzen, Christoph; Espy, Patrick J.; Hibbins, Robert E.; Djupvik, Amanda A.

    2017-04-01

    Vibrationally excited hydroxyl (OH) is produced in the mesosphere by the reaction of atomic hydrogen and ozone. This excited OH radiates a strong, near-infrared airglow emission in a thin ( 8 km thick) layer near 87 km. In the past, remote sensing of perturbations in the OH Meinel airglow has often been used to observe gravity, tidal and planetary waves travelling through this region. However, information on the highest frequency gravity waves is often limited by the temporal and spatial resolution of the available observations. In an effort to expand the wave scales present near the mesopause, we present a series of observations of the OH Meinel (9,7) transition that were executed with the Nordic Optical Telescope on La Palma (18°W, 29°N). These measurements are taken with a 10 s integration time (24 s repetition rate), and the spatial resolution at 87 km is as small as 10 m, allowing us to quantify the transition between the gravity and acoustic wave domains in the mesosphere.

  16. High Altitude Double Sodium Layer Observed at Low Latitude and Possible Formation: A Case Study

    NASA Astrophysics Data System (ADS)

    Xue, X.

    2012-12-01

    We report the lidar observations of a double sodium layer (DSL) at Lijiang, China, in March 2012, in which, a high-altitude secondary sodium layer (SeSL) and a sporadic sodium layer (SSL) are co-existed. The SeSL has a maximum density 120 cm-3, 6.7% of that of main layer, and starts at 122 km with full width at half maximum (FWHM) ~ 4-8 km. It does not merge into the main layer for whole observational period. The SSL with a peak density 5500 cm-3, 3 times larger than that of the main layer, appears at 105 km and accompanies with SeSL all the time. Further observations illustrate that multi-layer sporadic E (Es) layers are related to the formation of SeSL and SSL through meridional and zonal diurnal tidal (DT) wind shear mechanisms, respectively, whose efficiency differ at a given altitude according to the ratio of ion-neutral collision frequency to ion gyrofrequency.

  17. Ionospheric response to the 2006 sudden stratospheric warming event over the equatorial and low latitudes in the Brazilian sector using GPS observations

    NASA Astrophysics Data System (ADS)

    de Jesus, R.; Batista, I. S.; Fagundes, P. R.; Venkatesh, K.; de Abreu, A. J.

    2017-02-01

    The main purpose of this paper is to study the response of the ionospheric F-region using GPS-TEC measurements at equatorial and low latitude regions over the Brazilian sector during an sudden stratospheric warming (SSW) event in the year 2006. In this work, we present vertical total electron content (VTEC) and phase fluctuations derived from GPS network in Brazil. The continuous wavelet transform (CWT) was employed to check the periodicities of the ∆VTEC during the SSW event. The results show a strong decrease in VTEC and ∆VTEC values in the afternoon over low latitudes from DOY 05-39 (during the SSW event) mainly after the second SSW temperature peak. The ionospheric ∆VTEC pattern over Brazilian sector shows diurnal and semidiurnal oscillations during the 2006 SSW event. In addition, for the first time, variations in ∆VTEC (low latitude stations) with periods of about 02-08 day have been reported during an SSW event. Using GPS stations located in the Brazilian sector, it is reported for the first time that equatorial ionospheric irregularities were not suppressed by the SSW event.

  18. An atlas of low latitude 6300A (01) night airglow from OGO-4 observations

    NASA Technical Reports Server (NTRS)

    Reed, E. I.; Fowler, W. B.; Blamont, J. E.

    1972-01-01

    The atomic oxygen emission line at 6300 A, measured in the nadir direction by a photometer on the polar orbiting satellite OGO-4, was plotted between 40 deg N and 40 deg S latitude on a series of maps for the moon-free periods between 30 August 1967 and 10 January 1968 The longitudinal and local time variations which occur during the northern fall-winter season are indicated. The northern tropical arc is more widespread while the southern arc is not present at all longitudes. The conditions under which the observations were made are described, and four airglow maps were selected to show the local time variations.

  19. Ground-satellite conjugate observations of low-latitude travelling ionospheric disturbances

    NASA Astrophysics Data System (ADS)

    Moral, A. C.; Shiokawa, K.; Suzuki, S.; Liu, H.; Otsuka, Y.; Yatini, C. Y.

    2016-12-01

    Equatorial travelling ionospheric disturbances (TIDs) are studied by using three CHAMP satellite overpasses on ground-based 630-nm airglow images. The airglow images are obtained from Kototabang (KTB), Indonesia (geographic coordinates: 0.2S, 100.3E, geomagnetic latitude: 10.6S). From 7-year data from October 2002 to October 2009, April 30, 2006 (event 1), September 28, 2006 (event 2) and April 12, 2004 (event 3) are the only TID events found in both ground and satellite measurements. They show southward-moving structures in 630-nm airglow images. The events 1 and 2 are single pulse with horizontal scales of 500-1000 km and event 3 show three wave fronts with horizontal scale sizes of 500-700 km. For events 1 and 3, the neutral density in CHAMP shows out-of-phase variations with the airglow intensity, while event 2 is in-phase. For event 1, the relation between electron density and airglow intensity is out of phase, while relationships of event 2 and 3 are unclear. These unclear relationships suggest that ionospheric plasma variation is not the cause of the TIDs. In the case if gravity waves in the thermosphere is the source of the observed TIDs, in-phase and out-of-phase relationships of neutral density and airglow intensity can be explained by different vertical wavelengths of the gravity wave. We estimate possible vertical wavelengths for those events using observed wave parameters and modeled neutral winds.

  20. Ground-satellite conjugate observations of low-latitude travelling ionospheric disturbances

    NASA Astrophysics Data System (ADS)

    Ceren Moral, Aysegul; Shiokawa, Kazuo; Otsuka, Yuichi; Suzuki, Shin; Liu, Huixin; Yatini, Clara

    2016-07-01

    Equatorial travelling ionospheric disturbances (TIDs) are studied by using three CHAMP satellite overpasses on ground-based 630-nm airglow images. The airglow images are obtained from Kototabang (KTB), Indonesia (geographic coordinates: 0.2S, 100.3E, geomagnetic latitude: 10.6S). From 7-year data from October 2002 to October 2009, April 30, 2006 (event 1), September 28, 2006 (event 2) and April 12, 2004 (event 3) are the only TID events found in both ground and satellite measurements. They show southward-moving structures in 630-nm airglow images. The events 1 and 2 are single pulse with horizontal scales of ~500-1000 km and event 3 show three wave fronts with horizontal scale sizes of 500-700 km. For events 1 and 3, the neutral density in CHAMP shows out-of-phase variations with the airglow intensity, while event 2 is in-phase. For event 1, the relation between electron density and airglow intensity is out of phase, while relationships of event 2 and 3 are unclear. These unclear relationships suggest that ionospheric plasma variation is not the cause of the TIDs. In the case if gravity waves in the thermosphere is the source of the observed TIDs, in-phase and out-of-phase relationships of neutral density and airglow intensity can be explained by different vertical wavelengths of the gravity wave. We estimate possible vertical wavelengths for those events using observed wave parameters and modeled neutral winds.

  1. Studies of ionospheric variations during geomagnetic activities at the low-latitude station, Ile-Ife, Nigeria

    NASA Astrophysics Data System (ADS)

    Emmanuel, Ariyibi

    The dual frequency SCINDA NovAtel GSV 4004B GPS receiver installed at the Ile-Ife (low-latitude station) has been in operation since December 2009. Data records for the year 2010 were processed to obtain Total Electron Content (TEC) and S 4 index. These were interpreted to analyze the ionospheric condition during low geomagnetic activity period (when Dst is from -40 to 0 nT) and during geomagnetic storm events (with Dst about -100 nT). Seasonal variations of the TEC and S 4 index were also investigated. The occurrence of scintillations is closely linked to the peak value of TEC during the daytime; this is very evident during the equinox months when TEC ≥ 30 TECu. When the maximum TEC value is below 30 TECu, as shown by most of the days in the summer months, the scintillation phenomenon does not occur. During geomagnetic storms, the daytime segment of the TEC plot experiences fluctuations (even bifurcations) in values with the peak TEC value of about 40 TECu. From the interpreted data, the occurrence of geomagnetic storm does not necessarily suggest an increase in the level of scintillations at a low-latitude region. Also, there is a remarkable difference between the IRI 2007 model and the observed TEC values, as the daytime TEC peak differs in magnitude and time of occurrence from the observed TEC.

  2. Studies of ionospheric variations during geomagnetic activities at the low-latitude station, Ile-Ife, Nigeria

    NASA Astrophysics Data System (ADS)

    Ariyibi, Emmanuel; Joshua, Emanuel; Rabiu, Babatunde

    2013-02-01

    The dual frequency SCINDA NovAtel GSV 4004B GPS receiver installed at the Ile-Ife (low-latitude station) has been in operation since December 2009. Data records for the year 2010 were processed to obtain Total Electron Content (TEC) and S 4 index. These were interpreted to analyze the ionospheric condition during low geomagnetic activity period (when Dst is from -40 to 0 nT) and during geomagnetic storm events (with Dst about -100 nT). Seasonal variations of the TEC and S 4 index were also investigated. The occurrence of scintillations is closely linked to the peak value of TEC during the daytime; this is very evident during the equinox months when TEC ≥ 30 TECu. When the maximum TEC value is below 30 TECu, as shown by most of the days in the summer months, the scintillation phenomenon does not occur. During geomagnetic storms, the daytime segment of the TEC plot experiences fluctuations (even bifurcations) in values with the peak TEC value of about 40 TECu. From the interpreted data, the occurrence of geomagnetic storm does not necessarily suggest an increase in the level of scintillations at a low-latitude region. Also, there is a remarkable difference between the IRI 2007 model and the observed TEC values, as the daytime TEC peak differs in magnitude and time of occurrence from the observed TEC.

  3. Geomagnetically conjugate observations of ionospheric and thermospheric variations accompanied by a midnight brightness wave at low latitudes

    NASA Astrophysics Data System (ADS)

    Fukushima, D.; Shiokawa, K.; Otsuka, Y.; Kubota, M.; Yokoyama, T.; Nishioka, M.; Komonjinda, S.; Yatini, C. Y.

    2017-08-01

    We conducted geomagnetically conjugate observations of 630-nm airglow for a midnight brightness wave (MBW) at Kototabang, Indonesia [geomagnetic latitude (MLAT): 10.0°S], and Chiang Mai, Thailand (MLAT: 8.9°N), which are geomagnetically conjugate points at low latitudes. An airglow enhancement that was considered to be an MBW was observed in OI (630-nm) airglow images at Kototabang around local midnight from 2240 to 2430 LT on February 7, 2011. This MBW propagated south-southwestward, which is geomagnetically poleward, at a velocity of 290 m/s. However, a similar wave was not observed in the 630-nm airglow images at Chiang Mai. This is the first evidence of an MBW that does not have geomagnetic conjugacy, which also implies generation of MBW only in one side of the hemisphere from the equator. We simultaneously observed thermospheric neutral winds observed by a co-located Fabry-Perot interferometer at Kototabang. The observed meridional winds turned from northward (geomagnetically equatorward) to southward (geomagnetically poleward) just before the wave was observed. This indicates that the observed MBW was generated by the poleward winds which push ionospheric plasma down along geomagnetic field lines, thereby increasing the 630-nm airglow intensity. The bottomside ionospheric heights observed by ionosondes rapidly decreased at Kototabang and slightly increased at Chiang Mai. We suggest that the polarization electric field inside the observed MBW is projected to the northern hemisphere, causing the small height increase observed at Chiang Mai. This implies that electromagnetic coupling between hemispheres can occur even though the original disturbance is caused purely by the neutral wind.[Figure not available: see fulltext.

  4. Observations at Low Latitudes of Magnetic Merging Signatures Within a Flux Transfer Event During a Northward IMF

    NASA Technical Reports Server (NTRS)

    Chandler, M. O.; Avanov, L. A.

    2003-01-01

    Flux transfer events (FTE) have been postulated to result from transient magnetic merging. If so, the ion distributions within an event should exhibit features known to result from merging. Observations of a FTE by instruments on the Polar spacecraft revealed classical merging signatures that included: 1) D-shaped, accelerated, magnetosheath ion distributions, 2) a well defined de Hoffman-Teller frame, 3) local stress balance, and 4) a P-N magnetic field signature. This FTE was observed near the magnetic equator at approx. 13 MLT under conditions of a moderately northward interplanetary magnetic field (IMF) (clock angle of less than 10 deg). The nature of the ion distributions and the consistency of the measured cutoff speed with that calculated from the measured local magnetic field and the derived de Hoffman-Teller speed show the ion injection to be local. Coupled with the northward IMF these results lead to the conclusion that component merging in the low latitude region was responsible for the FTE.

  5. Observations at Low Latitudes of Magnetic Merging Signatures Within a Flux Transfer Event During a Northward IMF

    NASA Technical Reports Server (NTRS)

    Chandler, M. O.; Avanov, L. A.

    2003-01-01

    Flux transfer events (FTE) have been postulated to result from transient magnetic merging. If so, the ion distributions within an event should exhibit features known to result from merging. Observations of a FTE by instruments on the Polar spacecraft revealed classical merging signatures that included: 1) D-shaped, accelerated, magnetosheath ion distributions, 2) a well defined de Hoffman-Teller frame, 3) local stress balance, and 4) a P-N magnetic field signature. This FTE was observed near the magnetic equator at approx. 13 MLT under conditions of a moderately northward interplanetary magnetic field (IMF) (clock angle of less than 10 deg). The nature of the ion distributions and the consistency of the measured cutoff speed with that calculated from the measured local magnetic field and the derived de Hoffman-Teller speed show the ion injection to be local. Coupled with the northward IMF these results lead to the conclusion that component merging in the low latitude region was responsible for the FTE.

  6. Short- and Long-Timescale Thermospheric Variability as Observed from OI 630.0 nm Dayglow Emissions from Low Latitudes

    NASA Technical Reports Server (NTRS)

    Pallamraju, Duggirala; Das, Uma; Chakrabarti, Supriya

    2010-01-01

    We carried out high-cadence (5 min) and high-spatial resolution (2deg magnetic latitude) observations of daytime OI 630.0 nm airglow emission brightness from a low-latitude station to understand the behavior of neutral dynamics in the daytime. The results indicate that the wave periodicities of 12.20 min, and 2 h exist over a wide spatial range of around 8deg-12deg magnetic latitudes. The 20.80 min periodicities in the dayglow seem to appear more often in the measurements closer to the magnetic equator and not at latitudes farther away. Further, periodicities in that range are found to be frequent in the variations of the equatorial electrojet (EEJ) strength as well. We show that wave periodicities due to the neutral dynamics, at least until around 8deg magnetic latitude, are influenced by those that affect the EEJ strength variation as well. Furthermore, the average daily OI 630.0 nm emission brightness over 3 months varied in consonance with that of the sunspot numbers indicating a strong solar influence on the magnitudes of dayglow emissions.

  7. Short- and Long-Timescale Thermospheric Variability as Observed from OI 630.0 nm Dayglow Emissions from Low Latitudes

    NASA Technical Reports Server (NTRS)

    Pallamraju, Duggirala; Das, Uma; Chakrabarti, Supriya

    2011-01-01

    We carried out high-cadence (5 min) and high-spatial resolution (2deg magnetic latitude) observations of daytime OI 630.0 nm airglow emission brightness from a low-latitude station to understand the behavior of neutral dynamics in the daytime. The results indicate that the wave periodicities of 12.20 min, and 2 h exist over a wide spatial range of around 8deg-12deg magnetic latitudes. The 20.80 min periodicities in the dayglow seem to appear more often in the measurements closer to the magnetic equator and not at latitudes farther away. Further, periodicities in that range are found to be frequent in the variations of the equatorial electrojet (EEJ) strength as well. We show that wave periodicities due to the neutral dynamics, at least until around 8deg magnetic latitude, are influenced by those that affect the EEJ strength variation as well. Furthermore, the average daily OI 630.0 nm emission brightness over 3 months varied in consonance with that of the sunspot numbers indicating a strong solar influence on the magnitudes of dayglow emissions.

  8. Observations of the coupling efficiency of VLF lightning-generated whistlers into the low-latitude plasmasphere

    NASA Astrophysics Data System (ADS)

    Jacobson, A. R.; Holzworth, R. H., II; Pfaff, R. F., Jr.; Heelis, R. A.

    2014-12-01

    The C/NOFS satellite [de La Beaujardiere, 2004] has provided a vast archive of multi-sensor data on the low-latitude ionosphere/plasmasphere since 2008. As part of the project, the VEFI payload [Pfaff et al., 2010] has recorded the 3-D electric field from DC through 16 kHz with high fidelity. The relative calibrations track between the three E-field antennas with sufficient accuracy and stability to allow retrieval of the wave polarization for a wide range of lightning-generated whistler waves [Jacobson et al., 2014]. The wave polarization in turn allows retrieval of the wavevector (within a sign ambiguity), which in turn allows an inverse-raytrace of the whistler raypath from the satellite to the ionospheric entry point. We will compare the raytrace predictions with ground-truth from the WWLLN global lightning-monitoring system [Lay et al., 2004; Rodger et al., 2005; Rodger et al., 2004]. In addition to providing location and time of lightning strokes, WWLLN provides an estimate of the radiated radio energy in the whistler passband [Hutchins et al., 2012]. Finally, the CINDI payload [Heelis et al., 2009] on C/NOFS provides ion composition at the satellite, permitting the index of refraction to be inferred. We will compare these estimates to the Poynting fluence density observed by VEFI, thereby providing a direct test of the coupling of lightning radio energy into plasmaspheric whistlers.

  9. Statistical description of low-latitude plasma blobs as observed by DMSP F15 and KOMPSAT-1

    NASA Astrophysics Data System (ADS)

    Park, J.; Min, K. W.; Kim, V. P.; Kil, H.; Kim, H. J.; Lee, J. J.; Lee, E.; Kim, S. J.; Lee, D. Y.; Hairston, M.

    The global distribution of low-latitude plasma blobs was investigated by in-situ plasma density measurements from the Korea Multi-Purpose Satellite-1 (KOMPSAT-1) and Defense Meteorological Satellite Program (DMSP) F15. In the observations, blobs occurred in the longitude sector where the activity of the equatorial plasma bubble (EPB) was appreciable, and additional blobs were found at the lower (KOMPSAT-1) altitude as in the EPBs. However, several notable differences exist between the distributions of EPBs and blobs. First, KOMPSAT-1 found few blobs around 0°E in March and June, as did DMSP F15 from 30°W to 120°E for every season. Second, the overall occurrences in December and March at the DMSP F15 (840 km) altitude were somewhat lower than expected from those of the EBPs. Third, at the DMSP F15 altitude, the occurrence probability of plasma blobs was less controlled by yearly variations in the solar activity. These results imply that topside ionospheric conditions as well as the existence of EPBs control further development of blobs. Additionally, it was found that the blob latitudes became higher as the yearly solar activity increased. Moreover, most of the blobs were encountered in the winter hemisphere, possibly due to the low ambient density.

  10. Ground and satellite observations of the low-latitude onsets of auroral substorm during a major magnetic storm

    NASA Astrophysics Data System (ADS)

    Ievenko, Igor; Parnikov, Stanislav; Alexeyev, Valeriy

    It is known that the first onset of auroral substorm expansion is connected with the brightness increase and breakup of the most equatorial arc. The subsequent substorm activizations can be observed in the intensification of auroral arcs at higher latitudes. As a result a formation of auroral bulge and poleward shift of a westward electrojet maximum takes place. The development of auroral bulge maps the precipitation dynamics of energetic particles during magnetospheric substorms. In this work the research results of auroral substorm during the major magnetic storm on March 20, 2001 (Dst =-150 nT) are submitted. The aurorae were registered at the Yakutsk meridian (130ºE; 200ºE, geom.) by the meridian-scanning and zenith photometers in the 630, 557.7 [OI], 427.8 (N2+) and 486.1 nm (H beta) emissions. Before the substorm onset the equatorial arc is observed at low geomagnetic latitudes of 55-57ºN (the dipole L=3.0-3.3). The zenith photometer registers an intense H beta emission in the arc (~400 R). The fast increase of the 427.8, 557.7 nm emission intensity during two equatorial arc breakups is accompanied by the decrease of H beta intensity by a factor of ~5. The aurora dynamics is compared with the measurements of precipitating flux of electrons and protons aboard DMSP F15 satellite, substorm injections at a geosynchronous orbit, variations in the solar wind and IMF and also images of the auroral oval from the IMAGE satellite. The ground and satellite observations are considered from a position of change of the magnetic field configuration during the low-latitude substorm. We assume that the sharp drop of H beta emission intensity (precipitating protons flux) during the breakups of equatorial arc may testify to a connection of its location with a proton isotropic boundary in the inner magnetosphere.

  11. Study of solar flare induced D-region ionosphere changes using VLF amplitude observations at a low latitude site

    NASA Astrophysics Data System (ADS)

    Tan, L. M.; Thu, N. N.; Ha, T. Q.; Marbouti, M.

    2014-06-01

    About 26 solar flare events from C2.56 to X3.2 classes were obtained and analyzed at Tay Nguyen University, Vietnam (12.56°N, 108.02°E) during May - December 2013 using very low frequency remote sensing to understand the responses of low latitude D-region ionosphere during solar flares. The observed VLF amplitude perturbations are used as the input parameters for the simulated Long Wavelength Propagation Capability (LWPC) program, using Wait's model of lower ionosphere, to calculate two Wait's parameters, viz. the reflection height (H') and the sharpness factor (?). The results reveal that when X-ray irradiance is increased, ? increased from 0.3 to 0.506 km-1, while H' decreased from 74 to 60 km. The electron density increased at the height of 74 km with 1-3 orders of magnitude during solar flares. These phenomena can be explained as: the ionization due to X-ray irradiance becomes greater than that due to cosmic rays and Lyman-α radiation, which increases the electron density profile. The present results are in agreement with the earlier results. The 3D representation of the electron density changes with altitude and time supports to fully understand the shape of the electron density changes due to X-ray flares. The shape variation of electron density is roughly followed to the variation of the amplitude perturbation and keeps this rule for different altitudes. It is also found that the electron density versus the height in lower latitude D-region ionosphere increases more rapidly during solar flares.

  12. Analysis of ionosphere variability over low-latitude GNSS stations during 24th solar maximum period

    NASA Astrophysics Data System (ADS)

    Venkata Ratnam, D.; Sivavaraprasad, G.; Latha Devi, N. S. M. P.

    2017-07-01

    Global Positioning System (GPS) is a remote sensing tool of space weather and ionospheric variations. However, the interplanetary space-dependent drifts in the ionospheric irregularities cause predominant ranging errors in the GPS signals. The dynamic variability of the low-latitude ionosphere is an imperative threat to the satellite-based radio communication and navigation ranging systems. The study of temporal and spatial variations in the ionosphere has triggered new investigations in modelling, nowcasting and forecasting the ionospheric variations. Hence, in this paper, the dynamism in the day-to-day, month-to-month and seasonal variability of the ionospheric Total Electron Content (TEC) has been explored during the solar maximum period, January-December 2013, of the 24th solar cycle. The spatial and temporal variations of the ionosphere are analysed using the TEC values derived from three Indian low-latitude GPS stations, namely, Bengaluru, Guntur and Hyderabad, separated by 13-18° in latitude and 77-81° in longitude. The observed regional GPS-TEC variations are compared with the predicted TEC values of the International Reference Ionosphere (IRI-2012 and 2007) models. Ionospheric parameters such as Vertical TEC (VTEC), relative TEC deviation index and monthly variations in the grand-mean of ionosphere TEC and TEC intensity, along with the upper and lower quartiles, are adopted to investigate the ionosphere TEC variability during quiet and disturbed days. The maximum ionospheric TEC variability is found during March and September equinoxes, followed by December solstice while the minimum variability is observed during June solstice. IRI models are in reasonable agreement with GPS TEC but are overestimating during dawn hours (01:00-06:00 LT) as compared to the dusk hours. Higher percentage deviations are observed during equinoctial months than summer over EIA stations, Guntur and Hyderabad. GPS TEC variations are overestimated during dawn hours for all the

  13. Regional Arctic observations of TEC gradients and scintillations

    NASA Astrophysics Data System (ADS)

    Durgonics, Tibor; Høeg, Per; Benzon, Hans-Henrik

    2015-04-01

    In recent years, there has been growing scientific interest in Arctic ionospheric properties and variations. However our understanding of the fundamental ionospheric processes present in this area is still incomplete. GNSS networks present in Greenland today make it possible to acquire near-real time observations of the state and variations of the high-latitude ionosphere. This data can be employed to obtain relevant geophysical variables and statistics. In our study GPS-derived total electron content (TEC) measurements have been complemented with amplitude scintillation indices (S4), and phase scintillation indices (σφ). The investigation of the relationship between these geophysical variables will likely lead to new ways to study the underlying physical processes and to build tools for monitoring and predicting large-scale patterns in Arctic TEC and scintillations. A number of specific ionosphere events will be presented and the underlying geophysical process will be identified and described. In particular, results will be presented where large-scale gradients in the regional TEC are compared with the growth of scintillations. The statistics of the scintillations will be investigated, with emphasis on how well the scintillations follow the Nakagami-m distribution. The spectra of both the intensities and phase will be calculated, and the corner frequency of these spectra will also be determined. These corner frequencies will be used to compute a number of important geophysical and ionospheric parameters. Furthermore, we will discuss how the spectral characteristics of the scintillations during large TEC gradients vary, and how values of the power spectra slopes change during increasing scintillations. These values will be validated against values found in prior studies. TEC and scintillation time-series and maps will also be presented over the Greenlandic region. We will show how the expansion of the auroral oval during geomagnetic storms can be detected from

  14. Comparison of IRI-2012 with JASON-1 TEC and incoherent scatter radar observations during the 2008-2009 solar minimum period

    NASA Astrophysics Data System (ADS)

    Ji, Eun-Young; Jee, Geonhwa; Lee, Changsup

    2016-08-01

    The 2008-2009 solar minimum period was unprecedentedly deep and extended. We compare the IRI-2012 with global TEC data from JASON-1 satellite and with electron density profiles observed from incoherent scatter radars (ISRs) at middle and high latitudes for this solar minimum period. Global daily mean TECs are calculated from JASON-1 TECs to compare with the corresponding IRI TECs during the 2008-2009 period. It is found that IRI underestimates the global daily mean TEC by about 20-50%. The comparison of global TEC maps further reveals that IRI overall underestimates TEC for the whole globe except for the low-latitude region around the equatorial anomaly, regardless of season. The underestimation is particularly strong in the nighttime winter hemisphere where the ionosphere seems to almost disappear in IRI. In the daytime equatorial region, however, the overestimation of IRI is mainly due to the misrepresentation of the equatorial anomaly in IRI. Further comparison with ISR electron density profiles confirms the significant underestimation of IRI at night in the winter hemisphere.

  15. Electrodynamics of ionospheric weather over low latitudes

    NASA Astrophysics Data System (ADS)

    Abdu, Mangalathayil Ali

    2016-12-01

    The dynamic state of the ionosphere at low latitudes is largely controlled by electric fields originating from dynamo actions by atmospheric waves propagating from below and the solar wind-magnetosphere interaction from above. These electric fields cause structuring of the ionosphere in wide ranging spatial and temporal scales that impact on space-based communication and navigation systems constituting an important segment of our technology-based day-to-day lives. The largest of the ionosphere structures, the equatorial ionization anomaly, with global maximum of plasma densities can cause propagation delays on the GNSS signals. The sunset electrodynamics is responsible for the generation of plasma bubble wide spectrum irregularities that can cause scintillation or even disruptions of satellite communication/navigation signals. Driven basically by upward propagating tides, these electric fields can suffer significant modulations from perturbation winds due to gravity waves, planetary/Kelvin waves, and non-migrating tides, as recent observational and modeling results have demonstrated. The changing state of the plasma distribution arising from these highly variable electric fields constitutes an important component of the ionospheric weather disturbances. Another, often dominating, component arises from solar disturbances when coronal mass ejection (CME) interaction with the earth's magnetosphere results in energy transport to low latitudes in the form of storm time prompt penetration electric fields and thermospheric disturbance winds. As a result, drastic modifications can occur in the form of layer restructuring (Es-, F3 layers etc.), large total electron content (TEC) enhancements, equatorial ionization anomaly (EIA) latitudinal expansion/contraction, anomalous polarization electric fields/vertical drifts, enhanced growth/suppression of plasma structuring, etc. A brief review of our current understanding of the ionospheric weather variations and the

  16. Comparison of IRI-2001 With TOPEX TEC Measurements

    NASA Astrophysics Data System (ADS)

    Jee, G.; Schunk, R. W.; Scherliess, L.

    2003-12-01

    The International Reference Ionosphere (IRI) is an international joint project of the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI). As one of the most comprehensive empirical models of the ionosphere, the IRI provides the electron density, electron temperature, ion temperature, and ion composition in the altitude range from about 50 km to 2000 km, and also the TEC for a given location, time and date, based on the various measurements from the ground and space. During the last decade, the TOPEX/POSEIDON satellite mission has provided a wealth of data pertaining to TEC measurements over the oceans, and these data can be used to further improve the IRI. With this in mind, we compared a 10-year TOPEX TEC dataset with IRI predictions. The study covered solar cycle, seasonal, geomagnetic activity, and longitudinal variations. The resulting comparisons provide information on how to improve the IRI over the ocean areas. For low solar activity, IRI slightly overestimates TEC at low latitudes and underestimates it at middle and high latitudes at all local times. For the high solar activity, however, a large underestimate appears at low latitudes and extends to mid-latitudes, but with a smaller magnitude. At high latitudes, the IRI TEC shows an overestimate in both hemispheres for equinox and in the winter hemisphere for solstice, but it underestimates in the summer hemisphere. For the geomagnetic activity, we observed little effects on both TEC maps. Another important result is that right after sunrise, the IRI TEC at low latitudes starts to increase always earlier than the TOPEX TEC. At low latitudes, the TOPEX TEC shows stronger annual and semiannual variations than the IRI TEC, and at higher mid-latitudes, both TECs show the seasonal anomaly for high solar activity. Finally, both IRI and TOPEX TEC maps show significant longitudinal variations.

  17. Performance evaluation of linear time-series ionospheric Total Electron Content model over low latitude Indian GPS stations

    NASA Astrophysics Data System (ADS)

    Dabbakuti, J. R. K. Kumar; Venkata Ratnam, D.

    2017-10-01

    Precise modeling of the ionospheric Total Electron Content (TEC) is a critical aspect of Positioning, Navigation, and Timing (PNT) services intended for the Global Navigation Satellite Systems (GNSS) applications as well as Earth Observation System (EOS), satellite communication, and space weather forecasting applications. In this paper, linear time series modeling has been carried out on ionospheric TEC at two different locations at Koneru Lakshmaiah University (KLU), Guntur (geographic 16.44° N, 80.62° E; geomagnetic 7.55° N) and Bangalore (geographic 12.97° N, 77.59° E; geomagnetic 4.53° N) at the northern low-latitude region, for the year 2013 in the 24th solar cycle. The impact of the solar and geomagnetic activity on periodic oscillations of TEC has been investigated. Results confirm that the correlation coefficient of the estimated TEC from the linear model TEC and the observed GPS-TEC is around 93%. Solar activity is the key component that influences ionospheric daily averaged TEC while periodic component reveals the seasonal dependency of TEC. Furthermore, it is observed that the influence of geomagnetic activity component on TEC is different at both the latitudes. The accuracy of the model has been assessed by comparing the International Reference Ionosphere (IRI) 2012 model TEC and TEC measurements. Moreover, the absence of winter anomaly is remarkable, as determined by the Root Mean Square Error (RMSE) between the linear model TEC and GPS-TEC. On the contrary, the IRI2012 model TEC evidently failed to predict the absence of winter anomaly in the Equatorial Ionization Anomaly (EIA) crest region. The outcome of this work will be useful for improving the ionospheric now-casting models under various geophysical conditions.

  18. Observations and modeling of UHF-band scintillation occurrence probability over the low-latitude region of China during the maximum activity of solar cycle 24

    NASA Astrophysics Data System (ADS)

    Zhang, H.; Liu, Y.; Wu, J.; Xu, T.; Sheng, D.

    2015-01-01

    The climatological characteristics of UHF-band scintillations over the low-latitude region of China were investigated by analyzing the observations recorded at three stations of our regional network of satellite-beacon-based scintillation monitoring in 2013. The three stations are Hainan (geographic 20.0° N, 110.3° E; geomagnetic 10.1° N, 177.4° W, dip 28.2°), Guangzhou (geographic 23.0° N, 113.0° E; geomagnetic 13.1° N, 174.8° W, dip 33.9°) and Kunming (geographic 25.6° N, 103.7° E; geomagnetic 15.7° N, 176.4° E, dip 39.0°), located at low latitudes of China. The variations of UHF-band scintillation occurrence with latitude, time and season are presented in detail to understand the morphology and climatology of ionospheric scintillations over the low-latitude region of China. An equinoctial asymmetry in the occurrences of scintillation and an obvious difference of the onset time of scintillations between Hainan and Kunming is noted in this data set. Subsequently, the ionosonde data are utilized to study the possible causes of the asymmetry between two equinoxes. The observations suggest that the mean critical frequency (foF2) at 20:00 LT (12:00 UT) in the autumnal equinoctial months (September and October) and the vernal equinoctial months (March and April) has a similar asymmetry. The ratio of the mean foF2 between two equinoxes is proportional to the ratio between the maximum scintillation occurrence in the autumnal equinox and in the vernal equinox. Therefore, this ratio can act as a proxy for the equinoctial asymmetry in the occurrences of scintillation over the low-latitude region of China, and can be used to model the equinoctial asymmetry in our empirical climatological model of scintillation occurrence probability (CMSOP). The CMSOP can provide the predictions of the occurrences of scintillation over the low-latitude region of China and was validated in this study.

  19. Statistical description of low-latitude plasma blobs as observed by DMSP F15 and KOMPSAT-1

    NASA Astrophysics Data System (ADS)

    Park, J.; Min, K. W.; Kim, V. P.; Kil, H.; Kim, H. J.; Lee, J. J.; Lee, E.; Kim, S. J.; Lee, D. Y.; Hairston, M.

    We investigated the global distribution of low-latitude plasma blobs using in-situ plasma density measurements from Korea Multi-Purpose Satellite-1 KOMPSAT-1 and Defense Meteorological Satellite Program DMSP F15 The seasonal-longitudinal S L distribution of blobs is generally consistent with that of equatorial plasma bubbles EPBs but between them exist two notable differences First during equinoxes the blob activity is inhibited around the Atlantic region Second during the June solstice the African peak is rather suppressed in the distribution KOMPSAT-1 at the lower altitude encountered blobs more frequently than DMSP F15 The occurrence probability of plasma blobs is less subjected to the yearly variation of solar activity And the latitudinal distribution of the blobs shows strong asymmetry during solstices Most of them are concentrated on the winter hemisphere where the background density is low and the inter-hemispheric plasma transport is poleward along the geomagnetic field line And the asymmetry becomes weak as the solar activity decreases suggesting that the blob generation bears connection with the fountain effect inside EPBs and the poleward plasma transport

  20. Low Latitude Aurora: Index of Solar Activity

    NASA Astrophysics Data System (ADS)

    Bekli, M. R.; Aissani, D.; Chadou, I.

    2010-10-01

    Observations of aurora borealis at low latitudes are rare, and are clearly associated with high solar activity. In this paper, we analyze some details of the solar activity during the years 1769-1792. Moreover, we describe in detail three low latitude auroras. The first event was reported by ash-Shalati and observed in North Africa (1770 AD). The second and third events were reported by l'Abbé Mann and observed in Europe (1770 and 1777 AD).

  1. Low-latitude ionospheric effects on SBAS

    NASA Astrophysics Data System (ADS)

    Arenas, J.; Sardón, E.; Sainz, A.; Ochoa, B.; Magdaleno, S.

    2016-06-01

    Satellite-based augmentation systems (SBAS) provide augmentation to Global Navigation Satellite Systems (GNSS) users in three areas: (1) broadcasting accurate corrections to GNSS satellite ephemeris, (2) providing a real-time empirical ionospheric model in the service area, and (3) providing integrity information in the form of estimates of the confidence of the ephemeris corrections and ionospheric delays. Ionospheric effects on SBAS are twofold: (a) the input data used by the SBAS will be affected by ionospheric effects, and (b) the more perturbed the ionosphere is, the more difficult it will be to provide accurate and reliable ionospheric information to the users. The ionosphere at low latitudes presents larger variability and more intense phenomena than at midlatitudes. Therefore, SBAS providing service to low-latitude regions will be more affected than those at other latitudes. From the different low-latitude ionospheric effects, this paper will focus on those having the largest impact on SBAS, which are total electron content temporal and spatial gradients, ionospheric scintillations, and depletions. This paper will present the impact of these effects on EGNOS (European Global Navigation Overlay System), the European SBAS. Although EGNOS can be considered as a midlatitude SBAS, it has to provide coverage down to rather low latitudes, so sometimes low-latitude ionospheric effects are observed in the EGNOS data. It will be shown how EGNOS performs under nominal conditions and how its performance is degraded when low-latitude ionospheric phenomena occur. Real EGNOS data affected by low-latitude ionospheric phenomena will be used.

  2. Observation of a unique storm-time VLF chorus emissions at low latitude Indian ground station Jammu (L=1.17)

    NASA Astrophysics Data System (ADS)

    Singh, K. K.

    2017-08-01

    This paper summarizes a unique observation of storm-time VLF chorus emissions obtained during investigation of VLF data collected over a period of about one year at our low latitude ground-based Indian station Jammu (geomag. lat., 22 deg 26 min North, L=1.17) in order to connect the peculiarities of this ground-observed VLF chorus with the plasma processes in the magnetosphere during geomagnetic disturbances. The upper boundary frequency method (UBF-method) based on the measurement of the upper boundary frequency of the ground-observed chorus is used for the determination of the location of the source of storm-time chorus emissions observed at our ground station Jammu. Dynamics of chorus spectra of the emissions observed at Jammu on 18 February 1998 in daytime during magnetic storm period are investigated in detail in order to solve the physics of the ground-observed chorus emissions at low latitudes. To explain various temporal and spectral features of storm-time VLF chorus emissions observed at Jammu, a generation mechanism based on the backward wave oscillator (BWO) regime of the magnetospheric cyclotron maser has been presented. Using BWO model, the calculation of some magnetospheric parameters such as plasma density, large-scale electric field etc. are carried out. The conformity between the calculated and typical measured parameters is shown and discussed. Finally the conclusion of this study is reported.

  3. Persistent Longitudinal Variations of Plasma Density and DC Electric Fields in the Low Latitude Ionosphere Observed with Probes on the C/NOFS Satellite

    NASA Technical Reports Server (NTRS)

    Pfaff, R.; Freudenreich, H.; Klenzing, J.; Rowland, D.; Liebrecht, C.; Bromund, K.; Roddy, P.

    2010-01-01

    Continuous measurements using in situ probes on consecutive orbits of the C/N0FS satellite reveal that the plasma density is persistently organized by longitude, in both day and night conditions and at all locations within the satellite orbit, defined by its perigee and apogee of 401 km and 867 km, respectively, and its inclination of 13 degrees. Typical variations are a factor of 2 or 3 compared to mean values. Furthermore, simultaneous observations of DC electric fields and their associated E x B drifts in the low latitude ionosphere also reveal that their amplitudes are also strongly organized by longitude in a similar fashion. The drift variations with longitude are particularly pronounced in the meridional component perpendicular to the magnetic field although they are also present in the zonal component as well. The longitudes of the peak meridional drift and density values are significantly out of phase with respect to each other. Time constants for the plasma accumulation at higher altitudes with respect to the vertical drift velocity must be taken into account in order to properly interpret the detailed comparisons of the phase relationship of the plasma density and plasma velocity variations. Although for a given period corresponding to that of several days, typically one longitude region dominates the structuring of the plasma density and plasma drift data, there is also evidence for variations organized about multiple longitudes at the same time. Statistical averages will be shown that suggest a tidal "wave 4" structuring is present in both the plasma drift and plasma density data. We interpret the apparent association of the modulation of the E x B drifts with longitude as well as that of the ambient plasma density as a manifestation of tidal forces at work in the low latitude upper atmosphere. The observations demonstrate how the high duty cycle of the C/NOFS observations and its unique orbit expose fundamental processes at work in the low latitude

  4. Plasma drifts and polarization electric fields associated with TID-like disturbances in the low-latitude ionosphere: C/NOFS observations

    NASA Astrophysics Data System (ADS)

    Huang, Chao-Song

    2016-02-01

    Medium-scale traveling ionospheric disturbances are often observed at the magnetically conjugate points in the nighttime midlatitude ionosphere. It has been suggested that gravity waves disturb the ionosphere and induce electric fields in one hemisphere and that the electric fields are amplified by the Perkins instability and transmitted along the geomagnetic field lines to the conjugate ionosphere, creating similar disturbances there. However, direct observations of electric fields associated with traveling ionospheric disturbances (TIDs) are very few. In this study, we present low-latitude TID-like disturbances observed by the Communication/Navigation Outage Forecasting System (C/NOFS) satellite. It is found that ion velocity perturbations are generated in the directions parallel and perpendicular to the geomagnetic field within TIDs. Both the parallel and perpendicular ion velocity perturbations show an in-phase correlation with the ion density perturbations. For nighttime TIDs, the amplitude of both the parallel and meridional ion velocity perturbations increases almost linearly with the amplitude of the ion density perturbations, and the meridional ion drift is proportional to the parallel ion velocity. For daytime TIDs, the parallel ion velocity perturbation increases with the ion density perturbation, but the meridional ion velocity perturbation does not change much. The observations provide evidence that polarization electric field is generated within TIDs at low latitudes and maps along the geomagnetic field lines over a large distance.

  5. Evidence of low-latitude daytime large-scale traveling ionospheric disturbances observed by high-frequency multistatic backscatter sounding system during a geomagnetically quiet period

    NASA Astrophysics Data System (ADS)

    Zhou, Chen; Zhao, Zhengyu; Yang, Guobin; Chen, Gang; Hu, Yaogai; Zhang, Yuannong

    2012-06-01

    Observations from the high-frequency multistatic backscatter sounding radars on a geomagnetically quiet day (minimum Dst = -14 nT) captured the anti-equatorward propagation of daytime large-scale traveling ionospheric disturbance (LSTID) at the low-latitude regions. The observed LSTID was characterized approximately by a meridional propagation speed of 347 ± 78 m/s and azimuthal angle of -4.7 ± 27.6° (counterclockwise from north), with a period of 76 min and a wavelength of 1583 ± 354 km by means of maximum entropy cross-spectral analysis. Vertical phase velocity was also evaluated to be <˜42 m/s through the Doppler measurements. These results provide evidence that the low-latitude ionosphere can undergo large-scale perturbations even under geomagnetically quiet conditions. We suggest that this observed LSTID could be due to the secondary gravity waves from thermospheric body forces created from the dissipation of primary gravity waves from deep tropospheric convection.

  6. Salient features of the dayside low latitude ionospheric response to the main phase step-I of the 17 March 2015 geomagnetic storm

    NASA Astrophysics Data System (ADS)

    Bagiya, Mala S.; Sunil, A. S.; Chakrabarty, D.; Sunda, Surendra

    2017-10-01

    Based on TEC observations by India's GPS Aided GEO Augmented Navigation (GAGAN) GPS network, we report the dayside low latitude ionospheric variations over the Indian region during the moderate main phase step-I of the 17 March 2015 geomagnetic storm. In addition, we assess the efficacy of GPS inferred TEC maps by International GNSS service (IGS) in capturing large scale diurnal features of equatorial ionization anomaly (EIA) over the Indian region during this period. Following the prompt penetration electric field (PPE) at ∼0605 UT, equatorial electrojet (EEJ) enhances by ∼55 nT over 75 ± 3oE longitudes where main phase step-I is coincided with local noon. Initial moderate EIA gradually strengthens with the storm commencement. Although GAGAN TEC exhibits more intense EIA evolution compare to IGS TEC maps, latitudinal extent of EIA are comparable in both. The enhanced EEJ reverses by ∼0918 UT under the effect of overshielding electric field, the later is accompanied by northward turning of interplanetary magnetic field (IMF) Bz. The weakening of well evolved EIA reflects in IGS TEC maps after ∼45 min of the overshielding occurrence. In contrary, GAGAN TEC shows the corresponding feature after ∼0115 h. Resurgence of EIA, following the PPE ∼1115 UT, shows up in GAGAN TEC but IGS TEC maps fails in capturing this feature. The observed low latitude TEC variations and EIA modulations are explained in terms of the varying storm time disturbance electric fields. The anomalies between the GAGAN TEC and IGS TEC maps are discussed in terms of the possible limitations of the IGS TEC maps in capturing storm time EIA variability over the Indian region.

  7. Observations of the Tongue of Ionization with GPS TEC and SuperDARN

    DTIC Science & Technology

    2006-06-01

    RTO-MP-IST-056 7 - 1 UNCLASSIFIED/UNLIMITED UNCLASSIFIED/UNLIMITED Observations of the Tongue of Ionization with GPS TEC and SuperDARN Anthea...the southern hemisphere. In this paper, SuperDARN HF radar observations of the high-latitude convection pattern have been overlaid onto the GPS...TEC polar plots for multiple TOI events. It is observed quantitatively that the GPS TEC observations of the TOI match the SuperDARN estimates of the

  8. A study on the low-latitude daytime E region plasma irregularities using coordinated VHF radar, rocket-borne, and ionosonde observations

    NASA Astrophysics Data System (ADS)

    Patra, A. K.; Venkateswara Rao, N.; Phanikumar, D. V.; Chandra, H.; Das, U.; Sinha, H. S. S.; Pant, T. K.; Sripathi, S.

    2009-11-01

    In this paper we study the off-electrojet low-latitude daytime E region plasma irregularities using first multi-instrument observations in India made during July 2004 by the MST radar from Gadanki (13.5°N, 79.2°E, magnetic latitude 6.4°N), Langmuir probe on board the RH-300 Mk II rocket, and ionosonde from Sriharikota (13.6°N, 80.2°E, magnetic latitude 6.4°N). Radar echoes were confined to altitudes below 105 km and were observed in the form of a descending echoing layer with the descent rate of 1 km/h. Virtual height of the E layer, as observed by ionosonde, shows identical descending behavior. A detailed analysis based on the radar and ionosonde observations shows that the radar echoes are related to the range spread in the ionogram. Rocket observations made on 23 July 2004 revealed weak plasma irregularities with scale sizes more than 100 m and no noticeable irregularity at shorter scales. The spectral slope of the irregularities observed by the rocket probe is found to be -4 for scales in between 1 km and 100 m. During the rocket launch, radar did not detect any echo conforming that the small-scale irregularities were not present. Examination of concurrent observations of neutral wind made by TIMED Doppler interferometry suggests that zonal wind plays a crucial role in forming electron density layers, which become unstable via the gradient drift instability with background electric field or/and zonal neutral wind generating low-latitude E region plasma irregularities.

  9. Observing Traveling Ionospheric Disturbances Caused by Tsunamis Using GPS TEC Measurements

    NASA Technical Reports Server (NTRS)

    Galvan, David A.; Komjathy, Attila; Hickey, Michael; Foster, James; Mannucci, Anthony J.

    2010-01-01

    Ground-based Global Positioning System (GPS) measurements of ionospheric Total Electron Content (TEC) show variations consistent with atmospheric internal gravity waves caused by ocean tsunamis following two recent seismic events: the American Samoa earthquake of September 29, 2009, and the Chile earthquake of February 27, 2010. Fluctuations in TEC correlated in time, space, and wave properties with these tsunamis were observed in TEC estimates processed using JPL's Global Ionospheric Mapping Software. These TEC estimates were band-pass filtered to remove ionospheric TEC variations with wavelengths and periods outside the typical range of internal gravity waves caused by tsunamis. Observable variations in TEC appear correlated with the tsunamis in certain locations, but not in others. Where variations are observed, the typical amplitude tends to be on the order of 1% of the background TEC value. Variations with amplitudes 0.1 - 0.2 TECU are observable with periods and timing affiliated with the tsunami. These observations are compared to estimates of expected tsunami-driven TEC variations produced by Embry Riddle Aeronautical University's Spectral Full Wave Model, an atmosphere-ionosphere coupling model, and found to be in good agreement in some locations, though there are cases when the model predicts an observable tsunami-driven signature and none is observed. These TEC variations are not always seen when a tsunami is present, but in these two events the regions where a strong ocean tsunami was observed did coincide with clear TEC observations, while a lack of clear TEC observations coincided with smaller tsunami amplitudes. There exists the potential to apply these detection techniques to real-time GPS TEC data, providing estimates of tsunami speed and amplitude that may be useful for early warning systems.

  10. Analysis of local ionospheric variability based on SVD and MDS at low-latitude GNSS stations

    NASA Astrophysics Data System (ADS)

    Dabbakuti, J. R. K. Kumar; Devanaboyina, Venkata Ratnam; Kanchumarthi, S. Ramesh

    2016-06-01

    Investigation of ionospheric anomalies during equatorial and low latitude is of major concern for modeling and global navigation satellite system (GNSS) applications. Total electron content (TEC) varies with the ionospheric conditions, which will lead to the errors in the global positioning system (GPS) measurements. It is therefore a method that is necessary to characterize the ionospheric anomalies for satellite-based navigation systems. In this study, characterization of ionospheric variations based on the singular value decomposition (SVD) and classical multidimensional scaling (MDS) methods was studied. The yearly and daily variations are decomposed from the GPS-TEC, international reference ionosphere (IRI) 2007 and IRI 2012 models TEC over the three low-latitude GNSS stations located at Koneru Lakshmaiah University (KLU-Guntur), Hyderabad and Bangalore, respectively. From the results, it is found that there is a strong correlation between GPS-TEC and IRI models. The correlation coefficient for the first three singular values is more than 0.86. From this, it is possible to reconstruct more than 85 % of the variability contained in global GPS-derived VTEC data (for year 2013) by using only the first three modes. The semiannual variation has maximum value during March-April and September-October and has minimum value during June-July. It is observed that the annual variations have maximum value in summer and minimum value in winter, and the amplitudes decrease with increasing latitude. Further, opposite latitudinal asymmetry among annual and semiannual variations for three GNSS stations is noticed. SVD and MDS methods clearly show time-varying characteristics and the absence of the winter anomaly at low-latitude GNSS stations.

  11. TEC variations over Europe during the solar eclipse of March 20, 2015 using GLONASS observations

    NASA Astrophysics Data System (ADS)

    Shagimuratov, Irk; Cherniak, Iurii; Krankowski, Andrzej; Zakharenkova, Irina; Yakimova, Galina; Tepenitzina, Nadezhda

    2016-07-01

    We report the features of the ionospheric TEC variations derived from the GLONASS measurements during the partial solar eclipse of March 20, 2015. Over Europe the maximal phase of the eclipse was observed around 10 UT. The eclipse took place during period when the ionosphere changed from night to day conditions. This eclipse occurred on the recovery phase of the strong geomagnetic storm of March 17, 2015. The effect of the eclipse was detected in diurnal variations of TEC over the individual GNSS stations as a trough-like variation with a gradual decrease and a succeeding increase of TEC at the time of the eclipse. The eclipse effect on the TEC distribution was observed more distinctly along individual satellite passes. Over the Kaliningrad GNSS station (54N, 20E) we registered the maximal TEC depression of about 4-6 TECU along several satellite passes. We should note that analysis of the ionospheric effects of the solar eclipse was complicated by the geomagnetic storm of March 17. The superposition of the storm and the eclipse make it difficult to separate the absolute TEC changes caused by the eclipse. At the same time the strong changes of the spatial structure of the TEC distribution were registered on the TEC maps. To analyze the spatial TEC distribution during the eclipse the TEC maps with high spatial-temporal resolution were produced. We used the GLONSS measurements derived from 150-180 stations of the dense European GNSS network. Dynamics of the ionospheric plasma density was analyzed using the mixture GLONASS-GPS TEC maps produced with 5 min sampling rate. The spatial structure of the ionosphere changed essentially during the eclipse comparing with the control days. The occurred TEC gradients were quite different comparing with previous and subsequent days. The complex pattern in the spatial-temporal TEC distribution highlights the important role of the dynamic processes in the ionosphere during the eclipse.

  12. New radar observations of temporal and spatial dynamics of the midnight temperature maximum at low latitude and midlatitude

    NASA Astrophysics Data System (ADS)

    Hickey, Dustin A.; Martinis, Carlos R.; Erickson, Philip J.; Goncharenko, Larisa P.; Meriwether, John W.; Mesquita, Rafael; Oliver, William L.; Wright, Ashley

    2014-12-01

    Presented here are several cases of midnight temperature maximum (MTM) observations using the Millstone Hill incoherent scatter radar (ISR) and Arecibo ISR. The MTM, a temperature enhancement in the upper atmosphere (at ~300 km altitude), is a poorly understood phenomenon as observations are sparse. An upward propagating terdiurnal tide and coupling between atmospheric regions may play a large part in the generation of the MTM, yet this phenomenon and its implications are not fully understood. Two nights (6 March 1989 and 12 July 1988) show clear cases of the MTM occurring between 30 and 34°N with amplitudes of ~100 K and at ~18°N with amplitudes of ~40 K. The MTMs occurred later at the higher latitude. Experiments in 2013 also show a clear MTM at 34° and 36°N from 250 to 350 km altitude. The ionospheric measurements presented here demonstrate a new application of a well-established technique to study atmospheric parameters and allow us to study the latitudinal extent of the MTM. The results provide evidence of the phenomenon occurring at latitudes and altitudes not previously sampled by radar techniques, showing that the MTM is not just an equatorial process, but one that can easily reach midlatitudes. Simultaneous measurements with a Fabry-Perot interferometer allow us to compare the neutral temperatures with the ion temperature. Overall, these are key observations that point to large-scale effects that can help constrain model outputs at different heights and latitudes.

  13. Low-latitude variability of ice cloud properties and cloud thermodynamic phase observed by the Atmospheric Infrared Sounder (AIRS)

    NASA Astrophysics Data System (ADS)

    Kahn, B. H.; Yue, Q.; Davis, S. M.; Fetzer, E. J.; Schreier, M. M.; Tian, B.; Wong, S.

    2016-12-01

    We will quantify the time and space dependence of ice cloud effective radius (CER), optical thickness (COT), cloud top temperature (CTT), effective cloud fraction (ECF), and cloud thermodynamic phase (ice, liquid, or unknown) with the Version 6 Atmospheric Infrared Sounder (AIRS) satellite observational data set from September 2002 until present. We show that cloud frequency, CTT, COT, and ECF have substantially different responses to ENSO variations. Large-scale changes in ice CER are also observed with a several micron tropics-wide increase during the 2015-2016 El Niño and similar decreases during the La Niña phase. We show that the ice CER variations reflect fundamental changes in the spatial distributions and relative frequencies of different ice cloud types. Lastly, the high spatial and temporal resolution variability of the cloud fields are explored and we show that these data capture a multitude of convectively coupled tropical waves such as Kelvin, westward and eastward intertio-gravity, equatorial Rossby, and mixed Rossby-gravity waves.

  14. Investigation of Ionospheric Response to Geomagnetic Storms over a Low Latitude Station, Ile-Ife, Nigeria

    NASA Astrophysics Data System (ADS)

    Jimoh, Oluwaseyi E.; Yesufu, Thomas K.; Ariyibi, Emmanuel A.

    2016-06-01

    Due to several complexities associated with the equatorial ionosphere, and the significant role which the total electron content (TEC) variability plays in GPS signal transmission, there is the need to monitor irregularities in TEC during storm events. The GPS SCINDA receiver data at Ile-Ife, Nigeria, was analysed with a view to characterizing the ionospheric response to geomagnetic storms on 9 March and 1 October 2012. Presently, positive storm effects, peaks in TEC which were associated with prompt penetration of electric fields and changes in neutral gas composition were observed for the storms. The maximum percentage deviation in TEC of about 120 and 45% were observed for 9 March and 1 October 2012, respectively. An obvious negative percentage TEC deviation subsequent to sudden storm commencement (SSC) was observed and besides a geomagnetic storm does not necessarily suggest a high scintillation intensity (S4) index. The present results show that magnetic storm events at low latitude regions may have an adverse effect on navigation and communication systems.

  15. Low-latitude ionospheric height variation as observed by meridional ionosonde chain: Formation of ionospheric ceiling over the magnetic equator

    NASA Astrophysics Data System (ADS)

    Maruyama, Takashi; Uemoto, Jyunpei; Ishii, Mamoru; Tsugawa, Takuya; Supnithi, Pornchai; Komolmis, Taradol

    2014-12-01

    A multipoint ionosonde observation campaign was conducted along the magnetic meridional plane in Southeast Asia to study ionosphere-thermosphere coupling. One station was near the magnetic equator and two of the other stations were at off-equatorial latitudes (˜10° magnetic latitude). The daytime ionospheric peak height (hmF2) was analyzed for each season during the solar minimum years, 2006-2007 and 2009. The peak height increased for ˜3 h after sunrise at the magnetic equator and off-equatorial latitudes, as expected from the daytime upward E × B drift. The apparent upward drift at the magnetic equator ceased before noon, while the drift at the off-equatorial latitudes continued upward and the layer height exceeded the equatorial height around noon. The noontime limited layer peak height at the magnetic equator, which was termed the ionospheric ceiling, did not depend on the season, while the maximum peak height at the off-equatorial latitudes largely varied with each season. Numerical modeling using the SAMI2 code was conducted and the features of the ionospheric ceiling were reproduced quite well. The dynamical parameters provided by the SAMI2 modeling runs showed that the ionospheric ceiling is formed by the field-aligned plasma diffusion, which is a part of the fountain effect.

  16. DMSP F8 observations of the mid-latitude and low-latitude topside ionosphere near solar minimum

    SciTech Connect

    Greenspan, M.E.; Hughes, W.J. |; Burke, W.J.; Rich, F.J.; Heelis, R.A.

    1994-03-01

    The retarding potential analyzer on the DMSP F8 satellite measured ion density, composition, temperature, and ram flow velocity at 840-km altitude near the dawn and dusk meridians close to solar minimum. Nine days of data were selected for study to represent the summer and winter solstices and the autumnal equinox under quiet, moderately active, and disturbed geomagnetic conditions. The observations revealed extensive regions of light-ion dominance along both the dawn and dusk legs of the DMSP F8 orbit. These regions showed seasonal, longitudinal, and geomagnetic control, with light ions commonly predominating in places where the subsatellite ionosphere was relatively cold. Field-aligned plasma flows also were detected. In the morning, ions flowed toward the equator from both sides. In the evening, DMSP F8 detected flows that either diverged away from the equator or were directed toward the northern hemisphere. The effects of diurnal variations in plasma pressure gradients in the ionosphere and plasmasphere, momentum coupling between neutral winds and ions at the feet of field lines, and E {times} B drifts qualitatively explain most features of these composition and velocity measurements. 23 refs., 5 figs., 2 tabs.

  17. Seasonal variations of nighttime D-region ionosphere in 2013 solar maximum observed from a low-latitude station

    NASA Astrophysics Data System (ADS)

    Tan, Le Minh; Thu, Nguyen Ngoc; Ha, Tran Quoc; Nguyen-Luong, Quang

    2015-10-01

    We present the observation of tweek atmospherics with harmonics m = 1-8 during the solar maximum year, 2013, at Tay Nguyen University, Vietnam (Geog. 12.65° N, 108.02° E). The analysis of 33,690 tweeks on ten international quiet days during 2 months each season, summer (May, August), winter (February, November), and equinox (March, September), shows that tweeks occur about 51 % during summer, 22 % during winter, and 27 % during equinox. The D-region ionosphere is more sharply bounded for harmonics m = 5-6 around an altitude of 85.5 km. The environment of the D-region is more inhomogeneous during winter and equinox seasons. The mean electron density varies from 28.4-225 cm -3, which corresponds to the harmonics m = 1-8 at the mean reflection height of 81.5-87.7 km. The results reveal that the lower reference height in our work as compared to other works is due to the higher level of solar activity. The equivalent electron density profile of the nighttime D-region ionosphere using tweek method during summer, equinox, and winter seasons shows lower values of electron density by 12-58 %, 3-67 %, and 24-76 % than those obtained using the International Reference Ionosphere (IRI-2012) model.

  18. The relationship between high- and low-latitude Pi2 pulsations simultaneously observed by DE-1, AMPTE/CCE, and ground stations

    NASA Astrophysics Data System (ADS)

    Teramoto, M.; Nose, M.; Takahashi, K.; Sutcliffe, P. R.

    2007-12-01

    Pi2 pulsations (period from 40s to 150s) are observed at substorm onset. Cavity mode resonance is the possible scenario of low-latitude Pi2 pulsations. It is an open question whether the resonance boundary, plasmapause, is good reflector or not. We investigated Pi2 pulsations observed simultaneously by the polar orbiting DE-1 satellite (an apogee: about 3.6 Re altitude and a perigee: about 500km altitude), equatorial orbiting AMPTE/CCE satellite (an apogee: about 8.8 Re altitude and a perigee: about 1100km altitude), and ground stations at 1132-1136 UT on November 14, 1986. DE-1 was located at polar region (Geomagnetic latitude=-83.42 degrees). AMPTE/CCE was located at L=4.57 and 23.6 MLT. AMPTE/CCE might be located outside the plasmasphere. They are observed Pi2 pulsations in the compressional component. The AL index began to decrease at 1114 UT. It showed substorm onset. They had high coherence with that observed at Kakioka (KAK) in the H component, which was located at L=1.25 and 3.21 MLT. The phase difference between KAK and DE-1 and between KAK and AMPTE/CCE were 180 and 90 degrees at 14 mHz. These Pi2 pulsations had high coherence with that observed by Hermanus (HER), which was located at L=1.8 and 12.8 MLT. These observational results may support that the plasmapause is imperfect boundary. Pi2 pulsations at low latitude are excited by the plasmaspheric virtual resonance mode, in which the ambient magnetic fields outside plasmasphere oscillated with the cavity mode resonance. And Pi2 pulsations at the polar cap are also excited by PVR mode. In this presentation, we will show these Pi2 pulsations observed by DE-1, AMPTE/CCE, HER and KAK, in addition to other ground stations (Port Aux Francais, Furstenfeldbruck, and Wingst) and conduct statistical study of Pi2 pulsations, which were simultaneously observed by the DE-1 and AMPTE/CCE satellite.

  19. The Study of Ionospheric TEC Horizontal Gradient By Means of GPS Observations

    NASA Astrophysics Data System (ADS)

    Huang, Wengeng

    Dual-frequency GPS receiver can automatically correct time delay error caused by ionospheric total electron content (TEC), but it cannot eliminate signal scintillation error caused by iono-spheric irregularities. Even for differential GPS systems, ionospheric error is one of major sources in GPS positioning, for example, error from ionospheric TEC gradient, which will affect accuracy and degrade performance of GPS application system. In this paper, the magnitude of ionospheric TEC horizontal gradient is studied by means of GPS observations. The vertical TEC, and its horizontal gradient and ROTI index over Fuzhou, Xiamen, Guangzhou, Nanning, and Hainan station, which located in equatorial anomaly region in China, are derived from GPS observation. It is found that, between post-sunset and around midnight, the ionospheric TEC has great fluctuation, and intense radio waves scintillation which is represented by big ROTI index, takes place during this periods. Furthermore, the steep TEC horizontal gradient and its fast variation with respect to time appear. By comparison, the ROTI index, ionospheric TEC horizontal gradient and vertical TEC correlate very well with each other, and their variations characteristics result from density irregularities produced by plasma instability in ionosphere. It is suggested that when scintillation events are investigated ionospheric TEC gradient is also one of considerable parameters.

  20. Recent Plasma Observations Related to Magnetic Merging and the Low-Latitude Boundary Layer. Case Study by Polar, March 18, 2006

    NASA Technical Reports Server (NTRS)

    Chandler, M.; Avanov, L.; Craven, P.; Mozer, F.; Moore, T. E.

    2007-01-01

    We have begun an investigation of the nature of the low-latitude boundary layer in the mid-altitude cusp region using data from the Polar spacecraft. Magnetosheath-like plasma is frequently observed deep (in terms of distance from the magnetopause and in invariant latitude) in the magnetosphere. One such case, taken during a long period of northward interplanetary magnetic field (IMP) on March 18, 2006, shows injected magnetosheath ions within the magnetosphere with velocity distributions resulting from two separate merging sites along the same field lines. Cold ionospheric ions were also observed counterstreaming along the field lines, evidence that these field lines were closed. Our results support the idea of double reconnection under northward IMP on the same group of field lines can provide a source for the LLBL. However, the flow direction of the accelerated magnetosheath ions antiparallel to the local magnetic field and given location of the spacecraft suggest that these two injection sites are located northward of the spacecraft position. Observed convection velocities of the magnetic field lines are inconsistent with those expected for double post-cusp reconnection in both hemispheres. These observations favor a scenario in which a group of newly closed field lines was created by a combination of high shear merging at high latitudes in the northern hemisphere and low shear merging at lower latitudes at the dayside magnetopause.

  1. Case study of simultaneous observations of sporadic sodium layer, E-region field-aligned irregularities and sporadic E layer at low latitude of China

    NASA Astrophysics Data System (ADS)

    Xie, H. Y.; Ning, B. Q.; Zhao, X. K.; Hu, L. H.

    2017-03-01

    Using the Na lidar at Haikou (20.0°N, 110.3°E), the VHF coherent radar and the digital ionosonde both at Sanya (18.4°N, 109.6°E), cases of simultaneous observations of sporadic sodium layer (SSL), E-region field-aligned irregularities (FAI) and sporadic E layer (Es) in the mesosphere and lower thermosphere (MLT) region at low latitude of China are studied. It is found that SSL occurs simultaneously or follows the enhancement of Es and FAI. The Es, FAI and SSL descend slowly with time which is mostly controlled by the diurnal tide (DT). Besides, the interaction of gravity wave (GW) with tides can cause oscillations in FAI and SSL. Our observations support the neutralization of ions for SSL formation: when the metallic ions layer descents to the altitudes where models predict, the sodium ions convert rapidly to atomic Na that may form an SSL event. Moreover, the SSL peak density will increase (decrease) in the convergence (divergence) vertical shear region of zonal wind.

  2. In situ Electric Field Observations of Schumann Resonances in the Low Latitude Ionosphere and Their Implications for Tropospheric-Ionospheric Electromagnetic Coupling Mechanisms

    NASA Astrophysics Data System (ADS)

    Simoes, F.; Pfaff, R. F.; Freudenreich, H.; Bromund, K. R.; Martin, S. C.

    2010-12-01

    The Communications/Navigation Outage Forecasting System (C/NOFS) satellite investigates the electrodynamics of the low latitude ionosphere using a low inclination (13 degree) orbit with perigee and apogee of 401 and 867 km, respectively. The satellite is equipped with a three-axis double probe electric field detector, which provides continuous DC and AC electric field measurements. Among the most intriguing data acquired thus far, the electric field probe on C/NOFS unexpectedly detected Schumann resonances in the nightside ionosphere on a number of orbits within the satellite altitude sampling region. The Schumann resonance data are generally very low amplitude (~ 1 µV/m) signatures with a distinct spectral structure that corresponds precisely to the frequency modes predicted by the Schumman resonance waveguide cavity response; up to 10 ten peaks have been observed. These observations suggest major implications for Extremely Low Frequency (ELF) electromagnetic wave propagation in the earth’s ionosphere, namely the global electric circuit, lightning detection, and cavity leakage mechanisms. These findings provide a fresh approach to the study of tropospheric-space weather coupling mechanisms and transient luminous events, imply the need for a significant revision of the “standard” ELF wave propagation model, and offer a new, remote sensing technique for the investigation of planetary atmospheric electricity.

  3. X-rays and solar proton event induced changes in the first mode Schumann resonance frequency observed at a low latitude station Agra, India

    NASA Astrophysics Data System (ADS)

    Singh, Birbal; Tyagi, Rajesh; Hobara, Yasuhide; Hayakawa, Masashi

    2014-06-01

    Effects of two events of X-ray bursts followed by solar proton events (SPEs) occurred on 22 September, 2011 and 06 July, 2012 on the variation of first mode Schumann resonance (SR) frequency monitored at a low latitude station, Agra (Geograph. lat. 27.2°N, long. 78°E) India are examined. The variation of average first mode SR frequency shows a sudden increase in coincidence with the X-ray bursts and a decrease associated with the peak flux of SPE. The increases in the frequency in the two cases are 8.4% and 10.9% and corresponding decreases are 4.3% and 3.3% respectively. The increases in the frequency are interpreted in terms of growth of ionization in the upper part of D-region ionosphere due to X-ray bursts and decreases during SPE are caused by the high ionization in the lower D-region (altitude about 50-60 km) in the polar region. The variation of SR frequency is observed to be consistent with other observatories at middle and high latitudes. The effects of X-ray flares on the D-region of the ionosphere at low and equatorial latitudes are also examined by analyzing the amplitude data of VLF transmitter signal (NWC, f=19.8 kHz) monitored at Agra. The flare effect observed prior to sun-set hours shows increase of electron density above 60 km in the ionosphere.

  4. Accounting for observation uncertainties in an evaluation metric of low latitude turbulent air-sea fluxes: application to the comparison of a suite of IPSL model versions

    NASA Astrophysics Data System (ADS)

    Servonnat, Jérôme; Găinuşă-Bogdan, Alina; Braconnot, Pascale

    2017-09-01

    Turbulent momentum and heat (sensible heat and latent heat) fluxes at the air-sea interface are key components of the whole energetic of the Earth's climate. The evaluation of these fluxes in the climate models is still difficult because of the large uncertainties associated with the reference products. In this paper we present an objective metric accounting for reference uncertainties to evaluate the annual cycle of the low latitude turbulent fluxes of a suite of IPSL climate models. This metric consists in a Hotelling T 2 test between the simulated and observed field in a reduce space characterized by the dominant modes of variability that are common to both the model and the reference, taking into account the observational uncertainty. The test is thus more severe when uncertainties are small as it is the case for sea surface temperature (SST). The results of the test show that for almost all variables and all model versions the model-reference differences are not zero. It is not possible to distinguish between model versions for sensible heat and meridional wind stress, certainly due to the large observational uncertainties. All model versions share similar biases for the different variables. There is no improvement between the reference versions of the IPSL model used for CMIP3 and CMIP5. The test also reveals that the higher horizontal resolution fails to improve the representation of the turbulent surface fluxes compared to the other versions. The representation of the fluxes is further degraded in a version with improved atmospheric physics with an amplification of some of the biases in the Indian Ocean and in the intertropical convergence zone. The ranking of the model versions for the turbulent fluxes is not correlated with the ranking found for SST. This highlights that despite the fact that SST gradients are important for the large-scale atmospheric circulation patterns, other factors such as wind speed, and air-sea temperature contrast play an

  5. Accounting for observation uncertainties in an evaluation metric of low latitude turbulent air-sea fluxes: application to the comparison of a suite of IPSL model versions

    NASA Astrophysics Data System (ADS)

    Servonnat, Jérôme; Găinuşă-Bogdan, Alina; Braconnot, Pascale

    2016-11-01

    Turbulent momentum and heat (sensible heat and latent heat) fluxes at the air-sea interface are key components of the whole energetic of the Earth's climate. The evaluation of these fluxes in the climate models is still difficult because of the large uncertainties associated with the reference products. In this paper we present an objective metric accounting for reference uncertainties to evaluate the annual cycle of the low latitude turbulent fluxes of a suite of IPSL climate models. This metric consists in a Hotelling T 2 test between the simulated and observed field in a reduce space characterized by the dominant modes of variability that are common to both the model and the reference, taking into account the observational uncertainty. The test is thus more severe when uncertainties are small as it is the case for sea surface temperature (SST). The results of the test show that for almost all variables and all model versions the model-reference differences are not zero. It is not possible to distinguish between model versions for sensible heat and meridional wind stress, certainly due to the large observational uncertainties. All model versions share similar biases for the different variables. There is no improvement between the reference versions of the IPSL model used for CMIP3 and CMIP5. The test also reveals that the higher horizontal resolution fails to improve the representation of the turbulent surface fluxes compared to the other versions. The representation of the fluxes is further degraded in a version with improved atmospheric physics with an amplification of some of the biases in the Indian Ocean and in the intertropical convergence zone. The ranking of the model versions for the turbulent fluxes is not correlated with the ranking found for SST. This highlights that despite the fact that SST gradients are important for the large-scale atmospheric circulation patterns, other factors such as wind speed, and air-sea temperature contrast play an

  6. Low-latitude ionosphere response to super geomagnetic storm of 17/18 March 2015: Results from a chain of ground-based observations over Indian sector

    NASA Astrophysics Data System (ADS)

    Ramsingh; Sripathi, S.; Sreekumar, Sreeba; Banola, S.; Emperumal, K.; Tiwari, P.; Kumar, Burudu Suneel

    2015-12-01

    In this paper, we present unique results of equatorial and low-latitude ionosphere response to one of the major geomagnetic storms of the current solar cycle that occurred during 17-18 March 2015, where Dst reached its minimum of -228 nT. Here we utilized data from magnetometers, chain of ionosondes located at Tirunelveli (8.73°N, 77.70°E; geometry: 0.32°N), Hyderabad (17.36°N, 78.47°E; geometry 8.76°N), and Allahabad (25.45°N, 81.85°E; geometry 16.5°N) along with multistation GPS receivers over Indian sector. The observations showed a remarkable increase of h'F to as high as ~560 km over Tirunelveli (magnetic equator) with vertical drift of ~70 m/s at 13:30 UT due to direct penetration of storm time eastward electric fields which exactly coincided with the local time of pre-reversal enhancement (PRE) and caused intense equatorial spread F irregularities in ionosondes and scintillations in GPS receivers at wide latitudes. Plasma irregularities are so intense that their signatures are seen in Allahabad/Lucknow. Storm time thermospheric meridional winds as estimated using two ionosondes suggest the equatorward surge of gravity waves with period of ~2 h. Suppression of anomaly crest on the subsequent day of the storm suggests the complex role of disturbance dynamo electric fields and disturbance wind effects. Our results also show an interesting feature of traveling ionospheric disturbances possibly associated with disturbance meridional wind surge during recovery phase. In addition, noteworthy observations are nighttime westward zonal drifts and PRE-related total electron content enhancements at anomaly crests during main phase and counter electrojet signatures during recovery phase.

  7. Middle- and low-latitude ionosphere response to 2015 St. Patrick's Day geomagnetic storm

    NASA Astrophysics Data System (ADS)

    Nava, B.; Rodríguez-Zuluaga, J.; Alazo-Cuartas, K.; Kashcheyev, A.; Migoya-Orué, Y.; Radicella, S. M.; Amory-Mazaudier, C.; Fleury, R.

    2016-04-01

    This paper presents a study of the St Patrick's Day storm of 2015, with its ionospheric response at middle and low latitudes. The effects of the storm in each longitudinal sector (Asian, African, American, and Pacific) are characterized using global and regional electron content. At the beginning of the storm, one or two ionospheric positive storm effects are observed depending on the longitudinal zones. After the main phase of the storm, a strong decrease in ionization is observed at all longitudes, lasting several days. The American region exhibits the most remarkable increase in vertical total electron content (vTEC), while in the Asian sector, the largest decrease in vTEC is observed. At low latitudes, using spectral analysis, we were able to separate the effects of the prompt penetration of the magnetospheric convection electric field (PPEF) and of the disturbance dynamo electric field (DDEF) on the basis of ground magnetic data. Concerning the PPEF, Earth's magnetic field oscillations occur simultaneously in the Asian, African, and American sectors, during southward magnetization of the Bz component of the interplanetary magnetic field. Concerning the DDEF, diurnal magnetic oscillations in the horizontal component H of the Earth's magnetic field exhibit a behavior that is opposed to the regular one. These diurnal oscillations are recognized to last several days in all longitudinal sectors. The observational data obtained by all sensors used in the present paper can be interpreted on the basis of existing theoretical models.

  8. Equatorial and low-latitude ionospheric response due to 2009 sudden stratospheric warming, South American sector.

    NASA Astrophysics Data System (ADS)

    Fagundes, Paulo Roberto; Gende, Mauricio; De Jesus, Rodolfo; Goncharenko, Larisa; Coster, Anthea; Kavutarapu, Venkatesh; De Abreu, Alessandro; Pillat, ValdirGil; Pezzopane, Michael

    The equatorial and low-latitude ionosphere/thermosphere system is permanently disturbed by waves (MSTIDs, tides, and planetary waves), which are generated in the lower atmosphere or in situ, as well as electric fields and TIDs produced by geomagnetic storm and UV, EUV, and X-ray solar radiation. Until recently it was thought, that during geomagnetic quiet conditions the equatorial and low-latitude F-layer was mainly perturbed by waves that were generated not far away from the observed location or electric fields generated by electroject. On the contrary during geomagnetic storms when the energy sources are in high latitudes the waves (TIDs) travel a very long distance from high latitude to equatorial region and electric fields can be mapped via magnetic field lines. However, recently an unexpected coupling between high latitude, -mid latitude, and -equatorial/low-latitude was discovered during sudden stratospheric warming (SSW). The exploration of all aspects involved in this process must be investigated in order to improve our knowledge about the Earth's atmosphere. This investigation, studies the consequences of the vertical coupling from lower to upper atmosphere during a major Northern Hemisphere sudden stratospheric warming, which took place in January 2009, on the equatorial and low-latitude ionosphere in the Southern Hemisphere. Using 16 ground-based GPS stations over the Brazilian sector, spanning from latitude 2.8N to 30.1S and longitude 62.0W to 37.7W, it was possible to notice that the ionosphere was disturbed by SSW from the Equator to low latitude. The TEC at all 16 stations was severely disturbed during several days after the SSW temperature peak.

  9. Accounting for observational uncertainties in the evaluation of low latitude turbulent air-sea fluxes simulated in a suite of IPSL model versions

    NASA Astrophysics Data System (ADS)

    Servonnat, Jerome; Braconnot, Pascale; Gainusa-Bogdan, Alina

    2015-04-01

    Turbulent momentum and heat (sensible and latent) fluxes at the air-sea interface are key components of the whole energetic of the Earth's climate and their good representation in climate models is of prime importance. In this work, we use the methodology developed by Braconnot & Frankignoul (1993) to perform a Hotelling T2 test on spatio-temporal fields (annual cycles). This statistic provides a quantitative measure accounting for an estimate of the observational uncertainty for the evaluation of low-latitude turbulent air-sea fluxes in a suite of IPSL model versions. The spread within the observational ensemble of turbulent flux data products assembled by Gainusa-Bogdan et al (submitted) is used as an estimate of the observational uncertainty for the different turbulent fluxes. The methodology holds on a selection of a small number of dominating variability patterns (EOFs) that are common to both the model and the observations for the comparison. Consequently it focuses on the large-scale variability patterns and avoids the possibly noisy smaller scales. The results show that different versions of the IPSL couple model share common large scale model biases, but also that there the skill on sea surface temperature is not necessarily directly related to the skill in the representation of the different turbulent fluxes. Despite the large error bars on the observations the test clearly distinguish the different merits of the different model version. The analyses of the common EOF patterns and related time series provide guidance on the major differences with the observations. This work is a first attempt to use such statistic on the evaluation of the spatio-temporal variability of the turbulent fluxes, accounting for an observational uncertainty, and represents an efficient tool for systematic evaluation of simulated air-seafluxes, considering both the fluxes and the related atmospheric variables. References Braconnot, P., and C. Frankignoul (1993), Testing Model

  10. TEC, Trigger and Check, preparing LOFAR for Lunar observations

    NASA Astrophysics Data System (ADS)

    ter Veen, Sander; Mevius, Maaijke; Bonardi, Antonio; Buitink, Stijn; Corstanje, Arthur; Enriquez, J. Emilio; Falcke, Heino; Hörandel, Jörg R.; Mitra, Pragati; Mulrey, Katey; Nelles, Anna; Rachen, Jörg Paul; Rossetto, Laura; Schellart, Pim; Scholten, Olaf; Thoudam, Satyendra; Trinh, Gia; Winchen, Tobias

    2017-03-01

    One of the main ways to use radio to detect Ultra High Energy Neutrinos and Cosmic Rays is the Lunar Askaryan technique, that uses the Moon as a target and searches for nanosecond pulses with large radio telescopes. To use low frequency aperture arrays, such as LOFAR and the SKA, pose new challenges and possibilities in detection techniques of short radio pulses and to measure the Total Electron Content (TEC). As a prepatory work, we have used other measurements that use similar techniques, or that can answer a specific question, with the LOFAR radio telescope. This contribution reports on our work on triggering on short radio signals, post-event imaging of radio signals from buffered data and methods to determine the TEC-value.

  11. Statistical comparison of TEC derived from GPS and ISR observations at high latitudes

    NASA Astrophysics Data System (ADS)

    Makarevich, Roman A.; Nicolls, Michael J.

    2013-07-01

    A comprehensive data set collected with the Poker Flat Incoherent Scatter Radar (PFISR) and GPS receiver in Fairbanks, Alaska (magnetic latitude = 65.4°N) in 2007-2010 is employed to analyze and compare the total electron content (TEC) estimates derived from two radio techniques at high latitudes. The average TEC trends are shown to be largely similar and consistent with expectations based on solar conditions. The TEC residuals expressed as the difference and ratio between the PFISR- and GPS-derived TEC are evaluated to be below 2 total electron content units (TECU = 1016 electronsm-2) and 0.7-0.8, respectively, with some dependence on solar conditions. The agreement between TEC estimates is examined by limiting the difference between the GPS satellite and PFISR beam elevations to 2.5° and postintegrating GPS measurements over the period of each PFISR measurement. Factors controlling the agreement are investigated, including possible roles of GPS satellite bias, GPS elevation angle, and topside contribution to TEC. It is demonstrated that the best agreement, expressed as a linear correlation and a fraction of points consistent with the linear trend, is achieved with satellites at the largest elevation angles and smallest distances from PFISR, which are a possible effect of small spatial differences and unremoved differential biases. Estimates of the topside contribution to TEC range between 14% and 30% and are most consistent during daytime hours, while observations near the solar terminator and during the night suffer from large uncertainties.

  12. Statistical Characterization of Storm Enhanced Density based on GPS TEC Observations

    NASA Astrophysics Data System (ADS)

    Coster, A. J.; Colerico, M.; Rideout, W.; Taylor, B.; Foster, J.; Rich, F.

    2005-12-01

    Storm enhanced density (SED) involves the redistribution of plasmas in the ionosphere and magnetosphere driven by disturbance electric fields. Associated with SED events are large scale gradients in the total electron density (TEC) over relatively short distances. These TEC gradients can have direct impact on navigation and communication users. For example, marine users have horizontal positioning requirements of 2-5 meters at a 95 percent confidence level for safety of navigation in inland waterways. TEC gradients observed during 2003 SED events resulted in positioning errors larger than 20 meters on differential GPS baselines as short as 200 km. Large TEC gradients associated with SED events (greater than 100 TEC units per degree) have been observed near many large airports in the Northeast and Northwest continental US. Better SED characterization has been needed to improve current ionospheric models and to further our understanding of this phenomenon. The Madrigal database at MIT Haystack Observatory now contains TEC data with an unprecedented combination of global spatial coverage and high temporal resolution. Data from more than 2000 receivers are being incorporated into the daily 2005 TEC maps. This database has allowed for long term statistical studies of the presence of SED. In this paper, we present statistics of SED plumes observed during multiple storms during the 2000-2005 time period. The location, size of gradients, and time evolution of multiple SED events has been statistically characterized using an automated gradient analysis tool. Examples of magnetically conjugate SED plumes over northern Europe and the American longitude sectors will be discussed. Inter-hemispheric comparisons of the TEC magnitude of the observed SED events at the base, as well as within the plume, will be presented. The results also include observations of magnetically conjugate sub-auroral polarization streams (SAPS) which accompany the SED events using DMSP ion drift

  13. Large-scale traveling ionospheric disturbances observed by GPS dTEC maps over North and South America on Saint Patrick's Day storm in 2015

    NASA Astrophysics Data System (ADS)

    Figueiredo, C. A. O. B.; Wrasse, C. M.; Takahashi, H.; Otsuka, Y.; Shiokawa, K.; Barros, D.

    2017-04-01

    Large-scale traveling ionospheric disturbances (LSTIDs) were detected in both Northern and Southern Hemispheres over American sector during the geomagnetic storm on 17-18 March 2015, also known as the Saint Patrick's Day storm. Detrended total electronic content (dTEC) maps were made using dense GNSS network receiver data. The retrieved LSTIDs showed wavelengths of 1000 to 2000 km, phase velocity of 300-1000 m/s, and period of 30-50 min. Among them, three couples of LSTIDs were observed propagating from the polar regions to low latitudes. Two wave events observed in daytime showed the propagation direction of southwest in the Northern Hemisphere and northeast in the Southern Hemisphere, which means an asymmetric propagation against the geographic equator. The other wave event observed during the evening hour showed symmetric propagation direction, i.e., southwest in the Northern Hemisphere and northwest in the Southern Hemisphere, whereas their wavelength and phase velocity are significantly different between NH and SH. These observations indicate that the two groups of LSTID have different propagation conditions from polar to low-latitude regions. The observed asymmetric/symmetric propagation forms suggest asymmetric/symmetric auroral current activity between the northern and southern polar regions.

  14. Comparison of the observed topside ionospheric and plasmaspheric electron content derived from the COSMIC podTEC measurements with the IRI_Plas model results

    NASA Astrophysics Data System (ADS)

    Zhang, Man-Lian; Liu, Libo; Wan, Weixing; Ning, Baiqi

    2017-07-01

    In this paper, variations of the topside ionospheric and plasmaspheric electron contents (TPEC) in the altitude range of ∼800 to 20,200 km are compared with the IRI_Plas model results for the low (2008) and high (2012) solar activity years using TEC data (podTEC) derived from the upward-looking precise orbit determination antenna on board COSMIC low Earth orbit (LEO) satellites tracking the GPS signals. For each year, the dataset were divided into groups according to four seasons: M-Equinox (March, April), J-Solstice (May June, July and August), S-Equinox (September, October) and D-Solstice (January, February, November, and December). Our study showed that the IRI_Plas model is able to reproduce reasonably well the main features of the observational TPEC's latitudinal, diurnal as well as seasonal variation tendency when no longitudinal difference is taken into account. However, there exist discrepancies between the observational TPEC and the model results. Except for the daytime hours in the Equinoctial seasons of the high solar activity year 2012 when the IRI_Plas model results showed an overestimation, in general, the IRI_Plas model results underestimate the observational ones, in particular at nighttime hours in the low-latitude region. When the longitudinal difference is taken into account, the comparison study showed that the longitudinal dependence effect shown in the observational TPEC's seasonal variations was not captured by the IRI_Plas model result. Moreover, the IRI_Plas model results tend to show a double-peak structure in the low-latitude region, a feature not appearing in the observational results.

  15. Equatorial and Low-Latitudes Ionospheric Reaction to Solar Flares

    NASA Astrophysics Data System (ADS)

    Nicoli Candido, C. M.; Becker-Guedes, F.; Paula, E. R.; Takahashi, H.

    2015-12-01

    Solar X-ray and extreme ultraviolet (EUV) photons are responsible for ionizing the terrestrial atmosphere and create the ionosphere. During solar flares, a fast increase in the electron density at different altitude regions takes place due to the abrupt enhance of the X-ray and EUV fluxes reaching Earth. With these changes in the ionosphere, radio communication and navigation can be drastically affected. The magnitudes of these Space Weather events can be related to the X-ray peak brightness and duration, which drive the intensity of the ionosphere response when the associated electromagnetic wave hit the sunlit side of the Earth's atmosphere. Other aspects defining these changes in a particular region are the local time, the solar zenith angle, and the position of the flare in the solar disc for each event. In order to improve the understand of radio signal degradation and loss in the Brazilian sector due to solar abrupt electromagnetic emissions, total electron content (TEC) data obtained by a GPS network formed by tents of dual-frequency receivers spread all over Brazilian territory were analyzed. It was observed different ionospheric local changes during several X-ray events identified by GOES satellite regarding the 0.1-0.8 nm range, and some case studies were ponder for a more detailed analysis of these effects. Considering the results, we have made an estimation of the ionospheric disturbances range for a particular event with great chance to affect space based communications in the equatorial and low-latitude regions.

  16. TEC Variations Over Korean Peninsula During Magnetic Storm

    NASA Astrophysics Data System (ADS)

    Ji, E.-Y.; Choi, B.-K.; Kim, K.-H.; Lee, D.-H.; Cho, J.-H.; Chung, J.-K.; Park, J.-U.

    2008-03-01

    By analyzing the observations from a number of ground- and space-based instruments, including ionosonde, magnetometers, and ACE interplanetary data, we examine the response of the ionospheric TEC over Korea during 2003 magnetic storms. We found that the variation of vertical TEC is correlated with the southward turning of the interplanetary magnetic field B_z. It is suggested that the electric fields produced by the dynamo process in the high-latitude region and the prompt penetration in the low-latitude region are responsible for TEC increases. During the June 16 event, dayside TEC values increase more than 15%. And the ionospheric F2-layer peak height (hmF2) was ˜300km higher and the vertical E×B drift (estimated from ground-based magnetometer equatorial electrojet delta H) showed downward drift, which may be due to the ionospheric disturbance dynamo electric field produced by the large amount of energy dissipation into high-latitude regions. In contr! ast, during November 20 event, the nightside TEC increases may be due to the prompt penetration westward electric field. The ionospheric F2-layer peak height was below 200km and the vertical E×B drift showed downward drift. Also, a strong correlation is observed between enhanced vertical TEC and enhanced interplanetary electric field. It is shown that, even though TEC increases are caused by the different processes, the electric field disturbances in the ionosphere play an important role in the variation of TEC over Korea.

  17. Signature of ionospheric irregularities under different geophysical conditions on SBAS performance in the western African low-latitude region

    NASA Astrophysics Data System (ADS)

    Abe, Oladipo Emmanuel; Villamide, Xurxo Otero; Paparini, Claudia; Ngaya, Rodrigue Herbert; Radicella, Sandro M.; Nava, Bruno

    2017-01-01

    Rate of change of TEC (ROT) and its index (ROTI) are considered a good proxy to characterize the occurrence of ionospheric plasma irregularities like those observed after sunset at low latitudes. SBASs (satellite-based augmentation systems) are civil aviation systems that provide wide-area or regional improvement to single-frequency satellite navigation using GNSS (Global Navigation Satellite System) constellations. Plasma irregularities in the path of the GNSS signal after sunset cause severe phase fluctuations and loss of locks of the signals in GNSS receiver at low-latitude regions. ROTI is used in this paper to characterize plasma density ionospheric irregularities in central-western Africa under nominal and disturbed conditions and identified some days of irregularity inhibition. A specific low-latitude algorithm is used to emulate potential possible SBAS message using real GNSS data in the western African low-latitude region. The performance of a possible SBAS operation in the region under different ionospheric conditions is analysed. These conditions include effects of geomagnetic disturbed periods when SBAS performance appears to be enhanced due to ionospheric irregularity inhibition. The results of this paper could contribute to a feasibility assessment of a European Geostationary Navigation Overlay System-based SBAS in the sub-Saharan African region.

  18. Widespread Low-Latitude Diurnal CO2 Frost on Mars

    NASA Astrophysics Data System (ADS)

    Piqueux, S.; Kleinböhl, A.; Hayne, P. O.; Heavens, N. G.; Kass, D. M.; McCleese, D. J.; Schofield, J. T.; Shirley, J. H.

    2016-09-01

    We map and characterize MCS nighttime surface temperature observations consistent with the occurrence of CO2 frost on Mars. Low-latitude nighttime CO2 frost is widespread, with potential implications for the physical nature of the surface layer.

  19. Equatorial and Low-Latitude Ionospheric Response to the Extreme Space Weather Event of March 2015, in the Brazilian Sector.

    NASA Astrophysics Data System (ADS)

    Fagundes, P. R.; Cardoso, F. A.; Fejer, B. G.; Kavutarapu, V.; Ribeiro, B. A.; Pillat, V. G.

    2015-12-01

    Fagundes PR, Cardoso FA and Venkatesh KPhysics and Astronomy Laboratory, Universidade do Vale do Paraiba (UNIVAP), Sao Jose dos Campos, Sao Paulo, Brazil In the present investigation we discuss the results on the response of the ionosphere (F-region) in the Brazilian sector, during extreme space weather event of March 2015. This geomagnetic storm has been considered as one of strongest storms in the solar cycle 24 where, the Dst index reached a minimum of -227 nT at 23:00 UT (17/03/2015) with KP reaching to 8-, and the monthly mean F10.7 solar flux was 125 sfu. This space weather event was studied using a large network of 110 GPS stations. It has been noticed that the Total Electron Content (TEC) was severely disturbed during the geomagnetic storm main and recovery phases. A wavelike oscillation with three peaks is observed from equator to low latitudes during the storm main phase on 17th and 18th March, 2015. Using a latitudinal chain of 8 GPS stations from equatorial region to low latitudes the storm time behavior of the Equatorial Ionization Anomaly (EIA) is investigated. It was noticed that the wavelike oscillation peak latitudinal extent decreases from the beginning of main phase to the recovery phase. The first maximum extends beyond from 2oS to 20oS, the second one from 8oS to 18oS and the third one from 13oS to 17oS. In addition, a strong negative phase in TEC variations is observed during the recovery phase on March 18, 2015. This negative phase is found to be stronger at low-latitude compared to the equatorial region. An anomalous behavior of EIA caused by the wavelike oscillations is observed during the main phase on March 17, 2015. Also, due to the strong negative phase in TEC resulted in strong EIA suppression on March 18, 2015.

  20. Dynamical perturbations of the thermosphere at Southern midlatitudes inferred from satellite observations of O(1D) nightglow and TEC

    NASA Astrophysics Data System (ADS)

    Shepherd, Marianna; Shepherd, Gordon; Cho, Young-Min

    2017-04-01

    The Midnight Temperature Maximum (MTM) is a large scale neutral temperature anomaly with a wide-ranging effect on the nighttime thermospheric dynamics at low latitudes. The focus of the current study is an investigation of the extent of the MTM to southern midlatitudes (20°S - 40°S) employing multi-year observations of O(1D) airglow volume emission rates (VER), Doppler temperatures (DoT), and neutral winds over the altitude range of 190-300 km by the Wind Imaging Interferometer (WINDII) experiment on board the Upper Atmosphere Research Satellite. The MTM dependence on longitude, season, local time and altitude has been examined. Midnight maxima were observed both in the O(1D) VER and DoT with peaks at 21 LT and 2 LT during summer solstice; 21 LT, 24 LT and 2 LT in O(1D) VER and 21 LT and 1 LT in DoT for fall equinox; 21 LT and 2 LT in O(1D) VER and 21LT and 3LT in DoT for spring equinox. The observed perturbations in the O(1D) VER and temperature were out-of-phase with respect to longitude. Latitude/longitude maps of the VER and DoT revealed wave-1 signatures most persistently seen after local midnight in summer, with very little day-to-day variation in phase, while the amplitude varied with time. WINDII meridional wind observations, as well as correlative in time TOPEX TEC (Total Electron Content) data have been employed to investigate the mechanisms underlying the observed enhancement in O(1D) VER and DoT, including the possible relationship to the Weddell Sea Anomaly in the observed perturbations.

  1. GPS observation of continent-size traveling TEC pulsations at the start of geomagnetic storms

    NASA Astrophysics Data System (ADS)

    Pradipta, R.; Valladares, C. E.; Doherty, P. H.

    2014-08-01

    We report our experimental observation of continent-size traveling plasma disturbances using GPS measurements of total electron content (TEC) over the North American sector. These plasma disturbances occurred at the beginning of geomagnetic storms, immediately after the shock arrived, and prior to the appearance of large-scale traveling ionospheric disturbances (LSTIDs) from the auroral region. Specifically, these supersize TEC perturbations were observed when the interplanetary magnetic field Bz was oscillating between northward and southward directions. They were found to propagate zonally with a propagation speed of 2-3 km/s. We interpret these TEC pulsations as ion drift waves in the magnetosphere/plasmasphere that propagate azimuthally inside the GPS orbit.

  2. Assessment of IRI and IRI-Plas models over the African equatorial and low-latitude region

    NASA Astrophysics Data System (ADS)

    Adebiyi, S. J.; Adimula, I. A.; Oladipo, O. A.; Joshua, B. W.

    2016-07-01

    A reliable ionospheric specification by empirical models is important to mitigate the effects of the ionosphere on the operations of satellite-based positioning and navigation systems. This study evaluates the capability of the International Reference Ionosphere (IRI) and IRI extended to the plasmasphere (IRI-Plas) models in predicting the total electron content (TEC) over stations located in the southern hemisphere of the African equatorial and low-latitude region. TEC derived from Global Positioning System (GPS) measurements were compared with TEC predicted by both the IRI-Plas 2015 model and the three topside options of the IRI 2012 model (i.e., NeQuick (NeQ), IRI 2001 corrected factor (IRI-01 Corr), and the IRI 2001(IRI-01)). Generally, the diurnal and the seasonal structures of modeled TEC follow quite well with the observed TEC in all the stations, although with some upward and downward offsets observed during the daytime and nighttime. The prediction errors of both models exhibit latitudinal variation and these showed seasonal trends. The values generally decrease with increase in latitude. The TEC data-model divergence of both models is most significant at stations in the equatorial region during the daytime and nighttime. Conversely, both models demonstrate most pronounced convergence during the nighttime at stations outside the equatorial region. The IRI-Plas model, in general, performed better in months and seasons when the three options of the IRI model underestimate TEC. Factors such as the height limitation of the IRI model, the inaccurate predictions of the bottomside and topside electron density profiles were used to explain the data-model discrepancies.

  3. Comparison of Two IRI plasmasphere Extensions with GPS-TEC Observations

    NASA Technical Reports Server (NTRS)

    Gulyaeva, T. L.; Gallagher, Dennis L.

    2006-01-01

    Comparisons of two model results with Global Positioning System GPS-TEC measurements have been carried out for different latitudinal, solar activity, magnetic activity, diurnal and seasonal conditions. The models evaluated are the Global Core Plasma Model (GCPM-2000) and the IRI extension with Russian plasmasphere model (IRI*).Data of 23 observatories providing GPS-TEC and ionosonde data have been used. It is shown that IRI* plasmasphere electron density is greater than GCPM results by an order of magnitude at 6370 km altitude (one Earth's radius) with this excess growing to 2-3 orders of magnitude towards the GPS satellite altitude of 20000 km. Another source of model and GPS-TEC differences is a way of selection of the F2 layer peak parameters driving the models either with ITU-R (former CCIR) maps or ionosonde observations. Plasmasphere amendment to IRI improves accuracy of TEC model predictions because the plasmasphere contribution to the total TEC varies from 10% by daytime under quiet magnetic conditions to more than 50% by night under stormy conditions.

  4. Seismo-Ionospheric Coupling as Intensified EIA Observed by Satellite Electron Density and GPS-TEC Data

    NASA Astrophysics Data System (ADS)

    Ryu, K.; Jangsoo, C.; Kim, S. G.; Jeong, K. S.; Parrot, M.; Pulinets, S. A.; Oyama, K. I.

    2014-12-01

    Examples of intensified EIA features temporally and spatially related to large earthquakes observed by satellites and GPS-TEC are introduced. The precursory, concurrent, and ex-post enhancements of EIA represented by the equatorial electron density, which are thought to be related to the M8.7 Northern Sumatra earthquake of March 2005, the M8.0 Pisco earthquake of August 2007, and the M7.9 Wenchuan Earthquake of 12 May 2008, are shown with space weather condition. Based on the case studies, statistical analysis on the ionospheric electron density data measured by the Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions satellite (DEMETER) over a period of 2005-2010 was executed in order to investigate the correlation between seismic activity and equatorial plasma density variations. To simplify the analysis, three equatorial regions with frequent earthquakes were selected and then one-dimensional time series analysis between the daily seismic activity indices and the EIA intensity indices were performed for each region with excluding the possible effects from the geomagnetic and solar activity. The statistically significant values of the lagged cross-correlation function, particularly in the region with minimal effects of longitudinal asymmetry, indicate that some of the very large earthquakes with M > 7.0 in the low latitude region can accompany observable seismo-ionospheric coupling phenomena in the form of EIA enhancements, even though the seismic activity is not the most significant driver of the equatorial ionospheric evolution. The physical mechanisms of the seismo-ionospheric coupling to explain the observation and the possibility of earthquake prediction using the EIA intensity variation are discussed.

  5. Low-latitude daytime F region irregularities observed in two geomagnetically quiet days by the Hainan coherent scatter phased array radar (HCOPAR)

    NASA Astrophysics Data System (ADS)

    Chen, Gang; Jin, Han; Yan, Jingye; Zhang, Shaodong; Li, Guozhu; Yokoyama, Tatsuhiro; Yang, Guotao; Yan, Chunxiao; Wu, Chen; Wang, Jin; Zhong, Dingkun; Li, Yaxian; Wang, Zhihua

    2017-02-01

    Hainan coherent scatter phased array radar (HCOPAR) located at low latitude of China has recorded the extremely rare daytime F region irregularities at noon of 22 July 2013 and 23 May 2016. The two field-aligned irregularities (FAIs) appeared in the topside F2 layer and presented small Doppler velocities and narrow spectral widths. The fan sector maps show that the FAIs moved northward with almost no zonal speed. The irregularities emerged in the geomagnetically quiet condition and were irrelevant to the storm-induced eastward electric field as other daytime cases. More than 2 h after the emergency of the daytime irregularities over Hainan, the Shaoyang digisonde situated 870 km north to the HCOPAR recorded the spread-F in ionospheric F1 layer. According to the echo altitudes, the spread-F may connect the daytime bubbles via magnetic field line. The strong photoionization after sunrise made it difficult to generate the plasma bubbles in the sunlit ionosphere. Consequently, the two midday FAIs over Hainan may drift along the magnetic field lines from higher altitudes in the south and are most likely the remnant of previous night's bubbles.

  6. Latitudinal and Seasonal Investigations of Storm-Time TEC Variation

    NASA Astrophysics Data System (ADS)

    Adimula, I. A.; Oladipo, O. A.; Adebiyi, S. J.

    2016-07-01

    The ionosphere responds markedly and unpredictably to varying magnetospheric energy inputs caused by solar disturbances on the geospace. Knowledge of the impact of the space weather events on the ionosphere is important to assess the environmental effect on the operations of ground- and space-based technologies. Thus, global positioning system (GPS) measurements from the international GNSS service (IGS) database were used to investigate the ionospheric response to 56 geomagnetic storm events at six different latitudes comprising the northern and southern hemispheres in the Afro-European sector. Statistical distributions of total electron content (TEC) response show that during the main phase of the storms, enhancement of TEC is more pronounced in most of the seasons, regardless of the latitude and hemisphere. However, a strong seasonal dependence appears in the TEC response during the recovery phase. Depletion of TEC is majorly observed at the high latitude stations, and its appearance at lower latitudes is seasonally dependent. In summer hemisphere, the depletion of TEC is more pronounced in nearly all the latitudinal bands. In winter hemisphere, enhancement as well as depletion of TEC is observed over the high latitude, while enhancement is majorly observed over the mid and low latitudes. In equinoxes, the storm-time TEC distribution shows a fairly consistent characteristic with the summer distribution, particularly in the northern hemisphere.

  7. Comparison of GPS TEC variations with IRI-2007 TEC prediction at equatorial latitudes during a low solar activity (2009-2011) phase over the Kenyan region

    NASA Astrophysics Data System (ADS)

    Olwendo, O. J.; Baki, P.; Cilliers, P. J.; Mito, C.; Doherty, P.

    2013-11-01

    This work presents an analysis of the Total Electron Content (TEC) derived from the International GNSS Service (IGS) receivers at Malindi (mal2: 2.9oS, 40.1oE, dip -26.813o), Kasarani (rcmn: 36.89oE, 1.2oS, dip -23.970o), Eldoret (moiu: 35.3oE, 0.3oN, dip -21.037o) and GPS-SCINDA (36.8oE, 1.3oS, dip -24.117o) receiver located in Nairobi for the period 2009-2011. The diurnal, monthly and seasonal variations of the GPS derived TEC (GPS-TEC) and effects of space weather on TEC are compared with TEC from the 2007 International Reference Ionosphere model (IRI-TEC) using the NeQuick option for the topside electron density. The diurnal peaks in GPS-TEC is maximum during equinoctial months (March, April, October) and in December and minimum in June solstice months (May, June, July). The variability in GPS-TEC is minimal in all seasons between 0:00 and 04:00 UT and maximum near noon between 10:00 and 14:00 UT. Significant variability in TEC at post sunset hours after 16:00 UT (19:00 LT) has been noted in all the seasons except in June solstice. The TEC variability of the post sunset hours is associated with the occurrence of the ionization anomaly crest which enhances nighttime TEC over this region. A comparison between the GPS-TEC and IRI-TEC indicates that both the model and observation depicts a similar trend in the monthly and seasonal variations. However seasonal averages show that IRI-TEC values are higher than the GPS-TEC. The IRI-TEC also depicts a double peak in diurnal values unlike the GPS-TEC. This overestimation which is primarily during daytime hours could be due to the model overestimation of the equatorial anomaly effect on levels of ionospheric ionization over the low latitude regions. The IRI-TEC also does not show any response to geomagnetic activity, despite the STORM option being selected in the model; the IRI model generally remains smooth and underestimates TEC during a storm. The GPS-TEC variability indicated by standard deviation seasonal averages

  8. The Low-latitude Ionospheric Sensor Network: The Initial Campaigns

    NASA Astrophysics Data System (ADS)

    Doherty, P. H.; Valladares, C. E.; Carrano, C.

    2009-05-01

    The Low-latitude Ionospheric Sensor Network (LISN) is a distributed observatory designed to provide regional coverage in South America and high-temporal resolution measurements to diagnose the initiation and development of plasma structures and the state and dynamics of the low latitude ionosphere. It combines inexpensive GPS receivers and state-of-the-art radars such as the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) ionosondes and magnetometers. This paper describes the characteristics of the LISN distributed observatory and discusses the results of the first two campaigns. LISN will be comprised of nearly 70 GPS receivers with the capability to measure Total Electron Content (TEC), amplitude and phase scintillation and Traveling Ionospheric Disturbances (TIDs). LISN will also include 5 ionosondes able to measure nighttime E-region densities and 5 collocated magnetometers that will be placed along the same magnetic meridian. The first campaign was dedicated to detect medium-scale (~100 km) TIDs and was conducted at Huancayo, Peru in July 2008 using 3 GPS receivers spaced by 4-5 km arranged in a triangular configuration. TEC data corresponding to 3 consecutive days indicate that the TIDs phase velocity was close to 120 m/s and directed northward during the early evening hours. The second campaign was conducted in February 2009 using 3 GPS receivers installed near Ancon and coordinated with the VIPIR ionosonde running in an interferometer mode. We will discuss the implications of these new results within the frame of the current theories of plasma bubble onset.

  9. Geomorphological impacts of high-latitude storm waves on low-latitude reef islands - Observations of the December 2008 event on Nukutoa, Takuu, Papua New Guinea

    NASA Astrophysics Data System (ADS)

    Smithers, S. G.; Hoeke, R. K.

    2014-10-01

    Low-latitude reefs and reef islands usually experience relatively benign climatic and hydrodynamic conditions due to their location near to the equator, outside of the major storm belts, and they typically exhibit geomorphological traits that reflect the prevailing low-energy conditions. For example, algal ridges are poorly developed, reef flat boulder zones are modest or lacking, rubble banks are rare, and reef islands tend to be low and dominated by sand. Nukutoa is a low-lying triangular-shaped reef island of ~ 6 ha located on the eastern rim of Takuu atoll (4°45‧S, 157°2‧E), Papua New Guinea, approximately 300 km northeast of Bougainville. The approximately 450 residents of Takuu all live on Nukutoa. In December 2008 Takuu was struck by several days of very high water levels and waves, which washed completely over approximately 50% of Nukutoa. GPS shoreline mapping and topographic surveys of the island were undertaken in the days immediately prior to the event, and were repeated immediately after. Homes and village infrastructure were damaged during this event, which eroded around 60% of the shoreline, and deposited a sand sheet averaging around 50 mm thick over approximately 13% of the island. This event was generated by two distant storms - one located > 6000 km away near 50°N, and affected a wide area of the Western Pacific. Oral histories record at least five similar events since the 1940s. In this paper we document the geomorphic impacts of the December 2008 event and discuss the possible significance of similar events in the past, and in the future.

  10. Observed TEC Anomalies by GNSS Sites Preceding the Aegean Sea Earthquake of 2014

    NASA Astrophysics Data System (ADS)

    Ulukavak, Mustafa; Yal&ccedul; ınkaya, Mualla

    2016-11-01

    In recent years, Total Electron Content (TEC) data, obtained from Global Navigation Satellites Systems (GNSS) receivers, has been widely used to detect seismo-ionospheric anomalies. In this study, Global Positioning System - Total Electron Content (GPS-TEC) data were used to investigate ionospheric abnormal behaviors prior to the 2014 Aegean Sea earthquake (40.305°N 25.453°E, 24 May 2014, 09:25:03 UT, Mw:6.9). The data obtained from three Continuously Operating Reference Stations in Turkey (CORS-TR) and two International GNSS Service (IGS) sites near the epicenter of the earthquake is used to detect ionospheric anomalies before the earthquake. Solar activity index (F10.7) and geomagnetic activity index (Dst), which are both related to space weather conditions, were used to analyze these pre-earthquake ionospheric anomalies. An examination of these indices indicated high solar activity between May 8 and 15, 2014. The first significant increase (positive anomalies) in Vertical Total Electron Content (VTEC) was detected on May 14, 2014 or 10 days before the earthquake. This positive anomaly can be attributed to the high solar activity. The indices do not imply high solar or geomagnetic activity after May 15, 2014. Abnormal ionospheric TEC changes (negative anomaly) were observed at all stations one day before the earthquake. These changes were lower than the lower bound by approximately 10-20 TEC unit (TECU), and may be considered as the ionospheric precursor of the 2014 Aegean Sea earthquake

  11. Comparison of two IRI electron-density plasmasphere extensions with GPS-TEC observations

    NASA Astrophysics Data System (ADS)

    Gulyaeva, T. L.; Gallagher, D. L.

    Comparisons of two model results with Global Positioning System, GPS-TEC, measurements have been carried out for different latitudinal, solar activity, magnetic activity, diurnal, and seasonal conditions. The models evaluated are the Global Core Plasma Model (GCPM-2000) and the IRI extension (IRI ∗) with the Russian plasmasphere model. Data from 23 observatories providing GPS-TEC and ionosonde data have been used. It is shown that the IRI ∗ plasmasphere electron densities are greater than the GCPM results by an order of magnitude at 6370 km altitude (one Earth radius), becoming two to three orders of magnitude larger at the GPS satellite orbital altitude of 20,200 km. Another source of model and GPS-TEC differences is the selection of the ionospheric F2 layer peak parameters driving the models, either with ITU-R (former CCIR) maps or ionosonde observations. The plasmasphere model extension of IRI improves the accuracy of the TEC model predictions taking into account the plasmasphere contribution to the total electron content which could vary from 10% during daytime under quiet magnetic conditions to more than 50% during the night under storm-time conditions.

  12. A detection algorithm for scale analysis of post-sunset low-latitude plasma depletions as observed by the Swarm constellation mission

    NASA Astrophysics Data System (ADS)

    Kervalishvili, Guram; Stolle, Claudia; Xiong, Chao

    2016-04-01

    ESA's constellation mission Swarm was successfully launched on 22 November 2013. The three satellites achieved their final constellation on 17 April 2014 and since then Swarm-A and Swarm-C orbiting the Earth at about 470 km (flying side-by-side) and Swarm-B at about 520 km altitude. The satellites carry instruments to monitor the F-region electron density with a sampling frequency of 2 Hz. This paper will present a detection algorithm for low-latitude post-sunset plasma bubbles (depletions), which uses local minima and maxima to detect depletions directly from electron density readings from Swarm. Our analyses were performed in the magnetic latitude (MLat) and local time (MLT) coordinate system. The detection procedure also captures the amplitude of depletion, which is called depth in the following. The width of a bubble corresponds to the length the satellite is located inside a depletion. We discuss the global distribution of depth and width of plasma bubbles and its seasonal and local time dependence for all three Swarm satellites from April 2015 through September 2015. As expected, on global average the bubble occurrence rate is highest for combined equinoxes (Mar, Apr, Sep, and Oct) and smallest for June solstice (May, Jun, Jul, and Aug). MLT distribution of the bubble occurrence number shows a sharp increase at about 19 MLT and decreases towards post-midnight hours. Interestingly, there is an inverse relation between depth and width of bubbles as function of MLT. This is true for all seasons and for all Swarm satellites. The bubble depth (width) is decreasing (increasing) from post-sunset to post-midnight for December solstice (Jan, Feb, Nov, and Dec) and combined equinoxes with about the same amplitude values for bubbles depth (width). Therefore we suggest that at post midnight when the depletions are less steep the structures of the depletions is broader than early after sunset. However for June solstice the depletions are less deep and the bubble depth and

  13. Very low latitude (L = 1.08) whistlers

    NASA Astrophysics Data System (ADS)

    Singh, Rajesh; Cohen, Morris B.; Maurya, Ajeet K.; Veenadhari, B.; Kumar, Sushil; Pant, P.; Said, Ryan K.; Inan, Umran S.

    2012-12-01

    For decades, whistlers observed on the ground at mid and high latitudes have been used for diagnostics of Earth's plasmasphere. Whistlers have also been observed at low latitudes however, the propagation characteristics of low latitude whistlers are poorly understood thus they have not been used effectively as a diagnostic for the low latitude ionosphere. One key limitation with past studies has been lack of knowledge of the whistler source lightning location. Here we present the first cases of low latitude ground whistlers most likely linked with their causative lightning discharges in the conjugate zone. The Global Lightning Dataset 360 (GLD360) detected lightning discharges were found to be located close to the conjugate location of the recording stations, providing direct evidence of inter-hemispheric propagation at the low latitudes. A total of 864 whistlers were observed at Allahabad, India (Geomag. lat. 16.05°N Geomag. long. 155.34°E L = 1.08) during the night of 26 January 2011. Using GLD360 network data, we show the occurrence of thunderstorm activity between 200 and 450 km from the conjugate point of Allahabad. We also report the distribution of peak currents of whistler-producing lightning, which demonstrate a cutoff at 30 kA.

  14. AzTEC 1.1 mm OBSERVATIONS OF THE MBM12 MOLECULAR CLOUD

    SciTech Connect

    Kim, M. J.; Kim, S.; Youn, S.; Kang, Y.-W.; Yun, M. S.; Wilson, G. W.; Aretxaga, I.; Hughes, D. H.; Humphrey, A.; Williams, J. P.; Austermann, J. E.; Perera, T. A.; Mauskopf, P. D.; Magnani, L.

    2012-02-10

    We present 1.1 mm observations of the dust continuum emission from the MBM12 high-latitude molecular cloud observed with the Astronomical Thermal Emission Camera (AzTEC) mounted on the James Clerk Maxwell Telescope on Mauna Kea, Hawaii. We surveyed 6.34 deg{sup 2} centered on MBM12, making this the largest area that has ever been surveyed in this region with submillimeter and millimeter telescopes. Eight secure individual sources were detected with a signal-to-noise ratio of over 4.4. These eight AzTEC sources can be considered to be real astronomical objects compared to the other candidates based on calculations of the false detection rate. The distribution of the detected 1.1 mm sources or compact 1.1 mm peaks is spatially anti-correlated with that of the 100 {mu}m emission and the {sup 12}CO emission. We detected the 1.1 mm dust continuum emitting sources associated with two classical T Tauri stars, LkH{alpha}262 and LkH{alpha}264. Observations of spectral energy distributions (SEDs) indicate that LkH{alpha}262 is likely to be Class II (pre-main-sequence star), but there are also indications that it could be a late Class I (protostar). A flared disk and a bipolar cavity in the models of Class I sources lead to more complicated SEDs. From the present AzTEC observations of the MBM12 region, it appears that other sources detected with AzTEC are likely to be extragalactic and located behind MBM12. Some of these have radio counterparts and their star formation rates are derived from a fit of the SEDs to the photometric evolution of galaxies in which the effects of a dusty interstellar medium have been included.

  15. Comparison of Ionospheric TEC Derived from GPS and IRI 2012 Model during Geomagnetic Storms at Indonesia

    NASA Astrophysics Data System (ADS)

    Marlia, Dessi; Wu, Falin

    2016-07-01

    This paper investigates the variations of vertical Total Electron Content (VTEC) at Manado, Indonesia (geographic coordinates : lat 1.34 ° S and long 124.82 ° E) for period 2013. The GPS measured TEC is compared with the TEC derived from the IRI (International Reference Ionosphere) 2012 model. Vertical TEC measurements obtained from dual frequency GPS receiver that is GISTM (GPS Ionospheric Scintillations and TEC monitor). Variation of TEC validate to IRI 2012 model at Manado station has been compared with the model for three different topside of electron density namely NeQuick, IRI-01-Corr and IRI2001.There is a need to investigation on diurnal, seasonal variations, solar activity dependence of TEC and including effects of space weather related events to TEC and modeling of TEC. In this paper, diurnal and seasonal variations of VTEC and the effect of VTEC due to space weather events like Geomagnetic storms are analyzed. The result show that the TEC prediction using IRI-2001 model overestimated the GPS TEC measurements, while IRI-NeQuick and IRI-01-corr show a tendency to underestimates the observed TEC during the day time particularly in low latitude region in the maximum solar activity period (2013). The variations of VTEC during 17th March, 2013, 29th June, 2013 storms are analyzed. During 17th March,2013 storm enhancement in VTEC with Kp value 6 and Disturbance storm index (DST) -132 nT. During 29th June, 2013 storm VTEC depletion with value 7 and DST -98 nT. Significant deviations in VTEC during the main phase of the storms are observed. It is found that the response of ionospheric TEC consist of effects of both enhancement and depletions in ionospheric structures (positive and negative storm). Keywords: TEC ionosphere, GPS, GISTM, IRI 2012 model, solar activity, geomagnetic storm

  16. Observations of TEC fluctuations from an explosion on the Earth`s surface

    SciTech Connect

    Massey, R.S.; Carlos, R.C.; Jacobson, A.R.; Wu, G.

    1994-09-01

    The authors report observations of perturbations in the ionosphere total electron content (TEC) caused by acoustic waves propagating from a large chemical explosion in souther New Mexico at the earth`s surface. Fluctuations in TEC were measured by two arrays of receivers that monitor the phase of the 136 MHz beacons on two geostationary satellites. One array, located in northern New Mexico, observed fluctuations in the region where acoustic waves from the blast impinged directly on the ionosphere, while the second array, in Texas, was located to observe fluctuations caused by ducted acoustic waves. The TEC disturbance at the New Mexico array had an amplitude of about 2 {times} 10{sup 14} m{sup {minus}2} (more than 10 times the array noise level), while the amplitude at the Texas array, at a range of 900 km, was only a few times the instrumental noise level. Noise background analysis shows that the probability that a comparable or larger response at the New Mexico array might have been caused by a background noise event was less than 1%. The corresponding probability for the Texas array was 3%.

  17. Global features of ionospheric slab thickness derived from JPL TEC and COSMIC observations

    NASA Astrophysics Data System (ADS)

    Huang, He; Liu, Libo

    2016-04-01

    The ionospheric equivalent slab thickness (EST) is the ratio of total electron content (TEC) to F2-layer peak electron density (NmF2), describing the thickness of the ionospheric profile. In this study, we retrieve EST from Jet Propulsion Laboratory (JPL) TEC data and NmF2 retrieved from Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) ionospheric radio occultation data. The diurnal, seasonal and solar activity variations of global EST are analyzed as the excellent spatial coverage of JPL TEC and COSMIC data. During solstices, daytime EST in the summer hemisphere is larger than that in the winter hemisphere, except in some high-latitude regions; and the reverse is true for the nighttime EST. The peaks of EST often appear at 0400 local time. The pre-sunrise enhancement in EST appears in all seasons, while the post-sunset enhancement in EST is not readily observed in equinox. The dependence of EST on solar activity is very complicated. Furthermore, an interesting phenomenon is found that EST is enhanced from 0° to 120° E in longitude and 30° to 75° S in latitude during nighttime, just to the east of Weddell Sea Anomaly, during equinox and southern hemisphere summer.

  18. Highly Structured Plasma Density and Associated Electric and Magnetic Field Irregularities at Sub-Auroral, Middle, and Low Latitudes in the Topside Ionosphere Observed with the DEMETER and DMSP Satellites

    NASA Technical Reports Server (NTRS)

    Pfaff, Robert F.; Liebrecht, C; Berthelier, Jean-Jacques; Parrot, M.; Lebreton, Jean-Pierre

    2007-01-01

    Detailed observations of the plasma structure and irregularities that characterize the topside ionosphere at sub-auroral, middle, and low-latitudes are gathered with probes on the DEMETER and DMSP satellites. In particular, we present DEMETER observations near 700 km altitude that reveal: (1) the electric field irregularities and density depletions at mid-latitudes are remarkably similar to those associated with equatorial spread-F at low latitudes; (2) the mid-latitude density structures contain both depletions and enhancements with scale lengths along the spacecraft trajectory that typically vary from 10's to 100's of km; (3) in some cases, ELF magnetic field irregularities are observed in association with the electric field irregularities on the walls of the plasma density structures and appear to be related to finely-structured spatial currents and/or Alfven waves; (4) during severe geomagnetic storms, broad regions of nightside plasma density structures are typically present, in some instances extending from the equator to the subauroral regions; and (5) intense, broadband electric and magnetic field irregularities are observed at sub-auroral latitudes during geomagnetic storm periods that are typically associated with the trough region. Data from successive DEMETER orbits during storm periods in both the daytime and nighttime illustrate how enhancements of both the ambient plasma density, as well as sub-auroral and mid-latitude density structures, correlate and evolve with changes in the Dst. The DEMETER data are compared with near simultaneous observations gathered by the DMSP satellites near 840 km. The observations are related to theories of sub-auroral and mid-latitude plasma density structuring during geomagnetic storms and penetration electric fields and are highly germane to understanding space weather effects regarding disruption of communication and navigation signals in the near-space environment.

  19. LISN: Measurement of TEC values, and TID characteristics over South and Central America (Invited)

    NASA Astrophysics Data System (ADS)

    Valladares, C. E.

    2013-12-01

    The Low-latitude Ionospheric Sensor Network (LISN) is a distributed observatory designed to provide the climatology and weather of the low latitude ionosphere over the South American continent. Presently, the LISN observatory consists of 47 GPS receivers able to transmit TEC and scintillation values to a central server in a real-time basis. Historical TEC values from these receivers and from about 300 other GPSs that operated in South and Central America between 2008 and 2012 were used to derive regional maps of TEC and TIDs. A prominent feature of the TEC maps is the intense day-to-day variability that is observed during all seasons and under quiet and active magnetic conditions. To assess the TEC dependencies a non-linear least-square fit was conducted to simultaneously extract the solar flux, magnetic and seasonal variability for each square cell of the TEC maps and for each 30-min local time sector. It was found that TEC values and the anomaly intensity increase as a function of the solar flux. The latitudinal separation increases with magnetic activity, and TEC values in Central America become the largest when Kp is equal to 5o or more. TIDs are seen quite frequently over the Caribbean region and in the northern part of South America. To calculate the TIDs travel velocities, their propagation direction, and the scale-size of the disturbances a multi-site multi-dimension cross-correlation method was applied to the TEC database. Phase velocities of order 150 m/s and scale sizes between 100 and 400 km were typically observed. This paper will present the morphology and statistics of TIDs as a function of latitude, longitude, local time and season. It is also introduced the results of an investigation to correlate the appearance, phase velocity and angle of propagation of TIDs and tropospheric phenomena observed with the TRMM satellite.

  20. Modeling of ionospheric irregularities during geomagnetically disturbed conditions over African low-latitude region

    NASA Astrophysics Data System (ADS)

    Mungufeni, Patrick; Habarulema, John Bosco; Jurua, Edward

    2016-10-01

    In this study, station-specific models of ionospheric irregularities over low-latitude African region during geomagnetically disturbed days (Dst≤-50 nT) have been developed. Global Navigation Satellite Systems (GNSS)-derived ionospheric total electron content (TEC) data during 1998-2014 were used. Ionospheric irregularities were represented with the rate of change of TEC index (ROTI). The inputs for the models are the local time, solar flux index, day number of the year, auroral electrojet, and the disturbance storm time indices, while the output is the hourly median ROTI during these given conditions. To develop the models, the ROTI index values were binned based on the input parameters and cubic B splines were then fitted to the binned data. Developed models were validated with independent data over stations within 680 km radius. The models reproduced fairly well the inhibitions and the occurrences of ionospheric irregularities during geomagnetically disturbed days. The models even emulated these patterns in the various seasons, during medium and high solar activity conditions. During validations of the models, the percentages of the number of errors (difference between the observed and the modeled ROTI) <0.05 total electron content unit, 1TECU = 1016 el m-2 (TECU)/Min at all the stations were all >70% and the root-mean-square error were mostly < 0.1 TECU/Min. Furthermore, the correlation coefficients ranged from 0.47 to 0.76.

  1. Seasonal, longitude, local time, and altitude distributions of electric field, magnetic field, and plasma density components of depletions, structures, and waves observed on the C/NOFS satellite in the low latitude ionosphere

    NASA Astrophysics Data System (ADS)

    Pfaff, R. F., Jr.; Freudenreich, H. T.; Klenzing, J.; Liebrecht, M. C.

    2016-12-01

    The Vector Electric Field Investigation (VEFI) on the C/NOFS equatorial satellite provides simultaneous measurements of electric field, magnetic field, and plasma density irregularities associated with the equatorial ionosphere and in particular with plasma depletions. C/NOFS was launched into a 401 km by 857 km orbit with a 13 degree inclination in April, 2008 and gathered near continuous data until November, 2015. We present C/NOFS observations that address a variety of questions regarding the nature of equatorial plasma depletions, irregularities, and Alfven waves in the low latitude ionosphere. The presentation includes distributions of the different components with respect to longitude, latitude, local time, altitude, and season. Among the predominant findings are: (1) the strong tendency for plasma depletions to occur post-midnight in the April-September time frame where they were also observed predominantly in the African sector; (2) the prevalence of nighttime electric field structures to exist without corresponding density depletions or irregularities, particularly during solar minimum conditions; and (3) AC magnetic field observations which we interpret as signatures of small-scale filamentary currents and/or Alfven waves which were most pronounced in the South American sector after sunset. We focus in particular on the magnetic signatures which were strongest towards the end of the mission where solar activity (and hence the ambient plasma density) was moderately elevated and the satellite sampling altitudes were lowest. The AC magnetic field signatures appear in two categories: wave-like strutures, which do not appear to be organized about the magnetic equator and may represent Alfven waves propagating between hemispheres along closed magnetic field lines, and magnetic structures associated with plasma density depletions which, when combined with simultaneous electric field structures, reveal Poynting Flux energy flow in the poleward direction. The

  2. DEMETER Observations of Highly Structured Plasma Density and Associated ELF Electric Field and Magnetic Field Irregularities at Middle and Low Latitudes

    NASA Technical Reports Server (NTRS)

    Pfaff, R.; Liebrecht, C.; Berthelier, J.-J.; Parrot, M.; Lebreton, J.-P.

    2008-01-01

    The DEMETER spacecraft frequently encounters structured plasma and electric field irregularities associated with equatorial spread-F. However, during severe geonagnetic storms, the spacecraft detects broader regions of density structures that extend to higher latitudes, in some instances to the sub-auroral regions. In addition to the electric field irregularities, ELF magnetic field irregularities are sometimes observed. for example, on the walls of the density structures, and appear related to finely-structured spatial currents and/or Alfven waves. The mid-latitude irregularities are compared with those of equatorial spread-F as well as wit11 intense irregularities associated with the trough region observed at sub-auroral latitudes.

  3. DEMETER Observations of Highly Structured Plasma Density and Associated ELF Electric Field and Magnetic Field Irregularities at Middle and Low Latitudes

    NASA Technical Reports Server (NTRS)

    Pfaff, R.; Liebrecht, C.; Berthelier, J.-J.; Parrot, M.; Lebreton, J.-P.

    2008-01-01

    The DEMETER spacecraft frequently encounters structured plasma and electric field irregularities associated with equatorial spread-F. However, during severe geonagnetic storms, the spacecraft detects broader regions of density structures that extend to higher latitudes, in some instances to the sub-auroral regions. In addition to the electric field irregularities, ELF magnetic field irregularities are sometimes observed. for example, on the walls of the density structures, and appear related to finely-structured spatial currents and/or Alfven waves. The mid-latitude irregularities are compared with those of equatorial spread-F as well as wit11 intense irregularities associated with the trough region observed at sub-auroral latitudes.

  4. First Observations of Equatorial TEC and Scintillation With Multiple Dual-Frequency Software-Defined GPS Receivers

    NASA Astrophysics Data System (ADS)

    O'Hanlon, B.; Kintner, P. M.; de Paula, E. R.

    2009-05-01

    A dual-frequency software-defined GPS receiver has been developed and used for monitoring total electron content (TEC) and observing equatorial ionospheric scintillation. The Cornell University GPS Receiver Implemented on a DSP (GRID) utilizes the GPS L1 C/A and L2 C signals to measure TEC and observe scintillation. The GRID receiver measured TEC and GPS signal amplitude and phase at 10 Hz. Also employed were two similar GPS digital storage receivers (non-real-time) that made the same measurements at 50 Hz. These receivers were arranged in a linear array and utilized in January, 2009 in Natal, Brazil (magnetic latitude 2.42°) to make these observations. Mild scintillation of the L1 C/A and L2 C signals was observed. TEC measurements agreed well with those taken by a collocated GPSV 4004B Scintillation/TEC Monitor. We demonstrate the use of multiple receivers to measure drifts and report on the first fast (10Hz-50Hz) multiple receiver TEC measurements in the equatorial ionosphere.

  5. Hemispheric Asymmetry in Transition from Equatorial Plasma Bubble to Blob as Deduced from 630.0 nm Airglow Observations at Low Latitudes

    NASA Technical Reports Server (NTRS)

    Park, Jaeheung; Martinis, Carlos R.; Luehr, Hermann; Pfaff, Robert F.; Kwak, Young-Sil

    2016-01-01

    Transitions from depletions to enhancements of 630.0 nm nighttime airglow have been observed at Arecibo. Numerical simulations by Krall et al. (2009) predicted that they should occur only in one hemisphere, which has not yet been confirmed observationally. In this study we investigate the hemispheric conjugacy of the depletion-to-enhancement transition using multiple instruments. We focus on one event observed in the American longitude sector on 22 December 2014: 630.0 nm airglow depletions evolved into enhancements in the Northern Hemisphere while the evolution did not occur in the conjugate location in the Southern Hemisphere. Concurrent plasma density measured by low Earth orbit (LEO) satellites and 777.4 nm airglow images support that the depletions and enhancements of 630.0 nm night time airglow reflect plasma density decreases and increases (blobs), respectively. Characteristics of the airglow depletions, in the context of the LEO satellite data, further suggest that the plasma density depletion deduced from the airglow data represents equatorial plasma bubbles (EPBs) rather than medium-scale traveling ionospheric disturbances from midlatitudes. Hence, the event in this study can be interpreted as EPB-to-blob transition.

  6. Hemispheric asymmetry in transition from equatorial plasma bubble to blob as deduced from 630.0 nm airglow observations at low latitudes

    NASA Astrophysics Data System (ADS)

    Park, Jaeheung; Martinis, Carlos R.; Lühr, Hermann; Pfaff, Robert F.; Kwak, Young-Sil

    2016-01-01

    Transitions from depletions to enhancements of 630.0 nm nighttime airglow have been observed at Arecibo. Numerical simulations by Krall et al. (2009) predicted that they should occur only in one hemisphere, which has not yet been confirmed observationally. In this study we investigate the hemispheric conjugacy of the depletion-to-enhancement transition using multiple instruments. We focus on one event observed in the American longitude sector on 22 December 2014: 630.0 nm airglow depletions evolved into enhancements in the Northern Hemisphere while the evolution did not occur in the conjugate location in the Southern Hemisphere. Concurrent plasma density measured by low Earth orbit (LEO) satellites and 777.4 nm airglow images support that the depletions and enhancements of 630.0 nm nighttime airglow reflect plasma density decreases and increases (blobs), respectively. Characteristics of the airglow depletions, in the context of the LEO satellite data, further suggest that the plasma density depletion deduced from the airglow data represents equatorial plasma bubbles (EPBs) rather than medium-scale traveling ionospheric disturbances from midlatitudes. Hence, the event in this study can be interpreted as EPB-to-blob transition.

  7. Electric Field and Plasma Density Observations of Irregularities and Plasma Instabilities in the Low Latitude Ionosphere Gathered by the C/NOFS Satellite

    NASA Technical Reports Server (NTRS)

    Pfaff, Robert F.; Freudenreich, H.; Rowland, D.; Klenzing, J.; Liebrecht, C.

    2012-01-01

    The Vector Electric Field Investigation (VEFI) on the C/NOFS equatorial satellite provides a unique data set which includes detailed measurements of irregularities associated with the equatorial ionosphere and in particular with spread-F depletions. We present vector AC electric field observations gathered on C/NOFS that address a variety of key questions regarding how plasma irregularities, from meter to kilometer scales, are created and evolve. The talk focuses on occasions where the ionosphere F-peak has been elevated above the C/NOFS satellite perigee of 400 km as solar activity has increased. In particular, during the equinox periods of 2011, the satellite consistently journeyed below the F-peak whenever the orbit was in the region of the South Atlantic anomaly after sunset. During these passes, data from the electric field and plasma density probes on the satellite have revealed two types of instabilities which had not previously been observed in the C/NOFS data set: The first is evidence for 400-500km-scale bottomside "undulations" that appear in the density and electric field data. In one case, these large scale waves are associated with a strong shear in the zonal E x B flow, as evidenced by variations in the meridional (outward) electric fields observed above and below the F-peak. These undulations are devoid of smaller scale structures in the early evening, yet appear at later local times along the same orbit associated with fully-developed spread-F with smaller scale structures. This suggests that they may be precursor waves for spread-F, driven by a collisional shear instability, following ideas advanced previously by researchers using data from the Jicamarca radar. A second result is the appearance of km-scale irregularities that are a common feature in the electric field and plasma density data that also appear when the satellite is near or below the F-peak at night. The vector electric field instrument on C/NOFS clearly shows that the electric field

  8. Small spatial scale field aligned currents in middle and low latitudes as observed by the CHAMP satellite and verification of their current circuit model

    NASA Astrophysics Data System (ADS)

    Nakanishi, K.; Iyemori, T.; Luhr, H.

    2013-12-01

    The magnetic field observation by the CHAMP satellite shows the global and frequent appearance of small scale (1-5 nT) magnetic fluctuations with period around a few tens seconds along the satellites. They have the following characteristics. 1. The signal is perpendicular to the geomagnetic main field, and the amplitude of the zonal component is larger than that of the meridional component. 2. Around the dip equator, as the latitude becomes lower, the period and amplitudes of the two components perpendicular to the main field respectively tend to become longer and smaller (to nearly zero on the dip equator). 3. The amplitude of the magnetic fluctuations on the dayside is larger than that on the night side by around one order in magnitude, which highly correlates to the electric conductivity of the ionospheric dynamo layer. 4. The amplitude shows symmetry about the magnetic dip equator which indicates a magnetic conjugacy to a certain extent. 5. The amplitude shows almost no dependence on the solar wind parameters such as the IMF cone angle nor the solar wind speed, which strongly suggests no association with the Pc3 micro pulsation. 6. The amplitude also shows almost no dependence on the geomagnetic activity. 7. The amplitude has a clear seasonal dependence with topographical characteristics. They can be interpreted as the spatial structure of small scale field-aligned currents generated by the ionospheric dynamo driven by atmospheric gravity waves propagating from the lower atmosphere. The mechanism is the following; first, the gravity waves generated by the lower atmospheric disturbance propagate to the ionosphere; the neutral winds oscillate, cause ionospheric dynamo and Pedersen and Hall currents flow; because the dynamo region is finite, the currents cause polarized electric fields; and the polarized electric fields propagate along the geomagnetic filed as Alfven waves accompanied by field-aligned currents, at the same time, the ionospheric currents divert to

  9. Comparison of Two IRI Plasmasphere Extensions with GPS-TEC Observations

    NASA Technical Reports Server (NTRS)

    Gulyacva, Tamara; Gallagher, Dennis

    2005-01-01

    Two plasmasphere extensions of the International Reference Ionosphere are made available for the users. It is aimed to estimate the effect of charged particles on technical devices in the Earth's environment and to define the ionosphere-plasmasphere operational conditions compatible with existing and future systems of radio communication, radio navigation and other relevant radio technologies in the ranges of medium and higher frequencies. The Global Core Plasma Model (GCPM-2000) of Gallagher et al. (2000) is an empirical description of thermal plasma densities in the plasmasphere, plasmapause, magnetospheric trough, and polar cap. GCPM-2000 uses the Kp index and is coupled to IRI in the transition region 500-600 km. The IZMIRAN plasmasphere model (Chasovitin et al., 1998; Gulyaeva et al., 2002) is an empirical model based on whistler and satellite observations. It presents global vertical analytical profiles of electron density smoothly fitted to IRI electron density profile at 1000 km altitude and extended towards the plasmapause (up to 36,000 km). For the smooth fitting of the two models, the shape of the IRI topside electron density profile is improved using ISIS 1, ISIS 2, and IK19 satellite inputs (Gulyaeva, 2003). The plasmasphere model depends on solar activity and magnetic activity (kp-index). The two IRI plasmasphere extensions are compared in the present study with the total electron content derived from records of Global Positioning Satellites (GPS-TEC) observations for different latitudinal, solar activity, magnetic activity, diurnal and seasonal conditions. The differences of model TEC with observed TEC in the topside ionosphere and plasmasphere are discussed.

  10. Comparison of Two IRI Plasmasphere Extensions with GPS-TEC Observations

    NASA Technical Reports Server (NTRS)

    Gulyacva, Tamara; Gallagher, Dennis

    2005-01-01

    Two plasmasphere extensions of the International Reference Ionosphere are made available for the users. It is aimed to estimate the effect of charged particles on technical devices in the Earth's environment and to define the ionosphere-plasmasphere operational conditions compatible with existing and future systems of radio communication, radio navigation and other relevant radio technologies in the ranges of medium and higher frequencies. The Global Core Plasma Model (GCPM-2000) of Gallagher et al. (2000) is an empirical description of thermal plasma densities in the plasmasphere, plasmapause, magnetospheric trough, and polar cap. GCPM-2000 uses the Kp index and is coupled to IRI in the transition region 500-600 km. The IZMIRAN plasmasphere model (Chasovitin et al., 1998; Gulyaeva et al., 2002) is an empirical model based on whistler and satellite observations. It presents global vertical analytical profiles of electron density smoothly fitted to IRI electron density profile at 1000 km altitude and extended towards the plasmapause (up to 36,000 km). For the smooth fitting of the two models, the shape of the IRI topside electron density profile is improved using ISIS 1, ISIS 2, and IK19 satellite inputs (Gulyaeva, 2003). The plasmasphere model depends on solar activity and magnetic activity (kp-index). The two IRI plasmasphere extensions are compared in the present study with the total electron content derived from records of Global Positioning Satellites (GPS-TEC) observations for different latitudinal, solar activity, magnetic activity, diurnal and seasonal conditions. The differences of model TEC with observed TEC in the topside ionosphere and plasmasphere are discussed.

  11. Comment on 'Observations of Low-Latitude Electron Precipitation' by R. Lieu, J. Watermann, K. Wilhelm, J. J. Quenby, and W. I. Axford

    NASA Technical Reports Server (NTRS)

    Rassoul, H. K.; Hanson, W. B.

    1989-01-01

    Observations made by an electron spectrometer aboard Spacelab 1 and presented by Lieu et al. (1988) are examined critically. The precipitation of electrons in the energy range of 0.1-12.5 keV was measured on December 6 and 7, 1983. Data for 16 passes near 240 km altitude, between + and - 30 deg geographic latitude, outside the South Atlantic Anomaly were included. It is argued that there is no geophysical confirmation of the large electron fluxes reported by Lieu et al. In their response, Lieu et al. discuss the sampling bias in the Spacelab 1 data and the magnetic shielding deficiencies of the calibration facility below about 500 eV.

  12. Low latitude middle atmosphere ionization studies

    NASA Technical Reports Server (NTRS)

    Bassi, J. P.

    1976-01-01

    Low latitude middle atmosphere ionization was studied with data obtained from three blunt conductivity probes and one Gerdien condenser. An investigation was conducted into the effects of various ionization sources in the 40 to 65 Km altitude range. An observed enhancement of positive ion conductivity taking place during the night can be explained by an atmsopheric effect, with cosmic rays being the only source of ionization only if the ion-ion recombination coefficient (alpha sub i) is small(10 to the -7 power cu cm/s) and varies greatly with altitude. More generally accepted values of alpha sub i ( approximately equal to 3x10 to the -7 power cu cm/s) require an additional source of ionization peaking at about 65 Km, and corresponding approximately to the integrated effect of an X-ray flux measured on a rocket flown in conjunction with the ionization measurements. The reasonable assumption of an alpha sub i which does not vary with altitude in the 50-70 Km range implies an even greater value alpha sub i and a more intense and harder X-ray spectrum.

  13. Analysis of ionospheric TEC from GNSS observables over the Turkish region and predictability of IRI and SPIM models

    NASA Astrophysics Data System (ADS)

    Ansari, Kutubuddin; Corumluoglu, Ozsen; Panda, Sampad Kumar

    2017-04-01

    The present study investigates the ionospheric Total Electron Content (TEC) variations in the lower mid-latitude Turkish region from the Turkish Permanent GNSS Network (TPGN) and International GNSS Services (IGS) observations during the period from January 2015 to December 2015. The corresponding TEC predicted by the International Reference Ionosphere (IRI 2012) and Standard Plasmasphere-Ionosphere Model (SPIM), and interpolated from Global Ionosphere Maps (GIMs) are evaluated to realize their reliability over the region. We studied the diurnal and monthly behavior of TEC and the relative TEC deviations along with the upper and lower quartiles to represent its spatio-temporal variability. The diurnal variation of GNSS-derived TEC indicates its maximum peak value around 10.00 UT which decreases gradually to attain minimum value after midnight. The monthly maximum value of TEC is observed in March followed by May and August, and the lowest value is seen during September. Studies show that the monthly relative deviation of TEC variability lies in the range of -1 to 4 units for all stations with the maximum difference between positive and negative variability remaining around 5. The studies also cover seasonal variation, grand-mean of ionospheric TEC and TEC intensity from the TPGN. The seasonal ionospheric VTEC pattern over all stations depicts slight increment in VTEC distribution during March equinox compared to September equinox. The December solstice perceived relatively higher VTEC than June solstice. The overall of VTEC values enhanced at all stations towards end of the year 2015 compare to mid of year due the high solar activity. The maximum grand-mean of VTEC is registered in March equinox while the lowest value is seen in September irrespective of all stations. The measured grand-mean intensity variations of VTEC values are in ascending phase during March, May, August and November months, but in descending phase during February, April, June and September

  14. Low latitude electrodynamic plasma drifts - A review

    NASA Technical Reports Server (NTRS)

    Fejer, B. G.

    1991-01-01

    The characteristics and driving mechanisms of low latitude ExB plasma drifts and electric fields particularly at F-region heights are reviewed. It is concluded that the general characteristics of the quiet-time plasma can be explained as resulting from E- and F-region dynamo and interhemispheric coupling processes. The disturbance dynamo effects are found to be responsible for the drift perturbations following the periods of enhanced magnetic activity. The prompt penetration of high-latitude electric fields to lower latitudes produces large perturbations on the upward/poleward drifts, but has no significant effect on the low-latitude and the equatorial zonal drifts. Detailed low-latitude and global numerical models for studying the characteristics of plasma drifts are capable of reproducing the latitudinal variation of the perturbation electric fields and their diurnal variations.

  15. Low latitude ionospheric scintillation and zonal irregularity drifts observed with GPS-SCINDA system and closely spaced VHF receivers in Kenya

    NASA Astrophysics Data System (ADS)

    Olwendo, O. J.; Baluku, T.; Baki, P.; Cilliers, P. J.; Mito, C.; Doherty, P.

    2013-05-01

    In this study we have used VHF and GPS-SCINDA receivers located at Nairobi (36.8°E, 1.3°S, dip -24.1°) in Kenya, to investigate the ionospheric scintillation and zonal drift irregularities of a few hundred meter-scale irregularities associated with equatorial plasma density bubbles for the period 2011. From simultaneous observations of amplitude scintillation at VHF and L-band frequencies, it is evident that the scintillation activity is higher during the post sunset hours of the equinoctial months than at the solstice. While it is noted that there is practically no signatures of the L-band scintillation in solstice months (June, July, December, January) and after midnight, VHF scintillation does occur in the solstice months and show post midnight activity through all the seasons. VHF scintillation is characterized by long duration of activity and slow fading that lasts till early morning hours (05:00 LT). Equinoctial asymmetry in scintillation occurs with higher occurrence in March-April than in September-October. The occurrence of post midnight VHF scintillation in this region is unusual and suggests some mechanisms for the formation of scintillation structure that might not be clearly understood. Zonal drift velocities of irregularities were measured using cross-correlation analysis with time series of the VHF scintillation structure from two closely spaced antennas. Statistical analyses of the distribution of zonal drift velocities after sunset hours indicate that the range of the velocities is 30-160 m/s. This is the first analysis of the zonal plasma drift velocity over this region. Based on these results we suggest that the east-west component of the plasma drift velocity may be related to the evolution of plasma bubble irregularities caused by the prereversal enhancement of the eastward electric fields. The equinoctial asymmetry of the drift velocities and scintillation could be attributed to the asymmetry of neutral winds in the thermosphere that drives

  16. Foregrounds for observations of the cosmological 21 cm line. I. First Westerbork measurements of Galactic emission at 150 MHz in a low latitude field

    NASA Astrophysics Data System (ADS)

    Bernardi, G.; de Bruyn, A. G.; Brentjens, M. A.; Ciardi, B.; Harker, G.; Jelić, V.; Koopmans, L. V. E.; Labropoulos, P.; Offringa, A.; Pandey, V. N.; Schaye, J.; Thomas, R. M.; Yatawatta, S.; Zaroubi, S.

    2009-06-01

    Context: The cosmological 21 cm line promises to be a formidable tool for cosmology, allowing the investigation of the end of the so-called dark ages, when the first galaxies formed. Aims: Astrophysical foregrounds are expected to be about three orders of magnitude greater than the cosmological signal and therefore represent a serious contamination of the cosmological 21 cm line. Detailed knowledge of both their intensity and polarization structure on the relevant angular scale of 1-30 arcmin will be essential for extracting the cosmological signal from the data. Methods: We present the first results from a series of observations conducted with the Westerbork telescope in the 140-160 MHz range with a 2 arcmin resolution aimed at characterizing the properties of the foregrounds for epoch of reionization experiments. The polarization data were analysed through the rotation measure synthesis technique. We computed total intensity and polarization angular power spectra. Results: For the first time we have detected fluctuations in the Galactic diffuse emission on scales greater than 13 arcmin at 150 MHz, in the low Galactic latitude area known as Fan region, centred at α = 3^h10^m, δ = 65° 30'. Those fluctuations have an rms of 14 K. The total intensity power spectrum shows a power-law behaviour down to ℓ 900 with slope β^I_ℓ = -2.2 ± 0.3. The detection of diffuse emission at smaller angular scales is limited by residual point sources. We measured an rms confusion noise of 3 mJy beam-1. Diffuse polarized emission was also detected for the first time at this frequency. The polarized signal shows complex structure both spatially and along the line of sight. The polarization power spectrum is not affected by residual point sources and is only limited by the thermal noise. It shows a power-law behaviour down to ℓ 2700 with slope β^P_ℓ = -1.65 ± 0.15. The rms of polarization fluctuations is 7.2 K on 4 arcmin scales. Conclusions: The measured total intensity

  17. Simulation of nose whistlers: An application to low latitude whistlers

    NASA Astrophysics Data System (ADS)

    Singh, Kalpana; Singh, R. P.; Kumar Singh, Abhay; Singh, R. N.

    2006-05-01

    Simulation technique for whistler mode signal propagating through inhomogeneous plasma using WKB approximation has been developed (Singh, K., Singh, R.P., Ferencz, O.E., 2004. Simulation of whistler mode propagation for low latitude stations. Earth Planet Space 56, 979-987). In the present paper, we have used it for the analysis of recorded signals at low latitudes and estimated the nose frequency, which is not observed on the dynamic spectra. At low latitudes nose frequency is ˜100 kHz or more and therefore it is absent in the dynamic spectra due to attenuation of the signal at higher frequencies. The importance of nose frequency is in determining the exact path of propagation, which is required in probing of ambient medium. It is shown that the method permits to study the nose frequency variation, it can be used to deduce physical parameters as the global electric field. A case study permits to get a reasonable value of the electric field, which up to now could not be done at very low latitude.

  18. Response of the equatorial and low-latitude ionosphere over the West Pacific Ocean Sector to an X1.2 solar flare on 15 May 2013

    NASA Astrophysics Data System (ADS)

    Mao, Tian; Sun, Lingfeng; Wang, Yungang; She, Chengli; Xiong, Bo; Hu, Lianhuan

    2017-09-01

    On the basis of multiple observations of ionosondes, meteor radars, magnetometers and GNSS receivers, we present the response of the equatorial and low latitude ionosphere over the West Pacific Ocean Sector to an X1.2 solar flare that peaked at 1:48 UT on 15 May 2013. The geomagnetic H component observations indicate the equatorial electrojet strength over the East Asia region is obviously enhancement during the flare. After the end time of solar flare, the ionosonde observations at Guam, an ionosonde station near the geomagnetic equatorial region, show the decrease of the peak height of ionospheric F2 layer which is related to the decrease of the eastward electric field. Simultaneous strong southern wind is observed by meteor radar over Sanya, a geomagnetic low latitude station, which probably product the westward dynamo electric field and further result in the decrease of vertical drift velocity over the geomagnetic equatorial region. In addition, GNSS total electron content (TEC) observations from six stations in the researching region show the TEC enhancement only appears nearby the geomagnetic equator region.

  19. Ionospheric TEC Weather Map Over South America

    NASA Astrophysics Data System (ADS)

    Takahashi, H.; Wrasse, C. M.; Denardini, C. M.; Pádua, M. B.; Paula, E. R.; Costa, S. M. A.; Otsuka, Y.; Shiokawa, K.; Monico, J. F. Galera; Ivo, A.; Sant'Anna, N.

    2016-11-01

    Ionospheric weather maps using the total electron content (TEC) monitored by ground-based Global Navigation Satellite Systems (GNSS) receivers over South American continent, TECMAP, have been operationally produced by Instituto Nacional de Pesquisas Espaciais's Space Weather Study and Monitoring Program (Estudo e Monitoramento Brasileiro de Clima Especial) since 2013. In order to cover the whole continent, four GNSS receiver networks, (Rede Brasileiro de Monitoramento Contínuo) RBMC/Brazilian Institute for Geography and Statistics, Low-latitude Ionospheric Sensor Network, International GNSS Service, and Red Argentina de Monitoreo Satelital Continuo, in total 140 sites, have been used. TECMAPs with a time resolution of 10 min are produced in 12 h time delay. Spatial resolution of the map is rather low, varying between 50 and 500 km depending on the density of the observation points. Large day-to-day variabilities of the equatorial ionization anomaly have been observed. Spatial gradient of TEC from the anomaly trough (total electron content unit, 1 TECU = 1016 el m-2 (TECU) <10) to the crest region (TECU > 80) causes a large ionospheric range delay in the GNSS positioning system. Ionospheric plasma bubbles, their seeding and development, could be monitored. This plasma density (spatial and temporal) variability causes not only the GNSS-based positioning error but also radio wave scintillations. Monitoring of these phenomena by TEC mapping becomes an important issue for space weather concern for high-technology positioning system and telecommunication.

  20. Observations of the Weddell Sea Anomaly in the ground-based and space-borne TEC measurements

    NASA Astrophysics Data System (ADS)

    Zakharenkova, Irina; Cherniak, Iurii; Shagimuratov, Irk

    2017-08-01

    The Weddell Sea Anomaly (WSA) is a summer ionospheric anomaly, which is characterized by a greater nighttime ionospheric density than that in daytime in the region near the Weddell Sea. We investigate the WSA signatures in the ground-based TEC (vertical total electron content) by using GPS and GLONASS measurements of the dense regional GNSS networks in South America. We constructed the high-resolution regional TEC maps for December 2014-January 2015. The WSA effects of the TEC exceed the noontime values are registered starting from 17 LT, it reaches its maximum at 01-05 LT and starts to disappear after 09 LT. Maximal TEC enhancements were as large as a factor of 2.5-3.5 and were registered at 03-04 LT. This effect was mainly localized in the geographical region of 55°S-75°S latitude and 80°W-30°W longitude, close to the Antarctic Peninsula. Further, we examined the WSA occurrence in the topside ionosphere by using GPS measurements from a zenith-looking GPS antenna on board three Swarm satellites to determine topside TEC (above ∼500 km altitude) at the topside ionosphere-plasmasphere system. Global maps of the topside TEC indicated that the zone with significant WSA effect in the topside TEC (TEC increase ∼2-4 times the noontime level) had a large spatial extent over southern Pacific and Atlantic Ocean. It was observed around 150°W-20°W and between 40°S and 70°S during 23 LT - 06 LT. For the first time, the WSA signatures were shown in the topside TEC data derived from the GPS measurements onboard the Swarm constellation. Independently, two other instruments - FORMOSAT-3/COSMIC radio occultation electron density profiles and in situ measurements by the Langmuir Probe instrument onboard Swarm satellites - were able to confirm: (1) the same location of the WSA zone as revealed in Swarm TEC; (2) the most-pronounced WSA effect, as a maximal electron density exceed over the noontime values, corresponds to altitudes above 400-500 km.

  1. A study on the variability of ionospheric total electron content over the East African low-latitude region and storm time ionospheric variations

    NASA Astrophysics Data System (ADS)

    Olwendo, O. J.; Yamazaki, Yosuke; Cilliers, P. J.; Baki, P.; Doherty, P.

    2016-09-01

    The variation of total electron content (TEC) derived from the International Global Navigation Satellite Systems Service receiver (formerly IGS) over the East African low-latitude region from up to 12 observation stations for the period 2012 was analyzed. The diurnal and annual TEC contour plots generated from data over the region show that the equatorial anomaly crests manifest remarkable seasonal variations. The crest of the equatorial ionization anomaly is fully formed and yields the maximum values of TEC during the equinoxes (March/April and September/October) and minimum in the solstice (June/July and November/December). The results of this observation show that the crest develops between 12:00 and 16:00 LT and is greatly dependent on the time when the ionosphere is uplifted at the dip equator via the E × B drift force. The postsunset TEC enhancements at stations away from dip equator depict the ionospheric plasma density diffusion (flow) from the dip equator leading to the formation of ionization anomaly crests that lasts for few hours after the sunset local time. The ionospheric response to the strong geomagnetic storm of the March 2015 has also been examined. The ionospheric response to the geomagnetic storms has shown a strong thermosphere-ionosphere coupling. The negative storm effect that occurred over the anomaly crest region is more likely due to the composition disturbances associated with high energy deposits.

  2. Virtual array beamforming of GPS TEC observations of coseismic ionospheric disturbances near the Geomagnetic South Pole triggered by teleseismic megathrusts

    NASA Astrophysics Data System (ADS)

    Gómez, Demián.; Smalley, Robert; Langston, Charles A.; Wilson, Terry J.; Bevis, Michael; Dalziel, Ian W. D.; Kendrick, Eric C.; Konfal, Stephanie A.; Willis, Michael J.; Piñón, Diego A.; Cimbaro, Sergio R.; Caccamise, Dana

    2015-10-01

    We identified coseismic ionospheric disturbances (CID) in Antarctica generated by the 2010 Maule and the 2011 Tohoku-Oki earthquakes analyzing total electron content (TEC) data with a modified beamforming technique. Beamforming in Antarctica, however, is not straightforward due to the effects of array deformation and atmospheric neutral wave-ionospheric plasma coupling. We take these effects into account and present a method to invert for the seismically generated acoustic wave using TEC observations. The back azimuths, speeds, and waveforms obtained by the beamform are in excellent agreement with the hypothesis that the TEC signals are generated by the passage of Rayleigh waves from the Maule and Tohoku-Oki earthquakes. The Tohoku-Oki earthquake is ~12,500 km from Antarctica, making this the farthest observation of CIDs to date using GPS.

  3. GPS Observation of Fast-moving Continent-size Traveling TEC Pulsations at the Start of Geomagnetic Storms

    NASA Astrophysics Data System (ADS)

    Pradipta, R.; Valladares, C. E.; Doherty, P. H.

    2014-12-01

    Using network of GPS receiver stations in North and South America, we have recently observed fast-moving continent-size traveling plasma disturbances in the mapped total electron content (TEC) data. These space plasma disturbances occurred at the beginning of geomagnetic storms, immediately after the storm's suddent commencement (SSC) and prior to the appearance of large-scale traveling ionospheric disturbances (LSTIDs) from the auroral regions. More specifically, these supersize TEC perturbations were observed when the IMF Bz was oscillating between northward and southward directions. They were found to propagate zonally westward with a propagation speed of 2-3 km/s, if projected onto an ionospheric-equivalent altitude of 350 km. Based on their general characteristics and comparison with ground-based ionosonde data, we interpret these TEC pulsations as ion drift waves in the magnetosphere/plasmasphere that propagate azimuthally inside the GPS orbit.

  4. Global response of the low-latitude to midlatitude ionosphere due to the Bastille Day flare

    NASA Astrophysics Data System (ADS)

    Huba, J. D.; Warren, H. P.; Joyce, G.; Pi, X.; Iijima, B.; Coker, C.

    2005-08-01

    The first global simulation study and comparison to data of the ionospheric effects associated with the enhanced EUV irradiance of the Bastille Day flare are presented. This is done by incorporating a time-dependent EUV spectrum, based on data and hydrodynamic modeling, into the NRL ionosphere model SAMI3. The simulation results indicate that the total electron content (TEC) increases to over 7 TEC units in the daytime, low-latitude ionosphere. In addition, it is predicted that the maximum density in the F-layer (NmF2) increases by $\\lesssim$20% and that the height of the maximum electron density (HmF2) decreases by $\\lesssim$20%. These results are explained by the increased ionization at altitudes <400 km which increases TEC and NmF2 while decreasing HmF2. The results are in reasonably good agreement with data obtained from GPS satellites and the TOPEX satellite.

  5. Sources of Low-latitude Ionospheric Electric-field Disturbances

    NASA Astrophysics Data System (ADS)

    Richmond, A. D.; Peymirat, C.

    2001-12-01

    Storm-time ionospheric electric-field disturbances at middle and low latitudes are generated both by direct magnetosphere-ionosphere interactions at high latitudes and by secondary effects of disturbance thermospheric winds. These disturbance electric fields can have a strong influence on the equatorial ionosphere. Using the NCAR Magnetosphere-Thermosphere-Ionosphere-Electrodynamics General Circulation Model (MTIEGCM), we simulate low-latitude electric-field disturbances associated with direct penetration from high latitudes and with secondary generation by disturbed thermospheric winds. The simulations display a number of features observed in equatorial electric fields on time scales of minutes to hours. Models like the MTIEGCM, together with simultaneous global observations of ionospheric electric fields and thermospheric winds, can help clarify the relative importance of the different mechanisms that produce ionospheric disturbances.

  6. An Ionospheric Response to the 2013 Moore EF5 Tornad, Detected By High-Resolution GPS-TEC Observations

    NASA Astrophysics Data System (ADS)

    Kubota, M.; Nishioka, M.; Tsugawa, T.; Ishii, M.

    2014-12-01

    We observed clear concentric waves and short-period oscillations in the ionosphere after the EF5 tornado hit Moore, Oklahoma, USA, on 20 May 2013 using a dense wide-coverage ionospheric total electron content (TEC) observation in North America. These concentric waves were non-dispersive waves with a horizontal wavelength of ~120 km and a period of ~13 minutes. They were observed for more than seven hours throughout North America. TEC oscillations with a period of ~4 minutes were also observed in the south of Moore for more than eight hours. Comparison between the TEC observation and the infrared cloud image from the GOES satellite indicates that the concentric waves were caused by supercells rather than the tornados themselves. Backward ray-tracing analysis suggests that the leaking of atmospheric waves in a thermal duct excited AGWs in the ionosphere. The short-period TEC oscillation could be explained by the acoustic resonance triggered by strong long-lasting supercells. This observational result provides the first clear evidence of a severe meteorological event causing atmospheric waves propagating upward in the upper atmosphere and reaching the ionosphere.

  7. Space Weather Studies Using the Low-Latitude Ionospheric Sensor Network (LISN)

    NASA Astrophysics Data System (ADS)

    Valladares, C. E.; Pacheco, E.

    2014-12-01

    LISN is an array of small instruments that operates as a real-time distributed observatory to understand the complex day-to-day variability and the extreme state of disturbance that occurs in the South American low-latitude ionosphere nearly every day after sunset. The LISN observatory aims to forecast the initiation and transport of plasma bubbles across the South American continent. The occurrence of this type of plasma structures and their embedded irregularities poses a prominent natural hazard to communication, navigation and high precision pointing systems. As commercial and military aviation is increasingly reliant on Global Navigation Satellite Systems (GNSS) any interruption due to ionospheric irregularities or errors due to large density gradients constitutes a serious threat to passengers and crew. Therefore, it is important to understand the conditions and sources that contribute to the formation of these irregularities. To achieve high quality regional nowcasts and forecasts, the LISN system was designed to include a dense coverage of the South American landmass with 47 GPS receivers, 5 flux-gate magnetometers distributed on 2 base lines and 3 Vertical Incidence Pulsed Ionospheric Radar (VIPIR) ionosondes deployed along the same magnetic meridian that intersects the magnetic equator at 68° W. This presentation will provide a summary of recent instrument installations and new processing techniques that have been developed under the LISN project. We will also present the results of recent efforts to detect TIDs and TEC plasma depletions on a near real-time basis. We will describe a method to estimate the zonal velocity and tilt of the plasma bubbles/depletions by combining observations of TEC depletions acquired with adjacent receivers, making it possible to predict precisely their future locations.

  8. Modelling of ionospheric irregularities during geomagnetic storms over African low latitude region

    NASA Astrophysics Data System (ADS)

    Mungufeni, Patrick

    2016-07-01

    In this study, empirical models of occurrence of ionospheric irregularities over low latitude African region during geomagnetic storms have been developed. The geomagnetic storms considered consisted of Dst ≤ -50 nT. GNSS-derived ionospheric Total Electron Content (TEC) data over Libreville, Gabon (NKLG) (0.35° N, 9.68° E, geographic, 8.05° S, magnetic) and Malindi, Kenya (MAL2) (2.99° S, 40.19° E, geographic, 12.42° S, magnetic) during 2000 - 2014 were used. Ionospheric irregularities at scale- lengths of a few kilometers and ˜400 m were represented with the rate of change of TEC index (ROTI). The inputs for the models are the local time, solar flux index, Auroral Electrojet index, day of the year, and the Dst index, while the output is the median ROTI during these given conditions. To develop the models, the ROTI index values were binned based on the input parameters and cubic B splines were then fitted to the binned data. Developed models using data over NKLG and MAL2 were validated with independent data over stations within 510 km and 680 km radius, respectively. The models captured the enhancements and inhibitions of the occurrence of the ionospheric irregularities during the storm period. The models even emulated these patterns in the various seasons, during medium and high solar activity conditions. The correlation coefficients for the validations were statistically significant and ranged from 0.58 - 0.73, while the percentage of the variance in the observed data explained by the modelled data ranged from 34 - 53.

  9. The influence of ionospheric thin shell height on TEC retrieval from GPS observation

    NASA Astrophysics Data System (ADS)

    Wang, Xiao-Lan; Wan, Qing-Tao; Ma, Guan-Yi; Li, Jing-Hua; Fan, Jiang-Tao

    2016-07-01

    We investigate the influence of assumed height for the thin shell ionosphere model on the Total Electron Content (TEC) derived from a small scale Global Positioning System (GPS) network. TEC and instrumental bias are determined by applying a grid-based algorithm to the data on several geomagnetically quiet days covering a 10 month period in 2006. Comparisons of TEC and instrumental bias are made among assumed heights from 250 km to 700 km with an interval of 10 km. While the TEC variations with time follow the same trend, TEC tends to increase with the height of the thin shell. The difference in TEC between heights 250 km and 700 km can be as large as ˜ 8 TECU in both daytime and nighttime. The times at which the TEC reaches its peak or valley do not vary much with the assumed heights. The instrumental biases, especially bias from the satellite, can vary irregularly with assumed height. Several satellites show a large deviation of ˜ 3 ns for heights larger than 550 km. The goodness of fit for different assumed heights is also examined. The data can be generally well-fitted for heights from 350 km to 700 km. A large deviation happens at heights lower than 350 km. Using the grid-based algorithm, there is no consensus on assumed height as related to data fitting. A thin shell height in the range 350 - 500 km can be a reasonable compromise between data fitting and peak height of the ionosphere.

  10. On the mutual relationship of the equatorial electrojet, TEC and scintillation in the Peruvian sector

    NASA Astrophysics Data System (ADS)

    Khadka, Sovit M.; Valladares, Cesar; Pradipta, Rezy; Pacheco, Edgardo; Condor, Percy

    2016-06-01

    This paper presents the interrelationship between the equatorial electrojet (EEJ) strength, Global Positioning System (GPS)-derived total electron content (TEC), and postsunset scintillation from ground observations with the aim of finding reliable precursors of the occurrence of ionospheric irregularities. Mutual relationship studies provide a possible route to predict the occurrence of TEC fluctuation and scintillation in the ionosphere during the late afternoon and night respectively based on daytime measurement of the equatorial ionosphere. Data from ground based observations in the low latitudes of the west American longitude sector were examined during the 2008 solar minimum. We find a strong relationship exists between the noontime equatorial electrojet and GPS-derived TEC distributions during the afternoon mediated by vertical E × B drift via the fountain effect, but there is little or no relationship with postsunset ionospheric scintillation.

  11. The Low-latitude Ionospheric Sensor Network (LISN): Initial Measurements

    NASA Astrophysics Data System (ADS)

    Valladares, C. E.

    2008-12-01

    This paper describes the characteristics and illustrates the early measurements of the first distributed observatory that is being installed in the South American region to study the low-latitude ionosphere and upper atmosphere. The LISN distributed observatory will be comprised of nearly 70 GPS receivers with the capability to measure Total Electron Content (TEC), amplitude and phase scintillation and Traveling Ionospheric Disturbances (TIDs). The network will include 5 ionosondes able to measure nighttime E-region densities and 5 collocated magnetometers that will be placed along the same magnetic meridian. This network of GPS receivers and ionospheric sensors span from north to south in the South American continent west of the 55o West meridian. In addition to introducing the present capabilities of the LISN network, this paper will present the results of the first LISN campaign dedicated to detect medium-scale (~100 km) TIDs that was conducted at Huancayo using 3 closely-spaced GPS receivers. This paper also presents initial calculations of the vertical drift velocity using 3 magnetometers, two of them placed off the equator in opposite hemispheres and a detailed description of the measurements of the first LISN ionosonde that is presently operating near the magnetic equator.

  12. Seasonal variations of GPS derived TEC at three different latitudes of the southern hemisphere during geomagnetic storms

    NASA Astrophysics Data System (ADS)

    Adebiyi, S. J.; Adimula, I. A.; Oladipo, O. A.

    2014-04-01

    Data from the archive of the International GNSS Services (IGS) were used to study the seasonal variations of Total Electron Content (TEC) over three stations located at different latitudes in the southern hemisphere during the geomagnetic storms of 11 January, 6 April, 8 June, and 13 October 2000, representing storms that occurred in summer, autumn equinox, winter and spring equinox, respectively. The percentage TEC deviation with respect to reference values differs substantially from season to season. A strong seasonal anomaly and clear equinoctial asymmetry in TEC response to the storms were observed. Weak and short-lived positive TEC deviations as well as strong and long-lasting negative trends were observed in summer storm during the main and recovery phases respectively over the high and low latitudes whereas in winter storm, the highest positive TEC deviations was recorded during the main phase over the entire latitudes. TEC enhancement dominated all the stations during the autumn (March) equinox storm while TEC depletion was majorly observed during the spring (September) equinox. All these variations find their explanations in the thermospheric composition change and circulation. Future work with direct or modeled measurement of atomic Oxygen to molecular Nitrogen ratio (O/N2), large number of storms and other possible factors such as variations in storm’s intensity and local time dependence of the storm onset is expected to validate the observations in this study.

  13. The storm-time assessment of GNSS-SBAS performance within low latitude African region using a testbed-like platform

    NASA Astrophysics Data System (ADS)

    Abe, O. E.; Paparini, C.; Ngaya, R. H.; Otero Villamide, X.; Radicella, S. M.; Nava, B.

    2017-09-01

    A Satellite Based Augmentation System (SBAS) is designed to improve Global Navigation Satellite Systems (GNSS) in terms of integrity, accuracy, availability and continuity. The main limitation to SBAS performance optimization is the ionosphere, and this is more critical in low latitude. During geomagnetically disturbed periods the role of storm-time winds is important because they modify the atmospheric composition toward low latitudes. An index of ionospheric disturbance, the relative percentage of deviation of the vertical Total Electron Content (TEC) from the quiet level (DvTEC) at each station was evaluated to study positive and negative phases of the geomagnetic storms. The rate of change of TEC index (ROTI) over all the GNSS stations was estimated to evaluate equatorial ionospheric gradients and irregularities. From the study it is observed that the positive deviations are more frequent than negative ones. The availability map, which is the mean of the combine Vertical Protection Level (VPL) and Horizontal Protection Level (HPL) are used for the SBAS performance. The cases of moderate and minor storms studied during the months of July and October 2013 showed that the SBAS system performance during the disturbed periods depends on the local time in which the storm occurs, geographic longitude and other phenomena that need further study. During the storm-time conditions considered, three out of seven geomagnetic storms indicated good SBAS performance and exceed monthly average of the availability map, three geomagnetic storms reduced the system performance below monthly average while one does not have effect on SBAS system performance in respect to monthly average. The present study indicates ROTI as a better proxy than geomagnetic indices for the assessment of storm-time effects on GNSS-SBAS performance.

  14. Features of 3-7-day planetary-wave-type oscillations in F-layer vertical drift and equatorial spread F observed over two low-latitude stations in China

    NASA Astrophysics Data System (ADS)

    Zhu, Zhengping; Luo, Weihua; Lan, Jiaping; Chang, Shanshan

    2017-06-01

    role of planetary-wave-type oscillations in the initiation and development of ESF in the Chinese low-latitude region.

  15. Mapping the East African Ionosphere Using Ground-based GPS TEC Measurements

    NASA Astrophysics Data System (ADS)

    Mengist, Chalachew Kindie; Kim, Yong Ha; Yeshita, Baylie Damtie; Workayehu, Abyiot Bires

    2016-03-01

    The East African ionosphere (3°S-18°N, 32°E-50°E) was mapped using Total Electron Content (TEC) measurements from ground-based GPS receivers situated at Asmara, Mekelle, Bahir Dar, Robe, Arbaminch, and Nairobi. Assuming a thin shell ionosphere at 350 km altitude, we project the Ionospheric Pierce Point (IPP) of a slant TEC measurement with an elevation angle of >10° to its corresponding location on the map. We then infer the estimated values at any point of interest from the vertical TEC values at the projected locations by means of interpolation. The total number of projected IPPs is in the range of 24-66 at any one time. Since the distribution of the projected IPPs is irregularly spaced, we have used an inverse distance weighted interpolation method to obtain a spatial grid resolution of 1°×1° latitude and longitude, respectively. The TEC maps were generated for the year 2008, with a 2 hr temporal resolution. We note that TEC varies diurnally, with a peak in the late afternoon (at 1700 LT), due to the equatorial ionospheric anomaly. We have observed higher TEC values at low latitudes in both hemispheres compared to the magnetic equatorial region, capturing the ionospheric distribution of the equatorial anomaly. We have also confirmed the equatorial seasonal variation in the ionosphere, characterized by minimum TEC values during the solstices and maximum values during the equinoxes. We evaluate the reliability of the map, demonstrating a mean error (difference between the measured and interpolated values) range of 0.04-0.2 TECU (Total Electron Content Unit). As more measured TEC values become available in this region, the TEC map will be more reliable, thereby allowing us to study in detail the equatorial ionosphere of the African sector, where ionospheric measurements are currently very few.

  16. Scintillations and TEC gradients from Europe to Africa: a picture by the MISW project

    NASA Astrophysics Data System (ADS)

    Alfonsi, Lucilla; Spogli, Luca; Cesaroni, Claudio; Vadakke Veettil, Sreeja; Aquino, Marcio; Zin, Alberto; Wilhelm, Nicolas; Serant, Damien; Forte, Biagio; Mitchell, Cathryn N.; Grzesiak, Marcin; Kos, Timoslav; von Benzon, Hans-Henrik; Zurn, Martin; Enell, Carl-Fredrik; Haggstrom, Ingemar

    2016-04-01

    MISW (Mitigation of space weather threats to GNSS services) is an EU/FP7 project with the purpose of tackling the research challenges associated with Space Weather effects on GNSS (Global Navigation Satellite System). In particular, the objective of MISW is to develop suitable algorithms capable of enabling Satellite Based Augmentation Systems (e.g. EGNOS) in the low-latitude African sector. For this purpose, MISW has created a detailed picture of extreme space weather events that occurred in the past and in the current solar cycle. Despite its weakness, the current solar cycle exhibited two superstorms that happened during the descending phase, in March and in June 2015. The latter has been studied in detail through a careful analysis of GNSS data acquired by TEC (Total Electron Content) and scintillation monitors and by IGS and regional geodetic networks located in Europe and in Africa. The investigation enabled creating the actual scenarios of TEC gradients and scintillation that occurred over a wide latitudinal extent between 21 and 30 June 2015. The investigation is based on calibrated TEC from different receivers, aiming at the estimation of east-west and north-south TEC gradients and on the integration of calibrated TEC and TEC gradients with the scintillation data. The impact of the storm on GNSS performance has also been investigated in terms of losses of lock. The results of this study highlight the importance of assessing the latitudinal and the longitudinal TEC gradients as crucial information to identify to what extent different ionospheric sectors are severely affected by scintillation. On the other hand, this study also shows evidences of how TEC gradients are not always responsible for the observed scintillation. Finally, the outcomes of the study demonstrate the complex relation between scintillation, TEC gradients and losses of GNSS satellites lock.

  17. Variational electric fields at low latitudes and their relation to spread-F and plasma irregularities

    NASA Technical Reports Server (NTRS)

    Holtet, J. A.; Maynard, N. C.; Heppner, J. P.

    1977-01-01

    In situ measurements of variational electric fields at low latitudes, taken by OGO 6 satellite instruments, are analyzed. The observations are compared with other data on F region and spread-F structures. Conformity of the electric field fluctuations with the overall picture of low-latitude irregularities is examined empirically and theoretically, and candidate processes for generation of the observed irregularities are considered. Three distinct types of irregularities are delineated and compared.

  18. GNSS-TEC observations of infrasound excited by a volcanic eruption: Inference of wave front geometry and acoustic wave energy

    NASA Astrophysics Data System (ADS)

    Nakashima, Y.; Aoki, Y.; Nishida, K.; Heki, K.

    2016-12-01

    Volcanic eruptions excite various oscillations depending on various factors. Very low frequency infrasound (-5 mHz) can reach the upper atmosphere and are often observed as ionospheric fluctuations. We can see three types of ionospheric disturbances excited by volcanic eruptions: (1) acoustic trap mode, (2) atmospheric internal gravity wave and (3) traveling acoustic waves come from volcanic explosion. The third one is important to improve our knowledge of acoustic energy by volcanic explosions because we cannot detect infrasound propagating upward by barometers deployed on the ground. We found ionospheric disturbances made by the Kuchinoerabujima volcano eruption on 29 May, 2015, using the GNSS-TEC method. This case is similar to the 2004 Asamayama volcano explosion, and we tried to estimate the energy of the acoustic wave. In this presentation, we will compare TEC perturbations and surface pressure changes. We use 1 Hz-sampled GEONET data from stations in Southwest Japan. Conversion from slant-TEC to vertical-TEC was done using the minimum scalloping. We used apparent velocities calculated from cross correlation function of the TEC disturbances to infer the wave front geometry. Previous studies discussed wave fronts calculated by ray-tracing. However, ray tracing may be inappropriate to such a case because it assumes very short wave length compared with the scale height. Accordingly, we estimated the incident angles from the apparent wave velocities assuming that the ionosphere is a thin layer located at 300 km above the surface. We derived incident angles, and they were similar to those we obtained by ray tracing. In addition, we found that the incident angles inferred in this way had significant dependence on azimuths. We need to find out whether it is real or an apparent phenomenon caused by stations-satellites-wave geometry. We are now trying to estimate the energy, but it varies around the estimated value for the 2004 Asama volcano case. We will consider the

  19. Multifractal analysis of low-latitude geomagnetic fluctuations

    NASA Astrophysics Data System (ADS)

    Bolzan, M. J. A.; Rosa, R. R.; Sahai, Y.

    2009-02-01

    The technique of large deviation multifractal spectrum has shown that the high-latitude (77.5° N, 69.2° W) geomagnetic fluctuations can be described from direct dissipation process or loading-unloading regimes of the solar wind-magnetosphere coupling. In this paper, we analyze the H-component of low-latitude (22.4° S, 43.6° W) geomagnetic field variability observed during the month of July 2000 at the Geomagnetic Observatory, Vassouras, RJ, Brazil. The variability pattern during this period is a mixture of quiet and disturbed days including the Bastille Day intense geomagnetic storm on 15 July. Due to the complexity of this data, we pursue a detailed analysis of the geomagnetic fluctuations in different time scales including a multifractal approach using the singular power spectrum deviations obtained from the wavelet transform modulus maxima (WTMM). The results suggest, as observed from high-latitude data, the occurrence of low-latitude multifractal processes driving the intermittent coupling between the solar wind-magnetosphere and geomagnetic field variations. On finer scales possible physical mechanisms in the context of nonlinear magnetosphere response are discussed.

  20. The response of the ionosphere to intense geomagnetic storms in 2012 using GPS-TEC data from East Africa longitudinal sector

    NASA Astrophysics Data System (ADS)

    Tesema, F.; Damtie, B.; Nigussie, M.

    2015-12-01

    The response of the ionosphere to intense magnetic storms has been studied using total electron content (TEC). TEC data recorded by a series of GPS receivers at a longitude ∼ 35 ° E covering a wide range of latitudes (32 ° S to 68 ° N, geographic) is analyzed to study spatio-temporal modifications of the vertical TEC (vTEC) during storms on 07 and 09 March 2012 and on 14 July 2012. We have observed main phase positive response at equatorial ionization anomaly (EIA) crests and mid latitude regions in all the storms. These main phase positive responses are associated with vertical drift enhancement (intensified equatorial electrojet (EEJ)) and the mechanical effect of equatorward neutral wind after an auroral activity. A daytime substantial depletion of TEC at low latitude region was observed on 08 March 2012. This is due to the combined effects of oversheilding and disturbance dynamo electric field that drive large downward drifts during the day. The low latitude and equatorial ionospheric response in the recovery phase days of March storm is found to be largely associated with the disturbance dynamo field that suppressed the upward E × B drift from EEJ observations. The summer negative and winter positive response in July storm as well as mid latitude positive response in March storm was associated with the composition changes as depicted by the O to N2 ratio from GUVI measurements.

  1. Ionospheric disturbances detected by high-resolution GPS-TEC observations after an earthquake and a tornado

    NASA Astrophysics Data System (ADS)

    Tsugawa, Takuya; Otsuka, Yuichi; Saito, Akinori; Ishii, Mamoru; Nishioka, Michi

    Ionospheric disturbances following the 2011 Tohoku earthquake and the 2013 Moore tornado were observed by high-resolution GPS total electron content (TEC) observations using dense GPS receiver networks. After the 2011 Tohoku earthquake, concentric waves with short propagation distance propagated in the radial direction in the propagation velocity of 3,457, 783, 423 m/s for the first, second, third peak, respectively. Following these waves, concentric waves with long propagation distance appeared to propagate at the velocity of 138-288 m/s. In the vicinity of the epicenter, sudden TEC depletions and short-period oscillations with a period of approximately 4 minutes were also observed. The center of these ionospheric variations, termed the "ionospheric epicenter", corresponded to the tsunami source. Comparing to the results of a numerical simulation using non-hydrostatic compressible atmosphere-ionosphere model, the first peak of circular wave would be caused by the acoustic waves generated from the propagating Rayleigh wave. The second and third waves would be caused by atmospheric gravity waves excited in the lower ionosphere due to the acoustic wave propagations from the tsunami source. The fourth and following waves are considered to be caused by the atmospheric gravity waves induced by the wavefronts of traveling tsunami. After the EF5 tornado hit Moore, Oklahoma, USA, on 20 May 2013, clear concentric waves and short-period oscillations were observed. These concentric waves were non-dispersive waves with a horizontal wavelength of approximately 120 km and a period of approximately 13 minutes. They were observed for more than seven hours throughout North America. TEC oscillations with a period of approximately 4 minutes were also observed in the south of Moore for more than eight hours. Comparison between the GPS-TEC observations and the infrared cloud images from the GOES satellite indicates that the concentric waves and the short-period oscillations would be

  2. Two-dimensional ionospheric tomography over the low-latitude Indian region: An intercomparison of ART and MART algorithms

    NASA Astrophysics Data System (ADS)

    Das, Sukanta Kumar; Shukla, Ashish Kumar

    2011-04-01

    Single-frequency users of a satellite-based augmentation system (SBAS) rely on ionospheric models to mitigate the delay due to the ionosphere. The ionosphere is the major source of range and range rate errors for users of the Global Positioning System (GPS) who require high-accuracy positioning. The purpose of the present study is to develop a tomography model to reconstruct the total electron content (TEC) over the low-latitude Indian region which lies in the equatorial ionospheric anomaly belt. In the present study, the TEC data collected from the six TEC collection stations along a longitudinal belt of around 77 degrees are used. The main objective of the study is to find out optimum pixel size which supports a better reconstruction of the electron density and hence the TEC over the low-latitude Indian region. Performance of two reconstruction algorithms Algebraic Reconstruction Technique (ART) and Multiplicative Algebraic Reconstruction Technique (MART) is analyzed for different pixel sizes varying from 1 to 6 degrees in latitude. It is found from the analysis that the optimum pixel size is 5° × 50 km over the Indian region using both ART and MART algorithms.

  3. The low-latitude Rapitan glaciation (Invited)

    NASA Astrophysics Data System (ADS)

    MacDonald, F. A.; Schmitz, M. D.; Crowley, J. L.; Roots, C.; Maloof, A. C.; Jones, D. S.; Strauss, J.

    2009-12-01

    The snowball Earth hypothesis1 was developed in response to strong palaeomagnetic evidence for low-latitude glaciation from “Marinoan” glacial deposits in the Elatina Formation of Australia. An earlier Cryogenian glaciation, commonly referred to as the “Sturtian” glaciation, has been inferred from the ubiquity of pre-Marinoan glacial deposits; however, the synchroneity and global extent of this event have been questioned due to the lack of precise U/Pb ages and robust paleomagnetic data. Herein we provide new age constraints on the Franklin LIP with revised U/Pb ID-TIMS dates on the the Mt. Harper volcanic complex in the Yukon Territory and the Coronation sills of Victoria Island. Furthermore we present a new age from a tuff interbedded with diamictite in the Upper Mt. Harper Group. A glaciogenic origin of the diamictites is provided by striated clasts and laminae-penetrating dropstones. These glacial deposits can be traced from Alaska westward through the Yukon Territory and into the Northwest Territories, and are correlative to the Rapitan Group. Throughout the Cordillera, the Rapitan Group and its correlatives commonly host iron formation, are the lower of two Cryogenian glacial horizons, and globally are thought to be equivalent to the Sturtian glaciation. The age of the tuff interbedded with the glacial deposits in the Mt. Harper Group is within 1 million years of the revised age on the Franklin LIP. Several paleomagnetic studies on dikes, sills, and basalts spanning >2000 km of NW Canada have agreed that the Franklin LIP erupted when NW Laurentia was in an equatorial position2,3. Consequently, the Sturtian glaciation on Laurentia can now be confidently inferred to have occurred at a very low palaeolatitude. Thus, there were at least two Cryogenian glaciations of global extent. 1 Kirschvink, J.L., in The Proterozoic Biosphere, edited by J. W. Schopf and C. Klein (Cambridge University Press, Cambridge, 1992), pp. 51. 2 Park, J.K., Paleomagnetic

  4. Comparison with IRI-PLUS and IRI-2012-TEC values of GPS-TEC values

    NASA Astrophysics Data System (ADS)

    Atıcı, Ramazan; Saǧır, Selçuk

    2016-07-01

    This study presents a comparison with IRI-PLUS and IRI-2012 Total Electron Content (TEC) values of Total Electron Content (TEC) values obtained from Ankara station (39,7 N; 32,76 E) of Global Position System (GPS) of Turkey on equinox and solstice days of 2009 year. For all days, it is observed that GPS-TEC values are greater than IRI-2012-TEC values, while IRI-PLUS-TEC values are very close to GPS-TEC values. When GPS-TEC values for both equinoxes are compared, it is seen that TEC values on September equinox are greater than one on March equinox. However, it is observed that GPS-TEC values on June solstice are greater than one on December solstice. Also, the relationship between GPS-TEC values and geomagnetic indexes is investigated.

  5. Theory of the low-latitude boundary layer

    NASA Technical Reports Server (NTRS)

    Sonnerup, B. U. OE.

    1980-01-01

    A one-dimensional steady state fluid mechanical model is developed of the low-latitude plasma boundary layer inside the dawn and dusk magnetopause. Momentum transfer in the layer is produced by viscosity and/or mass diffusion. Coupling to the ionosphere is achieved via field-aligned currents, the magnitude of which is limited by parallel potential drops. These currents flow into and out of the ionosphere in the manner described by Iijima and Potemra. The higher-latitude (region 1) currents are associated with the boundary layer proper, while the lower-latitude (region 2) ones are associated with a region of sunward return flow adjacent to the boundary layer. The parallel potential drops have a magnitude of typically 2-3 kV and a north-south extent of 100-200 km. The calculated potential profile corresponds reasonably well to observed inverted V precipitation events.

  6. Low-latitude mountain glacier evidence for abrupt climate changes

    NASA Astrophysics Data System (ADS)

    Thompson, L. G.; Mosley-Thompson, E. S.; Lin, P.; Davis, M. E.; Mashiotta, T. A.; Brecher, H. H.

    2004-12-01

    Clear evidence that a widespread warming of Earth's climate system is now underway comes from low latitude mountain glaciers. Proxy temperature histories reconstructed from ice cores, and the rapidly accelerating loss of both the total ice area and ice volume on a near global scale suggest that these glaciers responding to increasing rates of melting. In situ observations reveal the startling rates at which many tropical glaciers are disappearing. For example, the retreat of the terminus of the Qori Kalis Glacier in Peru is roughly 200 meters per year, 40 times faster than its retreat rate in 1978. Similarly, in 1912 the ice on Mount Kilimanjaro covered 12.1 km2, but today it covers only 2.6 km2. If the current rate of retreat continues, the perennial ice fields may disappear within the next few decades, making this the first time in the past 11,700 years that Kilimanjaro will be devoid of the ice that shrouds its summit. Tropical glaciers may be considered ``the canaries in the coal mine'' for the global climate system as they integrate and respond to key climatological variables, such as temperature, precipitation, cloudiness, humidity, and incident solar radiation. A composite of the decadally-averaged oxygen isotopic records from three Andean and three Tibetan ice cores extending back over the last two millennia shows an isotopic enrichment in the 20th century that suggests a large-scale warming is underway at lower latitudes. Multiple lines of evidence from Africa, the Middle East, Europe and South America indicate an abrupt mid-Holocene climate event in the low latitudes. If such an event were to occur now with a global human population of 6.3 billion people, the consequences could be severe. Clearly, we need to understand the nature and cause of abrupt climate events.

  7. Thermal imbalance and shock wave effects on low latitude ionosphere : asymmetric case of a total solar eclipse

    NASA Astrophysics Data System (ADS)

    Vila, P. M.; Fleury, R.; Le Roux, Y.; Kone, E.

    2003-04-01

    frequency, as well as on GPS TEC values. Analogic analysis of these modes show that local atmospheric shock waves were generated by the rapid eclipse totality cooling from sources at troposphere, mesosphere and lower thermosphere levels. For the general case of low latitudes Africa, we infer that similar oscillations from atmospheric storms can disturb the equatorial ionosphere during daytime (e.g. electrojet shears, layer strata) and nighttime (the irregular ESF seed component).

  8. Comparison between IRI-2012 and GPS-TEC observations over the western Black Sea

    NASA Astrophysics Data System (ADS)

    Inyurt, Samed; Yildirim, Omer; Mekik, Cetin

    2017-07-01

    The ionosphere is a dynamic layer which generally changes according to radiation emitted by the sun, the movement of the earth around the sun, and sunspot activity. Variations can generally be categorized as regular or irregular variations. Both types of variation have a huge effect on radio wave propagation. In this study, we have focused on the seasonal variation effect, which is one of the regular forms of variation in terms of the ionosphere. We examined the seasonal variation over the ZONG station in Turkey for the year 2014. Our analysis results and IRI-2012 present different ideas about ionospheric activity. According to our analysed results, the standard deviation reached a maximum value in April 2014. However, the maximum standard deviation obtained from IRI-2012 was seen in February 2014. Furthermore, it is clear that IRI-2012 underestimated the VTEC values when compared to our results for all the months analysed. The main source of difference between the two models is the IRI-2012 topside ionospheric representation. IRI-2012 VTEC has been produced as a result of the integration of an electron density profile within altitudinal limits of 60-2000 km. In other words, the main problem with regard to the IRI-2012 VTEC representation is not being situated in the plasmaspheric part of the ionosphere. Therefore we propose that the plasmaspheric part should be taken into account to calculate the correct TEC values in mid-latitude regions, and we note that IRI-2012 does not supply precise TEC values for use in ionospheric studies.

  9. Response of nighttime equatorial and low latitude F-region to the geomagnetic storm of August 18, 2003, in the Brazilian sector

    NASA Astrophysics Data System (ADS)

    Sahai, Y.; Becker-Guedes, F.; Fagundes, P. R.; Lima, W. L. C.; Otsuka, Y.; Huang, C.-S.; Espinoza, E. S.; Pi, X.; de Abreu, A. J.; Bolzan, M. J. A.; Pillat, V. G.; Abalde, J. R.; Pimenta, A. A.; Bittencourt, J. A.

    This paper presents an investigation of geomagnetic storm effects in the equatorial and low latitude F-region in the Brazilian sector during the intense geomagnetic storm on 18 August, 2003 (SSC 14:21 UT on 17/08; ΣKp = 52+; Ap = 108; ∣Dst∣ max = 168 at 1600 UT on 18/08). Simultaneous ionospheric sounding measurements from two stations, viz., Palmas (10.2°S, 48.2°W; dip latitude 5.7°S) and Sao Jose dos Campos (23.2°S, 45.9°W; dip latitude 17.6°S), Brazil, are presented for the nights of 16-17, 17-18 and 18-19 August, 2003 (quiet, disturbed and recovery phases). Both stations are equipped with the Canadian Advanced Digital Ionosonde (CADI). Quiet and disturbed conditions of the F-region ionosphere are compared using data collected from the two stations. The relationship between magnetospheric disturbance and low-latitude ionospheric dynamics, and generation of ionospheric irregularities are discussed. On the disturbed nights (17-18 and 18-19 August), the low latitude station S. J. Campos showed strong enhancements in the F-region critical frequency (foF2), whereas the near equatorial station Palmas showed strong uplifting of the F-layer about 1 h earlier. Normally during the June solstice months (May-August) in the Brazilian sector, large-scale ionospheric irregularities in form of plasma bubbles are rarely observed. On the night of 17-18 August, ionsospheric sounding observations at Palmas showed the presence of bottomside spread-F, whereas on the night of 18-19 August, the observations at Palmas and S. J. Campos showed the presence of plasma bubbles when the storm recovery phase had just started. The complementary GPS data available from several stations in the "Rede Brasileira de Monitoramento Continuo de GPS (Brazilian Network for Continuous GPS Monitoring)" are used to obtain the vertical total electron content (VTEC) and the rate of change of TEC per minute on UT days 18 and 19 August, 2003 and presented. Also, several global ionospheric TEC maps

  10. Equatorial ionization anomaly development as studied by GPS TEC and foF2 over Brazil: A comparison of observations with model results from SUPIM and IRI-2012

    NASA Astrophysics Data System (ADS)

    Nogueira, P. A. B.; Abdu, M. A.; Souza, J. R.; Batista, I. S.; Bailey, G. J.; Santos, A. M.; Takahashi, H.

    2013-11-01

    The equatorial ionization anomaly (EIA) development is studied using the total electron content (TEC) observed by the Global Positioning System (GPS) satellites, the F2-layer critical frequency (foF2) as measured by digisondes operated in the Brazilian sector, and by model simulation using the SUPIM (Sheffield University Plasmasphere Ionosphere Model). We have used two indices based on foF2 and TEC to represent the strength of the EIA Southern Anomaly Crest (SAC), which are denoted, respectively, by SAC(foF2) and SAC(TEC). Significant differences in the local time variations of the EIA intensity, as represented by these two indices, are investigated. The observed SAC indices are compared with their values modeled by the SUPIM and also by the International Reference Ionosphere (IRI)-2012. The SUPIM simulations that use the standard E×B plasma drift and neutral air wind models are found to provide acceptable representations of the observed foF2 and TEC, and hence the indices SAC(foF2) and SAC(TEC) during daytime, whereas the IRI-2012 model is not, except during the post-midnight/sunrise hours. It is found that the differences in the local time variations between the SAC(foF2) and SAC(TEC) can be reduced by limiting the TEC integrations in height up to an altitude of 630 km in the SUPIM calculations. It is also found that when the EIA intensity is calculated for an intermediate dip latitude (12°S) the difference between the local time variation patterns of the two corresponding indices in the experimental data and in the SUPIM results is reduced. For the IRI-2012 values, the subequatorial station modification does not appear to have any effect.

  11. Studies of Total Electron Content variations at low-latitude stations within the Equatorial Ionization Anomaly zone

    NASA Astrophysics Data System (ADS)

    Lilian, Olatunbosun

    2016-07-01

    The total electron content (TEC) is an important parameter to monitor for possible space weather impacts. The radio waves that pass through the earth's ionosphere travel more slowly than their free space velocity due to group path delay of the ionosphere. This group path delay is directly proportional to the TEC of the ionosphere. Using dual frequency GPS receiver at low latitude stations of Ile-Ife (7.52oN, 4.28oE), Addis Ababa (9.04oN, 38.77oE) and Bangalore (13.03oE, 77.57oE), all located within 0 - 15oN of the equatorial anomaly region, the measurement of ionospheric TEC for 2012 has been carried out. The data from the three stations were used to study the diurnal, monthly and seasonal variations of TEC. The diurnal variations maximize between 10:00 - 16:00UT, 08:00 - 14:00UT and 06:00 - 12:00UT for Ile-Ife, Addis Ababa and Bangalore stations respectively. The diurnal variations showed wave-like pertubation during disturbed and quiet periods at Bangalore and Addis Ababa stations. The monthly average TEC variations showed that the month of March recorded the highest TEC value of ~59TECu at about 16:00UT in Ile-Ife station, while TEC at Addis Ababa and Bangalore maximize in October with ~72TECu and 65TECu at about 11:00UT and 09:00UT respectively. Seasonal variations showed that TEC maximizes during the equinoctial months and least in summer, over the three stations. Keywords: Total Electron Content, Equatorial Ionization Anomaly, Global Positioning System co-author:E.A. Ariyibi(Department of Physics and Engineering Physics, Obafemi Awolowo University, Ile-Ife, Nigeria)

  12. Penetrating of high-latitude-electric-field effects to low latitudes during SUNDIAL 1984

    NASA Technical Reports Server (NTRS)

    Spiro, R. W.; Wolf, R. A.; Fejer, B. G.

    1988-01-01

    Electric-field-penetration events have been identified using F-region vertical-drift measurements obtained in the October 6-13, 1984 period by the Jicamarcan incoherent-backscatter radar and corresponding h-prime F measurements from ionosondes at Fortaleza, Cachoeira Paulista, and Dakar. Predictions made using the Rice Convection Model for the pattern, strength, and duration of the low-latitude electric field occurring in response to an increasing high-latitude convection agree with observations. The observed 1-2 h duration of the low-latitude response to decreased convection can be explained by the fossil-wind theory of Richmond (1983).

  13. Comparative analysis of GPS-derived TEC estimates and foF2 observations during storm conditions towards the expansion of ionospheric forecasting capabilities over Europe

    NASA Astrophysics Data System (ADS)

    Tsagouri, Ioanna; Belehaki, Anna; Elias, Panagiotis

    2017-04-01

    This paper builts the discussion on the comparative analysis of the variations in the peak electron density at F2 layer and the TEC parameter during a significant number of geomagnetic storm events that occurred in the present solar cycle 24. The ionospheric disturbances are determined through the comparison of actual observations of the foF2 critical frequency and GPS-TEC estimates obtained over European locations with the corresponding median estimates, and they are analysed in conjunction to the solar wind conditions at L1 point that are monitored by the ACE spacecraft. The quantification of the storm impact on the TEC parameter in terms of possible limitations introduced by different TEC derivation methods is carefully addressed.The results reveal similarities and differences in the response of the two parameters with respect to the solar wind drivers of the storms, as well as the local time and the latitude of the observation point. The aforementioned dependences drive the storm-time forecasts of the SWIF model (Solar Wind driven autorgressive model for Ionospheric short-term Forecast), which is operationally implemented in the DIAS system (http://dias.space.noa.gr) and extensively tested in performance at several occassions. In its present version, the model provides alerts and warnings for upcoming ionospheric disturbances, as well as single site and regional forecasts of the foF2 characteristic over Europe up to 24 hours ahead based on the assesment of the solar wind conditions at ACE location. In that respect, the results obtained above support the upgrade of the SWIF's modeling technique in forecasting the storm-time TEC variation within an operational environment several hours in advance. Preliminary results on the evaluation of the model's efficiency in TEC prediction are also discussed, giving special attention in the assesment of the capabilities through the TEC-derivation uncertanties for future discussions.

  14. Real-time reconstruction of topside ionosphere scale height from coordinated GPS-TEC and ionosonde observations

    NASA Astrophysics Data System (ADS)

    Gulyaeva, Tamara; Poustovalova, Ljubov

    The International Reference Ionosphere model extended to the plasmasphere, IRI-Plas, has been recently updated for assimilation of total electron content, TEC, derived from observations with Global Navigation Satellite System, GNSS. The ionosonde products of the F2 layer peak density (NmF2) and height (hmF2) ensure true electron density maximum at the F2 peak. The daily solar and magnetic indices used by IRI-Plas code are compiled in data files including the 3-hour ap and kp magnetic index from 1958 onward, 12-monthly smoothed sunspot number R12 and Global Electron Content GEC12, daily solar radio flux F10.7 and daily sunspot number Ri. The 3-h ap-index is available in Real Time, RT, mode from GFZ, Potsdam, Germany, daily update of F10.7 is provided by Space Weather Canada service, and daily estimated international sunspot number Ri is provided by Solar Influences Data Analysis Center, SIDC, Belgium. For IRI-Plas-RT operation in regime of the daily update and prediction of the F2 layer peak parameters, the proxy kp and ap forecast for 3 to 24 hours ahead based on data for preceding 12 hours is applied online at http://www.izmiran.ru/services/iweather/. The topside electron density profile of IRI-Plas code is expressed with complementary half-peak density anchor height above hmF2 which corresponds to transition O+/H+ height. The present investigation is focused on reconstruction of topside ionosphere scale height using vertical total electron content (TEC) data derived from the Global Positioning System GPS observations and the ionosonde derived F2 layer peak parameters from 25 observatories ingested into IRI-Plas model. GPS-TEC and ionosonde measurements at solar maximum (September, 2002, and October, 2003) for quiet, positively disturbed, and negatively disturbed days of the month are used to obtain the topside scale height, Htop, representing the range of altitudes from hmF2 to the height where NmF2 decay by e times occurs. Mapping of the F2 layer peak parameters

  15. Longitudinal Variation in GPS -TEC and Topside Electron Density Associated with the Wave Number Four Structures over South American Sector

    NASA Astrophysics Data System (ADS)

    Nogueira, P. A.; Abdu, M. A.; Souza, J. R.; Bailey, G. J.; Shume, E. B.; Denardini, C. M.

    2012-12-01

    Recent observations of the low-latitude ionospheric electron density have revealed a longitudinal structure in the Equatorial Ionization Anomaly (EIA) intensity, which is characterized by a wave number-four pattern when plotted at a constant-local-time frame. It has been proposed that neutral wind driven dynamo electric fields from the E-region due to non migrating tidal modes are responsible for this pattern. In the present work we have used measurements from the Defense Meteorological Satellite Program (DMSP) to investigate the four peaks structure in the topside electron density of the low latitude ionosphere. We also compare the climatology of the Total Electron Content (TEC) as observed by GPS receivers in two equatorial stations over South America, São Luís (2.33 S, 315.8E, declination = -19 degree) in Brazil and Arequipa (16.5S, 288.5E, declination = 0.5 degree) in Peru. TEC variations for three solar activity levels (high, moderate and low) have been analyzed. TEC values over São Luís are found to be larger than that ones over Arequipa independent of the season, local time and solar cycle conditions. We estimated the vertical plasma drifts over these stations using magnetometer data during daytime and using ionosonde data for evening hours. We fed the Sheffield University Plasmasphere Ionosphere Model (SUPIM) with this drifts in an attempt to partially explain the differences in the TEC over these stations. The SUPIM was also used to evaluate the effect of thermospheric wind to cause the four peaks structure in the plasma density. Therefore, we analyze the equatorial ionospheric response to combined effects of thermospheric neutral winds and zonal electric field causing the longitudinal variation in TEC observed in the South American longitude sector.

  16. Low Latitude Aeronomy Study in Africa

    DTIC Science & Technology

    2016-02-09

    bubbles. Due to delay in deploying the instruments, we were not able to validate the TIEGCM results with new observations from Africa. However, the new...modeling and observational efforts on equatorial aeronomy study in Africa. During a recent International Symposium on Equatorial Aeronomy in Ethiopia...we met many colleagues from Africa, who are greatly interested in the new observational results from Africa. We are looking forward to more

  17. Virtual array beamforming of GPS TEC observations of co-seismic ionospheric disturbances near the Geomagnetic South Pole triggered by teleseismic megathrusts

    NASA Astrophysics Data System (ADS)

    Gomez, D.; Smalley, R., Jr.; Langston, C. A.; Wilson, T. J.; Bevis, M. G.; Dalziel, I. W. D.; Kendrick, E. C.; Konfal, S. A.; Willis, M. J.; Piñón, D. A.; Cimbaro, S.; Caccamise, D. J., II

    2015-12-01

    We identified co-seismic ionospheric disturbances (CID) in Antarctica generated by the Rayleigh waves of the 2010 Maule and the 2011 Tohoku-Oki earthquakes analyzing TEC data with a modified beamforming technique. Beamforming in Antarctica, however, is not straightforward due to the effects of array deformation and atmospheric neutral wave-ionospheric plasma coupling. We take these effects into account and present a method to invert for the seismically generated acoustic wave using TEC observations. The back azimuths, speeds and waveforms obtained by the beamform are in excellent agreement with the hypothesis that the TEC signals are generated by the passage of Rayleigh waves from the Maule and Tohoku-Oki earthquakes. The Tohoku-Oki earthquake is ~12,500 km from Antarctica, making this the farthest observation of CIDs to date using GPS.

  18. Analytical study of nighttime scintillations using GPS at low latitude station Bhopal

    SciTech Connect

    Maski, Kalpana; Vijay, S. K.

    2015-07-31

    Sporadically structured ionosphere (i.e. in-homogeneities in refractive index) can cause fluctuations (due to refraction effects) on the radio signal that is passing through it. These fluctuations are called ionospheric scintillations. Low latitude region is suitable for studying these scintillations. The influence of the ionosphere on the propagation of the radio wave becomes very marked with reference to communication or navigational radio system at very low frequency (VLF) to a high frequency (HF), which operate over the distances of 1000 km or more. Radio wave communication at different frequencies depends on structure of the ionosphere. With the advent of the artificial satellites, they are used as a prime mode of radio wave communication. Some natural perturbation termed as irregularities, are present in the form of electron density of the ionosphere that cause disruption in the radio and satellite communications. Therefore the study of the ionospheric irregularities is of practical importance, if one wishes to understand the upper atmosphere completely. In order to make these communications uninterrupted the knowledge of irregularities, which are present in the ionosphere are very important. These irregularities can be located and estimated with the help of Ionospheric TEC and Scintillation. Scintillation is generally confined to nighttime hours, particularly around equatorial and low latitudes.

  19. Characteristics of the low latitude ionospheric storm in the East-Asian region

    NASA Astrophysics Data System (ADS)

    Zhao, B.; Wan, W.; Liu, L.

    2009-04-01

    First, an classification of ionospheric storm effects in the sub-equatorial ionization anomaly(SEIA) region at 120°E has been performed through the analysis of ionogram data at two ionosonde stations, Wuhan (114.4°, 30.5°) and Chung-Li (121.2°, 25°), and total electron content (TEC) derived from GPS network distributed around 120°E during the year 1999-2004. Three types of negative phase are identified. One is shown to be varied in phase of F-layer height variation and the other two out of phase. Two types of positive phase are also found. The mechanisms to cause these types of ionospheric effects has been considered to be related with storm meridional thermospheric wind including traveling atmosphere disturbance(TAD), electric fields and composition changes. Then based on the 50 years of ionosonde and 8 years of global ionospheric maps (GIMs) data, features of low latitude ionospheric storm were obtained. Results shows that positive phases during 18:00-2400 LT with its center near the 21:00 LT and 24:00-08:00 LT with its center near the sunrise time have predomination in controlling the storm behavior at low latitudes. It is shown that the distortion of EIA under the effects of the above factors have significant influence on the behavior of SEIA ionogram parameters.

  20. Origin of high-frequency TEC disturbances observed by GPS over the European mid-latitude region

    NASA Astrophysics Data System (ADS)

    Wautelet, Gilles; Warnant, Rene

    2015-10-01

    High-frequency variability of the ionospheric Total Electron Content (TEC) can strongly affect precise positioning with GNSS. The occurrence rate as well as the amplitude of such disturbances has been extensively studied over the last decade. Mainly, one can distinguish disturbances due to space-weather events and the others, qualified as "quiet-time" as they are observed during quiet geomagnetic conditions. The latter, which represent more than 75% of the total number of disturbances over mid-latitudes, are then divided into two categories: the Winter Daytime (WD) and the Summer Nighttime (SN). The first category, representing the bulk of quiet-time disturbances, corresponds to classical Medium-Scale Traveling Ionospheric Disturbances (MSTIDs), that are the result of the interaction of gravity waves and the ionospheric plasma. On the other hand, SN disturbances are generally understood as non-classical MSTIDs of electrical origin. The paper investigates the origin of these two types of disturbance based on GPS measurements, ionospheric soundings and wind speed data at a tropospheric level. If one cannot exclude the solar terminator as a potential source of gravity waves responsible for WD events, it is thought that the major contribution comes from the lower atmosphere. More precisely, tropospheric jetstream is considered as the favorite candidate for daytime MSTIDs. Turning to SN disturbances, our analysis reveals that they are related to spread-F phenomenon, linked to the appearance of sporadic E-layers. The related instabilities are responsible for field-aligned irregularities in the F-region, which are thought to be responsible for noise-like fluctuations of the GPS TEC observed during SN events.

  1. TEC enhancements during HF ionospheric heating experiments

    NASA Astrophysics Data System (ADS)

    Fallen, C. T.; Watkins, B. J.; Secan, J. A.

    2009-12-01

    Experimental observations at the HAARP facility near Gakona, Alaska have shown TEC enhancements in association with powerful O-mode HF transmissions. Typical TEC enhancements are 10-20% of a TEC unit. A numerical ionospheric model has been used to simulate these events. HF energy is absorbed by the ionospheric electrons in a narrow range of altitudes below the F-region peak. Subsequent increases in electron temperature and ambipolar diffusion along the geomagnetic field act to redistribute existing ionization to higher altitudes where chemical recombination rates are lower. As a result the model shows TEC increases of about 10-20% of a TEC unit, similar to observations.

  2. An ensemble average method to estimate absolute TEC using radio beacon-based differential phase measurements: Applicability to regions of large latitudinal gradients in plasma density

    NASA Astrophysics Data System (ADS)

    Thampi, Smitha V.; Bagiya, Mala S.; Chakrabarty, D.; Acharya, Y. B.; Yamamoto, M.

    2014-12-01

    A GNU Radio Beacon Receiver (GRBR) system for total electron content (TEC) measurements using 150 and 400 MHz transmissions from Low-Earth Orbiting Satellites (LEOS) is fabricated in house and made operational at Ahmedabad (23.04°N, 72.54°E geographic, dip latitude 17°N) since May 2013. This system receives the 150 and 400 MHz transmissions from high-inclination LEOS. The first few days of observations are presented in this work to bring out the efficacy of an ensemble average method to convert the relative TECs to absolute TECs. This method is a modified version of the differential Doppler-based method proposed by de Mendonca (1962) and suitable even for ionospheric regions with large spatial gradients. Comparison of TECs derived from a collocated GPS receiver shows that the absolute TECs estimated by this method are reliable estimates over regions with large spatial gradient. This method is useful even when only one receiving station is available. The differences between these observations are discussed to bring out the importance of the spatial differences between the ionospheric pierce points of these satellites. A few examples of the latitudinal variation of TEC during different local times using GRBR measurements are also presented, which demonstrates the potential of radio beacon measurements in capturing the large-scale plasma transport processes in the low-latitude ionosphere.

  3. Low-Latitude Ethane Rain on Titan

    NASA Technical Reports Server (NTRS)

    Dalba, Paul A.; Buratti, Bonnie J.; Brown, R. H.; Barnes, J. W.; Baines, K. H.; Sotin, C.; Clark, R. N.; Lawrence, K. J.; Nicholson, P. D.

    2012-01-01

    Cassini ISS observed multiple widespread changes in surface brightness in Titan's equatorial regions over the past three years. These brightness variations are attributed to rainfall from cloud systems that appear to form seasonally. Determining the composition of this rainfall is an important step in understanding the "methanological" cycle on Titan. I use data from Cassini VIMS to complete a spectroscopic investigation of multiple rain-wetted areas. I compute "before-and-after" spectral ratios of any areas that show either deposition or evaporation of rain. By comparing these spectral ratios to a model of liquid ethane, I find that the rain is most likely composed of liquid ethane. The spectrum of liquid ethane contains multiple absorption features that fall within the 2-micron and 5-micron spectral windows in Titan's atmosphere. I show that these features are visible in the spectra taken of Titan's surface and that they are characteristically different than those in the spectrum of liquid methane. Furthermore, just as ISS saw the surface brightness reverting to its original state after a period of time, I show that VIMS observations of later flybys show the surface composition in different stages of returning to its initial form.

  4. Low-Latitude Ethane Rain on Titan

    NASA Technical Reports Server (NTRS)

    Dalba, Paul A.; Buratti, Bonnie J.; Brown, R. H.; Barnes, J. W.; Baines, K. H.; Sotin, C.; Clark, R. N.; Lawrence, K. J.; Nicholson, P. D.

    2012-01-01

    Cassini ISS observed multiple widespread changes in surface brightness in Titan's equatorial regions over the past three years. These brightness variations are attributed to rainfall from cloud systems that appear to form seasonally. Determining the composition of this rainfall is an important step in understanding the "methanological" cycle on Titan. I use data from Cassini VIMS to complete a spectroscopic investigation of multiple rain-wetted areas. I compute "before-and-after" spectral ratios of any areas that show either deposition or evaporation of rain. By comparing these spectral ratios to a model of liquid ethane, I find that the rain is most likely composed of liquid ethane. The spectrum of liquid ethane contains multiple absorption features that fall within the 2-micron and 5-micron spectral windows in Titan's atmosphere. I show that these features are visible in the spectra taken of Titan's surface and that they are characteristically different than those in the spectrum of liquid methane. Furthermore, just as ISS saw the surface brightness reverting to its original state after a period of time, I show that VIMS observations of later flybys show the surface composition in different stages of returning to its initial form.

  5. Low-Latitude Ethane Rain on Titan

    NASA Astrophysics Data System (ADS)

    Dalba, Paul; Buratti, B. J.; Brown, R. H.; Barnes, J. W.; Baines, K. H.; Sotin, C.; Clark, R. N.; Lawrence, K. J.; Nicholson, P. D.

    2012-10-01

    Cassini ISS observed multiple widespread changes in surface brightness in Titan's equatorial regions over the past three years (Barnes, J. W. et al. 2012, Icarus, submitted). These brightness variations are attributed to rainfall from cloud systems that appear to form seasonally (Turtle, E. P. et al. 2011, Science, 331, 1414-1417). Determining the composition of this rainfall is an important step in understanding the “methanological” cycle that dominates Titan's surface and atmosphere. In this study, we use data from Cassini VIMS to complete a thorough spectroscopic investigation of rain-wetted areas near Yalaing Terra, Hetpet Regio and central Adiri on Titan. We compute “before-and-after” spectral ratios of any areas that show either deposition or evaporation of rain at any point in the time span of August 2009 to January 2012. By comparing these spectral ratios to a model of liquid ethane that was calculated to match the resolution and sampling interval of VIMS (Brown, R. H. et al. 2008, Nature, 454, 607-610), we find that the rain is most likely composed of liquid ethane. The spectrum of liquid ethane contains multiple absorption features that fortunately fall within the 2-micron and 5-micron spectral windows in Titan's atmosphere. We show that these features are visible in the spectra taken of Titan's surface and that they are characteristically different than those in the spectrum of liquid methane. Furthermore, just as ISS saw the surface brightness reverting to its original state after a period of time, we show that VIMS observations of later flybys show the surface composition in different stages of returning to its initial form as well. This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology under contract to the National Aeronautics and Space Administration. Copyright 2012. All rights reserved.

  6. Low-latitude ethane rain on Titan

    NASA Astrophysics Data System (ADS)

    Dalba, P. A.; Buratti, B. J.; Brown, R. H.; Barnes, J. W.; Baines, K. H.; Sotin, C.; Clark, R. N.; Lawrence, K. J.; Nicholson, P. D.

    2012-12-01

    Cassini ISS observed multiple widespread changes in surface brightness in Titan's equatorial regions over the past three years (Barnes, J. W. et al. 2012, Icarus, submitted). These brightness variations are attributed to rainfall from cloud systems that appear to form seasonally (Turtle, E. P. et al. 2011, Science, 331, 1414-1417). Determining the composition of this rainfall is an important step in understanding the "methanological" cycle that dominates Titan's surface and atmosphere. In this study, we use data from Cassini VIMS to complete a thorough spectroscopic investigation of rain-wetted areas near Yalaing Terra, Hetpet Regio and central Adiri on Titan. We compute "before-and-after" spectral ratios of any areas that show either deposition or evaporation of rain at any point in the time span of August 2009 to January 2012. By comparing these spectral ratios to a model of liquid ethane that was calculated to match the resolution and sampling interval of VIMS (Brown, R. H. et al. 2008, Nature, 454, 607-610), we find that the rain is most likely composed of liquid ethane. The spectrum of liquid ethane contains multiple absorption features that fortunately fall within the 2-micron and 5-micron spectral windows in Titan's atmosphere. We show that these features are visible in the spectra taken of Titan's surface and that they are characteristically different than those in the spectrum of liquid methane. Furthermore, just as ISS saw the surface brightness reverting to its original state after a period of time, we show that VIMS observations of later flybys show the surface composition in different stages of returning to its initial form as well. Funded by NASA.

  7. Numerical weather prediction in low latitudes

    NASA Technical Reports Server (NTRS)

    Krishnamurti, T. N.

    1985-01-01

    Based on the results of a number of numerical prediction experiments, the differential heating between land and ocean is an important and critical factor for investigation of phenomenon such as the onset of monsoons over the Indian subcontinent. The pre-onset period during the month of May shows a rather persistent flow field in the monsoon region. At low levels the circulation exhibits anticyclonic excursions over the Arabian Sea, flowing essentially parallel to the west coast of India from the north. Over the Indian subcontinent the major feature is a shallow heat low over northern India. As the heat sources commence a rapid northwestward movement toward the southern edge of the Tibetan Plateau, an interesting configuration of the large-scale divergent circulation occurs. A favorable configuration for a rapid exchange of energy from the divergent to the rotational kinetic energy develops. Strong low level monsoonal circulations evolve, attendant with that the onset of monsoon rains occurs. In order to test this observational sequence, a series of short-range numerical prediction experiments were initiated to define the initial heat sources.

  8. Inner Plasma Structure of the Low-Latitude Reconnection Layer

    NASA Technical Reports Server (NTRS)

    Zhang, Q.-H.; Dunlop, M. W.; Lockwood, M.; Lavraud, B.; Bogdanova, Y. V.; Hasegawa, H.; Yang, H. -G.; Liu, R. -Y.; Hu, H. -Q.; Zhang, B. -C.; hide

    2012-01-01

    We report a clear transition through a reconnection layer at the low-latitude magnetopause which shows a complete traversal across all reconnected field lines during northwestward interplanetary magnetic field (IMF) conditions. The associated plasma populations confirm details of the electron and ion mixing and the time history and acceleration through the current layer. This case has low magnetic shear with a strong guide field and the reconnection layer contains a single density depletion layer on the magnetosheath side which we suggest results from nearly field-aligned magnetosheath flows. Within the reconnection boundary layer, there are two plasma boundaries, close to the inferred separatrices on the magnetosphere and magnetosheath sides (Ssp and Ssh) and two boundaries associated with the Alfvén waves (or Rotational Discontinuities, RDsp and RDsh). The data are consistent with these being launched from the reconnection site and the plasma distributions are well ordered and suggestive of the time elapsed since reconnection of the field lines observed. In each sub-layer between the boundaries the plasma distribution is different and is centered around the current sheet, responsible for magnetosheath acceleration. We show evidence for a velocity dispersion effect in the electron anisotropy that is consistent with the time elapsed since reconnection. In addition, new evidence is presented for the occurrence of partial reflection of magnetosheath electrons at the magnetopause current layer.

  9. Seasonal and solar cycle effects on TEC at 95°E in the ascending half (2009-2014) of the subdued solar cycle 24: Consistent underestimation by IRI 2012

    NASA Astrophysics Data System (ADS)

    Kakoti, Geetashree; Bhuyan, Pradip Kumar; Hazarika, Rumajyoti

    2017-07-01

    TEC measured at Dibrugarh (27.5°N, 94.9°E, 17.5°N Geomag.) from 2009 to 2014 is used to study its temporal characteristics during the ascending half of solar cycle 24. The measurements provide an opportunity to assess the diurnal, seasonal and longterm predictability of the IRI 2012 (with IRI Nequick, IRI01-corr, IRI 2001topside options) during this solar cycle which is distinctively low in magnitude compared to the previous cycles. The low latitude station Dibrugarh is normally located at the poleward edge of the northern EIA. A semi-annual variation in GPS TEC is observed with the peaks occurring in the equinoxes. The peak in spring (March, April) is higher than that in autumn (September, October) irrespective of the year of observation. The spring autumn asymmetry is also observed in IRI TEC. In contrast, the winter (November, December, January, February) anomaly is evident only in high activity years. TEC bears a distinct nonlinear relationship with 10.7 cm solar flux (F10.7). TEC increases linearly with F10.7 up to about 125 sfu beyond which it tends to saturate. The correlation between TEC and solar flux is found to be a function of local time and peaks at 10:00 LT. TEC varies nonlinearly with solar EUV flux similar to its variation with F10.7. The nonlinearity is well captured by the IRI. The saturation of TEC at high solar activity is attributed to the inability of the ionosphere to accommodate more ionization after it reaches the level of saturation ion pressure. Annual mean TEC increased from the minimum in 2009 almost linearly till 2012, remains at the same level in 2013 and then increased again in 2014. IRI TEC shows a linear increase from 2009 to 2014. IRI01-corr and IRI-NeQuick TEC are nearly equal at all local times, season and year of observation while IRI-2001 simulated TEC are always higher than that simulated by the other two versions. The IRI 2012 underestimates the TEC at about all local times except for a few hours in the midday in all

  10. Estimation of the Ionosphere and Plasmasphere Contribution to the GPS TEC under Solar Minimum Conditions

    NASA Astrophysics Data System (ADS)

    Zakharenkova, Irina; Cherniak, Iurii; Krankowski, Andrzej; Shagimuratov, Irk; Sieradzki, Rafal

    2012-07-01

    The plasmaspheric electron content (PEC) was estimated by comparison GPS observations and FORMOSAT-3/COSMIC radio occultation (RO) measurements at the extended solar minimum of cycle 23/24. GPS observations provide information about values of vertical total electron content (TEC) up to the metricconverterProductID20,200 km20,200 km. FORMOSAT-3/COSMIC now provides unprecedented global coverage of GPS RO measurements. Depending on the state of the constellation, COSMIC has been producing 1,500 -- 2,500 good soundings of the ionosphere and atmosphere per day, uniformly distributed around the globe. This number of RO is much higher than even before. In this study, COSMIC RO data for different seasons corresponded to equinoxes and solstices of 2007-2009 (March, June, September and December) were analyzed. All selected COSMIC RO electron density profiles were integrated up to the height of metricconverterProductID700 km700 km (altitude of COSMIC satellites), in that way the estimates of ionospheric electron content (IEC) were retrieved on a global scale. The final IGS combined global ionospheric maps (GIMs) were used to calculate the global maps of monthly medians of TEC values. As a result there were analyzed global distributions of GPS TEC and IEC estimates corresponded to the monthly median values for different seasons of 2007-2009. We consider the quantitative differences PEC = TEC -- IEC as a measure of the contribution of the PEC to GPS TEC. In order to analyze seasonal behaviour of PEC contribution to GPS TEC at the different regions we selected several specific points with coordinates, corresponded to the approximate positions of different, mid-latitude and low-latitude, ionospheric sounding stations. Such points were selected at Northern America, European and Asian regions, Southern America, Southern Africa and country-regionplaceAustralia. For each specific points GPS TEC, COSMIC IEC and PEC estimates were analyzed. Results of our comparative study revealed

  11. Longitudinal Variability of the Low Latitude Ionosphere and Scintillation Activity

    NASA Astrophysics Data System (ADS)

    Anderson, D. N.; Fedrizzi, M.; Coker, C.; Dymond, K.; Budzien, S.; Chua, D.; Basu, S.; Caton, R.

    2007-12-01

    Longitudinal variability of the post-sunset low latitude ionosphere is provided by Tiny Ionospheric Photometers (TIP) on the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites. The TIP sensor is a compact, nadir directed, narrow-band, ultraviolet photometer operating at the 135.6 nm wavelength. This emission is produced by radiative recombination of O+ ions and electrons. At night, the strength of the emission is proportional to the square of the peak electron density, Nmax. TIP measures the horizontal structure of the ionosphere with 15-30 km resolution and high sensitivity, providing detailed observations of the post-sunset equatorial anomaly even during solar minimum conditions. It has previously been demonstrated by Whalen, that the maximum value of the pre-reversal enhancement in vertical ExB drift is linearly related to the value of Nmax at the crest of the equatorial anomaly at 2000 LT. A linear relationship has also been established between the TIP 135.6 nm radiances at the crest of the equatorial anomaly at 2000 LT and the pre-reversal enhancement in vertical ExB drift velocities at 1900 LT in the Peruvian longitude sector. This relationship is independent of the magnitude of the daytime vertical ExB drift velocities. Based on this relationship and TIP 135.6 nm observations, a longitude variation in the pre-reversal enhancement in ExB drift reveals a 4-cell pattern that is attributed to non-migrating tides which arise from tropospheric weather patterns in the tropics. The longitudinal pattern of the pre-reversal enhancement suggests that certain longitudes are more favored than others for the subsequent development of scintillation activity. The strength of this relationship is investigated using scintillation observations from the SCINDA network of ground-based VHF and UHF receivers. A strong relationship implies a connection between tropospheric weather and the occurrence of scintillation activity.

  12. Ionosphere-Thermosphere Coupling in Jupiter's Low Latitudes

    NASA Astrophysics Data System (ADS)

    Stallard, T.; Melin, H.; Johnson, R.; O'Donoghue, J.; Moore, L.; Miller, S.; Tao, C.; Achilleos, N. A.; Smith, C.; Ray, L. C.; Yates, J. N.

    2015-12-01

    One of the leading problems in our understanding of Jupiter's atmosphere, known colloquially as the 'energy crisis', is that the upper atmosphere has global temperatures far in excess of that predicted by solar heating. Unlike the Earth, solar heating has only a small effect on the thermosphere, varying little in temperature with local time, and with equatorial neutrals co-rotating with the planet due to meridional advection. Within the auroral region, ionosphere-thermosphere coupling produces strong flows and results in huge Joule Heating from auroral currents. In this region, the temperature excess can be explained, but Jupiter's fast rotation means that Coriolis forces prevent energy in the poles from transferring equatorward, so there remains no explanation of why low latitudes are overheated by a factor of 3-5 over that predicted by solar heating alone.Despite this anomaly, although the past twenty years has seen a wealth of new data and results in Jupiter's auroral region, studies of the equatorial region have been somewhat limited. This lack of investigation comes partly from the apparent uniform nature of the equatorial region, and partly from the difficulty in observing this region. It is only in the past three years that observers begun to re-examine this region, revealing evidence of complex interactions between the thermosphere and ionosphere, including what appears to be thermospheric weather patterns at a fixed planetary longitudes, stable over two decades; perhaps caused by continuous flows from the auroral region. Here, we introduce our recent research, in order to compare and contrast what has been observed at Jupiter with the more well understood interactions between Earth's ionosphere and thermosphere. We hope that this will open a discussion between the communities that will improve our understanding of the underlying physical processes, as they occur at both planets.

  13. Counter-streaming magnetosheath ions in the dayside low latitude boundary layer

    NASA Technical Reports Server (NTRS)

    Fuselier, S. A.; Klumpar, D. M.; Shelley, E. G.

    1992-01-01

    Observations in the subsolar low-latitude boundary layer (LLBL) during reconnection show counterstreaming suprathermal ions on rare occasions. Composition measurements for one event presented here indicate that the ions are from the magnetosheath although no similar equal energy per charge suprathermal He(2+) and H(+) populations were observed there. Acceleration of magnetosheath ions either across the magnetopause or in the LLBL followed by magnetic mirroring of the initially parallel suprathermal ions best explains these observations.

  14. Impact of Penetration Electric Fields on the Mid and Low Latitude Ionosphere during some Magnetic Storms in Solar Cycle 24

    NASA Astrophysics Data System (ADS)

    Basu, Sunanda; Basu, Santimay; Gardner, Larry; Valladares, Cesar; Scherliess, Ludger; Ruohoniemi, J. Michael; Schunk, Robert

    2012-07-01

    Analysis of GPS phase fluctuations in conjunction with regional total electron content (TEC) maps, in-situ measurements of sub-auroral polarization streams (SAPS) from several DMSP spacecraft supported by ground-based SuperDARN measurements in North America indicate the considerable impact of even moderate magnetic storms in 2010-2011 on the mid and low latitude ionosphere. The prompt penetration of the magnetospheric electric field is found to reach the magnetic equator in the main phase of the storm as monitored by the C/NOFS in-situ measurements and the array of GPS receivers forming part of the Low Latitude Ionospheric Sensor Network (LISN) in South America. At mid latitudes the prompt penetration, if occurring during photo-production in the afternoon hours, is associated with storm enhanced density (SED) and TEC plumes that usually travel from the SE to NW across North America. The USU Global Assimilation of Ionospheric Measurements (GAIM) Model is utilized to model these SEDs so that their impact on space-based navigation systems such as the Wide Area Augmentation System (WAAS) can be estimated. It is found that SAPS, mostly occurring around dusk and containing regions of large westward flows, can also have a substantial impact on the WAAS system by generating large phase fluctuations at GPS frequencies.

  15. Turbulent oceanic western-boundary layers at low latitude

    NASA Astrophysics Data System (ADS)

    Quam Cyrille Akuetevi, Cataria; Wirth, Achim

    2013-04-01

    Low latitude oceanic western-boundary layers range within the most turbulent regions in the worlds ocean. The Somali current system with the Great Whirl and the Brazilian current system with its eddy shedding are the most prominent examples. Results from analytical calculations and integration of a one layer reduced-gravity fine resolution shallow water model is used to entangle this turbulent dynamics. Two types of wind-forcing are applied: a remote Trade wind forcing with maximum shear along the equator and a local Monsoon wind forcing with maximum shear in the vicinity of the boundary. For high values of the viscosity (> 1000m2s-1) the stationary solutions compare well to analytical predictions using Munk and inertial layer theory. When lowering the friction parameter time dependence results. The onset of instability is strongly influenced by inertial effects. The unstable boundary current proceeds as a succession of anti-cyclonic coherent eddies performing a chaotic dynamics in a turbulent flow. The dynamics is governed by the turbulent fluxes of mass and momentum. We determine these fluxes by analyzing the (potential) vorticity dynamics. We demonstrate that the boundary-layer can be separated in four sub-layers, which are (starting from the boundary): (1) the viscous sub-layer (2) the turbulent buffer-layer (3) the layer containing the coherent structures and (4) the extended boundary layer. The characteristics of each sub-layer and the corresponding turbulent fluxes are determined, as are the dependence on latitude and the type of forcing. A new pragmatic method of determining the eddy viscosity, based on Munk-layer theory, is proposed. Results are compared to observations and solutions of the multi-level primitive equation model (DRAKKAR).

  16. Large enhancements in low latitude total electron content during 15 May 2005 geomagnetic storm in Indian zone

    NASA Astrophysics Data System (ADS)

    Dashora, N.; Sharma, S.; Dabas, R. S.; Alex, S.; Pandey, R.

    2009-05-01

    Results pertaining to the response of the equatorial and low latitude ionosphere to a major geomagnetic storm that occurred on 15 May 2005 are presented. These results are also the first from the Indian zone in terms of (i) GPS derived total electron content (TEC) variations following the storm (ii) Local low latitude electrodynamics response to penetration of high latitude convection electric field (iii) effect of storm induced traveling atmospheric disturbances (TAD's) on GPS-TEC in equatorial ionization anomaly (EIA) zone. Data set comprising of ionospheric TEC obtained from GPS measurements, ionograms from an EIA zone station, New Delhi (Geog. Lat. 28.42° N, Geog. Long. 77.21° E), ground based magnetometers in equatorial and low latitude stations and solar wind data obtained from Advanced Composition Explorer (ACE) has been used in the present study. GPS receivers located at Udaipur (Geog. Lat. 24.73° N, Geog. Long. 73.73° E) and Hyderabad (Geog. Lat. 17.33° N, Geog. Long. 78.47° E) have been used for wider spatial coverage in the Indian zone. Storm induced features in vertical TEC (VTEC) have been obtained comparing them with the mean VTEC of quiet days. Variations in solar wind parameters, as obtained from ACE and in the SYM-H index, indicate that the storm commenced on 15 May 2005 at 02:39 UT. The main phase of the storm commenced at 06:00 UT on 15 May with a sudden southward turning of the Z-component of interplanetary magnetic field (IMF-Bz) and subsequent decrease in SYM-H index. The dawn-to-dusk convection electric field of high latitude origin penetrated to low and equatorial latitudes simultaneously as corroborated by the magnetometer data from the Indian zone. Subsequent northward turning of the IMF-Bz, and the penetration of the dusk-to-dawn electric field over the dip equator is also discernible. Response of the low latitude ionosphere to this storm may be characterized in terms of (i) enhanced background level of VTEC as compared to the mean

  17. On the Connection Between Solar Activity and Low-Latitude Aurorae in the Period 1715 - 1860

    NASA Astrophysics Data System (ADS)

    Vázquez, M.; Vaquero, J. M.; Curto, J. J.

    2006-11-01

    Observations of aurorae borealis at low latitudes are very rare and are clearly associated with strong geomagnetic storms. Morphologically, they are characterized by a diffuse red colour with no rapid motions. The main aim of this paper is to analyse two hitherto ignored aurorae that were observed at two low-latitude sites, Tenerife (28°N 18°W) and Mexico City (19°N 99°W), in 1770 and 1789, respectively. These observations can give supplementary information about the level of solar activity at those times where direct solar observations were rather scarce. Studying also the behaviour of the heliosphere during this period using different proxies, we find that the open magnetic field better describes auroral occurrences. The variation over time in geomagnetic latitude at the two sites is also calculated.

  18. Low-latitude gravity wave variances in the mesosphere and lower thermosphere derived from SABER temperature observation and compared with model simulation of waves generated by deep tropical convection

    NASA Astrophysics Data System (ADS)

    Walterscheid, R. L.; Christensen, A. B.

    2016-10-01

    A portion of waves generated by deep convection have scales and amplitudes large enough to be detected by spaceborne instruments. We have analyzed temperature data from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED) satellite for subtidal-scale fluctuations. Filtering was applied both vertically and horizontally to extract wave variances. We have analyzed the altitude region between 70 and 130 km and focus on the variances at equatorial latitudes for the altitude region between 70 and 120 km as a function of season, local time intervals, geographical location, and altitude. We find significant variances where convection is particularly prolific (Intertropical Convergence Zone) and at altitudes where wave trapping is known to be favored (e.g., the lower thermospheric duct). The locations of significant variances persist from year to year. Standard deviations of a few tens of kelvins are found. We have also performed simulations of the response to deep tropical convection with a time-dependent, high-resolution fully compressible dynamical model. Our simulations give wave amplitudes that agree reasonably well with the observed amplitudes and show layering that is consistent with the observations. Our main finding is that significant variations seen in TIMED/SABER temperature data have a convective wave source and are concentrated in layers where thermal ducts occur.

  19. E-region radar echoes from low-latitude field aligned irregularities due to gravity waves and tides: A case study using radar, lidar and radiosonde observations and simulations

    NASA Astrophysics Data System (ADS)

    Sundararajan, Sridharan; M, Sandhya

    On the night of 5 February 2011, the height-time plot of echo from E-region field aligned irregularities (FAI) received by the Indian MST radar at Gadanki (13.5(°) N, 79.2(°) E) shows a slow descending layered structures separated by nearly 8 km in the height region 80-100 km and vertically elongated structures with quasi-periodic (QP) variations at higher heights. The stratospheric thermal structure also shows similar slowly descending structures with nearly 8 km separation. Hodograph of 7-9 km band-pass filtered radiosonde zonal and meridional winds over Gadanki form clearly an ellipse and the dominant period obtained from the ratio of major to minor axes of the ellipse is nearly 24 h indicating the presence of a non-migrating diurnal tide with vertical wavelength 8 km. The two-dimensional spectrum of temperature perturbations, which are assumed to be due to gravity waves, shows dominant periods near 1 h and 1.5 h. The spectrum of FAI echo also shows similar dominant periods. Altitude-time cross section of vertical ion velocity is computed using winds comprising of observed gravity wave and tidal parameters. As the diurnal tide over Tirunelveli (8.7(°) N, 77.8(°) E) shows weaker amplitudes above 90 km, the removal of tidal contribution clearly reveals the observed FAI echo structure of vertically slanting plasma blob structures at higher heights and slowly descending continuous structures separated by 8 km at lower heights. The present study clearly demonstrates the role of tides and gravity waves in the formation of descending echo structures and quasi-periodic echoes respectively.

  20. E region radar echoes from low-latitude field-aligned irregularities due to gravity waves and tides: A case study using radar, lidar, and radiosonde observations and simulations

    NASA Astrophysics Data System (ADS)

    Sridharan, S.; Sandhya, M.

    2014-04-01

    On the night of 5 February 2011, the height-time plot of echo from E region field-aligned irregularities (FAI) received by the Indian mesosphere-stratosphere-troposphere radar at Gadanki (13.5°N, 79.2°E) shows a slow-descending layered structures separated by nearly 8 km in the height region 80-100 km and vertically elongated structures with quasiperiodic variations at higher heights. The stratospheric thermal structure also shows similar slowly descending structures with nearly 8 km separation. Hodograph of 7-9 km band-pass filtered radiosonde zonal and meridional winds over Gadanki form clearly an ellipse, and the dominant period obtained from the ratio of major to minor axes of the ellipse is nearly 24 h indicating the presence of a nonmigrating diurnal tide with vertical wavelength of 8 km. The two-dimensional spectrum of temperature perturbations, which are assumed to be due to gravity waves, shows dominant periods near 1 h and 1.5 h. The spectrum of FAI echo also shows similar dominant periods. Altitude-time cross section of vertical ion velocity is computed using winds composed of observed gravity wave and tidal parameters. As the diurnal tide over Tirunelveli (8.7°N, 77.8°E) shows weaker amplitudes above 90 km, the removal of tidal contribution clearly reveals the observed FAI echo structure of vertically slanting plasma blob structures at higher heights and slowly descending continuous structures separated by 8 km at lower heights. The present study clearly demonstrates the role of tides and gravity waves in the formation of descending echo structures and quasiperiodic echoes respectively.

  1. The Low-latitude Ionospheric Sensor Network (LISN)

    NASA Astrophysics Data System (ADS)

    Valladares, Cesar

    This paper describes the characteristics and illustrates the early measurements of the first distributed observatory that is being installed in the South American region to study the lowlatitude ionosphere and upper atmosphere. The LISN distributed observatory will be comprised of nearly 70 GPS receivers with the capability to measure Total Electron Content (TEC), amplitude and phase scintillation and Traveling Ionospheric Disturbances (TIDs). The network will include 5 ionosondes able to measure nighttime E-region densities and 5 collocated magnetometers that will be placed along the same magnetic meridian. This network of GPS receivers and ionospheric sensors span from north to south in the South American continent and west of the 55o West meridian. They will complement each other to provide new, time continuous and spatially extended observations of the background ionosphere, its motion and the embedded structures over this large dynamic region. The LISN network is being complemented with a physics-based data-inversion that incorporates a ionosphere model and a field-line-integrated electric field model to provide a consistent representation of the ionospheric electron density, conductivities, E×B plasma drifts, and neutral winds. This new distributed observatory will bring the opportunity to understand the day-to-day variability and the stability of the lowlatitude ionosphere and to make forecasts on a regional basis. This paper describes the instrumentation, presents the first measurements and discusses the scientific benefits of the LISN network.

  2. Magnetic field maxima in the low latitude boundary layer

    NASA Technical Reports Server (NTRS)

    Sonnerup, B.; Paschmann, G.; Phan, T.-D.; Luehr, H.

    1992-01-01

    The magnetic field often exhibits a maximum in the earth's low-latitude boundary layer. Examples of this behavior are shown using data from the AMPTE/IRM spacecraft, and it is argued that two fundamentally distinct causes exist for the excess field: (1) a depression, within the layer, of the population of medium-energy ions of magnetospheric origin and (2) field curvature effects associated with undulations of the magnetopause itself.

  3. On the Conjugacy of the Aurora: High and Low Latitudes.

    DTIC Science & Technology

    1988-02-05

    AD43 00 ON TNE COWJUGACY OF THE AROM : HIGH AND LOW LATITUDES vi1 (U) AROSPACE CORP EL SEGUNDO CR SPACE SCIENCES LAD P IZERM ET RAL. 05 FEB GO TR-*M...and reentry fluid mechanics, heat transfer and flight dynamics; chemical and electric propulsion, propellant chemistry , chemical dynamics...environmental chemistry , trace detection; spacecraft structural mechanics, contamination, thermal and structural control; high temperature thermomechanics, gas

  4. Continuum observations of M 51 and M 83 at 1.1 mm with AzTEC

    NASA Astrophysics Data System (ADS)

    Wall, W. F.; Puerari, I.; Tilanus, R.; Israel, F. P.; Austermann, J. E.; Aretxaga, I.; Wilson, G.; Yun, M.; Scott, K. S.; Perera, T. A.; Roberts, C. M.; Hughes, D. H.

    2016-06-01

    We observed the spiral galaxies M 51 and M 83 at 20 arscec spatial resolution with the bolometer array Aztronomical Thermal Emission Camera (AzTEC) on the JCMT in the 1.1 mm continuum, recovering the extended emission out to galactocentric radii of more than 12 kpc in both galaxies. The 1.1 mm-continuum fluxes are 5.6 ± 0.7 and 9.9 ± 1.4 Jy, with associated gas masses estimated at 9.4 × 109 M⊙ and 7.2 × 109 M⊙ for M 51 and M 83, respectively. In the interarm regions of both galaxies, the N(H2)/I(CO) (or X-factor) ratios exceed those in the arms by factors of ˜1.5-2. In the inner discs of both galaxies, the X-factor is about 1 × 1020 cm- 2 (K km s- 1)- 1. In the outer parts, the CO-dark molecular gas becomes more important. While the spiral density wave in M 51 appears to influence the interstellar medium and stars in a similar way, the bar potential in M 83 influences the interstellar medium and the stars differently. We confirm the result of Foyle et al. that the arms merely heighten the star formation rate (SFR) and the gas surface density in the same proportion. Our maps reveal a threshold gas surface density for an SFR increase by two or more orders of magnitude. In both galaxy centres, the molecular gas depletion time is about 1 Gyr climbing to 10-20 Gyr at radii of 6-8 kpc. This is consistent with an inside-out depletion of the molecular gas in the discs of spiral galaxies.

  5. Seasonal variations of GPS TEC and its comparison with IRI-2007 models for a period of low to moderate solar activity Equatorial Ionization Anomaly region.

    NASA Astrophysics Data System (ADS)

    Karia, Sheetal; Pathak, Kamlesh; Patel, Nilesh

    The present paper reports the analysis of GPS derived TEC from three different stations in the low latitude Indian region. The data are obtained in RINEX format from IGS stations IISC (13.02N 77.57E), HYDE(17.41N,78.55E) and a standalone GISTM receiver in operation at SURAT (21.16N, 72.78E) for three years (2010-2012) in a period of low to moderate solar activity. The seasonal variations of the obtained TEC are plotted for all the stations (situated at latitude interval of 4 degrees) and compared with those obtained by IRI-2007 Models. The analysis is done to check the IRI prediction of seasonal variations in TEC in Equatorial Ionization Anomaly (EIA) region. It is observed that the measured TEC matches well for all seasons for all stations with that predicted by Ne-Quick and IRI-corr.for the year 2010. However there are deviations in2011 and 2012.Overall as we go towards the higher latitude (i.e., Anomaly crest), with increase in solar activity the deviation increases from over estimation to underestimation of the TEC for anomaly crest station Surat.

  6. Similarity and differences in morphology and mechanisms of the foF2 and TEC disturbances during the geomagnetic storms on 26-30 September 2011

    NASA Astrophysics Data System (ADS)

    Klimenko, Maxim V.; Klimenko, Vladimir V.; Zakharenkova, Irina E.; Ratovsky, Konstantin G.; Korenkova, Nina A.; Yasyukevich, Yury V.; Mylnikova, Anna A.; Cherniak, Iurii V.

    2017-08-01

    This study presents an analysis of the ground-based observations and model simulations of ionospheric electron density disturbances at three longitudinal sectors (eastern European, Siberian and American) during geomagnetic storms that occurred on 26-30 September 2011. We use the Global Self-consistent Model of the Thermosphere, Ionosphere and Protonosphere (GSM TIP) to reveal the main mechanisms influencing the storm-time behavior of the total electron content (TEC) and the ionospheric F2 peak critical frequency (foF2) during different phases of geomagnetic storms. During the storm's main phase the long-lasting positive disturbances in TEC and foF2 at sunlit mid-latitudes are mainly explained by the storm-time equatorward neutral wind. The effects of eastward electric field can only explain the positive ionospheric storm in the first few hours of the initial storm phase. During the main phase the ionosphere was more changeable than the plasmasphere. The positive disturbances in the electron content at the plasmaspheric heights (800-20 000 km) at high latitudes can appear simultaneously with the negative disturbances in TEC and foF2. The daytime positive disturbances in foF2 and TEC occurred at middle and low latitudes and at the Equator due to n(O) / n(N2) enhancement during later stage of the main phase and during the recovery phase of the geomagnetic storm. The plasma tube diffusional depletion and negative disturbances in electron and neutral temperature were the main formation mechanisms of the simultaneous formation of the positive disturbances in foF2 and negative disturbances in TEC at low latitudes during the storm's recovery phase.

  7. Spatial analyses on seismo-ionospheric precursors observed by GIM TEC and DEMETER during the 2008 M8.0 Wenchuan earthquake

    NASA Astrophysics Data System (ADS)

    Liu, Jann-Yenq; Chen, Yuh-Ing; Huang, Ching-Chi; Parrot, Michel; Pulinets, Sergey; Ouzounov, Dimitar

    2015-04-01

    This paper examines seismo-ionospheric precursors (SIPs) in the total electron content (TEC) of the global ionosphere map (GIM) and observations in the French satellite DEMETER (Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions) during the 12 May 2008 M8.0 Wenchuan earthquake. The temporal and spatial analyses on the GIM TEC are used to search SIPs of the Wenchuan earthquake. Meanwhile, both daytime and nighttime electron density (Ne), electron temperature (Te), ion density (Ni) and ion temperature (Ti) probed by DEMETER are investigated. A statistical analysis of the box-and-whisker method is utilized to see if the four DEMETER data sets 1-6 days before and after the earthquake are significantly different. The analysis is employed to investigate the epicenter and three reference areas along the same magnetic latitude discriminating the SIPs from global effects. Results show that the nighttime Ne and Ni (daytime Ti) over the epicenter significantly decrease (increase) 1-6 days before the earthquake. The intersections of the global distribution of the significant differences (or anomalous changes) in the nighttime Ne, the nighttime Ni, and the daytime Ti 1-6 days before and after the earthquake specifically appear over the epicenter. The spatial analyses confirm that SIPs of GIM TEC and DEMETER observations appearing 2-6 days before are related to the 2008 M8.0 Wenchuan earthquake.

  8. Recent low-latitude freeze thaw on Mars

    NASA Astrophysics Data System (ADS)

    Page, David P.

    2007-07-01

    Outside polar latitudes, features corresponding to surface thaw have yet to be identified on Mars. The youthful gully landforms observed at mid-high latitude [Malin, M., Edgett, K., 2000. Science 288, 2330-2335] are the nearest candidate, but the source (and nature) of the gully carving agent remains controversial [e.g., Musselwhite, D.S., Swindle, T.D., Lunine, J.I., 2001. Geophys. Res. Lett. 28, 1283-1285; Mellon, M.T., Phillips, R.J., 2001. J. Geophys. Res. 106, 1-15; Knauth, L.P., Burt, D.M., 2002. Icarus 158, 267-271; Costard, F., Forget, F., Mangold, N., Peulvast, J.P., 2002. Science 295, 110-113; Christensen, P.R., 2003. Nature 422, 45-48; Treiman, A.H., 2003. J. Geophys. Res. 108]. At higher obliquity than the present epoch, near-surface ground ice should be present globally [Mellon, M.T., Jakosky, B.M., 1995. J. Geophys. Res. 100 (E6), 11781-11799], populated by condensation of atmospheric water vapour in the top few metres of the regolith, or emplaced as dusty ice sheets reaching down towards the equator. The latitudinal restriction of these gullies to regions poleward of ±30° appears to argue against a thaw component to their formation—since ground ice is present and stable at all latitudes at high obliquity, the current (low) obliquity regime should result in ground ice thaw at low latitudes, where insolation and daytime temperatures are currently greatest, and this is not observed. A previously undescribed meltwater sequence in the Cerberus plains, at 20° N/187° E, shows that comparable, but much more continuous, and mappable melting and surface runoff have occurred in the geologically recent past at near-equatorial latitudes on Mars. Polygonal ground in the Cerberus plains is seen by the Mars Global Surveyor Mars Orbiter Camera (MOC) to suffer sequential, regional-scale volatile-loss consistent with thaw of near-surface ground ice under periglacial conditions. This degradation is continuously sampled by a single MOC strip, showing an icy

  9. Modelling of ionospheric Medium Scale Travelling Disturbances and a comparison with simultaneous ground-based TEC measurements and DEMETER plasma observations at 650 kilometres

    NASA Astrophysics Data System (ADS)

    Onishi, Tatsuo; Wang, Xiaoni; Berthelier, Jean-Jacques

    2010-05-01

    Medium-scale Travelling Ionospheric Disturbances (MSTIDs) are quasi periodic ionospheric disturbances with typical periods of 15 to 60 minutes and wavelengths of several hundreds of kilometers. They are triggered by Atmospheric Gravity Waves (AGWs) mostly generated at high latitudes. Simultaneous measurements of the Total Electron Content (TEC) by the US dense GPS receiver network and of the thermal ions by the CNES DEMETER micro-satellite at 650 km altitude have provided several examples of MSTID and shown typical variations of the ion density and velocity component parallel to the Earth's magnetic field during these events. A quantitative interpretation of such ionospheric disturbances has been undertaken by means of the SAMI2 ionospheric model. A representative pattern of an Atmospheric Gravity with a wave velocity toward the equator was developed to infer the variations of the neutral density and of the meridional component of the neutral velocity. These variations are introduced in the model and directly couples with the ionospheric plasma in the collisional region of the ionosphere. At higher altitudes, when the neutral atmosphere is too faint to have a direct effect on the ions, the resulting plasma disturbance propagates along the magnetic field lines. The computed variations of the plasma parameters along the orbit of DEMETER and of the TEC are analyzed for various parameters of the Atmospheric Gravity Wave. They are compared to the GPS-TEC and DEMETER observations in order to retrieve the AGW characteristics and study the propagation mechanism of the ionospheric plasma disturbance.

  10. Observation of TEC perturbation associated with medium-scale traveling ionospheric disturbance and possible seeding mechanism of atmospheric gravity wave at a Brazilian sector

    NASA Astrophysics Data System (ADS)

    Jonah, O. F.; Kherani, E. A.; De Paula, E. R.

    2016-03-01

    In the present study, we document daytime total electron content (TEC) disturbances associated with medium-scale traveling ionospheric disturbances (MSTIDs), on few chosen geomagnetically quiet days over Southern Hemisphere of Brazilian longitude sector. These disturbances are derived from TEC data obtained using Global Navigation Satellite System (GNSS) receiver networks. From the keograms and cross-correlation maps, the TEC disturbances are identified as the MSTIDs that are propagating equatorward-eastward, having most of their average wavelengths longer in latitude than in longitude direction. These are the important outcomes of the present study which suggest that the daytime MSTIDs over Southern Hemisphere are similar to their counterparts in the Northern Hemisphere. Another important outcome is that the occurrence characteristics of these MSTIDs and that of atmospheric gravity wave (AGW) activities in the thermosphere are found to be similar on day-to-day basis. This suggests a possible connection between them, confirming the widely accepted AGW forcing mechanism for the generation of these daytime MSTIDs. The source of this AGW is investigated using the Geostationary Operational Environmental Satellite system (GOES) and Constellation Observing System for Meteorology, Ionosphere, and Climate satellite data. Finally, we provided evidences that AGWs are generated by convection activities from the tropospheric region.

  11. Using GPS-SCINDA observations to study the correlation between scintillation, total electron content enhancement and depletions over the Kenyan region

    NASA Astrophysics Data System (ADS)

    Olwendo, J. O.; Cilliers, P. J.; Baki, P.; Mito, C.

    2012-05-01

    This paper presents the first results of total electron content (TEC) depletions and enhancement associated with ionospheric irregularities in the low latitude region over Kenya. At the low latitude ionosphere the diurnal behavior of scintillation is driven by the formation of large scale equatorial depletions which are formed by post-sunset plasma instabilities via the Rayleigh-Taylor instability near the magnetic equator. Data from the GPS scintillation receiver (GPS-SCINDA) located at the University of Nairobi (36.8°E, 1.27°S) for March 2011 was used in this study. The TEC depletions have been detected from satellite passes along the line of sight of the signal and the detected depletions have good correspondence with the occurrence of scintillation patches. TEC enhancement has been observed and is not correlated with increases in S4 index and consecutive enhancements and depletions in TEC have also been observed which results into scintillation patches related to TEC depletions. The TEC depletions have been interpreted as plasma irregularities and inhomogeneities in the F region caused by plasma instabilities, while TEC enhancement have been interpreted as the manifestation of plasma density enhancements mainly associated with the equatorial ionization anomaly crest over this region. Occurrence of scintillation does happen at and around the ionization anomaly crest over Kenyan region. The presence of high ambient electron densities and large electron density gradients associated with small scale irregularities in the ionization anomaly regions have been linked to the occurrence of scintillation.

  12. Modeling of equatorial and low latitude ionosphere over Indian zone

    NASA Astrophysics Data System (ADS)

    Singh Dabas, Raj

    Ionospheric Modeling is important for both scientific and practical purposes. Since the largest variability occurs in the ionospheric F-region and for practical HF communication and other applications it would be suffice to model the changes in the F-region parameters of the ionosphere. Ionospheric Electron Content (IEC) is another parameter useful for the determination of phase path, group delay, dispersion, refraction and Faraday polarization rotation of transionospheric signals. In the present study, two HF prediction models for short and long term and one IEC model are developed for equatorial and low latitude ionosphere. Short term HF prediction model is based on Multiple Regression Analysis (MRA) for the dependence of F- region parameters namely foF2 and M(3000)F2, on solar 2800 MHz flux (F10), and geomagnetic index Ap whereas for long term prediction, Second Degree (SD) coefficients are generated by fitting monthly median foF2 and M(3000)F2 with corresponding 12 monthly mean sunspot numbers (R12) using data over three solar cycles. For MRA, daily foF2, M(3000)F2 values for each hour, obtained from Delhi (28.6N, 77.1E) digital ionosonde for about half a solar cycle are used. MRA coefficients, separately for quiet (Ap¡25) and disturbed (Ap˜25) periods, for foF2 and M(3000)F2, are obtained for every month over 24 UT times using daily F10 and Ap values. Whereas SD coefficients are obtained each month at all local times for all the 14 stations covering a geographic latitude range from about 0 to 45N. Similar to SD model, IEC model is also developed using monthly median foF2 and hmF2 values for each hours for all the 14 stations which are feed into IRI 2000 model to calculate respective IEC values for two altitudes namely 1000km and 2000km. Then second degree coefficients, separately for above two altitudes, are obtained for each month at all local times for all the 14 stations. In this way once appropriate coefficients for each hour for all the twelve months

  13. LION: A dynamic computer model for the low-latitude ionosphere

    NASA Astrophysics Data System (ADS)

    Bittencourt, J. A.; Pillat, V. G.; Fagundes, P. R.; Sahai, Y.; Pimenta, A. A.

    2007-11-01

    A realistic fully time-dependent computer model, denominated LION (Low-latitude Ionospheric) model, that simulates the dynamic behavior of the low-latitude ionosphere is presented. The time evolution and spatial distribution of the ionospheric particle densities and velocities are computed by numerically solving the time-dependent, coupled, nonlinear system of continuity and momentum equations for the ions O+, O2+, NO+, N2+ and N+, taking into account photoionization of the atmospheric species by the solar extreme ultraviolet radiation, chemical and ionic production and loss reactions, and plasma transport processes, including the ionospheric effects of thermospheric neutral winds, plasma diffusion and electromagnetic E×B plasma drifts. The Earth's magnetic field is represented by a tilted centered magnetic dipole. This set of coupled nonlinear equations is solved along a given magnetic field line in a Lagrangian frame of reference moving vertically, in the magnetic meridian plane, with the electromagnetic E×B plasma drift velocity. The spatial and time distribution of the thermospheric neutral wind velocities and the pattern of the electromagnetic drifts are taken as known quantities, given through specified analytical or empirical models. The model simulation results are presented in the form of computer-generated color maps and reproduce the typical ionization distribution and time evolution normally observed in the low-latitude ionosphere, including details of the equatorial Appleton anomaly dynamics. The specific effects on the ionosphere due to changes in the thermospheric neutral winds and the electromagnetic plasma drifts can be investigated using different wind and drift models, including the important longitudinal effects associated with magnetic declination dependence and latitudinal separation between geographic and geomagnetic equators. The model runs in a normal personal computer (PC) and generates color maps illustrating the typical behavior of the

  14. Investigation of the longitudinal structure of the nighttime low-latitude ionosphere by means of assimilative modeling

    NASA Astrophysics Data System (ADS)

    Solomentsev, Dmitry; Cherniak, Yakov; Vyacheslav, Khattatov; Titov, Anton; Khattatov, Boris

    Recent studies based mostly on experimental data reveal the plausible effect of atmospheric tides in the nighttime low-latitude ionosphere. The wavenumber-four structure of the Ionospheric Electron Content (IEC), i.e. the vertical integral of electron density between 100 and 500 km altitude, has been observed using radio occultation measurements of COSMIC/FORMOSAT-3 mission data. Current study aims to model this effect and to understand its presence in different time periods and under different forcing conditions. Within the current study, the ionospheric state is estimated using a three-dimensional physics-based ionospheric model, assimilating slant Total Electron Content (TEC) observations from a set of ground-based GPS receivers. The model results are validated using independent sources of measurements. Results of the modeling were compared to the COSMIC/FORMOSAT-3 mission results and to the TIMED-GUVI observations. Two time periods were studied. The first case concerns the autumn equinox at the solar minimum of 2006 and was chosen because of the availability of interpolated observations gathered by means of space-borne measurements. The second case is autumn equinox of 2012, representing conditions of the solar maximum. The state of the nighttime ionosphere was shown to have similar longitudinal structure in both cases. For the September equinox of 2006 the modeling results in general agree with COSMIC/FORMOSAT-3 measurements. However, the first-principle model allows one to study the effect of interest with high resolution. For example, the latitudinal asymmetry of electron density distribution was investigated using the modeling results. The electron density peaks over Africa and Oceania are shown to have asymmetry relative to the geomagnetic equator. Possible physical mechanism for the formation of these asymmetries is variations in neutral winds at the thermospheric heights. The altitudinal dependency of electron density over the geomagnetic equator was

  15. Impact of the 15 January 2010 annular solar eclipse on the equatorial and low latitude ionosphere over the Indian region

    NASA Astrophysics Data System (ADS)

    Panda, S. K.; Gedam, S. S.; Rajaram, G.; Sripathi, S.; Bhaskar, A.

    2015-12-01

    The annular solar eclipse of 15 January 2010 over southern India was studied with a multi-instrument network consisting of magnetometer, ionosonde and GPS receivers. The presence of a counter electrojet (weakened or westward zonal electric field) during the eclipse and adjacent days suggests the strong gravitational tidal effect associated with the exceptional Sun-Moon-Earth alignment around the eclipse day. With a strong backup of magnetometer recordings on the day of eclipse, its adjacent days and the normal electrojet day, it is argued that the regular eastward electric field for the whole day at the equator was not just weakened, but actually was flipped for several hours by the influence of enhanced lunar tides. The effect of flipping the electric field was clearly seen in the equatorial ionosonde data and through the large array of GPS receivers that produced the total electron content (TEC) data. The main impact of flipping the electric field was poor feeding of equatorial ionization anomaly (EIA) due to the severely weakened fountain effect on the eclipse day, with the regular anomaly crest shifting towards the equator. The equatorial ionosonde profile was also showing an enhanced F2 region peak in spite of a reduced vertical TEC. While the plasma density depletion at the lower F region altitude over the equator was due to the temporary lack of photo-ionization, the reductions in high altitude plasma density beyond the equator were caused by the electrodynamics taking place around the eclipse. The important finding of this analysis is that the electrodynamical consequences on the low latitude ionosphere were mainly due to the combination of eclipse and lunar tides which were far more significant and influenced the EIA density rather than eclipse alone. Based on these findings, it is argued that the prevailing lunar tidal impact also needs to be taken into account while seeking to understand the electrodynamical impact of the solar eclipse on the low

  16. Simulation of low latitude ionospheric response to 2015 St. Patrick's Day super geomagnetic storm over Indian longitude sector

    NASA Astrophysics Data System (ADS)

    Mohan Joshi, Lalit; Sripathi, Samireddipelle; Singh, Ram

    2016-07-01

    We present low latitude ionospheric response over Indian longitude to the recent super geomagnetic storm of 17 March 2015, using the SAMI2 model which incorporates ionosonde derived vertical drift impacted by prompt penetration eastward electric field occurring during the evening Prereversal Enhancement (PRE) in the vertical drift. The importance of this storm is that (a) Dst reaches as low as -228 nT and (b) prompt penetration of eastward electric field coincided with evening hours PRE. The daytime vertical EXB drifts in the SAMI2 model are, however, considered based on Scherliess-Fejer model. The simulations indicate a significant enhancement in F layer height and equatorial ionization anomaly (EIA) in the post sunset hours on 17 March 2015 vis-a-vis quiet day. The model simulations during recovery phase, considering disturbance dynamo vertical EXB drift along with equatorward disturbance wind, indicates suppression of the daytime EIA. SAMI2 simulations considering the disturbance wind during the recovery phase suggests that equatorward wind enhances the ionospheric density in the low latitude, however, its role in the formation of the EIA depends on the polarity of the zonal electric field. Comparison of model derived total electron content (TEC) with the TEC from ground GPS receivers indicate that model does reproduce enhancement of the EIA during the main phase and suppression of the EIA during the recovery phase of the super storm. However, peculiarities pertaining to the ionospheric response to prompt penetration electric field in the Indian sector vis-a-vis earlier reports from American sector will be discussed.

  17. Orbital control of low-latitude seasonality during the Eemian

    USGS Publications Warehouse

    Winter, Amos; Paul, A.; Nyberg, J.; Oba, T.; Lundberg, J.; Schrag, D.; Taggart, Bruce E.

    2003-01-01

    We used Sr/Ca and stable isotope data from well dated and preserved corals from the northeastern Caribbean to determine the seasonal environmental conditions for four continuous years during the Eemian, the last time the Earth was in a prolonged warm phase. We determined that the seasonal range in SST during the Eemian was 25°–30° C. This is ∼1–2° larger than at present and caused primarily by winter cooling and, only to a small degree, by summer warming. As climate modeling studies indicate, the bias towards colder winters can be explained by changes in low latitude insolation induced by altered orbital parameters, modulated by atmospheric CO2 levels that were lower than today. Milankovitch forcing at higher latitudes was probably less important.

  18. Large ionospheric TEC depletion induced by the 2016 North Korea rocket

    NASA Astrophysics Data System (ADS)

    Choi, Byung-Kyu; Kil, Hyosub

    2017-01-01

    A rocket called Kwangmyongsong-4 was launched from North Korea at 00:30 UT on February 7, 2016. We investigated ionospheric total electron content (TEC) depletions induced by the rocket using the Global Navigation Satellite System (GNSS) stations in South Korea. A sudden depletion in TEC variations appeared ∼6 min after the rocket launch. The drops in slant TEC exceeded 17 TEC unit (TECU) and those in vertical TEC were approximately 7 TECU. It is remarkable that the TEC drop by the 2016 Kwangmyongsong-4 rocket is larger (almost by three times) than that by the 2012 Unha-3 rocket. There are the differences of the background TEC values at the 2012 and the 2016 cases. These results suggest that the difference of the background electron density affects the magnitude of TEC depletion. The horizonal velocity of the rocket was 1.6 km/s, which was estimated from horizontal distances with an initial time of TEC disturbances. However, the 2012 Unha-3 rocket (∼2.5 km/s) moved faster horizontally than the 2016 Kwangmyongsong-4 rocket. Furthermore, when the rocket moved from high latitudes to low latitudes, TEC disturbances reduced gradually, and then, the depletion persisted for a longer time at the west side (the right side of southern direction).

  19. Very low latitude (L = 1.08) whistlers and correlation with lightning activity

    NASA Astrophysics Data System (ADS)

    Gokani, Sneha A.; Singh, Rajesh; Cohen, Morris B.; Kumar, Sushil; Venkatesham, K.; Maurya, Ajeet K.; Selvakumaran, R.; Lichtenberger, J.

    2015-08-01

    We present analysis of more than 2000 lightning-generated whistlers recorded at a low-latitude station, located at Allahabad (geographic latitude, 25.40°N; geographic longitude, 81.93°E; L = 1.081), India, during December 2010 to November 2011. The main focus of this work is on the correlation between observed low-latitude whistlers and lightning activity detected by the World-Wide Lightning Location Network near the conjugate point (geography 9.87°S, 83.59°E) of Allahabad. Whistler occurrence is higher in the postmidnight period as compared to the premidnight period. Whistlers were observed in the daytime only on 2 days that too before 8:30 LT (morning). Seasonally, occurrence is maximum during winter months, which is due to more lightning activity in the conjugate region and favorable ionospheric conditions. About 63% of whistlers were correlated with lightning strokes in the vicinity of the conjugate point within spatial extent of 1000 km (conjugate area). Most (about 53%) whistlers were found to be associated with lightning strokes that were offset to the southeast of the conjugate point. The results indicate that an energy range of 7.5-17.5 kJ of lightning strokes generate most of whistlers at this station. The L shell calculations show that propagation paths of the observed whistlers were embedded in the topside ionosphere. Based on these results we suggest a possibility of ducted mode of propagation even for such very low latitude whistlers.

  20. Climatology of ionospheric scintillations and TEC trend over the Ugandan region

    NASA Astrophysics Data System (ADS)

    Amabayo, Emirant Bertillas; Edward, Jurua; Cilliers, Pierre J.; Habarulema, John Bosco

    2014-03-01

    This study presents results on the investigation of the diurnal, monthly and seasonal variability of Total Electron Content (TEC), phase (σΦ) and amplitude (S4) scintillation indices over Ugandan (Low latitude) region. Scintillation Network Decision Aid (SCINDA) data was obtained from Makerere (0.34°N, 32.57°E) station, Uganda for two years (2011 and 2012). Data from two dual frequency GPS receivers at Mbarara (0.60°S, 30.74°E) and Entebbe (0.04°N, 32.44°E) was used to study TEC climatology during the same period of scintillation study. The results show that peak TEC values were recorded during the months of October-November, and the lowest values during the months of July-August. The diurnal peak of TEC occurs between 10:00 and 14:00 UT hours. Seasonally, the ascending and descending phases of TEC were observed during the equinoxes (March and September) and solstice (June and December), respectively. The scintillations observed during the study were classified as weak (0.1≤S4,σΦ≤0.3) and strong (0.3TEC pattern mentioned above. Amplitude scintillation was more dominant than phase scintillation during the two years of the study. Scintillation peaks occur during the months of March-April and September-October, while the least scintillations occur during the months of June-July. Therefore, the contribution of this study is filling the gap in the current documentation of amplitude scintillation without phase scintillation over the Ugandan region. The scintillations observed have been attributed to wave-like structures which have periods of about 2-3 h, in the range of that of large scale travelling ionospheric disturbances (LSTIDs).

  1. Ionospheric turbulence from TEC variations and VLF/LF transmitter signal observations before and during the destructive seismic activity of August and October 2016 in Central Italy

    NASA Astrophysics Data System (ADS)

    Contadakis, Michael E.; Arabelos, Demetrios N.; Vergos, George; Spatala, Spyrous; Skeberis, Christos; Xenos, Tomas D.; Biagi, Pierfrancesco; Scordilis, Emmanuel M.

    2017-04-01

    In this paper we investigate the ionospheric turbulence from TEC variations and VLF/LF transmitter signal observations before and during the disastrous seismic activity of August and October 2016 in Central Italy . The Total Electron Content (TEC) data of 8 Global Positioning System (GPS) stations of the EUREF network, which are being provided by IONOLAB (Turkey), were analysed using Discrete Fourier Analysis in order to investigate the TEC variations (Contadakis et al. 2009, Contadakis et al. 2012, Contadakis et al. 2015). The data acquired for VLF/LF signal observations are from the receiver of Thessaloniki(40.59N, 22,78E), Greece (Skeberis et al. 2015) which monitor the VLF/LF transmitters of the International Network for Frontier Research on Earthquake Precursors (INFREP). A method of normalization according to the distance between the receiver and the transmitter is applied on the above data and then they are processed by the Hilbert Huang Transform (HHT) to produce the corresponding spectra for visual analysis. The results of this investigation indicate that the High- Frequency limit fo, of the ionospheric turbulence content, increases as the site and the moment of the earthquake occurrence is approaching, pointing to the earthquake locus. In accordence ,the analyzed data from the receiver of INFREP network in Thessaloniki, Greece show that the signals from the two VLF European transmitters, Tavolara ( Italy) and Le Blanc (France), for wich the transmission path crosses the epicentral zones, indicate enhanced high frequency variations, the last ten days before the moment of the earthquake occurrence. We conclude that the LAIC mechanism through acoustic or gravity wave could explain this phenomenology. Reference Contadakis, M.E., Arabelos, D.N., Asteriadis, G., Spatalas, S.D., Pikridas, C. TEC variations over the Mediterranean during the seismic activity period of the last quarter of 2005 in the area of Greece, Nat. Hazards and Earth Syst. Sci., 8, 1267

  2. Velocity fields in a low-latitude coronal hole - Results from the Solar Maximum Mission

    NASA Technical Reports Server (NTRS)

    Mullan, D. J.; Waldron, W. L.

    1987-01-01

    The Solar Maximum Mission (SMM) satellite has been used to observe Doppler signatures in C IV in a low-latitude coronal hole as it crossed the central meridian (1985 February 2-8). Scatter plots of C IV emission intensity versus velocity do not show the pronounced positive correlation which has been reported in other regions on the sun. These data suggest that the coronal hole may control the gross velocity field in the solar atmosphere at the level where C IV is formed. Some localized regions of upflow coincide with EUV bright points in the coronal hole.

  3. Low-Latitude Auroras: The Magnetic Storm of 14-15 May 1921

    NASA Technical Reports Server (NTRS)

    Silverman, S. M.; Cliver, E. W.

    2001-01-01

    We review solar geophysical data relating to the great magnetic storm of 14-15 May 1921, with emphasis on observations of the low-latitude visual aurora. From the reports we have gathered for this event the lowest geomagnetic latitude of definite overhead aurora (coronal form) was 40 deg and the lowest geomagnetic latitude from which auroras were observed on the poleward horizon in the northern hemisphere was 30 deg. For comparison, corresponding overhead/low-latitude values of 48 deg/32 deg and 41 deg/20 deg were reported for the great auroras on 28-29 August and 1-2 September 1859, respectively. However for the 1921 event, there is a report of aurora from Apia, Samoa, in the southern hemisphere, within 13 deg of the geomagnetic equator. This report by professional observers appears to be credible, based on the aurora description and timing, but is puzzling because of the discrepancy with the lowest latitude of observation in the northern hemisphere and the great implied aurora height (approximately 2000 km, assuming overhead aurora at Auckland, New Zealand). We discuss various possibilities that might account for this observation.

  4. Longitudinal variation in Global Navigation Satellite Systems TEC and topside ion density over South American sector associated with the four-peaked wave structures

    NASA Astrophysics Data System (ADS)

    Nogueira, P. A. B.; Abdu, M. A.; Souza, J. R.; Bailey, G. J.; Batista, I. S.; Shume, E. B.; Denardini, C. M.

    2013-12-01

    observations of the low-latitude ionospheric electron density revealed a four-peaked longitudinal structure in the equatorial ionization anomaly when plotted at a constant-local-time frame. It was proposed that neutral wind-driven E region dynamo electric fields due to nonmigrating tidal modes are responsible for this pattern. We examine the four-peaked structure in the observed topside ion density and its manifestation as longitudinal structures in total electron content (TEC) over South America. The strong longitudinal variation in TEC characterized by larger value over Brazilian eastern longitude sector as compared to that over the Peruvian western longitude is modeled using the Sheffield University plasmasphere-ionosphere model (SUPIM) aiming to identify the control factors responsible for the longitude variation. We found that the SUPIM runs using as input the existing standard models of vertical drift, and thermospheric winds do not explain the TEC longitudinal structure. Realistic values of these control parameters were generated based on the strong vertical drift longitudinal variation as determined from magnetometer and Digisonde data and appropriately adjusted winds (horizontal wind model). These realistic vertical drifts together with the modified thermospheric wind, when used as input to the SUPIM, are found to satisfactorily explain the longitudinal differences in the TEC and topside ion density (Ni) over South America. The study shows that the TEC in the whole latitude distribution is larger over the east coast than over the west coast of South America and that the vertical drift and thermospheirc winds control the longitudinal four wave structure in the TEC and Ni.

  5. A high-latitude, low-latitude boundary layer model of the convection current system

    SciTech Connect

    Siscoe, G.L. ); Lotko, W.; Sonnerup, B.U.O. )

    1991-03-01

    Observations suggest that both the high- and low-latitude boundary layers contribute to magnetospheric convection, and that their contributions are linked. In the interpretation pursued here, the high-latitude boundary layer (HBL) generates the voltage while the low-latitude boundary layer (LBL) generates the current for the part of the convection electric circuit that closes through the ionosphere. This paper gives a model that joins the high- and low-latitude boundary layers consistently with the ionospheric Ohm's law. It describes an electric circuit linking both boundary layers, the region 1 Birkeland currents, and the ionospheric Pedersen closure currents. The model works by using the convection electric field that the ionosphere receives from the HBL to determine two boundary conditions to the equations that govern viscous LBL-ionosphere coupling. The result provides the needed self-consistent coupling between the two boundary layers and fully specifies the solution for the viscous LBL-ionosphere coupling equations. The solution shows that in providing the current required by the ionospheric Ohm's law, the LBL needs only a tenth of the voltage that spans the HBL. The solution also gives the latitude profiles of the ionospheric electric field, parallel currents, and parallel potential. It predicts that the plasma in the inner part of the LBL moves sunward instead of antisunward and that, as the transpolar potential decreases below about 40 kV, reverse polarity (region 0) currents appear at the poleward border of the region 1 currents. A possible problem with the model is its prediction of a thin boundary layer ({approximately}1000 km), whereas thicknesses inferred from satellite data tend to be greater.

  6. Climatology of low latitude ionosphere under effect of varying solar flux during solar cycle 23 and 24

    NASA Astrophysics Data System (ADS)

    Dashora, Nirvikar; Suresh, Sunanda

    2016-07-01

    The characteristics of quiet time equatorial and low latitude total electron content (TEC) over the Indian sector using GIM data (1998-2014) is obtained. For the first time the analysis is carried filtering out the solar flare and storm effects and time series of quiet time VTEC data from three locations namely dip equator and two low latitude conjugate locations in Indian sector are obtained. It is well known that a complex interplay among drivers of equatorial electrodynamics like Solar flux, dynamo electric field and meridional winds determine the daytime ionization and distribution in equatorial ionization anomaly zone. In this study, we have critically examined the role of varying solar flux and response of low latitude ionosphere with new and standardized definitions. The results are examined and interpreted in the context of large number of previous studies. The newly found features from this study are as follows. Marked difference in nature of equinoctial asymmetry is noted between solar cycle 23 and 24. Long absence of winter anomaly both during low and high solar activity (HSA) in LL (low latitude) regions is found. Climatology of the diurnal cycle is provided in four categories using new criteria for demarcation of solar activity levels. Highest correlation (~77%) between GIM ionospheric electron content (IEC) and PI (solar EUV proxy index) is noted over equator in contrast to previous studies. The minimum positive contribution of PI in variation of IEC requires minimum of 2 years of data and if more than 7-8 years of data is used, it saturates. RMS (root mean square) width of PI can be used to define the HSA. Strong QBO (quasi biennial oscillations) in IEC is noted in tune with the one in PI over both the LL location but QBO remains surprisingly subdued over equator. The semi-annual oscillations in GIM-IEC are found to be stronger at all locations during high solar activity and weaker between 2005 and 2011, whereas, the annual oscillations are found to

  7. Investigation of low-latitude E and valley region irregularities: Their relationship to equatorial plasma bubble bifurcation

    NASA Astrophysics Data System (ADS)

    Li, Guozhu; Ning, Baiqi; Patra, A. K.; Wan, Weixing; Hu, Lianhuan

    2011-11-01

    The low-latitude E, valley and F region 3 m scale irregularities are studied with the Sanya (18.4°N, 109.6°E, dip latitude 12.8°N) VHF coherent scatter radar. The observations show that the E region irregularities (ERIs) often weaken or disappear during the development of postsunset equatorial plasma bubbles (EPBs) in equinoctial months. However, the valley region irregularities (VRIs) are found to occur during the EPB development and show structures with close relation to those of EPBs. The interesting aspect is that the ERI disruption and VRI generation are simultaneously detected. In terms of the electric field coupling from the equatorial F region down to low-latitude E and valley regions, the polarization electric fields (PEFs) associated with the EPB bifurcation are suggested to play key roles in the evolution of ERIs and VRIs. It is shown that the mapping of upward and eastward PEFs generated within the equatorial west tilted bubble would inhibit the occurrence of low-latitude ERIs. However, for the east tilted bubble structure, the associated downward PEFs might map to the low-latitude valley region and play an active role for the development of 3 m scale irregularities through gradient drift instability.

  8. Analysis of the atmospheric upward radiation in low latitude area

    NASA Astrophysics Data System (ADS)

    Li, Haiying; Wu, Zhensen; Lin, Leke; Lu, Changsheng

    2016-10-01

    Remote sensing using THz wave has irreplaceable advantage comparing to the microwave and the infrared waves, and study on the THz remote sensing become more and more popular in recent years. The major applications of the remote sensing in THz wavelengths are the retrieval of the atmospheric parameters and the microphysical information of the ice cloud. The remote sensing of the atmosphere is based on the radiation of THz wave along the earth-space path of which the most significant part is the upward radiation of the atmosphere. The upward radiation of the atmosphere in sunny day in the low latitude area is computed and analyzed in this paper. The absorption of THz wave by the atmosphere is calculated using the formulations illustrated in the Recommendation ITU-R P.676 to save machine hour, the frequency range is then restricted below 1THz. The frequencies used for the retrieval of atmospheric parameters such as temperature and water content are usually a few hundred GHz, at the lower end of THz wavelengths, so this frequency range is sufficient. The radiation contribution of every atmospheric layer for typical frequencies such as absorption window frequencies and peak frequencies are analyzed. Results show that at frequencies which absorption is severe, information about lower atmosphere cannot reach the receiver onboard a satellite or other high platforms due to the strong absorption along the path.

  9. Mathematical modeling of plasma drifts over equatorial low latitude regions

    NASA Astrophysics Data System (ADS)

    Sundaresan, S.; Nageswara Rao, B.

    2010-09-01

    This paper presents a mathematical model to simulate ionospheric plasma drifts at equatorial low latitude regions by coupling of E- and F-regions. The governing non-linear differential equations (of elliptic and parabolic nature) are solved numerically through finite-difference schemes and obtained neutral winds and electric fields. The temperature and electron density profiles are generated utilizing MSIS-86 atmospheric model. The continuity equation is employed to obtain night-time E-region density profile using measured ionograms at Trivandrum (India). The computed vertical and zonal plasma drifts are comparable with measured Jacamarca plasma drifts with little variations during noon and evening times. The plasma drifts at Trivandrum (8.5° N, 76.5° E, dip 0.5° N) are compared with those of Jicamarca (12° S, 76.9° W, dip 2° N). Neutral wind simulations of present model agree well with those of horizontal wind model (HWM-93). The post-sunset enhancement and its reversal are also discussed.

  10. Magnetic field draping at the low-latitude magnetopause

    NASA Technical Reports Server (NTRS)

    Richard, R. L.; Lotko, W.

    1991-01-01

    Magnetohydrodynamic simulations are used to investigate the structure of the low-latitude magnetopause for interplanetary magnetic field conditions with a dominant southward component. The structure is self-consistently calculated as an initial-value problem in which the system is allowed to evolve into a quasi-steady state. All components of the 3D velocity and magnetic field as well as compressibility, resistivity, and viscosity are included in the 2D calculation. The simulation model shows that magnetic field draping can occur at the magnetopause boundary when magnetic merging takes place in the presence of a tangential shear flow. For 'normal' (positive Bx) draping, the higher-latitude portion of the field lines are curved toward the sun on the magnetospheric side of the magnetopause and away from the sun on the magnetosheath side. The thickness of the normal draping structure scales with the viscosity. The field-aligned current system that accompanies normal magnetic draping is consistent with the sense of the region 1 currents that flow into the dayside ionosphere.

  11. Ionospheric electron content over brazilian low latitude and its comparison with the IRI and supim models

    NASA Astrophysics Data System (ADS)

    de Paula, E. R.; de Souza, J. R.; Abdu, M. A.; Bailey, G. J.; Batista, I. S.; Bittencourt, J. A.; Bonelli, E.

    1996-01-01

    Ionospheric Electron Content (IEC), measured at the Brazilian low latitude station of Cachoeira Paulista (22.5 degS, 45 degW, dip latitude 14 degS) was compared with IEC calculated using the IRI-90 and Sheffield University Plasmaphere-Ionosphere (SUPIM) models for a large range of solar fluxes at 10.7 cm (F10.7). The analysis of the F10.7 influence on IEC showed that, for Cachoeira Paulista (CP) at 05 LT (around the diurnal minimum), there is a good agreement between the IEC measurements and the models. At 16 LT (around the diurnal maximum), substantial discrepancies were observed in the IRI-90 results while SUPIM presents a good agreement up to F10.7 ~ 200. The measured IEC diurnal variation is better represented by SUPIM, mainly during higher F10.7. At low latitudes two critical parametric inputs for the SUPIM mathematical model are the equatorial exB plasma drifts and thermospheric neutral winds. Therefore, updated values of these parameters should be introduced in the model calculations everytime they are available. At CP, a station located close to the southern Appleton anomaly peak due to the fountain effect, where large discrepancies between IRI-90 and observations were found mainly with high F10.7, this model should include adequately this effect, which we believe is the main reason for the discrepancies.

  12. Simulation of the low latitude ionosphere response to disturbed winds and electric fields: Brazilian region

    NASA Astrophysics Data System (ADS)

    Batista, Inez S.; Souza, Jonas; Bailey, Graham; Bravo, Manuel

    2016-07-01

    Modeling the ionosphere during disturbed periods is one of the most challenging tasks due to the complexity of the phenomena that affect the electric fields and the thermosphere environment as whole. It is well known that depending on the direction of the interplanetary magnetic field disturbance electric fields (undershielding or overshielding) can penetrate from high to low latitudes causing significant disturbances in the electron density distribution and in the equatorial ionization anomaly (EIA) development. Besides that, the large amount of energy deposited in the polar region during disturbed periods will be responsible for the generation of disturbed winds that will flow towards the equator where they produce a disturbance dynamo which also affects the EIA density distribution. The TIDs and TADs are also sources of disturbances that propagate at high velocity reaching the equator 2-3 hours after the beginning of the magnetic storm. In this work we use the Sheffield University Plasmasphere-Ionosphere Model at INPE (SUPIM-INPE), to simulate the drastic effects that were observed at the low latitude ionosphere in the Brazilian region during a very intense magnetic storm event. A few models are tested for the disturbed electric field and wind. The simulation results showed that the observations are better explained when considering a traveling waveform disturbance propagating from north to south at a velocity equal to 200 m/s.

  13. Discovery of a widespread low-latitude diurnal CO2 frost cycle on Mars

    NASA Astrophysics Data System (ADS)

    Piqueux, Sylvain; Kleinböhl, Armin; Hayne, Paul O.; Heavens, Nicholas G.; Kass, David M.; McCleese, Daniel J.; Schofield, John T.; Shirley, James H.

    2016-07-01

    While the detection of CO2 ice has only been reported outside the Martian polar regions at very high elevation (i.e., Elysium, Olympus Mons, and the Tharsis Montes), nighttime surface observations by the Mars Climate Sounder on board the Mars Reconnaissance Orbiter document the widespread occurrence of atmospherically corrected ground temperatures consistent with the presence of extensive carbon dioxide frost deposits in the dusty low thermal inertia units at middle/low latitudes. Thermal infrared emissivities, interpreted in conjunction with mass balance modeling, suggest micrometer size CO2 ice crystals forming optically thin layers never exceeding a few hundreds of microns in thickness (i.e., 10-2 kg m-2) locally, which is insufficient to generate a measurable diurnal pressure cycle (<<0.1% of the Martian atmosphere). Atmospheric temperatures at middle/low latitudes are not consistent with precipitation of CO2 ice, suggesting that condensation occurs on the surface. The recurring growth and sublimation of CO2 ice on Martian dusty terrains may be an important process preventing soil induration and promoting dynamic phenomena (soil avalanching and fluidization and regolith gardening), maintaining a reservoir of micrometer size dust particles that are mobile and available for lifting. The discovery of this diurnal CO2 cycle represents an important step forward in our understanding of the way the Martian atmosphere interacts with the surface.

  14. Land-lake breezes at low latitudes: The case of Tonle Sap Lake in Cambodia

    NASA Astrophysics Data System (ADS)

    Tsujimoto, Kumiko; Koike, Toshio

    2013-07-01

    Tonle Sap Lake is the largest freshwater lake in Southeast Asia. During the postmonsoon season, a small linear cloud system has been observed over this lake in early morning, while the sky above the surrounding land is clear. Although this cloud system is apparently generated by land breezes, previous studies on land-lake (sea) circulation have suggested that environmental factors at low latitudes inhibit development of nocturnal land breezes. In this study, we investigate the mechanism of these early morning clouds through numerical simulation. The simulations show a linear updraft system over the lake, forming along the southwest lakeshore around 22:00 and moving northeast to the middle of the lake. The heavier air mass from the land meets the extraordinarily warm and humid air mass over the lake, triggering updrafts under the conditionally convective instability. The characteristic high surface water temperature was favorable for generation of the land breeze and updraft systems. That high surface water temperature of the lake is produced by the tropical climate along with efficient energy absorption because of the shallowness of the water body. This unique feature can generate a clear nocturnal land breeze circulation accompanying a migrating updraft system over the lake despite its low latitude.

  15. Possible precipitation of ice at low latitudes of Mars during periods of high obliquity

    USGS Publications Warehouse

    Jakosky, B.M.; Carr, M.H.

    1985-01-01

    Most of the old cratered highlands of Mars are dissected by branching river valleys that appear to have been cut by running water1,2 yet liquid water is unstable everywhere on the martian surface. In the equatorial region, where most of the valleys are observed, even ice is unstable3,4. It has been suggested, therefore, that Mars had an early denser atmosphere with sufficient greenhouse warming to allow the existence of liquid water 5. Here, we suggest instead that during periods of very high obliquities, ice could accumulate at low latitudes as a result of sustained sublimation of ice from the poles and transport of the water vapour equatorwards. At low latitudes, the water vapour would saturate the atmosphere and condense onto the surface where it would accumulate until lower obliquities prevailed. The mechanism is efficient only at the very high obliquities that occurred before formation of Tharsis very early in the planet's history, but limited equatorial ice accumulation could also have occurred at the highest obliquities during the rest of the planet's history. Partial melting of the ice could have provided runoff to form the channels or replenish the groundwater system. ?? 1985 Nature Publishing Group.

  16. The domination of Saturn's low-latitude ionosphere by ring 'rain'.

    PubMed

    O'Donoghue, J; Stallard, T S; Melin, H; Jones, G H; Cowley, S W H; Miller, S; Baines, K H; Blake, J S D

    2013-04-11

    Saturn's ionosphere is produced when the otherwise neutral atmosphere is exposed to a flow of energetic charged particles or solar radiation. At low latitudes the solar radiation should result in a weak planet-wide glow in the infrared, corresponding to the planet's uniform illumination by the Sun. The observed electron density of the low-latitude ionosphere, however, is lower and its temperature higher than predicted by models. A planet-to-ring magnetic connection has been previously suggested, in which an influx of water from the rings could explain the lower-than-expected electron densities in Saturn's atmosphere. Here we report the detection of a pattern of features, extending across a broad latitude band from 25 to 60 degrees, that is superposed on the lower-latitude background glow, with peaks in emission that map along the planet's magnetic field lines to gaps in Saturn's rings. This pattern implies the transfer of charged species derived from water from the ring-plane to the ionosphere, an influx on a global scale, flooding between 30 to 43 per cent of the surface of Saturn's upper atmosphere. This ring 'rain' is important in modulating ionospheric emissions and suppressing electron densities.

  17. Signatures of Self-Organized Criticality in Low-Latitude Magnetosphere

    NASA Astrophysics Data System (ADS)

    Wanliss, J.; Uritsky, V.

    2007-05-01

    In a series of previous publications (see e.g. Consolini, 1997; Uritsky and Pudovkin, 1998; Uritsky et al., 2002, 2006; Lui et al., 2000; Chapman and Watkins, 2001) it has been shown that spatiotemporal activity in high-latitude magnetosphere exhibits signatures of self-organized criticality (SOC) - a robust multiscale stochastic regime observed in driven nonlinear systems with many couple degrees of freedom. This regime has been identified by a set of mutually consistent scaling laws describing dynamical and statistical properties of magnetospheric substorms. Here, we report the existence of SOC in the dynamics of SYM-H index, whish is a marker of space storms and other types of low-latitude geomagnetic disturbances. The ensemble average dynamics of activity bursts in the SYM-H index are scale-free and are characterized by spreading critical scaling exponents whose values are consistent with the shape of the scaling of burst sizes versus bust lifetimes. The probability distributions for the lifetime and the size of the SYM-H bursts show robust power laws which are essentially independent of the lower activity threshold used to detect the bursts, and they extend over many orders of magnitude. The avalanche distribution exponents are also in agreement with theoretical predictions for SOC systems. All of these results, together with previous studies (Wanliss and Weygand, [2007]), begin to paint a coherent picture of critical state in the inner magnetosphere possibly associated with SOC dynamics of the ring current and other constituents of low-latitude activity.

  18. Variabilities of low latitude mesospheric and E region echoes: linked to common sources?

    NASA Astrophysics Data System (ADS)

    Dharmalingam, Selvaraj; Patra, Amit; Sathishkumar, Sundararaman; Narayana Rao, D.

    2016-07-01

    Variability in dynamics of the mesospheric and E region echoes have been studied in isolation. Both echoing phenomena are directly or indirectly coupled with each other through neutral dynamics. This is especially so for the low-latitudes outside the equatorial electrojet belt, where E region plasma irregularities causing radar echoes are governed by neutral dynamics, such as tides and gravity waves. Although these regions are close to each other, no effort has been made yet to understand the dynamical coupling processes manifesting the observed variabilities in the two echoing phenomena. To investigate linkage between the two phenomena, if any, we conducted systematic observations of low latitude mesospheric and E region echoes during 2011-2012 using the Gadanki MST radar and used these in conjunction with SABER temperature, MF radar wind, and sporadic E observations. Both echoes are found to occur in the height regions where temperature observations show negative gradients. Mesospheric echoes are collocated with temperature gradient associated with mesospheric temperature inversion while the E region echoes are collocated with negative temperature gradient close to the mesopause. Observations have revealed a common signature of semi-annual variations in the occurrence of both mesospheric and ionospheric E-region - occurrences peak in the equinoxes. The E region echoes have an additional peak occurring in the summer and this occurrence is well correlated with the enhancement in the diurnal tidal amplitude. We surmise that the enhancement in the diurnal tidal amplitude is linked with non-migrating tide of tropospheric weather phenomena in summer. Intriguingly, mesospheric echoing layers display descending pattern quite similar to the E region echoes and sporadic E layer, which have been used to invoke tidal dynamics in manifesting similar morphology in both mesospheric and E region echoes. These results will be presented and the role of tidal dynamics on the

  19. Relation Between Low Latitude Pc3 Magnetic Micropulsations and Solar Wind (P6)

    NASA Astrophysics Data System (ADS)

    Ansari, I. A.

    2006-11-01

    iaaamphysics@yahoo.co.in iaaphysicsamu@yahoo.com.au Geomagnetic pulsations recorded on the ground are the signatures of the integrated signals from the magnetosphere. Pc3 Geomagnetic pulsations are quasi-sinusoidal variations in the Earth’s Magnetic field in the period range 10-45 seconds. The magnitude of these pulsations ranges from fraction of a nT (nano Tesla) to several nT. These pulsations can be observed in a number of ways. However the application of ground based magnetometer arrays has proven to be one of the most successful methods of studying the spatial structure of hydromagnetic waves in the Earth’s Magnetosphere. The solar wind provides the energy for the Earth’s magnetospheric processes. Pc3-5 geomagnetic pulsations can be generated either externally or internally with respect to the magnetosphere. The Pc3 studies undertaken in the past have been confined to middle and high latitudes. The spatial and temporal variations observed in Pc3 occurrence are of vital importance because they provide evidence which can be directly related to wave generation mechanisms both inside and external to the magnetosphere. At low latitudes (L < 3) wave energy predominates in the Pc3 band and the spatial characteristics of these pulsations have received little attention in the past. An array of four low latitude induction coil magnetometers was established in south-east Australia over a longitudinal range of 17 degrees at L=1.8 to 2.7 for carrying out the study of the effect of the solar wind velocity on these pulsations. Digital dynamic spectra showing Pc3 pulsation activity over a period of about six months have been used to evaluate Pc3 pulsation occurrence. Pc3 occurrence probability at low latitudes has been found to be dominant for the solar wind velocity in the range 400-700 Km/sec. The results suggest that solar wind controls Pc3 occurrence through a mechanism in which Pc3 wave energy is convected through the magnetosheath and coupled to the standing

  20. LSWS linked with the low-latitude Es and its implications for the growth of the R-T instability

    NASA Astrophysics Data System (ADS)

    Joshi, L. M.

    2016-07-01

    A comprehensive investigation of spread F irregularities over the Indian sector has been carried out using VHF radar and ionosonde observations. Two different categories of spread F observations, one where the onset of the range spread F (RSF) was concurrent with the peak h'F (category 1) and another where the RSF onset happened ~90 min after the peak h'F time (category 2), are presented. RSF in category 2 was preceded by the presence of oblique echoes in ionograms, indicating the irregularity genesis westward of Sriharikota. The average peak h'F in category 1 was ~30 km higher than that in category 2 indicating the presence of standing large-scale wave structure (LSWS). Occurrence of the blanketing Es during 19:30 to 20:30 Indian Standard Time in category 1 (category 2) was 0% (>50%). Model computation is also carried out to further substantiate the observational results. Model computation indicates that zonal variation of low-latitude Es can generate zonal modulation in the F layer height rise. It is found that the modulation of the F layer height, linked with the low-latitude Es, assists the equatorial spread F onset by modifying both the growth rate of the collisional Rayleigh-Taylor (R-T) instability and also its efficiency. A predominant presence of low-latitude Es has been observed, but the increase in the F layer height and the R-T instability growth in the evening hours will maximize with complete absence of low-latitude Es. A new mechanism for the generation of LSWS and its implications on R-T instability is discussed.

  1. Evaluation of the performance of ionospheric models at solar maximum using COSMIC slant TEC measurements

    NASA Astrophysics Data System (ADS)

    Dymond, K. F.; Coker, C.; Metzler, C.; McDonald, S. E.

    2017-03-01

    We report the results of a model validation study that assessed how well several ionospheric models captured the slant total electron content, especially at low latitudes near the equatorial ionization anomaly, where horizontal and vertical density gradients are large. We assessed NeQuick, IRI-2007, IRI-2012, SAMI-3, and the Utah State University version of the Global Assimilation of Ionospheric Measurements (GAIM) model. We used slant total electron content measurements made by the Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) constellation during 5 May to 20 June 2012 to test GAIM, NeQuick, IRI-2007, and IRI-2012 and during 1 October 2011 to 31 December 2011 to test SAMI-3, as the SAMI-3 model runs were not available for the 2012 time frame. We found that the GAIM data assimilation model showed the lowest biases, although all of the models typically agreed with the COSMIC measurements to 8% in the worst case. One area of concern with all of the models was that the mean percentage difference between the COSMIC measurements and the calculated total electron content (TEC) showed significant scatter, >15% at the 1 sigma level; this was attributed to all of the models not capturing the density gradients near the equatorial ionization anomaly (EIA). All of the models underestimated the topside electron density and thus also the ionospheric slab thickness. Since ionospheric models are often validated using near-vertical TEC measurements and the vertical TEC is the product of the electron density at the F region peak and the slab thickness, our results suggest that the peak density values in the models may be too high.

  2. Role of Geomagnetic Disturbances on VLF Whistler Wave Activity at Low Latitudes (P32)

    NASA Astrophysics Data System (ADS)

    Patel, R. P.; Singh, S.; Singh, A. K.; Singh, R. P.

    2006-11-01

    rppatel123@rediffmail.com The disturbances on solar surfaces lead to the enhanced injection of energetic charged particles in to the inner magnetosphere, which modifies the electrodynamic features of ionosphere and magnetosphere. The electrodynamic properties control the generation and propagation characteristics of VLF waves. At Varanasi station, which is one of the low latitude stations in India, we have recorded VLF waves from 1992 onwards. The source of VLF wave is natural lightning discharges. Whistler activity varies with latitude having maximum around 500 geomagnetic latitude. The occurrence rate is low at low latitudes and also depends on the solar and geomagnetic conditions. In this paper, we report the results derived from the statistical analysis of whistler waves recorded at Varanasi during the period January 1990 December 1999. The monthly occurrence rate is obtained which shows maximum during January to March. Seasonal variation of the occurrence rate is also studied. In order to study the role of geomagnetic disturbances on the occurrence rate, we have used Kp index and its variation. It is observed that the occurrence probability monotonically increases with ∑Kp values. It is observed that when ∑Kp > 10, the occurrence rate is greater than the average value. This tendency is found to be in good agreement with those reported by other workers. In addition, we also present the probability of observation of whistler waves during the weak/intense geomagnetic storm. Detailed result of occurrence of whistler waves during the main phase and recovery phase of geomagnetic storms will also be presented. An attempt will be made to present explanation of these statistical results.

  3. Ionosphere/thermosphere dynamics as deduced from meridional ionosonde chain at low latitudes

    NASA Astrophysics Data System (ADS)

    Maruyama, Takashi; Uemoto, Junpei; Tsugawa, Takuya; Hidekatsu, Jin; Kubota, Minoru; Saito, Susumu

    Multipoint ionosonde observation was conducted to study ionosphere/thermosphere dynamics in Southeast Asia. For this study, data from three ionosonde stations aligned at 100° E were analyzed. Two of them are near the magnetic conjugate points in Northern Thailand and West Sumatra, Indonesia, and the other is near the magnetic equator in the Malay Peninsula; these are three of five SEALION (Southeast Asia low latitude ionospheric network) ionosonde stations. The F-layer critical frequency (foF2), the propagation factor (M3000F2), and the F-layer virtual height (h'F) were scaled from quarter hourly ionograms at the three stations. The ionospheric height is closely related to the thermospheric dynamics (neutral drags) as well as electromagnetic forces (EXB drifts) and chemical processes (productions and losses). Because h'F strongly depends on the chemical process during daytime, it is better to examine the peak height (hmF2) for the discussion of dynamics. hmF2 was determined from M3000F2 using the modified Shimazaki's formula. The peak height obtained by this method is less accurate near the magnetic equator because the electron density vertical profile around the peak is greatly distorted from the parabolic distribution and the formation of an additional layer (sometimes called the F3 layer). The peak heights at the low-latitude conjugate points were compared for the whole day. The height difference between the two stations is a good indicator of the transequatorial neutral wind as the equatorward wind lifts the ionosphere up, while the poleward wind lowers it. The diurnal variation of the transequatorial wind inferred from the height differences is examined for various seasons. We found that there exists a strong terdiurnal component, which was most significant around the March equinox. Also the results were compared with the HWM model wind. The terdiurnal amplitude inferred in this study was larger than the model wind and the phases of the diurnal variation

  4. An Investigation of the Low-Latitude Boundary Layer at Mid-Altitudes in the Cusp

    NASA Technical Reports Server (NTRS)

    Chandler, Michael O.; Avanov, Levon A.

    2006-01-01

    We have begun an investigation of the nature of the low-latitude boundary layer in the mid-altitude cusp region using data from the Polar spacecraft. This region has been routinely sampled for about three months each year for the periods 1999-2001 and 2004-2006. The low-to-mid-energy ion instruments frequently observed dense, magnetosheath-like plasma deep (in terms of distance from the magnetopause and in invariant latitude) in the magnetosphere. We seek to understand the morphology of the LLBL as it projects from the sub-solar region into the cusp and determine the influences on this morphology. An initial survey of the data is ongoing and we present here an overview of our intended study and some preliminary results.

  5. Simulation of PPEF effects in dayside low-latitude ionosphere for the October 30, 2003, Superstorm

    NASA Astrophysics Data System (ADS)

    Verkhoglyadova, Olga P.; Tsurutani, Bruce T.; Mannucci, Anthony J.; Saito, Akinori; Araki, Tohru; Anderson, David; Abdu, M.; Sobral, J. H. A.

    One of the important signatures during strong magnetic storms is prompt penetrating electric fields (PPEFs) into the ionosphere, which causes the dayside ionospheric superfountain (DIS). Interplanetary-ionosphere coupling for the October 30, 2003, superstorm is analyzed by using ACE and ground-based measurements. The relationships between the interplanetary magnetic field Bz component, ionospheric vertical velocities above Jicamarca, and horizontal magnetic field components measured at Huancayo are presented. DIS is associated with uplift, displacement, and enhancement of the equatorial ionospheric anomalies. We apply an extended SAMI-2 ionospheric model to simulate DIS effects above Jicamarca for this superstorm. An agreement between our results and observed ƒ0F2 during the main phase of the storm is reported. It is shown that the PPEF approach and corresponding modeling results capture the main physics of the dayside low-latitude ionospheric response during the first couple hours of the magnetic superstorm.

  6. Linear theory of the Kelvin-Helmholtz instability in the low-latitude boundary layer

    NASA Technical Reports Server (NTRS)

    Rajaram, R.; Sibeck, D. G.; Mcentire, R. W.

    1991-01-01

    The feasibility is examined of establishing characteristic profiles across the magnetospheric low-latitude boundary layer for the Kelvin-Helmholtz mode so that these profiles can be compared with satellite observations or a latitudinal chain of ground stations. An anisotropic collisionless fluid model is used instead of conventional MHD, and the finite thickness of the boundary layer and the thickness and position of the current layer are taken into account. The instability is found to be enhanced by a decrease in the thickness of the shear layer and of the current layer and by the proximity of the 'current layer' to the outer edge of the shear layer. The velocity threshold for the onset of instability is insensitive to the thickness. Characteristic profiles of the variation of plasma and field parameters across the boundary are obtained, and the importance of parameters specifying the current layer position and thickness is demonstrated.

  7. Regional GPS TEC modeling; Attempted spatial and temporal extrapolation of TEC using neural networks

    NASA Astrophysics Data System (ADS)

    Habarulema, John Bosco; McKinnell, Lee-Anne; Opperman, Ben D. L.

    2011-04-01

    In this paper, the potential extrapolation capabilities and limitations of artificial neural networks (ANNs) are investigated. This is primarily done by generating total electron content (TEC) predictions using the regional southern Africa total electron content prediction (SATECP) model based on the Global Positioning System (GPS) data and ANNs with the aid of multiple inputs intended to enable the software to learn and correlate the relationship between their variations and the target parameter, TEC. TEC values are predicted over regions that were not covered in the model's development, although it is difficult to validate their accuracy in some cases. The SATECP model is also used to forecast hourly TEC variability 1 year ahead in order to assess the forecasting capability of ANNs in generalizing TEC patterns. The developed SATECP model has also been independently validated by ionosonde data and TEC values derived from the adapted University of New Brunswick Ionospheric Mapping Technique (UNB-IMT) over southern Africa. From the comparison of prediction results with actual GPS data, it is observed that ANNs extrapolate relatively well during quiet periods while the accuracy is low during geomagnetically disturbed conditions. However, ANNs correctly identify both positive and negative storm effects observed in GPS TEC data analyzed within the input space.

  8. A high-altitude balloon experiment to probe stratospheric electric fields from low latitudes

    NASA Astrophysics Data System (ADS)

    Gurubaran, Subramanian; Shanmugam, Manu; Jawahar, Kaliappan; Emperumal, Kaliappan; Mahavarkar, Prasanna; Buduru, Suneel Kumar

    2017-02-01

    The Earth's electrical environment hosts a giant electrical circuit, often referred to as the global electric circuit (GEC), linking the various sources of electrical generators located in the lower atmosphere, the ionosphere and the magnetosphere. The middle atmosphere (stratosphere and mesosphere) has been traditionally believed to be passively transmitting electric fields generated elsewhere. Some observations have reported anomalously large electric fields at these altitudes, and the scientific community has had to revisit the earlier hypothesis time and again. At stratospheric altitudes and especially at low latitudes, horizontal electric fields are believed to be of ionospheric origin. Though measurements of these fields from a balloon platform are challenging because of their small magnitudes (around a few mV m-1), a suitably designed long-duration balloon experiment capable of detecting such small fields can provide useful information on the time evolution of ionospheric electric fields, which is otherwise possible only using radar or satellite in situ measurements. We present herein details of one such experiment, BEENS (Balloon Experiment on the Electrodynamics of Near Space), carried out from a low-latitude site in India. The instrument package for this experiment is comprised of four deployable booms for measurements of horizontal electric fields and one inclined boom for vertical electric field measurements, all equipped with conducting spheres at the tip. The experiment was conducted from Hyderabad (17.5° N, 78.6° E) during the post-midnight hours on 14 December 2013. In spite of a few shortcomings we report herein, a noticeable feature of the observations has been the detection of horizontal electric fields of ˜ 5 mV m-1 at the stratospheric altitudes of ˜ 35 km.

  9. Are the basins of Titan's Hotei Regio and Tui Regio sites of former low latitude seas?

    NASA Astrophysics Data System (ADS)

    Moore, Jeffrey M.; Howard, Alan D.

    2010-11-01

    Features observed in the low-latitude basins of Hotei Regio and Tui Regio on Titan have attracted the attention of the Cassini-era investigators. At both locations, Visual Infrared Mapping Spectrometer (VIMS) observed isolated 5-μm bright ˜500 km wide features described as lobate in shape. Several studies have proposed that these materials are cryo-volcanic flows. We propose an alternative explanation. Recently published topographic profiles across Hotei Regio and Tui Regio indicate these features appear to occur in large regional basins, at least along the direction of the profiles. Cassini Synthetic Aperture Radar (SAR) images show that the terrains surrounding both topographically low-lying 5-μm bright features exhibit fluvial networks that appear to converge into the probable basins. The 5-μm bright features themselves correspond to fields of discrete radar-bright depressions whose bounding edges are commonly rounded and cumulate in planform in SAR images. These fields of discrete radar-bright depressions strongly resemble fields of features seen at Titan's high latitudes identified as dry lakes. Thus the combination of (1) the resemblance to high-latitude dry lakes, (2) location in the centers of regional depressions, and (3) convergence of fluvial networks are inferred by us to best explain the features of Hotei Regio and Tui Regio as sites of paleolake clusters (and perhaps former, now dry seas). These low-latitude paleolake clusters or former seas, if real, may be evidence of substantially larger inventories of liquid alkanes in Titan's past.

  10. Analysis of low latitude Noctilucent Cloud occurrences using satellite data and modeling

    NASA Astrophysics Data System (ADS)

    Russell, J. M.; Rong, P.; Hervig, M. E.; Bailey, S. M.; Gumbel, J.; Lambert, A.

    2012-12-01

    Numerous NLC sightings have occurred in recent years at latitudes as low as ~ 40N in the skies over Chicago, Illinois, Boulder, Colorado, Omaha, Nebraska, Logan, Utah, Seattle, Washington, Calar Alto, Spain and Paris, France. While no confirming evidence has come forth thus far, such sightings raise the natural question about whether there are systematic NLC increases occurring at these low latitudes. This question is investigated using observations of temperature made by the SABER instrument on the TIMED satellite over the 2002 to 2011 time period, a 7-year water vapor climatology developed from data collected by the MLS instrument on the Aura satellite for 2005 to 2011, and Polar Mesospheric Cloud (PMC) measurements made by the OSIRIS instrument on the Odin satellite for the 2002 to 2011 period. These data are used in conjunction with a 0-D thermodynamic equilibrium model [Hervig et al., 2009] that assumes mesospheric ice is in equilibrium with available water vapor. The 0-D model has proven to be effective in reproducing variations of the observed ice mass density, ice layer centroid height [Russell et al., 2010], and daily PMC occurrence frequency on intra-seasonal scales. Inter-annual and decadal scale variations in the northern PMC season are examined in this paper. All analyses were performed on the 0.00464 hPa surface or ~ 84 km, which is the northern hemisphere mean cloud height. Both MLS 0-D and OSIRIS measured PMCs agree well with the SABER 0-D results for 2005 to 2011. Results show a statistically significant upward trend in the number of 0-D derived PMCs per season in the latitude range 40-55N for 2002 - 201l. The long-term increases in cloud number are accompanied by temperature decreases over the same time period. Analysis of temperature and cloud number anomaly data indicates that the low latitude cloud number changes are driven by temperature. Solar cycle effects have not yet been considered in this analysis.

  11. Ionospheric Storm Reconstructions with a Multimodel Ensemble Prdiction System (MEPS) of Data Assimilation Models: Mid and Low Latitude Dynamics

    NASA Astrophysics Data System (ADS)

    Schunk, R. W.; Scherliess, L.; Eccles, V.; Gardner, L. C.; Sojka, J. J.; Zhu, L.; Pi, X.; Mannucci, A. J.; Komjathy, A.; Wang, C.; Rosen, G.

    2016-12-01

    As part of the NASA-NSF Space Weather Modeling Collaboration, we created a Multimodel Ensemble Prediction System (MEPS) for the Ionosphere-Thermosphere-Electrodynamics system that is based on Data Assimilation (DA) models. MEPS is composed of seven physics-based data assimilation models that cover the globe. Ensemble modeling can be conducted for the mid-low latitude ionosphere using the four GAIM data assimilation models, including the Gauss Markov (GM), Full Physics (FP), Band Limited (BL) and 4DVAR DA models. These models can assimilate Total Electron Content (TEC) from a constellation of satellites, bottom-side electron density profiles from digisondes, in situ plasma densities, occultation data and ultraviolet emissions. The four GAIM models were run for the March 16-17, 2013, geomagnetic storm period with the same data, but we also systematically added new data types and re-ran the GAIM models to see how the different data types affected the GAIM results, with the emphasis on elucidating differences in the underlying ionospheric dynamics and thermospheric coupling. Also, for each scenario the outputs from the four GAIM models were used to produce an ensemble mean for TEC, NmF2, and hmF2. A simple average of the models was used in the ensemble averaging to see if there was an improvement of the ensemble average over the individual models. For the scenarios considered, the ensemble average yielded better specifications than the individual GAIM models. The model differences and averages, and the consequent differences in ionosphere-thermosphere coupling and dynamics will be discussed.

  12. Disturbance dynamo effects over low-latitude F region: A study by network of VHF spaced receivers

    NASA Astrophysics Data System (ADS)

    Kakad, B.; Surve, G.; Tiwari, P.; Yadav, V.; Bhattacharyya, A.

    2017-05-01

    Generation of equatorial spread F (ESF) irregularities resulting from magnetic disturbance is known for past few decades. However, better prediction models for this phenomenon are still lacking. Magnetic storms also affects the F region plasma drifts. In this work we examined variability in (i) occurrence of such freshly generated ESF and (ii) low-latitude F region zonal plasma drifts over Indian longitude. For this purpose simultaneous observations of amplitude scintillations on 251 MHz signal, recorded by a network of spaced receivers located at low-latitude stations, are utilized. Observational stations are situated such that it longitudinally (latitudinally) covers an area of 5.6° (13°). Here effect of disturbance dynamo (DD) electric field at low-latitude F region and its variability are studied for three magnetic storms occurring in 2011. These magnetic storms are having nearly similar type characteristics except their start time. We find that as time difference (i.e., ΔT) between local sunset and start of magnetic activity decreases, the DD effects seen at low-latitude F region zonal irregularity drift around midnight becomes stronger. For a given magnetic storm the DD effect on F region zonal irregularity drifts is found to be only marginally stronger at dip equator in comparison to off-equatorial stations. Although effect of DD on F region zonal irregularity drifts are felt simultaneously, generation of fresh ESF is variable within a smaller longitudinal belt of 5.6°. It is attributed to the presence of LSWS at the bottomside of F region, as initiation of ESF is highly likely (unlikely) in the vicinity of crest (trough) of such LSWS.

  13. Is high obliquity a still plausible explanation for Neoproterozoic low-latitude glaciations ?

    NASA Astrophysics Data System (ADS)

    Levrard, B.; Laskar, J.

    2003-04-01

    The observations of enigmatic low-latitude glaciogenic sequences during the Sturtian (˜ 750-700 Ma) and the Varanger (˜ 620-570 Ma) glacial intervals of the Neopropterozo& uml;i c era remain the subject of controversial debates concerning possible scenarii. Of the many models that have been proposed to account for theses paradoxical features, the high-obliquity scenario invoked by G.E. Williams (1975) appears to be still largely considered. However, a such scenario requires a dissipative mechanism to bring back the Earth's obliquity from a value higher than 55o to the present value (˜ 23.5o) in less than 200 Ma. Williams (1993) suggested that core-mantle friction could have explained this substantial decrease. However, it was demonstrated by Néron de Surgy and Laskar (1997) and confirmed by Pais et al. (1999) that, not only it requires abnormal values of effective viscosity, but due to the conservation of the angular momentum, it is also largely conflicting with the paleorotation data. D.M Williams et al. (Nature, 1998) recently proposed that ``climate friction" could have produced this decrease in less than 100 Ma between ˜ 600 Ma and 500 Ma. We have revisited in details this mecanism (Levrard and Laskar, submitted to Geo. J. Int., 2002) for the Neoproterozoic glaciations. In response to periodic variations in the obliquity, the redistribution of ice/water mass and the isostatic adjusment to the surface loading affect the dynamical ellipticity of the Earth. Delayed responses in the mass redistribution may introduce a secular term in the obliquity evolution (Bills, 1994; Rubincam, 1995; Ito et al, 1995). We analyze the obliquity-oblateness feedback using non-linear response of ice sheets (Imbrie and Imbrie, 1980) to insolation forcing and layered models with Maxwell visco-elastic rheology. Possible increase in the non-linear response of ice sheets to insolation forcing and latitudinal changes in this forcing strongly limit the contribution of the obliquity

  14. Latitudinal GRBR-TEC estimation in Southeast Asia region based on the two-station method

    NASA Astrophysics Data System (ADS)

    Watthanasangmechai, Kornyanat; Yamamoto, Mamoru; Saito, Akinori; Tsugawa, Takuya; Yokoyama, Tatsuhiro; Supnithi, Pornchai; Yatini, Clara Yono

    2014-10-01

    Total electron content (TEC) is an important parameter for revealing latitudinal ionospheric structures, such as the equatorial ionization anomaly (EIA) in Southeast Asia. Understanding the EIA is beneficial for studying equatorial spread F. To reveal the structures, the absolute TEC as a function of latitude must be accurately determined. In early 2012, we expanded a GNU Radio Beacon Receiver (GRBR) network to provide latitudinal coverage in the Thailand-Indonesia sector. We employed the GRBR network to receive VHF and UHF signals from polar low-Earth-orbit satellites. The TEC offset is an unknown parameter in the absolute TEC estimation process. We propose a new technique based on the two-station method to estimate the offset for the latitudinal TEC estimation, and it works better than the original method for a sparse network. The TEC estimation system requires two iterations to minimize the root-mean-square error (RMSE). Once the RMSE reaches the global minimum, the absolute TECs are estimated simultaneously over five GRBR stations. GPS-TECs from local stations are used as the initial guess of the offset estimation. The height of the ionospheric pierce point is determined from the ionosonde hmF2. As a result, the latitudinal GRBR-TEC was successfully estimated from the polar orbit satellites. The two EIA humps were clearly captured by the GRBR-TEC. The result was well verified with the TEC reconstructed from the C/NOFS density data and the ionosonde bottomside data. This is a significant step showing that the GRBR is a useful tool for the study of low-latitude ionospheric features.

  15. Relations Between Traveling Convection Vortex (TCV) Signatures in Near-Cusp Ground Data, at Geosynchronous Orbit, and in Low Latitude Ground Data

    NASA Astrophysics Data System (ADS)

    Camarena, H.; Engebretson, M. J.; Posch, J. L.; Murr, D.; Singer, H. J.; Sibeck, D. G.

    2015-12-01

    Traveling Convection Vortices (TCVs) occur as solitary localized ~2- 5 mHz transients near the ionospheric footpoint of the dayside magnetopause. Ion foreshock instabilities are now understood to drive all or nearly all TCVs; they generate localized changes in dynamic pressure at the dayside magnetospheric boundary, resulting in transient magnetic field variations that generate field-aligned currents that propagate to the high latitude ionosphere and also compressional waves that produce signatures at geosynchronous orbit and lower latitudes on the ground. In this work we extend earlier multistation event studies by means of a statistical study comparing isolated TCV events observed between 2010 and 2014 by the MACCS array (Magnetometer Array for Cusp and Cleft Studies) in Arctic Canada, GOES-13, and several low-latitude INTERMAGNET magnetic observatories, all in the same longitude sector. We found essentially no correlation between the amplitude of TCV events and the amplitude of magnetic field compressions (ΔB) at GOES-13 and low-latitude ground stations. Comparing TCV amplitudes to time derivatives (dB/dt) at geosynchronous orbit and low latitudes, as suggested by published approximate theoretical analyses, resulted in modest correlations. Consistent with earlier studies, the low latitude response was strongest at stations under or very near the equatorial electrojet. We also analyzed a set of sudden impulse (SI) events with bipolar high-latitude signatures; the geostationary and low latitude compressions associated with them were relatively higher than those for TCVs.

  16. Ionospheric variations over Indian low latitudes close to the equator and comparison with IRI-2012

    NASA Astrophysics Data System (ADS)

    Pavan Chaitanya, P.; Patra, A. K.; Balan, N.; Rao, S. V. B.

    2015-08-01

    In this paper, we analyze daytime observations of the critical frequencies of the F2 (foF2) and F3 (foF3) layers based on ionosonde observations made from Indian low latitudes close to the magnetic equator and study their local time, seasonal, planetary-scale variations (including the solar rotation effect), and solar activity dependence. Given the occurrence of the F3 layer, which has remarkable local time, seasonal and solar activity dependences, variations in foF2 have been evaluated. Local time variations in foF2 and foF3 show noon "bite-out" in all seasons and in all solar activity conditions, which are attributed to vertically upward plasma transport by the zonal electric field and meridional neutral wind. Comparison of observed foF2 with those of the IRI-2012 model clearly shows that the model values are always higher than observed values and the largest difference is observed during noontime owing to the noon bite-out phenomenon. Peak frequency of the F layer (foF2 / foF3), however, is found to have better agreement with IRI-2012 model. Seasonal variations of foF2 and foF3 show stronger asymmetry at the solstices than at the equinoxes. The strong asymmetry at the solstice is attributed to the asymmetry in the meridional neutral wind with a secondary contribution from E × B drifts, and the relatively weak asymmetry observed at the equinox is attributed to the asymmetry in E × B drifts. Variations in foF2 and foF3 with solar flux clearly show the saturation effect when F10.7 exceeds ~ 120 sfu, which is different from that of the mid-latitudes. Irrespective of solar flux, both foF2 and foF3 in summer, however, are found to be remarkably lower than those observed in other seasons. Variations in foF2 show dominant periods of ~ 27, ~ 16 and ~ 6 days. Intriguingly, amplitudes of ~ 27-day variations in foF2 are found to be maximum in low solar activity (LSA), moderate in medium solar activity (MSA) and minimum in high solar activity (HSA), while the amplitudes of

  17. A simplified indexing of F-region geophysical noise at low latitudes

    NASA Technical Reports Server (NTRS)

    Aggarwal, S.; Lakshmi, D. R.; Reddy, B. M.

    1979-01-01

    A simple method of deriving an F-region index that can warn the prediction users at low latitudes as to the specific months when they have to be more careful in using the long term predictions is described.

  18. Prompt GPS TEC response to magnetospheric compression

    NASA Astrophysics Data System (ADS)

    Hao, Yongqiang; Huang, Jianping; Liu, Wenlong; Zhang, Donghe; Xiao, Zuo

    2017-04-01

    A new type of total electron content (TEC) variation was observed when an interplanetary shock impacted on the Earth's magnetosphere on 17 March 2015. With hundreds of ground-based Global Positioning System (GPS) receivers, instantaneous TEC impulses were detected in the signals from four GPS satellites cruising in the dayside equatorial magnetosphere. Despite the small amplitude (0.3 TECU), the impulses were synchronously registered by receivers spreading from low to middle latitudes on the ground; they lasted for several minutes, during which period the interplanetary magnetic field (IMF) kept northward. The TEC impulses therefore discriminate themselves from usual traveling ionospheric disturbances and exclude possible effects from the southward IMF-driven magnetic storm. We suggest that the TEC variation is caused by shock-induced magnetospheric compression, which moves plasma earthward in the dayside plasmasphere. As a result, some plasma outside of GPS orbit (4.2 RE) is moved to the inside and contributes to the plasma content traversed by GPS raypath. The GPS TEC technique thus exhibits an unprecedented capability to capture small tremor of the magnetosphere, and with the dense receiver network on the ground it can be a feasible tool for remote sensing of the plasma dynamics around 4.2 RE.

  19. Early science with the Large Millimetre Telescope: Deep LMT/AzTEC millimetre observations of ɛ Eridani and its surroundings

    NASA Astrophysics Data System (ADS)

    Chavez-Dagostino, M.; Bertone, E.; Cruz-Saenz de Miera, F.; Marshall, J. P.; Wilson, G. W.; Sánchez-Argüelles, D.; Hughes, D. H.; Kennedy, G.; Vega, O.; De la Luz, V.; Dent, W. R. F.; Eiroa, C.; Gómez-Ruiz, A. I.; Greaves, J. S.; Lizano, S.; López-Valdivia, R.; Mamajek, E.; Montaña, A.; Olmedo, M.; Rodríguez-Montoya, I.; Schloerb, F. P.; Yun, Min S.; Zavala, J. A.; Zeballos, M.

    2016-11-01

    ɛ Eridani is a nearby, young Sun-like star that hosts a ring of cool debris analogous to the Solar system's Edgeworth-Kuiper belt. Early observations at (sub-)mm wavelengths gave tentative evidence of the presence of inhomogeneities in the ring, which have been ascribed to the effect of a putative low eccentricity planet, orbiting close to the ring. The existence of these structures has been recently challenged by high-resolution interferometric millimetre observations. Here, we present the deepest single-dish image of ɛ Eridani at millimetre wavelengths, obtained with the Large Millimetre Telescope Alfonso Serrano (LMT). The main goal of these LMT observations is to confirm (or refute) the presence of non-axisymmetric structure in the disc. The dusty ring is detected for the first time along its full projected elliptical shape. The radial extent of the ring is not spatially resolved and shows no evidence, to within the uncertainties, of dust density enhancements. Additional features of the 1.1 mm map are: (i) the presence of significant flux in the gap between the ring and the star, probably providing the first exo-solar evidence of Poynting-Robertson drag, (ii) an unambiguous detection of emission at the stellar position with a flux significantly above that expected from ɛ Eridani's photosphere, and (iii) the identification of numerous unresolved sources which could correspond to background dusty star-forming galaxies.

  20. Low-Latitude Solar Wind During the Fall 1998 SOHO-Ulysses Quadrature

    NASA Technical Reports Server (NTRS)

    Poletto, G.; Suess, Steven T.; Biesecker, D.; Esser, R.; Gloeckler, G.; Zurbuchen, T.; Whitaker, Ann F. (Technical Monitor)

    2001-01-01

    The Fall 1998 SOlar-Heliospheric Observatory (SOHO) - Ulysses quadrature occurred when Ulysses was at 5.2 AU, 17.4 deg South of the equator, and off the West line of the Sun. SOHO coronal observations, at heliocentric distances of a few solar radii, showed that the line through the solar center and Ulysses crossed, over the first days of observations, a dark, weakly emitting area and through the northern edge of a streamer complex during the second half of the quadrature campaign. Ulysses in situ observations showed this transition to correspond to a decrease from higher speed wind typical of coronal hole flow to low speed wind. Physical parameters (density, temperature, flow speed) of the low latitude coronal plasma sampled over the campaign are determined using constraints from what is the same plasma measured later in situ and simulating the intensities of the Hydrogen Lyman-alpha and OVI 1032 and 1037 Angstrom lines, measured by the Ultra Violet Coronagraph Spectrometer (UVCS) on SOHO. The densities, temperatures and outflow speed are compared with the same characteristic flow parameters for high-latitude fast wind streams and typical slow solar wind.

  1. Low-Latitude Solar Wind During the Fall 1998 SOHO-Ulysses Quadrature

    NASA Technical Reports Server (NTRS)

    Poletto, G.; Suess, Steven T.; Biesecker, D.; Esser, R.; Gloeckler, G.; Zurbuchen, T.; Whitaker, Ann F. (Technical Monitor)

    2001-01-01

    The Fall 1998 SOlar-Heliospheric Observatory (SOHO) - Ulysses quadrature occurred when Ulysses was at 5.2 AU, 17.4 deg South of the equator, and off the West line of the Sun. SOHO coronal observations, at heliocentric distances of a few solar radii, showed that the line through the solar center and Ulysses crossed, over the first days of observations, a dark, weakly emitting area and through the northern edge of a streamer complex during the second half of the quadrature campaign. Ulysses in situ observations showed this transition to correspond to a decrease from higher speed wind typical of coronal hole flow to low speed wind. Physical parameters (density, temperature, flow speed) of the low latitude coronal plasma sampled over the campaign are determined using constraints from what is the same plasma measured later in situ and simulating the intensities of the Hydrogen Lyman-alpha and OVI 1032 and 1037 Angstrom lines, measured by the Ultra Violet Coronagraph Spectrometer (UVCS) on SOHO. The densities, temperatures and outflow speed are compared with the same characteristic flow parameters for high-latitude fast wind streams and typical slow solar wind.

  2. Response of nighttime equatorial and low latitude F-region to the geomagnetic storm of August 18, 2003, in the Brazilian sector

    NASA Astrophysics Data System (ADS)

    Sahai, Y.; Univap Team

    This paper presents an investigation of geomagnetic storm effects in the equatorial and low latitude F-region in the Brazilian sector during the intense geomagnetic storm on 18 August 2003 SSC 14 21 UT on 17 08 Sigma Kp 52 Ap 108 vert Dst vert max 168 at 1600 UT on 18 08 Simultaneous ionospheric sounding measurements from two stations viz Palmas 10 2 o S 48 2 o W dip latitude 5 7 o S and S a o Jos e dos Campos 23 2 o S 45 9 o W dip latitude 17 6 o S Brazil are presented for the nights of 16-17 17-18 18-19 19-20 August 2003 quiet disturbed and recovery phases Both stations are equipped with the Canadian Advanced Digital Ionosonde CADI Quiet and disturbed conditions of the F-region ionosphere are compared using data collected from the two stations The relationship between magnetospheric disturbance and low-latitude ionospheric dynamics and generation of ionospheric irregularities will be discussed The GPS data available from several stations in Rede Brasileira de Monitoramento Cont i nuo de GPS Brazilian Network for Continuous GPS Monitoring are used to obtain the vertical total electron content VTEC and the rate of change of TEC per minute on UT days 18 and 19 August 2003 During the disturbed nights the low latitude station S J Campos showed strong positive phase whereas the near equatorial station Palmas showed strong uplifting of the F-layer Normally during the winter months May to August in the Brazilian sector large-scale ionospheric irregularities in form of plasma

  3. Isotopic composition of low-latitude paleoprecipitation during the Early Cretaceous

    USGS Publications Warehouse

    Suarez, M.B.; Gonzalez, Luis A.; Ludvigson, Greg A.; Vega, F.J.; Alvarado-Ortega, J.

    2009-01-01

    likely underestimated tropical to subtropical precipitation and evaporation fluxes. The limited latitudinal constraints for earlier isotope mass balance modeling of the Albian hydrologic cycle of the Northern Hemisphere Americas resulted in extrapolated low-latitude precipitation ??18O values that were much heavier (up to 3???) than the values observed in this study. The lighter values identified in this study indicate a more pronounced rainout effect for tropical regions and quite possibly a more vigorous evaporation effect. These and additional low-latitude data are required to better constrain changes in the hydrologic cycle during the Cretaceous greenhouse period, and to reduce the uncertainties resulting from limited geographic coverage of proxy data. ?? 2009 Geological Society of America.

  4. Low-Latitude Noctilucent Cloud of 15 June 1963.

    PubMed

    Meinel, A B; Middlehurst, B; Whitaker, E

    1963-09-20

    A bright noctilucent cloud was observed and photographed north-west of Tucson on 15 June 1963. Results of computations indicate that the cloud was at a height of 71 kilometers. The cloud appears to have resulted from the launching of a Scout space vehicle.

  5. A Semi-Empirical Low-Latitude Ionospheric Model

    DTIC Science & Technology

    1988-10-06

    the atmosphere, P., a value of marca incoherent scatter radar observations reported 5.5 x 10- s ’ represents solar cycle maximum con- by Fejer et al...bottomside components of the overall profiles with N(h) = N ex [c -:. - e)] good results. It was decided, therefore, to define a (h- h...) "generalized" or

  6. Statistical Evaluation of Forecasts Over the Low-Latitudes.

    DTIC Science & Technology

    1985-08-01

    o o .x 1. Introduction .. . ........ .. .- 2. The Models . . . . . . . . . . . ... ... 4 2.1 Filtered barotropic modelo . ....... ... 4 2.1.1...and Dynamic Meteorology. John Wiley and Sons, Inc., 477 pp . Hebert, P.J., 1980: Atlantic hurricane season of 1979. Mon. Wea. Rev., 108, 973-990. Julian...for the special observing periods. NOAA Data Report ERL GFDL-6, 237 pp . Mathur, M.B., 1970: A note on an improved quasi-Lagrangian advective scheme

  7. Jellyfish Patch Detecting Using Low Latitude Remote Sensing System

    NASA Astrophysics Data System (ADS)

    Lee, J. S.; Jo, Y. H.

    2015-12-01

    Jellyfish can be asexual and sexual reproduction depending on the environment, and it has excellent environmental adaptability and reproduction than other sea creatures. If the marine environment become worse, jellyfish can take advantage in the competition for survival. Marine environmental changes caused by rapid climate change, dyke construction and land reclamation will increase the amount of jellyfish and as a result can lead to a various social and economic problems. In this study, jellyfish were observed in coastal area using a low-altitude Helikite remote sensing system for the first time. Helikite is a type of helium balloon plus a kite that can get the data with optical sensors for the desired spatial resolutions by adjusting the altitudes. In addition, it has an advantage that can monitor any objects for a long time at one place as long as the electric power and helium last. In this study, we observed the jellyfish patches using a digital camera in the Chesapeake Bay and estimate populations and size of jellyfish patches through image processing. Research results suggests that we can have long-term real-time observations for not only jellyfish, but also other harmful marine creatures.

  8. Day-to-day changes in ionospheric electron content at low latitudes

    NASA Astrophysics Data System (ADS)

    Dabas, R. S.; Bhuyan, P. K.; Tyagi, T. R.; Bhardwaj, R. K.; Lal, J. B.

    1984-06-01

    For a number of years, the ionospheric electron content (IEC) over the Indian subcontinent has been determined on the basis of the Faraday rotation of satellite radio beacon transmissions. In these determinations, use was made of the orbiting satellites BE-B and BE-C, and, for a limited period, of the geostationary satellite ATS 6. A large variability in day-to-day values of IEC was reported, and it was tried to correlate this phenomenon with magnetic activity, solar flux, or the effect of neutral winds. Tyagi (1978) observed that the day-to-day changes in IEC occur in the form of single day abnormality, and alternate day abnormality. Long-term fluctuations were found with a periodicity of about 45 days. The present investigation is concerned with a more detailed study of the observed variations. An analysis is conducted of IEC data recorded during the low phase of the solar cycle, taking into account data from six low-latitude stations covering a latitude range from approximately 15.0 deg N to 30.0 deg N.

  9. Low-latitude Pi2 pulsations during intervals of quiet geomagnetic conditions (Kp≤1)

    NASA Astrophysics Data System (ADS)

    Kwon, H.-J.; Kim, K.-H.; Jun, C.-W.; Takahashi, K.; Lee, D.-H.; Lee, E.; Jin, H.; Seon, J.; Park, Y.-D.; Hwang, J.

    2013-10-01

    It has been reported that Pi2 pulsations can be excited under extremely quiet geomagnetic conditions (Kp=0). However, there have been few comprehensive reports of Pi2 pulsations in such a near ground state magnetosphere. To understand the characteristics of quiet-time Pi2 pulsations, we statistically examined Pi2 events observed on the nightside between 1800 and 0600 local time at the low-latitude Bohyun (BOH, L = 1.35) station in South Korea. We chose year 2008 for analysis because geomagnetic activity was unusually low in that year. A total of 982 Pi2 events were identified when Kp≤1. About 80% of the Pi2 pulsations had a period between 110 and 300 s, which significantly differs from the conventional Pi2 period from 40 to 150 s. Comparing Pi2 periods and solar wind conditions, we found that Pi2 periods decrease with increasing solar wind speed, consistent with the result of Troitskaya (1967). The observed wave properties are discussed in terms of plasmaspheric resonance, which has been proposed for Pi2 pulsations in the inner magnetosphere. We also found that Pi2 pulsations occur quasi-periodically with a repetition period of ˜23-38 min. We will discuss what determines such a recurrence time of Pi2 pulsations under quiet geomagnetic conditions.

  10. Statistical Study of Low Latitude Pc3 Geomagnetic Pulsations: Diurnal and Seasonal Characteristics

    NASA Astrophysics Data System (ADS)

    Ansari, I. A.

    2006-05-01

    Using an array of four low latitude induction coil magnetometers, geomagnetic pulsations in the Pc3 period range were recorded simultaneously in southeast Australia. The data recording stations were situated at Woomera, Broken Hill, Newcastle and Launceston over a latitudinal range of 100 and a longitudinal range of 170 at L=1.8 to 2.7. Geomagnetic data for a period of about six months were digitized and analysis was carried out employing digital sonagram technique. The Pc3 occurrence was found to be evenly distributed with magnetic activity over the Kp = 2 - 5 range. Launceston, however, showed lower occurrences at low Kp values (Kp <2+) than the other three stations. The Pc3 occurrence probability normalized with respect to Kp occurrence was maximum for Kp = 4+ at all the four stations. It is also worth noting that Pc3 in winter was observed during intense magnetic activity when 5 < Kp <9. Finally, a gradual increase in the Pc3 average, lower and upper frequencies limit with Kp was observed during the local winter months for Kp <5. Key words - Pc3 geomagnetic pulsations, digital sonagram, diurnal variation of Pc3 occurrence, seasonal variation of Pc3 occurrence, Kp dependence.

  11. Determining the Current and Future Health of Low-Latitude Andean Glaciers Using an Equilibrium Line Altitude Model and Hypsometric Data from the Randolph Glacier Inventory

    NASA Astrophysics Data System (ADS)

    Malone, A.; MacAyeal, D. R.

    2015-12-01

    Mountain glaciers have been described as the water towers of world, and for many populations in the low-latitude South American Andes, glacial runoff is vital for agricultural, industrial, and basic water needs. Previous studies of low-latitude Andean glaciers suggest a precarious future due to contemporary warming. These studies have looked at trends in freezing level heights or observations of contemporary retreat. However, regional-scale understanding of low-latitude glacial responses to present and future climate change is limited, in part due to incomplete information about the extent and elevation distribution of low-latitude glaciers. The recently published Randolph Glacier Inventory (RGI) (5.0) provides the necessary information about the size and elevation distribution of low-latitude glaciers to begin such studies. We determine the contemporary equilibrium line altitudes (ELAs) for low-latitude Andean glaciers in the RGI, using a numerical energy balance ablation model driven with reanalysis and gridded data products. Contemporary ELAs tend to fall around the peak of the elevation histogram, with an exception being the southern-most outer tropical glaciers whose modeled ELAs tend to be higher than the elevation histogram for that region (see below figure). Also, we use the linear tends in temperature and precipitation from the contemporary climatology to extrapolate 21stcentury climate forcings. Modeled ELAs by the middle on the century are universally predicted to rise, with outer tropical ELAs rising more than the inner tropical glaciers. These trends continue through the end of the century. Finally, we explore how climate variables and parameters in our numerical model may vary for different warming scenarios from United Nation's IPCC AR5 report. We quantify the impacts of these changes on ELAs for various climate change trajectories. These results support previous work on the precarious future of low latitude Andean glaciers, while providing a richer

  12. Anomalous variations of tropopause height in low latitude regions

    NASA Astrophysics Data System (ADS)

    Ramkumar, T. K.; Narayana Rao, D.

    Successful attempts have been made in early 1990s to link the possible influence of equatorial stratospheric quasi-biennial-oscillation QBO on tropopause dynamics at longer period scales leading ultimately to the evolution of strong El Nino events of global economic importance Gray et al 1992 GRL JMSJ As a result of this influence it is possible that the tropopause height may be increasing instead of decreasing with latitude from the equator in tropical regions during particular phase of QBO say westerly phase over the equator In the present work we report such observations using radiosonde data obtained from fourteen different tropical 30 N to 30 S radio-sounding stations located in the wide longitudinal zone of South East Asia South Pacific and Africa in the year 2004 The daily tropopause height determined at 00 00 and 12 00 hrs GMT is averaged for each month separately The tropopause height Cold Point Tropopause CPT is determined by noting the height of minimum temperature between 12 and 20 km It is observed further that the latitude variation of tropopause height in southern Hemisphere is much less when compared to that in the Northern Hemisphere The reason for this asymmetric characteristic of tropopause about the equator may be that greater fraction of the northern hemisphere is covered with land and it is ocean in the southern hemisphere Because of large variations in topography of the land and the associated thermal conductivity it is possible that convection activities of the

  13. Assessment of Modeling Capability for Reproducing Storm Impacts on TEC

    NASA Astrophysics Data System (ADS)

    Shim, J. S.; Kuznetsova, M. M.; Rastaetter, L.; Bilitza, D.; Codrescu, M.; Coster, A. J.; Emery, B. A.; Foerster, M.; Foster, B.; Fuller-Rowell, T. J.; Huba, J. D.; Goncharenko, L. P.; Mannucci, A. J.; Namgaladze, A. A.; Pi, X.; Prokhorov, B. E.; Ridley, A. J.; Scherliess, L.; Schunk, R. W.; Sojka, J. J.; Zhu, L.

    2014-12-01

    During geomagnetic storm, the energy transfer from solar wind to magnetosphere-ionosphere system adversely affects the communication and navigation systems. Quantifying storm impacts on TEC (Total Electron Content) and assessment of modeling capability of reproducing storm impacts on TEC are of importance to specifying and forecasting space weather. In order to quantify storm impacts on TEC, we considered several parameters: TEC changes compared to quiet time (the day before storm), TEC difference between 24-hour intervals, and maximum increase/decrease during the storm. We investigated the spatial and temporal variations of the parameters during the 2006 AGU storm event (14-15 Dec. 2006) using ground-based GPS TEC measurements in the selected 5 degree eight longitude sectors. The latitudinal variations were also studied in two longitude sectors among the eight sectors where data coverage is relatively better. We obtained modeled TEC from various ionosphere/thermosphere (IT) models. The parameters from the models were compared with each other and with the observed values. We quantified performance of the models in reproducing the TEC variations during the storm using skill scores. This study has been supported by the Community Coordinated Modeling Center (CCMC) at the Goddard Space Flight Center. Model outputs and observational data used for the study will be permanently posted at the CCMC website (http://ccmc.gsfc.nasa.gov) for the space science communities to use.

  14. Low-latitude particle precipitation and associated local magnetic disturbances

    NASA Technical Reports Server (NTRS)

    Rassoul, H. K.; Rohrbaugh, R. P.; Tinsley, B. A.

    1992-01-01

    The paper investigates the O(+) ion and NO(+) ion temperature and temperature anisotropy for a spatially homogeneous auroral F region in the presence of large electric fields perpendicular to the geomagnetic field. The maximum optical emissions at midlatitudes occur in concert with the maximum positive (northward) excursions in the H trace and with rapid fluctuations in the D trace of nearby magnetograms. The source of the particles is inferred to be the ring current, with precipitation occurring when the Dst index is large at the time of the large short-term excursions in the local magnetic field. This result is consistent with the finding of Voss and Smith (1979), derived from a series of rocket measurements of precipitating heavy particles, that the flux correlates better with the product of Dst and the exponential of Kp than with either alone. It is shown that the product of Dst and the amplitude of the short-term excursions in the horizontal component in local magnetograms has better time resolution and better correlation with the observed emission rates than the index using Kp.

  15. Low-latitude Ionospheric Research using the CIRCE Mission

    NASA Astrophysics Data System (ADS)

    Dymond, K.; Nicholas, A. C.; Budzien, S. A.; Stephan, A. W.

    2016-12-01

    The Coordinated Ionospheric Reconstruction Cubesat Experiment (CIRCE) is a dual-satellite mission consisting of two 6U CubeSats actively maintaining a lead-follow configuration in the same orbit with a launch planned for the 2018-2019 time frame. These nano-satellites will each feature two 1U ultraviolet photometers, observing the 135.6 nm emission of atomic oxygen at nighttime. The primary objective is to characterize the two-dimensional distribution of electrons in the Equatorial Ionization Anomaly (EIA). The methodology used to reconstruct the nighttime ionosphere employs continuous UV photometry from four distinct viewing angles in combination with an additional data source, such as in situ plasma density measurements or a wide-band beacon data, with advanced image space reconstruction algorithm tomography techniques. The COSMIC/FORMOSAT-3 (CF3) constellation featured six Tiny Ionospheric Photometers, a compact UV sensor design which served as the pathfinder for the CIRCE instruments. The TIP instruments on the CF3 satellites demonstrated detection of ionospheric bubbles before they had penetrated the peak of the F-region ionosphere. We present our mission concept, simulations illustrating the imaging capability of the sensor suite, and a range of science questions addressable using such a system.

  16. Solar activity dependence of the topside ionosphere at low latitudes

    NASA Astrophysics Data System (ADS)

    Chen, Yiding; Liu, Libo; Wan, Weixing; Yue, Xinan; Su, Shin-Yi

    2009-08-01

    We investigated the solar activity dependence of the topside ionosphere with ROCSAT-1 observations. The distribution of the plasma density at 600 km altitude shows features with considerable local time, season, and solar activity differences. In the daytime, plasma density peaks around the dip equator. This peak is more distinct in equinoxes and weaker in May-July, and it enhances with solar activity in all seasons. The seasonal behavior of this peak is primarily controlled by the seasonal variations of neutral density and E × B vertical drift. The enhancement of the peak with solar activity is related to the effect of E × B vertical drift. Around sunset, double peaks are found in the latitudinal distribution of plasma density in solar maximum equinoxes and December solstice, which are mainly attributed to the effects of strong prereversal enhancement (PRE) vertical drift. Moreover, the plasma density at 600 km altitude strongly depends on the solar proxy P = (F 107 + F 107A)/2. At higher altitudes, e.g., 800 km, the amplification trend prevails in the solar activity variations of plasma density. In contrast, the plasma density at 600 km altitude presents three kinds of patterns (linear, amplification, and saturation), which has not been reported. Saturation effect is found at equinox sunset around the dip equator. This saturation effect is attributed to the increase in the PRE vertical drift with solar activity. Solar activity effects of ROCSAT-1 plasma density are argued to be the combined effects induced by the changes in the peak height, the scale height, and the peak electron density, respectively. Among these factors, the rise of the F 2 peak is more important for the equatorial plasma density at 600 km altitude.

  17. Variational electric fields at low latitudes and their relation to spread F and plasma irregularities

    NASA Technical Reports Server (NTRS)

    Holtet, J. A.; Maynard, N. C.; Heppner, J. P.

    1976-01-01

    Recordings from OGO 6 show that electric field irregularities are frequently present between + or - 35 deg geomagnetic latitude in the 2000 - 0600 local time sector. The signatures are very clear, and are easily distinguished from the normal AC background noise, and whistler and emission activity. The spectral appearance of the fields makes it meaningful to distinguish between 3 different types of irregularities: strong irregularities, weak irregularities, and weak irregularities with a rising spectrum. Strong irregularities seem most likely to occur in regions where gradients in ionization are present. Changes in plasma composition, resulting in an increase in the mean ion mass, are also often observed in the irregularity regions. Comparison with ground based ionosondes indicates a connection between strong irregularities and low latitude spread F. A good correlation is also present between strong fields and small scale fluctuations in ionization, delta N/N 1 percent. From the data it appears as if a gradient driven instability is the most likely source of the strong irregularities.

  18. Insights into low-latitude cloud feedbacks from large-eddy simulations

    NASA Astrophysics Data System (ADS)

    Bretherton, Christopher; Blossey, Peter

    2017-04-01

    Cloud feedbacks are a leading source of uncertainty in the climate sensitivity simulated by global climate models (GCMs). Low-latitude boundary-layer and cumulus cloud regimes are particularly problematic, because they are sustained by tight interactions between clouds and unresolved turbulent circulations. Large-eddy simulations (LES) using sub-100 m grid spacings better simulate such cloud regimes without need for complex models of subgrid variability of cloud and turbulence. Recently, multiday LES over small computational domains have elucidated marine boundary layer cloud response to specified aspects of greenhouse warming and the associated changes in large-scale dynamics and atmospheric radiative heating. The focus will be the CGILS LES intercomparisons and subsequent related work. Four primary contributing mechanisms of subtropical low cloud response are implicated, all with observational support. These are (1) thermodynamic: cloudiness reduction from warming and moistening of the atmosphere-ocean column, (2) radiative: cloudiness reduction from CO2 and H2O-induced increase in atmospheric emissivity aloft, (3) stability-induced: low cloud increase from increased lower-tropospheric stratification, and (4) dynamical: low cloud increase from reduced subsidence. LES as a group robustly suggest that the cloudiness reduction mechanisms typically dominate, giving positive shortwave cloud feedback in the subtropics consistent with the range simulated by conventional global climate models. Finally, a possible approach for better bridging the scale gap between LES and global models will be noted.

  19. Insights into low-latitude cloud feedbacks from high-resolution models.

    PubMed

    Bretherton, Christopher S

    2015-11-13

    Cloud feedbacks are a leading source of uncertainty in the climate sensitivity simulated by global climate models (GCMs). Low-latitude boundary-layer and cumulus cloud regimes are particularly problematic, because they are sustained by tight interactions between clouds and unresolved turbulent circulations. Turbulence-resolving models better simulate such cloud regimes and support the GCM consensus that they contribute to positive global cloud feedbacks. Large-eddy simulations using sub-100 m grid spacings over small computational domains elucidate marine boundary-layer cloud response to greenhouse warming. Four observationally supported mechanisms contribute: 'thermodynamic' cloudiness reduction from warming of the atmosphere-ocean column, 'radiative' cloudiness reduction from CO2- and H2O-induced increase in atmospheric emissivity aloft, 'stability-induced' cloud increase from increased lower tropospheric stratification, and 'dynamical' cloudiness increase from reduced subsidence. The cloudiness reduction mechanisms typically dominate, giving positive shortwave cloud feedback. Cloud-resolving models with horizontal grid spacings of a few kilometres illuminate how cumulonimbus cloud systems affect climate feedbacks. Limited-area simulations and superparameterized GCMs show upward shift and slight reduction of cloud cover in a warmer climate, implying positive cloud feedbacks. A global cloud-resolving model suggests tropical cirrus increases in a warmer climate, producing positive longwave cloud feedback, but results are sensitive to subgrid turbulence and ice microphysics schemes. © 2015 The Author(s).

  20. Solar Cycle Effects on Equatorial Electrojet Strength and Low Latitude Ionospheric Variability (P10)

    NASA Astrophysics Data System (ADS)

    Veenadhari, B.; Alex, S.

    2006-11-01

    veena_iig@yahoo.co.in The most obvious indicators of the activity of a solar cycle are sunspots, flares, plages, and soon. These are intimately linked to the solar magnetic fields, heliospheric processes which exhibit complex but systematic variations. The changes in geomagnetic activity, as observed in the ground magnetic records follow systematic correspondence with the solar activity conditions. Thus the transient variations in the magnetic field get modified by differing solar conditions. Also the solar cycle influences the Earth causing changes in geomagnetic activity, the magnetosphere and the ionosphere. Daily variations in the ground magnetic field are produced by different current systems in the earth’s space environment flowing in the ionosphere and magnetosphere which has a strong dependence on latitude and longitude of the location. The north-south (Horizontal) configuration of the earth’s magnetic field over the equator is responsible for the narrow band of current system over the equatorial latitudes and is called the Equatorial electrojet (EEJ) and is a primary driver for Equatorial Ionization anomaly (EIA). Equatorial electric fields and plasma drifts play the fundamental roles on the morphology of the low latitude ionosphere and strongly vary during geomagnetically quiet and disturbed periods. Quantitative study is done to illustrate the development process of EEJ and its influence on ionospheric parameters. An attempt is also made to examine and discuss the response of the equatorial electrojet parameters to the fast varying conditions of solar wind and interplanetary parameters.

  1. TEC variation during high and low solar activities over South American sector

    NASA Astrophysics Data System (ADS)

    Jonah, O. F.; de Paula, E. R.; Muella, M. T. A. H.; Dutra, S. L. G.; Kherani, E. A.; Negreti, P. M. S.; Otsuka, Y.

    2015-12-01

    Using dual frequency GPS receivers in the South American sector, the measurement of absolute ionospheric Total Electron Content (TEC) has been estimated applying the Nagoya ionospheric model for both the years of 2009 and 2001, which represent low and high solar activities, respectively. The diurnal, day-to-day, monthly, seasonal, latitudinal and longitudinal variations of TEC were studied for equatorial and low latitude regions of South America. The strength and characteristics of the Equatorial Ionization Anomaly (EIA) were equally analyzed. The analyses reveal the diurnal, seasonal and semidiurnal TEC variation, as well as the nighttime variability during the low and high solar activities. Wavelet power spectra analysis was employed to check the periodicities of the TEC data, F10.7 and zonal and meridional wind velocities measured by Meteor radar at ∼100 km altitude. Periods such as 27, 16, 8-10, 1-5 days were found to be dominant in the zonal and meridional wind velocity corresponding with those of TEC periodicities. Hence, besides the solar radiation, we suggest that there are contributions of tides and planetary waves in spatial and temporal TEC enhancement and variations during the geomagnetic quiet periods of both solar activities.

  2. Anomalous Variations of Ionosphere Associated with the Strong Earthquake at Pakistan-Iran Border at a Low Latitude Station Agra, India

    NASA Astrophysics Data System (ADS)

    Pundhir, Devbrat; Singh, Birbal; Singh, O. P.; Gupta, Saral K.

    2016-06-01

    In this paper, we analyze the TEC data for April 2013 observed at Agra station, India (geogr. lat. 27.2° N, long. 78° E) to examine the effect of earthquake of magnitude M = 7.8 which occurred on 16 April 2013 at Pakistan-Iran border region. We process the TEC data using the s statistical criterion to find out anomalous variation in TEC data. We also study the VLF propagation signal from NPM, Hawaii (21.42° N, 158° W), which is monitored at the same station (Agra station) in the light of this earthquake as well as solar flares. The nighttime fluctuation method is used to analyze the VLF data for the period of ±5 days from the day of earthquake (11-21 April 2013). The anomalous enhancements and depletions are found in TEC data on 1-9 days before the occurrence of event.

  3. Attenuation of lightning-produced sferics in the Earth-ionosphere waveguide and low-latitude ionosphere

    NASA Astrophysics Data System (ADS)

    Burkholder, Brian S.; Hutchins, Michael L.; McCarthy, Michael P.; Pfaff, Robert F.; Holzworth, Robert H.

    2013-06-01

    We compare radio atmospherics (sferics) detected by the World Wide Lightning Location Network (WWLLN) to very low frequency (VLF) whistler waves observed in the low-latitude ionosphere by the Vector Electric Field Instrument of the Communications/Navigation Outage Forecasting System (C/NOFS) satellite. We also model the propagation of these sferics through the Earth-ionosphere waveguide to the subsatellite point using the Long-Wavelength Propagation Capability software and compare this result to the same C/NOFS data set. This unprecedentedly expansive data set allows comparison to theory and prior observation of VLF radio wave propagation in the Earth-ionosphere waveguide and low-latitude ionosphere. We show that WWLLN and C/NOFS observe the well-known effect of variable attenuation with direction within the Earth-ionosphere waveguide. Propagation within the ionosphere is also examined, and a lack of attenuation above 400 km is observed. Finally, in comparison to recent works using Detection of Electro-Magnetic Emissions Transmitted from Earthquake Regions (DEMETER) data by Fiser et al. and Chum et al., we find that C/NOFS successfully detects whistlers with comparable amplitudes at much greater distances, compared to those reported for DEMETER.

  4. Modeling storm-time electrodynamics of the low-latitude ionosphere thermosphere system: Can long lasting disturbance electric fields be accounted for?

    NASA Astrophysics Data System (ADS)

    Maruyama, Naomi; Sazykin, Stanislav; Spiro, Robert W.; Anderson, David; Anghel, Adela; Wolf, Richard A.; Toffoletto, Frank R.; Fuller-Rowell, Timothy J.; Codrescu, Mihail V.; Richmond, Arthur D.; Millward, George H.

    2007-07-01

    Storm-time ionospheric disturbance electric fields are studied for two large geomagnetic storms, March 31, 2001 and April 17 18, 2002, by comparing low-latitude observations of ionospheric plasma drifts with results from numerical simulations based on a combination of first-principles models. The simulation machinery combines the Rice convection model (RCM), used to calculate inner magnetospheric electric fields, and the coupled thermosphere ionosphere plasmasphere electrodynamics (CTIPe) model, driven, in part, by RCM-computed electric fields. Comparison of model results with measured or estimated low-latitude vertical drift velocities (zonal electric fields) shows that the coupled model is capable of reproducing measurements under a variety of conditions. In particular, our model results suggest, from theoretical grounds, a possibility of long-lasting penetration of magnetospheric electric fields to low latitudes during prolonged periods of enhanced convection associated with southward-directed interplanetary magnetic field, although the model probably overestimates the magnitude and duration of such penetration during extremely disturbed conditions. During periods of moderate disturbance, we found surprisingly good overall agreement between model predictions and data, with penetration electric fields accounting for early main phase changes and oscillations in low-latitude vertical drift, while the disturbance dynamo mechanism becomes increasingly important later in the modeled events. Discrepancies between the model results and the observations indicate some of the difficulties in validating these combined numerical models, and the limitations of the available experimental data.

  5. High latitude regulation of low latitude thermocline ventilation and planktic foraminifer populations across glacial-interglacial cycles

    NASA Astrophysics Data System (ADS)

    Sexton, Philip F.; Norris, Richard D.

    2011-11-01

    One of the earliest discoveries in palaeoceanography was the observation in 1935 that the (sub)tropical planktic foraminifer Globorotalia menardii became absent or extremely rare in the Atlantic Ocean during glacials of the late Pleistocene. Yet a mechanistic explanation for G. menardii's extraordinary biogeographic behaviour has eluded palaeoceanographers for 75 years. Here we show that modern G. menardii, along with two other species that also suffer Atlantic population collapses during glacials, track poorly ventilated waters globally in their thermocline habitats. The ventilation states of low latitude thermoclines are 'set', to a first order, by intermediate water masses originating at high latitudes. In the modern Atlantic this control on low latitude thermocline ventilation is exerted by relatively poorly ventilated, southern-sourced Antarctic Intermediate Water (AAIW) and sub-Antarctic Mode Water (SAMW). We suggest that the glacial Atlantic foraminifer population collapses were a consequence of a low latitude thermocline that was better ventilated during glacials than it is today, in line with geochemical evidence, and driven primarily by a well-ventilated, northern-sourced intermediate water mass. A ventilation mechanism driving the glacial population collapses is further supported by our new constraints on the precise timing of these species' Atlantic proliferation during the last deglaciation — occurring in parallel with a wholesale, bipolar reorganisation of the Atlantic's thermocline-to-abyssal overturning circulation. Our findings demonstrate that a bipolar seesaw in the formation of high latitude intermediate waters has played an important role in regulating the population dynamics of thermocline-dwelling plankton at lower latitudes.

  6. Multispecies spawning sites for fishes on a low-latitude coral reef: spatial and temporal patterns.

    PubMed

    Claydon, J A B; McCormick, M I; Jones, G P

    2014-04-01

    Spawning sites used by one or more species were located by intensively searching nearshore coral reefs of Kimbe Bay (New Britain, Papua New Guinea). Once identified, the spawning sites were surveyed repeatedly within fixed 5 m radius circular areas, for  > 2000 h of observations ranging from before dawn to after dusk spanning 190 days between July 2001 and May 2004. A total of 38 spawning sites were identified on the seven study reefs distributed at an average of one site every 60 m of reef edge. Pelagic spawning was observed in 41 fish species from six families. On three intensively studied reefs, all 17 spawning sites identified were used by at least three species, with a maximum of 30 different species observed spawning at a single site. Spawning was observed during every month of the study, on all days of the lunar month, at all states of the tide and at most hours of the day studied. Nevertheless, the majority of species were observed spawning on proportionately more days from December to April, on more days around the new moon and in association with higher tides. The strongest temporal association, however, was with species-specific diel spawning times spanning < 3 h for most species. While dawn spawning, afternoon spawning and dusk spawning species were differentiated, the time of spawning for the striated surgeonfish Ctenochaetus striatus also differed significantly among sites. The large number of species spawning at the same restricted locations during predictable times suggests that these sites are extremely important on this low-latitude coral reef.

  7. HF radar signatures of the cusp and low-latitude boundary layer

    NASA Technical Reports Server (NTRS)

    Baker, K. B.; Dudeney, J. R.; Greenwald, R. A.; Pinnock, M.; Newell, P. T.; Rodger, A. S.; Mattin, N.; Meng, C.-I.

    1995-01-01

    Continuous ground-based observations of ionospheric and magnetospheric regions are critical to the Geospace Environmental Modeling (GEM) program. It is therefore important to establish clear intercalibrations between different ground-based instruments and satellites in order to clearly place the ground-based observations in context with the corresponding in situ satellite measurements. HF-radars operating at high latitudes are capable of observing very large spatial regions of the ionosphere on a nearly continuous basis. In this paper we report on an intercalibration study made using the Polar Anglo-American Conjugate Radar Experiment radars located at Goose Bay, Labrador, and Halley Station, Antarctica, and the Defense Meteorological Satellite Program (DMSP) satellites. The DMSP satellite data are used to provide clear identifications of the ionospheric cusp and the low-latitude boundary layer (LLBL). The radar data for eight cusp events and eight LLBL events have been examined in order to determine a radar signature of these ionospheric regions. This intercalibraion indicates that the cusp is always characterized by wide, complex Doppler power spectra, whereas the LLBL is usually found to have spectra dominated by a single component. The distribution of spectral widths in the cusp is of a generally Gaussian form with a peak at about 220 m/s. The distribution of spectral widths in the LLBL is more like an exponential distribution, with the peak of the distribution occurring at about 50 m/s. There are a few cases in the LLBL where the Doppler power spectra are strikingly similar to those observed in the cusp.

  8. Some evidence of ground power enhancements at frequencies of global magnetospheric modes at low latitude

    NASA Astrophysics Data System (ADS)

    Francia, P.; Villante, U.

    1997-01-01

    A statistical analysis of the power spectra of the geomagnetic field components H and D for periods ranging between 3 min and 1 h was conducted at a low-latitude observatory (LÁquila, L=1.6) at the minimum and maximum of the solar cycle. For both components, during daytime intervals, we found evidence of power enhancements at frequencies predicted for global modes of the Earthś magnetosphere and occasionally observed at auroral latitudes in the F-region drift velocities (approximately at 1.3, 1.9, 2.6, and 3.4 mHz). Nighttime observations reveal a relative low frequency H enhancement associated with the bay occurrence together with a peak in the H/D power ratio which sharply emerges at 1.2 mHz in the premidnight sector. The strong similarity between solar minimum and maximum suggests that these modes can be considered permanent magnetospheric features. A separate analysis on a two-month interval shows that the observed spectral characteristics are amplified by conditions of high-velocity solar wind. Acknowledgements. The authors are grateful to Prof. D. J. Southwood (Imperial College, London), J. C. Samson (University of Alberta, Edmonton), L. J. Lanzerotti (AT&T Bell Laboratories), A. Wolfe (New York City Technical College) and to Dr. M. Vellante (University of LÁquila) for helpful discussions. They also thank Dr. A. Meloni (Istituto Nazionale di Geofisica, Roma) who made available geomagnetic field observations from LÁquila Geomagnetic Observatory. This research activity at LÁquila is supported by MURST (40% and 60% contracts) and by GIFCO/CNR. Topical Editor K.-H. Glaßmeier thanks C. Waters and S. Fujita for their help in evaluating this paper.->

  9. The Kinetic Scale Structure of the Low Latitude Boundary Layer: Initial MMS Results

    NASA Astrophysics Data System (ADS)

    Dorelli, John; Gershman, Dan; Avanov, Levon; Pollock, Craig; Giles, Barbara; Gliese, Ulrik; Barrie, Alexander; Holland, Matthew; Salo, Chad; Dickson, Charles; Coffey, Victoria; Chandler, Michael; Sato, Yoshifumi; Strangeway, Robert; Russell, Christopher; Baumjohann, Wolfgang; Khotyainstev, Yuri; Torbert, Roy; Burch, James

    2016-04-01

    Since its launch in March of 2015, NASA's Magnetospheric Multiscale (MMS) mission has captured thousands of high resolution magnetopause crossings, routinely resolving the sub-Larmor radius structure of the magnetopause boundary layer for the first time. The primary goal of MMS is to understand the microphysics of magnetic reconnection, and it is well on its way to achieving this objective. However, MMS is also making routine measurements of the electron and ion gyroviscous and heat flux tensors with unprecedented resolution and accuracy. This opens up the possibility of directly observing the physical processes that facilitate momentum and energy transport across the magnetopause boundary layer under arbitrary conditions (e.g., magnetic field geometry and flow shear) far from the reconnection X line. Currently, our global magnetosphere fluid models (e.g., resistive or Hall MHD) do not include accurate descriptions of viscosity and heat flow, both of which are known to be critical players at the magnetopause (not just at the reconnection sites), and several groups are attempting to make progress on this difficult fluid closure problem. In this talk, we will address the fluid closure problem in the context of MMS observations of the Low Latitude Boundary Layer (LLBL), focusing on high resolution particle observations by the Fast Plasma Investigation (FPI). FPI electron bulk velocities are accurate enough to compute current density in both the high density magnetosheath and low density magnetosphere and have already revealed that the LLBL has a complex parallel current structure on the proton Larmor radius scale. We discuss the relationship between these parallel currents and the Hall electric field structures predicted by kinetic models. We also present first observations of the ion and electron gyroviscous and heat flux tensors in the LLBL and discuss implications for the fluid closure problem at Earth's magnetopause.

  10. Responses of the low-latitude ionosphere to very intense geomagnetic storms

    NASA Astrophysics Data System (ADS)

    Sobral, J. H. A.; Abdu, M. A.; Yamashita, C. S.; Gonzalez, W. D.; de Gonzalez, A. C.; Batista, I. S.; Zamlutti, C. J.; Tsurutani, B. T.

    2001-01-01

    In this work, we investigate the ionospheric responses to exceptionally high-intensity and long-duration magnetic storms over Brazil. Disturbed ionospheric F-region vertical drifts and peak electron density changes observed at the equatorial station Fortaleza - Fz (/3°55'S /38°25'W /dip-3.5°) and the low-latitude station Cachoeira Paulista - CP /(22°41'S /45°00'W dip24°S), for three magnetospheric storm events that occurred in December 1980, April 1981 and September 1982, are analyzed. These storms had minimum Dst indexes /-240,-311 and /-289nT, respectively. The interplanetary magnetic field (Bz) data from the ISEE-3 satellite, the auroral activity index AE, and the ring current index Dst are used as indicators of the magnetospheric conditions. The ionospheric response features are analyzed using the F-layer critical parameters h'F, hpF2 and foF2, from ionograms obtained at Fz and CP. The Bz and the AE index variations were much higher than those in many previous studies. Therefore, many of the observations reported here either have not been observed or are not readily explained by current models for predicting the penetration//dynamo disturbance electric fields. The altitude of the nocturnal ionospheric F-layer at low latitudes may undergo significant variations during storm-time, caused by magnitude variations on the local zonal component of the F-region electric field intensity. During the period studied here, clear association of the F-layer rise (vertical velocity and altitude) and spread-F occurrence is observed. It is shown that the storm-time layer rise has a dominant role on the equatorial spread-F. An attempt is made to identify the origin of electric fields responsible for the disturbed F-layer alterations. The main conclusions of this study are that (a) some effects on the F-layer height and peak electron concentrations are consistent with model predictions. Some others are in discrepancy or have not been either predicted by model studies or

  11. Equatorial and low-latitude ionospheric response to the 17-18 March 2015 great storm over Southeast Asia longitude sector

    NASA Astrophysics Data System (ADS)

    Jiang, Chunhua; Yang, Guobin; Liu, Jing; Yokoyama, Tatsuhiro; Liu, Tongxin; Lan, Ting; Zhou, Chen; Zhang, Yuannong; Zhao, Zhengyu; Komolmis, Tharadol; Supnithi, Pornchai; Yatini, Clara Y.

    2017-05-01

    This study mainly investigates equatorial and low-latitude ionospheric response to a great geomagnetic storm that occurred on 17 March 2015. We found that there were some interesting ionospheric phenomena, e.g., short-term ionospheric positive effect, daytime spread F, and morning Equatorial Ionization Anomaly (EIA) in the topside ionosphere, emerged at equatorial and low-latitude region along the longitude of about 100°E. Ground-based ionosondes and in situ satellite (Swarm) were utilized to study the possible mechanisms for these ionospheric phenomena. We found that vertical downward transport of plasma or neutral induced by traveling ionospheric disturbances (TIDs) or traveling atmospheric disturbances (TADs) might make a contribution to the short-term ionospheric positive effect at the main stage of this great storm. Additionally, results suggested that the occurrence of daytime spread F at low latitudes might be due to the diffusion of equatorial ionospheric irregularities in the topside ionosphere along geomagnetic field lines. Moreover, observational evidence shows that TIDs also might be the main driver for morning EIA-like feature recorded by Swarm B satellite in the topside ionosphere. These ionospheric phenomena mentioned above could make us gain a better understanding of ionospheric storm effects at equatorial and low-latitude region.

  12. Characteristics of low-latitude whistlers and their relation with f0F2 and magnetic activity

    SciTech Connect

    Bao, Z.; Wang, T.; Xu, J.; Chen, S.; Liang, B.

    1982-01-01

    Characteristics of low-latitude whistlers observed with instrumentation at three locations in China over a three-year period are discussed. The whistler maxima were always detected after midnight, local time, and none occurred during the daytime or evening. The rate of occurrence at the three sites was less than 1/min. A similarity was found between the diurnal variation of the median value of the critical frequency of the F2 region and that of the whistler dispersion. A set of algebraic equations was employed to calculate the electron concentration in the equatorial plane at the top of the propagation path (Neq) and the tube content (NT). The whistler dispersion and f0F2 (the critical frequency) were then converted into Neq and NT, resulting in calculated median values which agreed with satellite observations. Finally, a positive correlation was obtained between the daily variation of Kp and whistler dispersion, supporting the hypothesis that the whistler occurrence rate at low latitudes peaks one or two days after a geomagnetic storm day.

  13. On the seasonal variations of reflectivity and turbulence characteristics of low-latitude mesospheric echoes over Gadanki

    NASA Astrophysics Data System (ADS)

    Selvaraj, D.; Patra, A. K.; Narayana Rao, D.

    2016-06-01

    Gadanki radar observations of the low-latitude mesospheric echoes studied earlier have shown that while both occurrence rate and signal-to-noise ratio of the mesospheric echoes peak in the equinoxes turbulent kinetic energy (TKE) dissipation rate and eddy diffusivity, estimated using spectral width of these echoes, peak in the summer. This seasonal difference is apparently inconsistent with the understanding that the mesospheric echoes are generated by turbulence. In this paper, we analyze Gadanki radar observations of mesospheric echoes made during 2011 and 2012 and study seasonal variations in reflectivity and TKE dissipation rate in an attempt to address the aforementioned puzzle. We show that both reflectivity and TKE dissipation rate in the mesosphere show semiannual variations peaking in the equinoxes, which are vastly different from those reported earlier. We also show that seasonal variations in reflectivity and TKE dissipation rate have a close correspondence with gravity wave activity. These results are found to be consistent with the gravity wave breaking hypothesis generating turbulence and radar echoes in the low-latitude mesosphere.

  14. Investigation of ionospheric TEC over China based on GNSS data

    NASA Astrophysics Data System (ADS)

    Xiong, Bo; Wan, Weixing; Yu, You; Hu, Lianhuan

    2016-09-01

    In this paper, the ionospheric total electron content (TEC) is derived from 250 Global Navigation Satellite Systems (GNSS) receivers over China. The GNSS TEC data are utilized to study the diurnal and day-to-day variability of ionosphere, ionospheric east-west differences and to construct regional ionospheric map. The GNSS-TEC curves clearly show sunrise and sunset enhancements in the diurnal variation. The peak value of TEC is lower in January 2015 than in May 2014. There is 2 h difference in the occurrence time of TEC maximum/minimum between May and January. Compared with the observations of Global Positioning System (GPS) and Global Navigation Satellite System (GLONASS), the measurements from the Geostationary Earth Orbit (GEO) satellites of BeiDou Navigation Satellite System (BDS) clearly present the ionospheric day-to-day variability and east-west differences in a region with small longitude differences (3.52-11.31°). The east-west differences in TEC are more obvious in larger longitude differences at 11:30 local time on 23 January 2015. The maximum east-west difference in TEC is about 7 total electron content unit (TECU, 1 TECU = 1016 el m-2) in longitude difference of 11.31°. Our analysis shows that the TEC for east-west small longitude differences may be associated with the east-west gradient of geomagnetic declination. Based on 250 GNSS stations, a regional TEC map constructed by Kriging method can well capture the main spatial structure of ionosphere in China. A comparison between TEC maps obtained by Kriging method and provided by Jet Propulsion Laboratory displays that there are large deviations in the North of China, which is mainly caused by the difference in the number of used GNSS stations. In addition, comprehensive investigation presents that GNSS has more advantages over GPS and GLONASS in the ionosphere research over China.

  15. Source Region Identification for Low Latitude Whistlers (L=1.08)

    NASA Astrophysics Data System (ADS)

    Gokani, S. A.; Singh, R.; Maurya, A. K.; Bhaskara, V.; Cohen, M.; Kumar, S.; Lichtenberger, J.

    2014-12-01

    Though whistlers are known and studied from past one century, the scientific community still strives to understand the generation and propagation mechanism of whistlers in very low latitude region. One of the solutions comes from locating the causative lightning discharges and source region of low latitude whistlers. In the present study, ~ 2000 whistlers recorded during period of one year (Dec, 2010 to Jan, 2011) at Allahabad (Geomag. lat. 16.79o N; L=1.08), India are correlated with lightning activity detected by World Wide Lightning Location Network (WWLLN) at and around conjugate region. About 63% of whistlers are correlated with the lightning strikes around conjugate region. Further to confirm this correlation, arrival azimuths of causative sferics are determined and the obtained azimuths points towards conjugate region of Allahabad. The characteristics of thunder cloud generating these whistlers are examined and found that the clouds with South-East alignment are more prone to trigger whistler waves. The seasonal and diurnal variation of whistler parameters such as occurrence rate, power spectral density and dispersion are also studied and explained on the basis of ionospheric conditions in low latitudes. The results obtained open a new window to look for the propagation mechanism of low latitude whistlers.

  16. Investigations of Penetration Electric Fields and Low-Latitude Ionospheric Disturbances During Intense Geomagnetic Storms

    DTIC Science & Technology

    2012-09-11

    Rayleigh - Taylor instability process over a large longitudinal range. These plasma bubbles grow and merge to form a wide bubble...Investigations of penetration electric fields and low-latitude ionospheric disturbances during intense geomagnetic storms Principal Investigator: Dr. Sunanda... ionospheric dynamics during intense geomagnetic storms. Summary of Progress: We have

  17. Response of ionospheric electric fields at mid-low latitudes during sudden commencements

    NASA Astrophysics Data System (ADS)

    Takahashi, N.; Kasaba, Y.; Shinbori, A.; Nishimura, Y.; Kikuchi, T.; Ebihara, Y.; Nagatsuma, T.

    2015-06-01

    Using in situ observations from the Republic of China Satellite-1 spacecraft, we investigated the time response and local time dependence of the ionospheric electric field at mid-low latitudes associated with geomagnetic sudden commencements (SCs) that occurred from 1999 to 2004. We found that the ionospheric electric field variation associated with SCs instantaneously responds to the preliminary impulse (PI) signature on the ground regardless of spacecraft local time. Our statistical analysis also supports the global instant transmission of electric field from the polar region. In contrast, the peak time detected in the ionospheric electric field is earlier than that of the equatorial geomagnetic field (~20 s before in the PI phase). Based on the ground-ionosphere waveguide model, this time lag can be attributed to the latitudinal difference of ionospheric conductivity. However, the local time distribution of the initial excursion of ionospheric electric field shows that dusk-to-dawn ionospheric electric fields develop during the PI phase. Moreover, the westward electric field in the ionosphere, which produces the preliminary reverse impulse of the geomagnetic field on the dayside feature, appears at 18-22 h LT where the ionospheric conductivity beyond the duskside terminator (18 h LT) is lower than on the dayside. The result of a magnetohydrodynamic simulation for an ideal SC shows that the electric potential distribution is asymmetric with respect to the noon-midnight meridian. This produces the local time distribution of ionospheric electric fields similar to the observed result, which can be explained by the divergence of the Hall current under nonuniform ionospheric conductivity.

  18. Distinct responses of the low-latitude ionosphere to CME and HSSWS: The role of the IMF Bz oscillation frequency

    NASA Astrophysics Data System (ADS)

    Rodríguez-Zuluaga, J.; Radicella, S. M.; Nava, B.; Amory-Mazaudier, C.; Mora-Páez, H.; Alazo-Cuartas, K.

    2016-11-01

    In this work an attempt to identify the role of the interplanetary magnetic field (IMF) in the response of the ionosphere to different solar phenomena is presented. For this purpose, the day-to-day variability of the equatorial ionospheric anomaly (EIA) and the main ionospheric disturbances are analyzed during one coronal mass ejection (CME) and two high-speed solar wind streams (HSSWSs). The EIA parameters considered are the zonal electric field and both the strength and position of its northern crest. The disturbances being the prompt penetration of magnetospheric electric field (PPMEF) and disturbance dynamo electric field (DDEF) are studied using the magnetic response of their equivalent current systems. In accordance, ground-based Global Navigation Satellite Systems receivers and magnetometers at geomagnetic low latitudes in the American sector are used. During both phenomena, patterns of PPMEF related to fluctuations of the IMF are observed. Diurnal and semidiurnal magnetic oscillations are found to be likely related to DDEF. Comparisons among the EIA parameters and the DDEF magnetic response exhibit poor relation during the CME in contrast to good relation during the HSSWSs. It is concluded that the response of the low-latitude ionosphere to solar phenomena is largely determined through the oscillation frequency of the IMF Bz by affecting the generation of the PPMEF and DDEF differently. This is seen as an effect of how the energy from the solar wind is transferred into the magnetosphere-ionosphere system.

  19. The magnetic storms of 3-4 August 2010 and 5-6 August 2011: 1. Ground- and space-based observations

    NASA Astrophysics Data System (ADS)

    Valladares, C. E.; Eccles, J. V.; Basu, Su.; Schunk, R. W.; Sheehan, R.; Pradipta, R.; Ruohoniemi, J. M.

    2017-03-01

    We have used total electron content (TEC) values from low, middle, and high latitudes recorded over the American continent and density and ion temperature measured in situ by the DMSP-F15 and F17 satellites during the geomagnetic storms of 3-4 August 2010 and 5-6 August 2011 to study the formation and dynamics of plasma density enhancements that developed during these two storms. Common to both storms are the timing of the main phase that extends between 20 and 24 UT and their seasonality with both storms occurring near the end of the Northern Hemisphere summer solstice. During both storms, TEC data show incipient equatorial anomalies lacking a poleward expansion beyond 20° magnetic latitude. Two large-scale TEC enhancements were observed at middle latitudes showing a complicated pattern of structuring and merging. The first TEC enhancement corresponds to a storm-enhanced density (SED) seen between 21 and 01 UT on the following day. The second TEC enhancement was observed over Central America, located equatorward of the SED and apparently moving northward. However, careful analysis of the TEC values indicates that this second TEC enhancement is not transported from lower latitudes through a superfountain effect. Instead, the enhanced plasma has a local origin and is driven by a southward directed meridional wind that moves plasma up the tilted magnetic field lines. DMSP flights passing over the second TEC enhancement show a reduction of the ion temperature, confirming an adiabatic expansion of the plasma as it moves up the field lines. It is concluded that the midlatitude TEC enhancements do not arise from a low-latitude ionospheric fountain effect.

  20. Equatorial anisotropy in the Earth's inner-inner core: evidence from seismic interferometry at low latitudes

    NASA Astrophysics Data System (ADS)

    Wang, T.; Song, X.; Wu, B.; Wang, L.; Xu, M.

    2016-12-01

    Anisotropy of Earth's inner core plays a key role on understanding the evolution and dynamics of the core. The previous inner core anisotropy models have assumed a cylindrical anisotropy with the symmetry axis (nearly) parallel to the Earth's spin axis. Recently, by using autocorrelations from earthquake's coda, we found the equatorial anisotropy of the inner inner core (IIC) with a fast axis near the Central America and the Southeast Asia, in apparent contrast to the polar anisotropy of the outer inner core (OIC). To exclude possible contaminations from large Fresnel zone of the PKIKP2 and PKIIKP2 phases at low periods, we processed the coda (10,000 40,000 s after Mw>=7.0 earthquakes) from stations at low latitudes (<=±35º) during 1990 2013. By imposing an automatic grouping strategy, the standard deviation normalization and a selection filter, we extracted more stable empirical Green's functions from 52 arrays. All arrival times were automatically picked by waveform cross correlation. Large variation (up to 10.9 s) of the residuals near the equator cannot be explained by the fast axis along the north-south direction. The interpolation of observed residuals shows a similar pattern with previous global dataset and the prediction of OIC+IIC model, including the fast axis and two low-velocity open rings. Even considering the influence of the Fresnel zone, our results support the IIC has an equatorial anisotropy. From the IIC to the OIC, large transition of the fast axis may shed light on the growth process of the inner core.

  1. Climatology of low latitude F-region irregularities using GPS radio occultation data

    NASA Astrophysics Data System (ADS)

    Kepkar, Ankur; Arras, Christina; Wickert, Jens

    2017-04-01

    Since the presence of electrons in the upper atmosphere affects radio waves, GNSS signals which operate at radio frequencies result in weak signal strength at reception. Such affected signals can be associated with the Signal-to-Noise (SNR) to study disturbances which are caused by irregularities in the Earth's ionosphere. For this study we use data obtained by the GPS radio occultation method, a satellite-satellite remote sensing technique. The general idea of this method is to track the GPS radio signal, as it passes through the Earth's atmosphere crossing Earth's limb. This weather independent method provides global coverage, high accuracy and a high vertical resolution. Due to the refraction of the GPS electromagnetic waves induced by electron density gradients in ionospheric altitudes, the GPS signals contain information on current ionospheric conditions. The study mainly focuses on providing a climatology of disturbances at low latitude region of the ionosphere taking into account the measurements obtained from FORMOSAT-3/COSMIC (2006-2013). We use GPS L1 profiles tracked in 1Hz mode which scans the Earth's atmosphere with an altitude resolution of 2km. Strong SNR fluctuations are referred to vertical changes in the electron density. The six FORMOSAT-3/COSMIC satellites provide in total about 2,000 radio occultation profiles per day on an average. About 5 millions of profiles were processed for this study, of which 0.16% contain strong disturbances in the ionospheric F-region. We observed that the F-region irregularity phenomenon occurs mainly at night time close to the Earth's magnetic equator during years with high solar activity. Distinctive seasonal variations can be seen from such investigations when analyzed for different years of data. This phenomenon is traditionally explained as a consequence of plasma instability.

  2. Causative mechanisms for the occurrence of a triple layered mesospheric inversion event over low latitudes

    NASA Astrophysics Data System (ADS)

    Ramesh, K.; Sridharan, S.; Vijaya Bhaskara Rao, S.

    2014-05-01

    The temperature profile obtained from the space borne instrument "Sounding of Atmosphere by Broadband Emission Radiometry" instrument onboard "Thermosphere Ionosphere Mesosphere Energetics and Dynamics" shows a triple layered mesospheric inversion event on the night of 23 September 2011, when there is an overpass near the low-latitude sites Gadanki (13.5°N, 79.2°E) and Tirunelveli (8.7°N, 77.8°E). The three mesospheric inversion layers (MILs) are formed in the height region around ~70 (lower), ~80 (middle), and ~90 km (upper) with amplitudes ~11, ~44, and ~109 K and thickness of 3.4, 4.9, and 6.6 km, respectively. The formation of the lower and middle MILs can only be observed in the Rayleigh lidar temperature profiles over Gadanki due to upper height limitation of the system. Nearly all the dominant causative mechanisms are examined for the occurrence of the MIL event. The lower MIL at ~70 km is inferred to be due to planetary wave dissipation, as there is a sudden decrease of planetary wave activity above 70 km. Further, it is demonstrated that the middle MIL at ~80 km is due to the turbulence generated by gravity wave breaking which is in turn due to gravity wave-semi-diurnal tidal interaction, though the height of the middle MIL descends at the rate of ~1 km/h, which is nearly the vertical phase speed of diurnal tide, whereas the upper MIL at above 90 km is due to the large chemical heating rate (~45 K/day) generated by the dominant exothermic reaction O + O + M → O2 + M.

  3. IRI-vTEC versus GPS-vTEC for Nigerian SCINDA GPS stations

    NASA Astrophysics Data System (ADS)

    Okoh, Daniel; McKinnell, Lee-Anne; Cilliers, Pierre; Okere, Bonaventure; Okonkwo, Chinelo; Rabiu, Babatunde

    2015-04-01

    Following the recent proliferation of dual-frequency GPS (Global Positioning System) receiver systems across the African continent, there is a growing number of papers that compare vertical Total Electron Content (vTEC) values derived from the International Reference Ionosphere (IRI) model with those obtained from the GPS receiver measurements. In this work we report an investigation of IRI-vTEC versus GPS-vTEC comparisons for three Nigerian SCINDAGPS stations (Nsukka, Ilorin, and Lagos) for which data are available in the year 2012, and present a further review of the differences/similarities observed between them. Since a major interest in this work is to use the GPS measurements to improve the predictions of the IRI model for the region, we present a detailed regression analysis of differences between the two sources in a manner that will benefit this application.

  4. Coronal streamers and fine scale structures of the low latitude corona as detected with Spartan 201-01 white light coronagraph

    SciTech Connect

    Guhathakurta, M.; Fisher, R.R.

    1995-07-15

    The solar corona was observed with an externally occulted White Light Coronagraph carried on the SPARTAN 201-01 spacecraft for a 47 hour period beginning on April 11, 1993. At this phase of the descending solar magnetic activity cycle there were well-developed coronal helmet streamers located over both the east and west limbs of the sun. Of additional interest in the SPARTAN data are the finer scale streamer structures observed in the low latitude corona which are partially resolved by the SP201-01 instrument. The purpose of this investigation was to determine the physical and morphological characteristics of the streamers and the fine scale ray structures observed in the region between streamers. A comparison of these low latitude rays with the polar rays observed in the north and south polar holes during the same flight suggest that they have similar morphology and physical characteristics. 9 refs., 5 figs.

  5. Post-Storm Middle and Low-Latitude Ionospheric Electric Fields Effects

    NASA Astrophysics Data System (ADS)

    Fejer, B. G.; Blanc, M.; Richmond, A. D.

    2017-03-01

    The Earth's upper atmosphere and ionosphere undergoes large and complex perturbations during and after geomagnetic storms. Thermospheric winds driven by enhanced energy and momentum due to geomagnetic activity generate large disturbance electric fields, plasma drifts and currents with a broad range of temporal and spatial scales from high to equatorial latitudes. This disturbance dynamo mechanism plays a fundamental role on the response of the middle and low-latitude ionosphere to geomagnetic activity. In this review, we initially describe the early evidence for the importance of this process and the first simulation study which already was able to explain its main effects on the electrodynamics of the middle and low-latitude ionosphere. We then describe the results of more recent simulations and the extensive experimental work that highlights the importance of this mechanism for ionospheric space weather studies extending to post-storms periods, and present some suggestions for future studies.

  6. Post-Storm Middle and Low-Latitude Ionospheric Electric Fields Effects

    NASA Astrophysics Data System (ADS)

    Fejer, B. G.; Blanc, M.; Richmond, A. D.

    2016-12-01

    The Earth's upper atmosphere and ionosphere undergoes large and complex perturbations during and after geomagnetic storms. Thermospheric winds driven by enhanced energy and momentum due to geomagnetic activity generate large disturbance electric fields, plasma drifts and currents with a broad range of temporal and spatial scales from high to equatorial latitudes. This disturbance dynamo mechanism plays a fundamental role on the response of the middle and low-latitude ionosphere to geomagnetic activity. In this review, we initially describe the early evidence for the importance of this process and the first simulation study which already was able to explain its main effects on the electrodynamics of the middle and low-latitude ionosphere. We then describe the results of more recent simulations and the extensive experimental work that highlights the importance of this mechanism for ionospheric space weather studies extending to post-storms periods, and present some suggestions for future studies.

  7. Why Do Model Tropical Cyclones Intensify More Rapidly at Low Latitudes?

    DTIC Science & Technology

    2015-05-01

    low latitudes. The answer to this question touches on practically all facets of the dynamics and thermodynamics of tropical cyclones. The answer invokes...practically all facets of the dynamics and thermodynamics of tropical cyclones. The answer invokes the conventional spin-up mechanism, as articulated in...the dependence of the intensification rate and mature intensity on latitude touches on practically all facets of the dynamics and thermodynamics of

  8. Signatures of solar event at middle and low latitudes in the Europe-African sector, during geomagnetic storms, October 2013

    NASA Astrophysics Data System (ADS)

    Azzouzi, I.; Migoya-Orué, Y.; Amory Mazaudier, C.; Fleury, R.; Radicella, S. M.; Touzani, A.

    2015-11-01

    This paper presents the variability of the total electron content, VTEC, the ROTI index (proxy of the scintillation index) and the transient variations of the Earth's magnetic field associated to the impacts of solar events during October 2013. The observations are from middle and low latitudes in European African longitude sector. During October 2013, there are four solar events reaching the Earth. The two first events, on October 2 and October 8 are CME, the third event on October 14, is a jet of fast solar wind flowing from a solar coronal hole, and the last event on October 30 is a slow solar wind with southward excursions of the Bz component of the interplanetary magnetic field, associated to CME passing near the Earth. For the four events, the variation of VTEC at middle latitudes is the same and presents an increase of VTEC at the time of the impact followed by a decrease of VTEC, lasting one or several days. At low latitudes, no clear common pattern for all the events appears. For the four events the variation of the ROTI index over Africa is different showing the asymmetry between West and East Africa. For the first event, on October 2, the scintillations are not inhibited, for the second and the fourth events on October 8 and 30, the scintillations are inhibited on East Africa and for the third event (high speed solar wind stream), on October 14, the scintillations are inhibited over the whole Africa. The available data allow the full explanation of the observations of October 14, indeed, on this day, there is no post sunset increase of the virtual height h‧F2 at Ascension Island. There is no Pre Reversal Enhancement (PRE) of the eastward electric field; it is this electric field which moves up the F layer, the necessary condition for the existence of scintillation. The analysis of the variations of the Earth's magnetic field at low latitudes highlights the presence of the ionospheric disturbance dynamo on October 14, which produces a decrease of the

  9. Biotic turnover driven by eutrophication before the Sturtian low-latitude glaciation

    NASA Astrophysics Data System (ADS)

    Nagy, Robin M.; Porter, Susannah M.; Dehler, Carol M.; Shen, Yanan

    2009-06-01

    Reconstructions of the diversity of Precambrian microorganisms suggest a pronounced biotic turnover coinciding with the onset of Neoproterozoic low-latitude glaciation, in which diverse assemblages of organic-walled microfossils known as acritarchs were replaced by assemblages of simple, smooth-walled forms called leiosphaerids, and the remnants of bacterial blooms. This turnover has been interpreted as the mass extinction of eukaryotic phytoplankton and the subsequent proliferation of bacteria. However, the causes of this mass extinction and its exact temporal relationship to the glaciations remain unclear. Here we present palaeontological data from the >742+/-6-Myr-old Chuar Group from Arizona, which indicate that the biotic turnover occurred before the first low-latitude (Sturtian) glaciation, constrained to be between 726 and 660Myr in age. In our record, the turnover is associated with the appearance of abundant and diverse protozoan fossils and a shift to rising total organic carbon, suggestive of increased primary productivity spurred by the influx of nutrients. This is followed by an increase in the ratio of highly reactive iron to total iron, which we interpret as persistent water column anoxia. We therefore conclude that the biotic turnover recorded in the Chuar Group was driven by widespread eutrophication of surface waters, rather than low-latitude glaciation.

  10. Induction effects of geomagnetic disturbances in the geo-electric field variations at low latitudes

    NASA Astrophysics Data System (ADS)

    Doumbia, Vafi; Boka, Kouadio; Kouassi, Nguessan; Didier Franck Grodji, Oswald; Amory-Mazaudier, Christine; Menvielle, Michel

    2017-01-01

    In this study we examined the influences of geomagnetic activity on the Earth surface electric field variations at low latitudes. During the International Equatorial Electrojet Year (IEEY) various experiments were performed along 5° W in West Africa from 1992 to 1995. Among other instruments, 10 stations equipped with magnetometers and telluric electric field lines operated along a meridian chain across the geomagnetic dip equator from November 1992 to December 1994. In the present work, the induced effects of space-weather-related geomagnetic disturbances in the equatorial electrojet (EEJ) influence area in West Africa were examined. For that purpose, variations in the north-south (Ex) and east-west (Ey) components of telluric electric field were analyzed, along with that of the three components (H, D and Z) of the geomagnetic field during the geomagnetic storm of 17 February 1993 and the solar flare observed on 4 April 1993. The most important induction effects during these events are associated with brisk impulses like storm sudden commencement (ssc) and solar flare effect (sfe) in the geomagnetic field variations. For the moderate geomagnetic storm that occurred on 17 February 1993, with a minimum Dst index of -110 nT, the geo-electric field responses to the impulse around 11:00 LT at LAM are Ex = 520 mV km-1 and Ey = 400 mV km-1. The geo-electric field responses to the sfe that occurred around 14:30 LT on 4 April 1993 are clearly observed at different stations as well. At LAM the crest-to-crest amplitude of the geo-electric field components associated with the sfe are Ex = 550 mV km-1 and Ey = 340 mV km-1. Note that the sfe impact on the geo-electric field variations decreases with the increasing distance of the stations from the subsolar point, which is located at about 5.13° N on 4 April. This trend does not reflect the sfe increasing amplitude near the dip equator due the high Cowling conductivity in the EEJ belt.

  11. New historical records and relationships among 14C production rates, abundance and color of low latitude auroras and sunspot abundance

    NASA Astrophysics Data System (ADS)

    Abbott, Dallas; Juhl, Robert

    2016-12-01

    Incursions of high-energy particles from space, specifically solar energetic particles and galactic cosmic rays, have significant effects on the Earth, including disruption of the Earth's magnetic field, generation of electric fields strong enough to damage electronic devices as well as the production of auroras at low-latitudes, within 45° of the magnetic equator. We examine the relationships among 14C production, auroral abundance, auroral color and sunspot abundance using existing data supplemented by a new dataset. The new dataset, based on Chinese and Korean records from A.D. 1100-1700, includes 46 new or revised records of sunspots and 279 records of low-latitude auroras. Low-latitude auroras are predominantly red (66%, 835 events) with lesser proportions of white (20%, 253 events) and black auroras (6%, 67 events). All other auroral colors (green, yellow, multicolored, blue and purple) aggregate to a total of 100 events (8%). Overall, white auroras are more frequent during times of higher 14C production. We use two empirical methods of evaluating the flux of high-energy particles: modeled peaks and lows in 14C production and peaks and lows in the 14C calibration curve. We find that comparison to modeled 14C production gives significant results. White auroras are significantly more abundant (98% probability) at times of high production of 14C. Red auroras are somewhat more abundant (88% probability) at times of low production of 14C. The abundances of black, multicolored, green, yellow, and blue auroras between times of low and high 14C production are not significantly different. Violet/purple auroras are significantly more abundant (98% probability) at times of low 14C production. The positive correlation of violet/purple auroras with times of low14C production rate and the lack of correlation of blue auroras with times of high14C production is surprising, for this portion of the visible spectrum contains strong emission lines and some lines with high

  12. Stratospheric ozone and temperature responses to short-term solar ultraviolet variations - Reproducibility of low-latitude response measurements

    NASA Technical Reports Server (NTRS)

    Hood, L. L.; Cantrell, S.

    1988-01-01

    Two independent 22-month time intervals of NIMBUS 7 solar backscattered UV (SBUV) ozone and stratospheric and mesospheric sounder (SAMS) temperature measurements for the upper stratosphere at low latitudes are analyzed to calculate mean responses to observed changes in solar ultraviolet spectral irradiance occurring on the time scale of the solar rotation period. Average cross-correlation functions of both SBUV ozone and SAMS temperature versus the solar 205 nm flux are in substantial agreement for these two intervals. Linear regression methods are applied to estimate response amplitudes or sensitivities. The derived sensitivities and phase lags relative to the 205 nm flux are also in approximate agreement for the two separate intervals although the temperature response measurements exhibit larger deviations. These results support the validity of previously reported measurements on the 27-day time scale, and impose firmer constraints on proposed theoretical models for the response of the stratosphere to solar UV forcing on both short and long time scales.

  13. CoLiTec software - detection of the near-zero apparent motion

    NASA Astrophysics Data System (ADS)

    Khlamov, Sergii V.; Savanevych, Vadym E.; Briukhovetskyi, Olexandr B.; Pohorelov, Artem V.

    2017-06-01

    In this article we described CoLiTec software for full automated frames processing. CoLiTec software allows processing the Big Data of observation results as well as processing of data that is continuously formed during observation. The scope of solving tasks includes frames brightness equalization, moving objects detection, astrometry, photometry, etc. Along with the high efficiency of Big Data processing CoLiTec software also ensures high accuracy of data measurements. A comparative analysis of the functional characteristics and positional accuracy was performed between CoLiTec and Astrometrica software. The benefits of CoLiTec used with wide field and low quality frames were observed. The efficiency of the CoLiTec software was proved by about 700.000 observations and over 1.500 preliminary discoveries.

  14. Modelling the probability of ionospheric irregularity occurrence over African low latitude region

    NASA Astrophysics Data System (ADS)

    Mungufeni, Patrick; Jurua, Edward; Bosco Habarulema, John; Anguma Katrini, Simon

    2015-06-01

    This study presents models of geomagnetically quiet time probability of occurrence of ionospheric irregularities over the African low latitude region. GNSS-derived ionospheric total electron content data from Mbarara, Uganda (0.60°S, 30.74°E, geographic, 10.22°S, magnetic) and Libreville, Gabon (0.35°N, 9.68°E, geographic, 8.05°S, magnetic) during the period 2001-2012 were used. First, we established the rate of change of total electron content index (ROTI) value associated with background ionospheric irregularity over the region. This was done by analysing GNSS carrier-phases at L-band frequencies L1 and L2 with the aim of identifying cycle slip events associated with ionospheric irregularities. We identified at both stations a total of 699 events of cycle slips. The corresponding median ROTI value at the epochs of the cycle slip events was 0.54 TECU/min. The probability of occurrence of ionospheric irregularities associated with ROTI ≥ 0.5 TECU / min was then modelled by fitting cubic B-splines to the data. The aspects the model captured included diurnal, seasonal, and solar flux dependence patterns of the probability of occurrence of ionospheric irregularities. The model developed over Mbarara was validated with data over Mt. Baker, Uganda (0.35°N, 29.90°E, geographic, 9.25°S, magnetic), Kigali, Rwanda (1.94°S, 30.09°E, geographic, 11.62°S, magnetic), and Kampala, Uganda (0.34°N, 32.60°E, geographic, 9.29°S, magnetic). For the period validated at Mt. Baker (approximately, 137.64 km, north west), Kigali (approximately, 162.42 km, south west), and Kampala (approximately, 237.61 km, north east) the percentages of the number of errors (difference between the observed and the modelled probability of occurrence of ionospheric irregularity) less than 0.05 are 97.3, 89.4, and 81.3, respectively.

  15. Contributions of the low-latitude boundary layer to the finite width magnetotail convection model

    NASA Technical Reports Server (NTRS)

    Spence, H. E.; Kivelson, M. G.

    1993-01-01

    Convection of plasma within the terrestrial nightside plasma sheet contributes to the structure and, possibly, the dynamical evolution of the magnetotail. In order to characterize the steady state convection process, we have extended the finite tail width model of magnetotail plasma sheet convection. The model assumes uniform plasma sources and accounts for both the duskward gradient/curvature drift and the earthward E x B drift of ions in a two-dimensional magnetic geometry. During periods of slow convection (i.e., when the cross-tail electric potential energy is small relative to the source plasma's thermal energy), there is a significant net duskward displacement of the pressure-bearing ions. The electrons are assumed to be cold, and we argue that this assumption is appropriate for plasma sheet parameters. We generalize solutions previously obtained along the midnight meridian to describe the variation of the plasma pressure and number density across the width of the tail. For a uniform deep-tail source of particles, the plasma pressure and number density are unrealistically low along the near-tail dawn flank. We therefore add a secondary source of plasma originating from the dawnside low-latitude boundary layer (LLBL). The dual plasma sources contribute to the plasma pressure and number density throughout the magnetic equatorial plane. Model results indicate that the LLBL may be a significant source of near-tail central plasma sheet plasma during periods of weak convection. The model predicts a cross-tail pressure gradient from dawn to dusk in the near magnetotail. We suggest that the plasma pressure gradient is balanced in part by an oppositely directed magnetic pressure gradient for which there is observational evidence. Finally, the pressure to number density ratio is used to define the plasma 'temperature.' We stress that such quantities as temperature and polytropic index must be interpreted with care as they lose their nominal physical significance in

  16. Variability of Ionospheric TEC and the Performance of the IRI-2012 Model at the BJFS Station, China

    NASA Astrophysics Data System (ADS)

    Li, Shuhui; Li, Lihua; Peng, Junhuan

    2016-10-01

    We studied variation characteristics of ionospheric total electron contents (TEC) and performance of the International Reference Ionosphere (IRI)-2012 model in predicting TEC at the BJFS (Beijing Fangshan station), China. Diurnal and seasonal variations were analyzed with TEC data derived from dual-frequency global positioning system (GPS) observations along with the solar activity dependence of TEC at the BJFS station. Data interpolated with information from IGS Global Ionosphere Maps (GIMs) were also used in the analysis. Results showed that the IRI-2012 model can reflect the climatic characteristics and solar activity dependence of ionospheric TEC. By using time series decomposition method, ionospheric daily averaged TEC values were divided into the periodic components, geomagnetic activity component, solar activity component and secular trend. Solar activity component and periodic components are supposed to be the main reasons which account for the difference between the GIMs TEC and the TEC from the IRI-2012 model.

  17. Magnetic Field Measurement on the C/NOFS Satellite: Geomagnetic Storm Effects in the Low Latitude Ionosphere

    NASA Technical Reports Server (NTRS)

    Le, Guan; Pfaff, Rob; Kepko, Larry; Rowland, Doug; Bromund, Ken; Freudenreich, Henry; Martin, Steve; Liebrecht, C.; Maus, S.

    2010-01-01

    The Vector Electric Field Investigation (VEFI) suite onboard the Communications/Navigation Outage Forecasting System (C/NOFS) spacecraft includes a sensitive fluxgate magnetometer to measure DC and ULF magnetic fields in the low latitude ionosphere. The instrument includes a DC vector measurement at 1 sample/sec with a range of +/- 45,000 nT whose primary objective is to provide direct measurements of both V x B and E x B that are more accurate than those obtained using a simple magnetic field model. These data can also be used for scientific research to provide information of large-scale ionospheric and magnetospheric current systems, which, when analyzed in conjunction with the C/NOFS DC electric field measurements, promise to advance our understanding of the electrodynamics of the low latitude ionosphere. In this study, we use the magnetic field data to study the temporal and local time variations of the ring currents during geomagnetic storms. We first compare the in situ measurements with the POMME (the POtsdam Magnetic Model of the Earth) model in order to provide an in-flight "calibration" of the data as well as compute magnetic field residuals essential for revealing large scale external current systems. We then compare the magnetic field residuals observed both during quiet times and during geomagnetic storms at the same geographic locations to deduce the magnetic field signatures of the ring current. As will be shown, the low inclination of the C/NOFS satellite provides a unique opportunity to study the evolution of the ring current as a function of local time, which is particularly insightful during periods of magnetic storms. This paper will present the initial results of this study.

  18. TEC enhancement immediately before M9 mega-thrust earthquakes

    NASA Astrophysics Data System (ADS)

    Heki, Kosuke

    2012-07-01

    Earthquakes are often preceded by electromagnetic precursors, e.g. electric currents in the ground and propagation anomalies of radio waves. By monitoring the differences of the L1 and L2 carrier phases from GPS satellites, we can infer ionospheric Total Electron Content (TEC). Here I report that positive anomalies of ionospheric TEC appeared immediately before the 2011 Tohoku-Oki (Mw9.0), 2010 Chile (Maule) (Mw8.8), 2007 Bengkulu (Mw8.6), and 2004 Sumatra-Andaman (Mw9.2) earthquakes. Coseismic vertical movements of the surface excite acoustic and internal gravity waves, causing coseismic ionospheric disturbances (CID), and GPS-TEC data showed that they occurred about ten minutes after these earthquakes. In addition to them, positive TEC anomalies were found to start 60-40 minutes before these earthquakes above the focal regions, and to last until the onsets of CID. In the Tohoku-Oki case, the anomaly was reached about one tenth of the background TEC immediately before the earthquake. TEC enhancements often occur irrespective of earthquakes, for example, sudden increase of TEC due to solar flares and large-scale traveling ionospheric disturbances (LSTID) propagating from the auroral oval to mid-latitude regions. These disturbances can be distinguished by carefully observing their spatial extents and movements. Geomagnetic activities were relatively high in the 2004 Sumatra-Andaman and 2011 Tohoku-Oki events, but were low in the 2007 Bengkulu and 2010 Chile events. For the Tohoku-Oki and the Bengkulu earthquakes, we analyzed the TEC time series of the same satellite and receiver pair over 120 days before and after the earthquakes, and confirmed that the precursory anomalies of the earthquakes were the largest in these periods. We also investigated three M8 class earthquakes, the 1994 Hokkaido-Toho-Oki (Mw8.3), 2006 Kuril (Mw8.2), and the 2003 Tokachi-Oki (Mw8.0) earthquakes. However, only weak precursory TEC anomalies were seen in the 1994 event, and not in the 2003

  19. Quantitative Evaluation of Ionosphere Models for Reproducing Regional TEC During Geomagnetic Storms

    NASA Astrophysics Data System (ADS)

    Shim, J. S.; Kuznetsova, M.; Rastaetter, L.; Bilitza, D.; Codrescu, M.; Coster, A. J.; Emery, B.; Foster, B.; Fuller-Rowell, T. J.; Goncharenko, L. P.; Huba, J.; Mitchell, C. N.; Ridley, A. J.; Fedrizzi, M.; Scherliess, L.; Schunk, R. W.; Sojka, J. J.; Zhu, L.

    2015-12-01

    TEC (Total Electron Content) is one of the key parameters in description of the ionospheric variability that has influence on the accuracy of navigation and communication systems. To assess current TEC modeling capability of ionospheric models during geomagnetic storms and to establish a baseline against which future improvement can be compared, we quantified the ionospheric models' performance by comparing modeled vertical TEC values with ground-based GPS TEC measurements and Multi-Instrument Data Analysis System (MIDAS) TEC. The comparison focused on North America and Europe sectors during selected two storm events: 2006 AGU storm (14-15 Dec. 2006) and 2013 March storm (17-19 Mar. 2013). The ionospheric models used for this study range from empirical to physics-based, and physics-based data assimilation models. We investigated spatial and temporal variations of TEC during the storms. In addition, we considered several parameters to quantify storm impacts on TEC: TEC changes compared to quiet time, rate of TEC change, and maximum increase/decrease during the storms. In this presentation, we focus on preliminary results of the comparison of the models performance in reproducing the storm-time TEC variations using the parameters and skill scores. This study has been supported by the Community Coordinated Modeling Center (CCMC) at the Goddard Space Flight Center. Model outputs and observational data used for the study will be permanently posted at the CCMC website (http://ccmc.gsfc.nasa.gov) for the space science communities to use.

  20. A robust TEC depletion detector algorithm for satellite based navigation in Indian zone and depletion analysis for GAGAN

    NASA Astrophysics Data System (ADS)

    Dashora, Nirvikar

    2012-07-01

    Equatorial plasma bubble (EPB) and associated plasma irregularities are known to cause severe scintillation for the satellite signals and produce range errors, which eventually result either in loss of lock of the signal or in random fluctuation in TEC, respectively, affecting precise positioning and navigation solutions. The EPBs manifest as sudden reduction in line of sight TEC, which are more often called TEC depletions, and are spread over thousands of km in meridional direction and a few hundred km in zonal direction. They change shape and size while drifting from one longitude to another in nighttime ionosphere. For a satellite based navigation system, like GAGAN in India that depends upon (i) multiple satellites (i.e. GPS) (ii) multiple ground reference stations and (iii) a near real time data processing, such EPBs are of grave concern. A TEC model generally provides a near real-time grid based ionospheric vertical errors (GIVEs) over hypothetically spread 5x5 degree latitude-longitude grid points. But, on night when a TEC depletion occurs in a given longitude sector, it is almost impossible for any system to give a forecast of GIVEs. If loss-of-lock events occur due to scintillation, there is no way to improve the situation. But, when large and random depletions in TEC occur with scintillations and without loss-of-lock, it affects low latitude TEC in two ways. (a) Multiple satellites show depleted TEC which may be very different from model-TEC values and hence the GIVE would be incorrect over various grid points (ii) the user may be affected by depletions which are not sampled by reference stations and hence interpolated GIVE within one square would be grossly erroneous. The most general solution (and the far most difficult as well) is having advance knowledge of spatio-temporal occurrence and precise magnitude of such depletions. While forecasting TEC depletions in spatio-temporal domain are a scientific challenge (as we show below), operational systems

  1. Analysis of propagation delays of compressional Pi 2 waves between geosynchronous altitude and low latitudes

    NASA Astrophysics Data System (ADS)

    Imajo, Shun; Yumoto, Kiyohumi; Uozumi, Teiji; Kawano, Hideaki; Abe, Shuji; Ikeda, Akihiro; Koga, Kiyokazu; Matsumoto, Haruhisa; Obara, Takahiro; Marshall, Richard; Akulichev, Victor A.; Mahrous, Ayman; Liedloff, Adam; Yoshikawa, Akimasa

    2014-12-01

    The propagation of compressional Pi 2 waves in the inner magnetosphere is investigated by analyzing the onset delay times between the ground and the geosynchronous altitude. We use the compressional component (northward) of magnetic data from low-latitude stations and the geosynchronous satellite ETS-VIII (GMLat. = -10.8°, GMLon. = 217.5°). The onset delays are determined by a cross-correlation analysis, and we analyzed the events with high waveform correlations (correlation coefficient greater than 0.75). Some of these high-correlation events have the properties of propagating waves; Pi 2 waveforms at the ground stations and the satellite were synchronized with each other when the data were shifted by onset delays. The results of the statistical analysis show that 87% of the Pi 2 onsets at a ground station (Kuju, GMLat. = 26.13°, GMLon. = 202.96°) were delayed from the Pi 2 onsets at ETS-VIII, and the average of the delay times was 29 sec. This clearly shows Pi 2 onsets (initial perturbations of Pi 2) propagated from the geosynchronous altitude to the low-latitude ground. The delay times tended to be larger around the midnight sector than around the dawn and dusk sectors. These results are consistent with two-dimensional propagation of fast waves estimated by the model of Uozumi et al. (J Geophys Res 114:A11207, 2009). The delay times are nearly identical to the travel time of fast waves from geosynchronous altitude to the low-latitude ground, and the local time variation of the delay shows the azimuthal propagation along the geosynchronous orbit. We conclude that the initial compressional perturbations of Pi 2 waves propagate radially and longitudinally as a fast wave in the inner magnetosphere.

  2. Day-to-day variability in ionospheric electron content at low latitudes

    NASA Astrophysics Data System (ADS)

    Aravindan, P.; Iyer, K. N.

    1990-06-01

    Published values of ionospheric electron content (IEC) are used to study its day-to-day variability at a number of low latitude stations in Indian, American, and Pacific sectors for varying levels of solar activity. It is seen that the variability is larger at night than day, higher in winter compared with summer, and higher in low-sunspot periods than in high-sunspot periods. From a correlative study of variability in IEC with possible causative factors such as solar 10.7-cm flux, sunspot activity, magnetic activity, and electrojet strength, it is seen that the dominant factor contributing to the variability in IEC is the electrojet strength.

  3. High and Low Latitude types of the Downstream Influences of the North Atlantic Oscillation

    NASA Astrophysics Data System (ADS)

    Song, J.

    2013-12-01

    Using reanalysis data, we find that the downstream-propagating quasi-stationary Rossby wave train associated with the North Atlantic Oscillation (NAO) generally propagates along a high (low)-latitude pathway during warm (cold) El Niño-Southern Oscillation (ENSO) boreal winters. Consistent with the different propagation directions of the NAO-related downstream wave train, during the warm (cold) ENSO winters, the NAO is associated with significant 300 hPa geopotential height anomalies over eastern Siberia (the Arabian Sea, the east coast of Asia at around 40N, and the North Pacific), and the near-surface air temperature (SAT) perturbations associated with the NAO over the high latitudes of Asia are relatively strong (weak). Based on these differences, we argue that the NAO has two distinct types of downstream influence: a high-latitude type and a low-latitude type. Furthermore, we argue that the two types of NAO's downstream influence are modulated by the intensity of the subtropical potential vorticity (PV) meridional gradient over Africa. When this gradient is weak (strong), as in the warm (cold) ENSO winters, the NAO's downstream influence tends to be of the high (low)-latitude type. These results are further supported by analysis of intraseasonal NAO events. We separate NAO events into two categories in terms of the intensity of the subtropical PV gradient over Africa. Composites of the NAO events accompanied by a weak (strong) subtropical PV gradient show that the NAO-related downstream wave train tends to propagate along a high (low)-latitude pathway. Fig. 1 Regressed monthly anomalous meridional wind at 300 hPa (V 300hPa, thin contours, interval is 1 m/s) onto the monthly NAO index and the corresponding stationary wave activity fluxes (vectors, unit is m2s-2) during a) the warm ENSO winters and b) cold ENSO winters. Solid (dashed) contours represent positive (negative) values and the zero contours are omitted. The regressed results at the 95% confidence level

  4. Frequency dependence of arrival direction and polarization of low-latitude whistlers and their ducted propagation

    SciTech Connect

    Ohta, K. ); Hayakawa, M. ); Shimakura, S. )

    1989-06-01

    The combined use of digital recorders and a fast Fourier transform analyzer made it possible for them to carry out wideband field analysis direction finding measurements of whistlers at low latitude (geomagnetic latitude 25{degree}N). It is found that there is a negligibly small frequency dependence of the ionospheric exit point and polarization of daytime whistlers during each occurrence peak with duration of about 2 hours, which lends further support to their propagation in a field-aligned duct in the magnetosphere for each occurrence peak.

  5. Some studies of zonal and meridional wind characteristics at low latitude Indian stations

    NASA Technical Reports Server (NTRS)

    Nagpal, O. P.; Kumar, S.

    1985-01-01

    At the beginning of the Indian Middle Atmosphere Programme (IMAP), it was decided that the preparation of consolidation reports of already available parameters for the middle atmosphere would be useful. Atmospheric wind data obtained by rockets and balloons constituted one such parameter which had to be consolidated. The present paper summaries the results of this consolidation study. Both zonal and meridional components of winds at four low latitude Indian stations namely Thumba, Shar, Hyderabad, and Balasore, have been analyzed to yield reference wind profiles for each month. The montly mean values have been used to bring out the amplitudes and phases of the annual, semiannual and quasi-biennial oscillations.

  6. Low-latitude arc–continent collision as a driver for global cooling

    PubMed Central

    Jagoutz, Oliver; Macdonald, Francis A.; Royden, Leigh

    2016-01-01

    New constraints on the tectonic evolution of the Neo-Tethys Ocean indicate that at ∼90–70 Ma and at ∼50–40 Ma, vast quantities of mafic and ultramafic rocks were emplaced at low latitude onto continental crust within the tropical humid belt. These emplacement events correspond temporally with, and are potential agents for, the global climatic cooling events that terminated the Cretaceous Thermal Maximum and the Early Eocene Climatic Optimum. We model the temporal effects of CO2 drawdown from the atmosphere due to chemical weathering of these obducted ophiolites, and of CO2 addition to the atmosphere from arc volcanism in the Neo-Tethys, between 100 and 40 Ma. Modeled variations in net CO2-drawdown rates are in excellent agreement with contemporaneous variation of ocean bottom water temperatures over this time interval, indicating that ophiolite emplacement may have played a major role in changing global climate. We demonstrate that both the lithology of the obducted rocks (mafic/ultramafic) and a tropical humid climate with high precipitation rate are needed to produce significant consumption of CO2. Based on these results, we suggest that the low-latitude closure of ocean basins along east–west trending plate boundaries may also have initiated other long-term global cooling events, such as Middle to Late Ordovician cooling and glaciation associated with the closure of the Iapetus Ocean. PMID:27091966

  7. Low-latitude arc-continent collision as a driver for global cooling.

    PubMed

    Jagoutz, Oliver; Macdonald, Francis A; Royden, Leigh

    2016-05-03

    New constraints on the tectonic evolution of the Neo-Tethys Ocean indicate that at ∼90-70 Ma and at ∼50-40 Ma, vast quantities of mafic and ultramafic rocks were emplaced at low latitude onto continental crust within the tropical humid belt. These emplacement events correspond temporally with, and are potential agents for, the global climatic cooling events that terminated the Cretaceous Thermal Maximum and the Early Eocene Climatic Optimum. We model the temporal effects of CO2 drawdown from the atmosphere due to chemical weathering of these obducted ophiolites, and of CO2 addition to the atmosphere from arc volcanism in the Neo-Tethys, between 100 and 40 Ma. Modeled variations in net CO2-drawdown rates are in excellent agreement with contemporaneous variation of ocean bottom water temperatures over this time interval, indicating that ophiolite emplacement may have played a major role in changing global climate. We demonstrate that both the lithology of the obducted rocks (mafic/ultramafic) and a tropical humid climate with high precipitation rate are needed to produce significant consumption of CO2 Based on these results, we suggest that the low-latitude closure of ocean basins along east-west trending plate boundaries may also have initiated other long-term global cooling events, such as Middle to Late Ordovician cooling and glaciation associated with the closure of the Iapetus Ocean.

  8. Airborne pseudolite aiding BeiDou system to improve positioning precision in low latitude areas

    NASA Astrophysics Data System (ADS)

    Ma, Weihua; Yuan, Jianping; Luo, Jianjun

    2005-11-01

    The BeiDou System (BDS), which has three satellites in Geostationary Earth Orbit (GEO), is a regional satellite navigation system of China and its positioning performance is notorious in low latitude areas. The two mending plans using Airborne Pseudolite (APL) aiding BDS to improve navigation precision in such areas are put forward. Plan I uses three BDS satellites and one APS to supply navigation data and Plan II employs two BDS satellites, one APS and altimeter to work. Both of the plans adopt point positioning with code pseudo-range algorithm. Geometric Dilution of Precision (GDOP), which is calculated by Positioning Error Transfer Coefficient Matrix (PETCM), is used to evaluate the positioning performance of new plans. PETCM is predigested when user is in low latitude areas. The key elements of predigested PETCM that effect the Geometric Dilution of Precision (GDOP) are analyzed. The character of GDOP is forecasted easily with the predigested PETCM. The simulations show that the precision of plans are expected to be hundreds of meters except some region where the latitudes of user and APL are close to each other and Plan II is better than plan I. The phenomenal consists with the theoretical analysis.

  9. C/NOFS Measurements of Stormtime Magnetic Perturbations in the Low-latitude Ionosphere

    NASA Technical Reports Server (NTRS)

    Le, Guan; Burke, William J.; Pfaff, Robert F.; Freudenreich, Henry; Maus, Stefan; Luehr, Hermann

    2012-01-01

    The Vector Electric Field Investigation suite on the C/NOFS satellite includes a fluxgate magnetometer to monitor the Earth's magnetic fields in the low-latitude ionosphere. Measurements yield full magnetic vectors every second over the range of +/- 45,000 nT with a one-bit resolution of 1.37 nT (16 bit AID) in each component. The sensor's primary responsibility is to support calculations of both VxB and ExB with greater accuracy than can be obtained using standard magnetic field models. The data also contain information about large-scale current systems, that, when analyzed in conjunction with electric field measurements, promise to significantly expand understanding of equatorial electrodynamics. We first compare in situ measurements with the POMME (POtsdam Magnetic Model of the Earth) model to establish in-flight sensor "calibrations" and to compute magnetic residuals. At low latitudes the residuals are predominately products of the stormtime ring current. Since C/NOFS provides a complete coverage of all local times every 97 minutes, magnetic field data allow studies of the temporal evolution and local-time variations of stormtime ring current. The analysis demonstrates the feasibility of using instrumented spacecraft in low-inclination orbits to extract a timely proxy for the provisional Dst index and to specify the ring current's evolution.

  10. Low-latitude arc-continent collision as a driver for global cooling

    NASA Astrophysics Data System (ADS)

    Jagoutz, Oliver; Macdonald, Francis A.

    2016-05-01

    New constraints on the tectonic evolution of the Neo-Tethys Ocean indicate that at ˜90-70 Ma and at ˜50-40 Ma, vast quantities of mafic and ultramafic rocks were emplaced at low latitude onto continental crust within the tropical humid belt. These emplacement events correspond temporally with, and are potential agents for, the global climatic cooling events that terminated the Cretaceous Thermal Maximum and the Early Eocene Climatic Optimum. We model the temporal effects of CO2 drawdown from the atmosphere due to chemical weathering of these obducted ophiolites, and of CO2 addition to the atmosphere from arc volcanism in the Neo-Tethys, between 100 and 40 Ma. Modeled variations in net CO2-drawdown rates are in excellent agreement with contemporaneous variation of ocean bottom water temperatures over this time interval, indicating that ophiolite emplacement may have played a major role in changing global climate. We demonstrate that both the lithology of the obducted rocks (mafic/ultramafic) and a tropical humid climate with high precipitation rate are needed to produce significant consumption of CO2. Based on these results, we suggest that the low-latitude closure of ocean basins along east-west trending plate boundaries may also have initiated other long-term global cooling events, such as Middle to Late Ordovician cooling and glaciation associated with the closure of the Iapetus Ocean.

  11. C/NOFS Measurements of Magnetic Perturbations in the Low-Latitude Ionosphere During Magnetic Storms

    NASA Technical Reports Server (NTRS)

    Le, Guan; Burke, William J.; Pfaff, Robert F.; Freudenreich, Henry; Maus, Stefan; Luhr, Hermann

    2011-01-01

    The Vector Electric Field Investigation suite on the C/NOFS satellite includes a fluxgate magnetometer to monitor the Earth s magnetic fields in the low-latitude ionosphere. Measurements yield full magnetic vectors every second over the range of +/-45,000 nT with a one-bit resolution of 1.37 nT (16 bit A/D) in each component. The sensor s primary responsibility is to support calculations of both V x B and E x B with greater accuracy than can be obtained using standard magnetic field models. The data also contain information about large-scale current systems that, when analyzed in conjunction with electric field measurements, promise to significantly expand understanding of equatorial electrodynamics. We first compare in situ measurements with the POMME (Potsdam Magnetic Model of the Earth) model to establish in-flight sensor "calibrations" and to compute magnetic residuals. At low latitudes the residuals are predominately products of the storm time ring current. Since C/NOFS provides a complete coverage of all local times every 97 min, magnetic field data allow studies of the temporal evolution and local time variations of storm time ring current. The analysis demonstrates the feasibility of using instrumented spacecraft in low-inclination orbits to extract a timely proxy for the provisional Dst index and to specify the ring current s evolution.

  12. Detailed study of Pi2 damped oscillations from low latitude magnetic observatory

    NASA Astrophysics Data System (ADS)

    Bulusu, Jayashree; Arora, Kusumita; Nagarajan, Nandini

    2017-03-01

    The study of low latitude damped Pi2 oscillations (40-150 s) are investigated using archived data from Choutuppal (CPL), geomagnetic observatory, operated by CSIR-NGRI, Hyderabad, India. The period of investigation is during solar cycle 21 (1975-1983). All the Pi2 events identified during this period are subjected to detailed analysis for their association with substorm and non-substorm events. It is interesting to note that there is equal probability of occurrence of Pi2s with or without a substorm. The Pi2 frequencies associated with substorms showed an increased value in the post-midnight sector than compared to the Pre-midnight sectors. The non-substorm Pi2s are seen to be associated with lower levels of geomagnetic activity. The corresponding period of Pi2s decreases with increasing level of activity. While the generation of Pi2s are generally attributed to substorm onset, it is seen that the quiet time non-substorm events are related to plasmaspheric cavity mode resonances at low latitudes.

  13. C/NOFS Measurements of Magnetic Perturbations in the Low-Latitude Ionosphere During Magnetic Storms

    NASA Technical Reports Server (NTRS)

    Le, Guan; Burke, William J.; Pfaff, Robert F.; Freudenreich, Henry; Maus, Stefan; Luhr, Hermann

    2011-01-01

    The Vector Electric Field Investigation suite on the C/NOFS satellite includes a fluxgate magnetometer to monitor the Earth s magnetic fields in the low-latitude ionosphere. Measurements yield full magnetic vectors every second over the range of +/-45,000 nT with a one-bit resolution of 1.37 nT (16 bit A/D) in each component. The sensor s primary responsibility is to support calculations of both V x B and E x B with greater accuracy than can be obtained using standard magnetic field models. The data also contain information about large-scale current systems that, when analyzed in conjunction with electric field measurements, promise to significantly expand understanding of equatorial electrodynamics. We first compare in situ measurements with the POMME (Potsdam Magnetic Model of the Earth) model to establish in-flight sensor "calibrations" and to compute magnetic residuals. At low latitudes the residuals are predominately products of the storm time ring current. Since C/NOFS provides a complete coverage of all local times every 97 min, magnetic field data allow studies of the temporal evolution and local time variations of storm time ring current. The analysis demonstrates the feasibility of using instrumented spacecraft in low-inclination orbits to extract a timely proxy for the provisional Dst index and to specify the ring current s evolution.

  14. Transformation of Chlorinated Benzenes and Toluenes by Ralstonia sp. Strain PS12 tecA (Tetrachlorobenzene Dioxygenase) and tecB (Chlorobenzene Dihydrodiol Dehydrogenase) Gene Products

    PubMed Central

    Pollmann, Katrin; Beil, Stefan; Pieper, Dietmar H.

    2001-01-01

    The tecB gene, located downstream of tecA and encoding tetrachlorobenzene dioxygenase, in Ralstonia sp. strain PS12 was cloned into Escherichia coli DH5α together with the tecA gene. The identity of the tecB gene product as a chlorobenzene dihydrodiol dehydrogenase was verified by transformation into the respective catechols of chlorobenzene, the three isomeric dichlorobenzenes, as well as 1,2,3- and 1,2,4-trichlorobenzenes, all of which are transformed by TecA into the respective dihydrodihydroxy derivatives. Di- and trichlorotoluenes were either subject to TecA-mediated dioxygenation (the major or sole reaction observed for the 1,2,4-substituted 2,4-, 2,5-, and 3,4-dichlorotoluenes), resulting in the formation of the dihydrodihydroxy derivatives, or to monooxygenation of the methyl substituent (the major or sole reaction observed for 2,3-, 2,6-, and 3,5-dichloro- and 2,4,5-trichlorotoluenes), resulting in formation of the respective benzyl alcohols. All of the chlorotoluenes subject to dioxygenation by TecA were transformed, without intermediate accumulation of dihydrodihydroxy derivatives, into the respective catechols by TecAB, indicating that dehydrogenation is no bottleneck for chlorobenzene or chlorotoluene degradation. However, only those chlorotoluenes subject to a predominant dioxygenation were growth substrates for PS12, confirming that monooxygenation is an unproductive pathway in PS12. PMID:11526005

  15. Where does the plasmasphere begin? Revisit to topside ionospheric profiles in comparison with plasmaspheric TEC from Jason-1

    NASA Astrophysics Data System (ADS)

    Lee, Han-Byul; Kim, Yong Ha; Kim, Eunsol; Hong, Junseok; Kwak, Young-Sil

    2016-10-01

    Topside ionospheric profiles have been measured by Alouette 1 and ISIS 1/2 in the periods of 1962-1972 and 1972-1979, respectively. The profiles cover from the orbital altitude of 1000 km to the F2 peak and show large variations over local time, latitude, and seasons. We here analyze these variations in comparison with plasmaspheric total electron contents (pTECs) that were measured by Jason-1 satellite from the altitude of 1336 km to 20,200 km (GPS orbit). The scale heights of the profiles are generally smaller in the daytime than nighttime but show large day-to-day variations, implying that the ionospheric profiles at 1000 km are changing dynamically, rather than being in diffusive equilibrium. We also derived transition heights between O+ and H+, which show a clear minimum at dawn for low-latitude profiles due to decreasing O+ density at night. To compare with pTEC, we compute topside ionospheric total electron content (tiTEC) by integrating over 800-1336 km using the slope of the profiles. The tiTEC varies in a clear diurnal pattern from 0.3 to 1 and 3 total electron content unit (TECU, 1 TECU = 1016 el m-2) for low and high solar activity, respectively, whereas Jason-1 pTEC values are distributed over 2-6 TECU and 4-8 TECU for low and high solar activity, respectively, with no apparent diurnal modulation. Latitudinal variations of tiTEC show distinctive hemispheric asymmetry while that of Jason-1 pTEC is closely symmetric about the magnetic equator. The local time and latitudinal variations of tiTEC basically resemble those of the ionosphere but are characteristically different from those of Jason-1 pTEC. Based on the difference between tiTEC and pTEC variations, we propose that the region above 1300 km should be considered as the plasmasphere. Lower altitudes for the base of "plasmaspheric TEC," as used in some studies, would cause contamination of ionospheric influence.

  16. Latitudinal TEC gradients over polar ionosphere using high latitude GPS measurements

    NASA Astrophysics Data System (ADS)

    Shagimuratov, Irk; Cherniak, Iurii; Zakharenkova, Irina; Tepenitsyna, Nadezhda; Yakimova, Galina; Ephishov, I. I.

    The GPS observations of Greenland network were used to analyze the latitudinal variations of TEC at the high-latitudes ionosphere. This network provides unique opportunity to monitor TEC variability in polar ionosphere on a regular base. GPS stations are arranged along the latitude over the range 60-83°N (65°-87° Corrected Geomagnetic Latitude) near of 30°-40° longitudes. More than 20 GPS stations are located closely with one another along latitude. The distance between stations is about 1°-2°.Such spatial resolution provides the possibility to analyze the detailed structure of latitudinal TEC profiles. The standard procedure of processing GPS observations was used for TEC estimation. On this base it was obtained the diurnal TEC variations over all Greenland stations. The TEC data is used to form latitudinal profiles (TEC section) covered subauroral, auroral and polar ionosphere. In the report the observations of TEC for quiet and disturbed ionosphere during several geomagnetic storms occurred in September 2011 are presented. During quiet conditions in the night-time TEC profiles demonstrated invariable values about of 4-6 TECU in latitudinal region of 60°-75°N; then it presented THE increase towards the higher latitude and reached the value of 10 TECU near 80°N. The daytime profiles revealed TEC decrease toward high latitude in keeping with 0.8 TECU/degree. During storm the structure of latitudinal TEC profiles was essentially changed with agreement to the development of geomagnetic storm. The positive effect was observed at subauroral and auroral latitudes, negative effect was prevailed at the polar region. During the night time the ionospheric trough can be observed. In the report features of the behavior of latitudinal profiles at high-latitude ionosphere for September 2011 events were discussed.

  17. Preliminary analyses of solar flare effects on geomagnetic H component at equatorial and low latitudes

    NASA Astrophysics Data System (ADS)

    Ugonabo, Obiageli Josephine; Ugwu, Ernest Benjamin Ikechukwu; Nneka Okeke, Francisca

    The study of solar flare effect (SFE) on geomagnetic H component at mid latitudes was carried out using data from INTERMAGNET website. M and X solar flare effects on three stations, Addis Ababa (AAE), Bangui (BNG), and Tamanrasset (TAM) were investigated. It was found that the ratio is greater than zero for all the three stations used, hence SFE enhances geomagnetic field in the equatorial and low latitudes. It was equally noted that the SFE on geomagnetic field is not just a simple augmentation at the pre-flare ionospheric currents over these stations. It is concluded that both pre-flare and solar flare amplitude variations of H are high in low and equatorial stations. Keywords: Solar flare, geomagnetic component, latitudes.

  18. A Unified Fluid Model for Low-latitude Ionosphere Turbulence Causes Radiowave Scintillations

    NASA Astrophysics Data System (ADS)

    Hassan, E.; Horton, W.

    2012-12-01

    Nonlinear dynamics of the low latitudes E-layer simulated with a systems of differential equations describing the neutral wind driven Farley-Buneman instability and the density-gradient-drift instability as rising bubbles and falling higher electron density spikes. The simulations extent earlier nonlinear studies by using empirical models for the atmosphere and ionosphere backgrounds to give realistic local time-altitude parameters within a Python wrapped F90 simulations. New equations that keep both the compressional and rotational ion flows that apply in the lower F layer are analyzed to describe plumes extending to the peak of the F layer. A ray-tracing technique is used to describe the small angle scattering at high frequency [Gigahertz] GNSS signals treated as rays in the turbulent ionospheric plasma.

  19. E- and F- region incoherent scatter radar spectral measurements at mid and low-latitudes

    NASA Astrophysics Data System (ADS)

    Kudeki, Erhan; Milla, Marco

    2016-07-01

    In this talk we will contrast and compare incoherent scatter radar spectral measurements conducted using the Arecibo, ALTAIR, and Jicamarca incoherent scatter radars at ionospheric heights ranging from E-region into the topside F-region. Arecibo measurements from mid-latitudes exemplify high SNR ISR techniques utilized with large magnetic aspect angles. Low-latitude measurements at ALTAIR and Jicamarca make use of and combine large and small magnetic aspect angle techniques. Examples presented will include both natural and naturally enhanced electron and ion lines detected in the lower F region near the geomagnetic equator as well as the results of search for proton gyro-resonance peaks in the Jicamarca topside spectra.

  20. Color-magnitude diagrams for six metal-rich, low-latitude globular clusters

    NASA Technical Reports Server (NTRS)

    Armandroff, Taft E.

    1988-01-01

    Colors and magnitudes for stars on CCD frames for six metal-rich, low-latitude, previously unstudied globular clusters and one well-studied, metal-rich cluster (47 Tuc) have been derived and color-magnitude diagrams have been constructed. The photometry for stars in 47 Tuc are in good agreement with previous studies, while the V magnitudes of the horizontal-branch stars in the six program clusters do not agree with estimates based on secondary methods. The distances to these clusters are different from prior estimates. Redding values are derived for each program cluster. The horizontal branches of the program clusters all appear to lie entirely redwards of the red edge of the instability strip, as is normal for their metallicities.

  1. The Zambezi Channel: A new perspective on submarine channel evolution at low latitudes

    NASA Astrophysics Data System (ADS)

    Wiles, E.; Green, A.; Watkeys, M.; Jokat, W.

    2017-06-01

    Submarine channels are not stand-alone systems. They are long-lived systems modified by imperceptibly slow processes and rapid gravity flows, in some part controlled by hinterland dynamics. The submarine Zambezi Channel, within the Mozambique Channel, receives sediment from the Zambezi River catchment which has a dynamic tectonic and morphological history. Using recently collected multibeam bathymetry and PARASOUND data we discuss the geomorphology of the Zambezi Channel. Results show this system to be distinct in geomorphologic character when compared to other low-latitude submarine channels, sharing similarities with high-latitude systems. We propose a new, source-to-sink, hypothesis for the evolution of the Zambezi Channel, taking in to consideration hinterland tectonics, palaeo-lake development, river capture and rapid gravity flows. This hypothesis accounts for the unique present-day anatomy of the Zambezi Channel within the dynamic framework of the systems regional setting.

  2. Ion-neutral coupling effects on low-latitude thermospheric evening winds

    NASA Astrophysics Data System (ADS)

    Evonosky, W.; Richmond, A. D.; Fang, T.-W.; Maute, A.

    2016-05-01

    We examine the forces that determine zonal wind structure in the low-latitude evening thermosphere and its relation with ion-neutral coupling. These winds drive the evening F region dynamo that affects the equatorial ionization anomaly (EIA) and the generation of plasma irregularities. Forces are calculated using the Thermosphere-Ionosphere-Electrodynamics General Circulation Model coupled with the Global Ionosphere-Plasmasphere model. At 19 LT, the horizontal pressure gradient dominates the net acceleration of neutral winds below ˜220 km, while it tends to be offset by ion drag and viscosity higher up. The eastward pressure-gradient acceleration above 200 km increases approximately linearly with height and tends to be similar for different latitudes and different levels of solar activity. The pressure-gradient and ion-drag forces in the central F region approximately balance for field lines that pass through the EIA. Viscosity is an important additional force at non-EIA latitudes and in the bottomside and topside EIA ionosphere. An increase in E region drag on plasma convection due to increased nighttime ionization causes both the ion and neutral velocities in the F region to decrease, while the velocity difference tends to be maintained. The presence of a low-latitude evening time vertical shear in the zonal wind is associated primarily with a strong eastward pressure-gradient acceleration at high altitude that reverses the daytime westward wind and a weak low-altitude pressure-gradient acceleration of either eastward or westward direction that fails to reverse the low-altitude westward wind present in the afternoon.

  3. Thermospheric meridional wind at low latitude from measurements of F layer peak height

    NASA Astrophysics Data System (ADS)

    de Medeiros, R. T.; Abdu, M. A.; Batista, I. S.

    1997-07-01

    Thermospheric meridional winds are calculated in this paper for the low-latitude station, Cachoeira Paulista, in Brazil using ionospheric F layer peak height hmax as the primary database for different seasons and solar activity conditions. A servo model is used that expresses the ionospheric F layer peak height displacements as a function of chemical loss, diffusion, thermospheric meridional wind and vertical plasma drift. The method used is similar to, and an extension of, the one used in recent years for midlatitudes, where the effect of vertical plasma drift is considered negligible. We have included in our analysis the effect of vertical plasma drift on hmax over Cachoeira Paulista by using a vertical drift model which is a field line extension of an equatorial electric field model developed for our longitude, for which the radar data from Jicamarca, Peru, are complemented, in the sunset sector, by vertical drift from ionosonde data over Fortaleza. A numerical model on the electrodynamic coupling of the E and F regions is used to obtain the equatorial height dependence of the vertical drift needed for its field line mapping to low latitude. Meridional winds were calculated using the servo equations in which the vertical plasma drifts, and hmax values deduced from Cachoeira Paulista ionograms, were the main inputs. The magnetic meridional winds calculated for the summer equinoctial and winter months of high and low solar activity epochs are compared with the HWM-90 and with the measurements by Fabri-Perot technique available for Cachoeira Paulista. The results show varying agreements. The paper presents details of the method of analysis and the comparison of the results.

  4. A new method for estimation of TEC from GNSS receivers during multiple cycle slips and data loss due to ionospheric irregularities

    NASA Astrophysics Data System (ADS)

    Dashora, Nirvikar

    2012-07-01

    Estimation of total electron content (TEC) is a must to utilize the GNSS signal for ionospheric research. The estimation of absolute ionospheric TEC from raw GNSS data itself is a lengthy and complex task and requires knowledge of sophisticated computer programming skills, satellite orbital geometry and many other aspects like GNSS signal structure, satellite and receiver specific information etc. Not many software are available that automatize the complex task of cycle slip detection and correction. The estimation of satellite and receiver biases is one more step left before getting true TEC. In equatorial and low latitudes, where the nighttime ionospheric irregularities are oft-occurring phenomena, estimation of TEC becomes a challenge. This is because, during TEC depletions and scintillations, the loss of lock in the receiver results either in data loss or sharp gradients in the recorded delays; more often for phase than code. Raw data in form of accumulated phase in radians and total code-range in meters show random occurrences of multiple cycle slips and data loss of several minutes in the data file. Thus, such phase and code data has to be corrected first before processing it to obtain TEC. Almost all available software/algorithms suggest flagging such data for no further use. Hence, TEC cannot be estimated during multiple cycle slip events within few minutes from raw GPS data. But, for ionospheric research the time of occurrence, evolution and drift of depletions are very useful. This paper details a complete new software for pre-processing the raw RINEX (receiver independent exchange format) data and retrieval of TEC using code and carrier-phase measurements from an stand alone dual frequency GPS receiver. We use modified and new sets of algorithms a GNSS receiver. It is significant to note that we are able to retrieves almost all the the corrupted TEC data points due to random multiple cycle slip events which are oft-occurring phenomena during ESF in

  5. Risk assessment of the extreme interplanetary shock of 23 July 2012 on low-latitude power networks

    NASA Astrophysics Data System (ADS)

    Zhang, J. J.; Wang, C.; Sun, T. R.; Liu, Y. D.

    2016-03-01

    Geomagnetic sudden commencements (SCs), characterized by a rapid enhancement in the rate of change of the geomagnetic field perturbation (dB/dt), are considered to be an important source of large geomagnetically induced currents (GICs) in middle- and low-latitude power grids. In this study, the extreme interplanetary shock of 23 July 2012 is simulated under the assumption that it had hit the Earth with the result indicating the shock-caused SC would be 123 nT. Based on statistics, the occurrence frequency of SCs with amplitudes larger than the simulated one is estimated to be approximately 0.2% during the past 147 years on the Earth. During this extreme event, the simulation indicates that dB/dt, which is usually used as a proxy for GICs, at a dayside low-latitude substation would exceed 100 nT/min; this is very large for low-latitude regions. We then assess the GIC threat level based on the simulated geomagnetic perturbations by using the method proposed by Marshall et al. (2011). The results indicate that the risk remains at "low" level for the low-latitude power network on a global perspective. However, the GIC risk may reach "moderate" or even "high" levels for some equatorial power networks due to the influence of the equatorial electrojet. Results of this study feature substantial implications for risk management, planning, and design of low-latitude electric power networks.

  6. Revisiting the question: Does high-latitude solar activity lead low-latitude solar activity in time phase?

    SciTech Connect

    Kong, D. F.; Qu, Z. N.; Guo, Q. L.

    2014-05-01

    Cross-correlation analysis and wavelet transform methods are used to investigate whether high-latitude solar activity leads low-latitude solar activity in time phase or not, using the data of the Carte Synoptique solar filaments archive from 1919 March to 1989 December. From the cross-correlation analysis, high-latitude solar filaments have a time lead of 12 Carrington solar rotations with respect to low-latitude ones. Both the cross-wavelet transform and wavelet coherence indicate that high-latitude solar filaments lead low-latitude ones in time phase. Furthermore, low-latitude solar activity is better correlated with high-latitude solar activity of the previous cycle than with that of the following cycle, which is statistically significant. Thus, the present study confirms that high-latitude solar activity in the polar regions is indeed better correlated with the low-latitude solar activity of the following cycle than with that of the previous cycle, namely, leading in time phase.

  7. Theoretical scaling laws for the spectrum of density irregularities in the high- and low-latitude ionosphere

    SciTech Connect

    Keskinen, M.J.

    1990-05-03

    Constraints on and scaling laws associated with the spectrum of density irregularities in the high and low latitude ionosphere are derived using conservation laws implied by the fundamental nonlinear plasma fluid equations describing low frequency, long wavelength ionospheric plasma dynamics and structure. For the high latitude case we discuss the spectrum implied by interchange-like plasma instabilities and apply our results to convecting ionospheric plasma enhancements, blobs, patches, and polar cap arcs. For the low latitude ionosphere, we derive scaling laws for the density spectrum associated with the Rayleigh-Taylor instability and make applications to equatorial spread-F. For both the high and low latitude cases, we distinguish the spectral behavior to be expected in both the lower and upper ionosphere.

  8. Study of TEC fluctuation via stochastic models and Bayesian inversion

    NASA Astrophysics Data System (ADS)

    Bires, A.; Roininen, L.; Damtie, B.; Nigussie, M.; Vanhamäki, H.

    2016-11-01

    We propose stochastic processes to be used to model the total electron content (TEC) observation. Based on this, we model the rate of change of TEC (ROT) variation during ionospheric quiet conditions with stationary processes. During ionospheric disturbed conditions, for example, when irregularity in ionospheric electron density distribution occurs, stationarity assumption over long time periods is no longer valid. In these cases, we make the parameter estimation for short time scales, during which we can assume stationarity. We show the relationship between the new method and commonly used TEC characterization parameters ROT and the ROT Index (ROTI). We construct our parametric model within the framework of Bayesian statistical inverse problems and hence give the solution as an a posteriori probability distribution. Bayesian framework allows us to model measurement errors systematically. Similarly, we mitigate variation of TEC due to factors which are not of ionospheric origin, like due to the motion of satellites relative to the receiver, by incorporating a priori knowledge in the Bayesian model. In practical computations, we draw the so-called maximum a posteriori estimates, which are our ROT and ROTI estimates, from the posterior distribution. Because the algorithm allows to estimate ROTI at each observation time, the estimator does not depend on the period of time for ROTI computation. We verify the method by analyzing TEC data recorded by GPS receiver located in Ethiopia (11.6°N, 37.4°E). The results indicate that the TEC fluctuations caused by the ionospheric irregularity can be effectively detected and quantified from the estimated ROT and ROTI values.

  9. How Strong is the Case for Proterozoic Low-Latitude Glaciation?

    NASA Astrophysics Data System (ADS)

    Evans, D. A.

    2004-05-01

    The most recent global compilations of paleomagnetic depositional latitudes for Proterozoic glaciogenic formations indicate a dominant mode near the paleo-equator (Evans 2000 AJS; Evans 2003 Tectonophysics). This result would therefore support either the snowball Earth or the large-obliquity hypotheses for Precambrian ice ages, but would reject the uniformitarian comparison to polar-temperate-restricted Phanerozoic glaciogenic deposits. The most reliable low-latitude results come from the Australian Marinoan succession, but a recent summary of these units has suggested that a glaciogenic origin is not yet demonstrated (Eyles and Januszczak 2004 Earth-Sci Reviews). It becomes useful, then, to review the global evidence for Proterozoic low-latitude glaciation. Eyles and Januszczak (ibid.) identified 13 Neoproterozoic deposits with "demonstrated" glacial influence. Among these, poor age constraints and lack of paleomagnetic data prohibit estimation of depositional paleolatitudes for the Fiq, Sturtian, Vreeland, Taoudeni, East Greenland, Port Askaig, and Zhengmuguan units. Moderate paleolatitudes are reasonably well supported for the South China, Gaskiers, Smalfjord, and Moelv units. Among the three remaining units, the Rapitan Group can be assigned a near-equatorial paleolatitude indirectly through use of the Galeros and Franklin-Natkusiak paleomagnetic results, as long as the Rapitan age lies within 750-720 Ma as generally expected. The Moonlight Valley Formation in northern Australia may be assigned a tropical paleolatitude according to high-quality paleomagnetic results from compellingly correlated Marinoan strata in southern Australia. Those strata, including the famous Elatina Formation, have yielded a robust paleomagnetic signature that is commonly interpreted to imply frigid climate (manifest in part by frost-wedge polygons) at near-equatorial latitudes. Concerns that the Neoproterozoic geomagnetic field was either nonaxial or nondipolar are valid in principle

  10. Middle- and low-latitude emissions from energetic neutral atom precipitation seen from ATLAS 1 under quiet magnetic conditions

    NASA Technical Reports Server (NTRS)

    Tinsley, B. A.; Rohrbaugh, R. P.; Ishimoto, M.; Torr, M. R.; Torr, D. G.

    1994-01-01

    During the ATLAS 1 mission spectral observations were made at middle and low latitudes of features expected from the precipitation of energetic neutral atoms. The Imaging Spectrometric Observatory was used at night in the UV and visible with maximum gain. The tangent ray heights of the look directions ranged from near 100 km to near 200 km, and the geomagnetic conditions were quiet during the observations, which were made March 28 to April 3, 1992. The N2(+) 1N 391.4-nm and O I 130.4 and 135.6-nm emissions were observed at all latitudes, with lower emission rates at lower magnetic dip latitudes, except that enhancements in the O I lines were seen within 30 deg of the dip equator to radiative recombination of ionospheric plasma. The latitude profile observed for the N2(+) 1N emission did not show an equatorial or midlatitude peak. This implies that the source of energetic neutrals is more consistent with prompt charge exchange loss of freshly injected trapped ions with relatively low mirror heights (i.e., ions on higher L shells with equatorial pitch angle distributions nearly isotropic to the loss cone) than loss of highly eroded populations of particles with high mirror heights (i.e., ions on lower L shells with pancake equatorial pitch angle distributions). The N2(+) 1N emission rates have been compared with models of atmospheric emission due to fluxes of O/O(+) and H/H(+) in the thermosphere, as produced by energetic neutral oxygen or hydrogen atom precipitation. Energy deposition rates are inferred.

  11. Difficulties in the study of cosmic radio noise absorption at 30 MHz using riometer at low latitude station, Kolhapur (Lat-16.8°N, Long-74.25°E)

    NASA Astrophysics Data System (ADS)

    Nikte, S. S.; Sharma, A. K.; Nade, D. P.; Rokade, M. V.; Ghodpage, R. N.; Patil, P. T.; Bhonsle, R. V.

    2014-01-01

    A dual dipole antenna has been installed at low latitude station Kolhapur (Geographic 16.8°N, 74.25°E), Maharashtra, India for the study of cosmic radio noise absorption using Solid State Riometer (which operates at 30 MHz) during pre phase of 24th solar maxima. The aim for this type of study over Kolhapur was to know the response of lower (D region) ionosphere over low latitude by cosmic radio noise absorption using riometer technique during quite period as well as sudden ionospheric disturbances (SID). The observations are being taken for 3 years. Two different sites (˜40 km away from each other) were used for the installation of riometer equipment assuming minimum local noise. It is found that solar noise to cosmic radio noise hence resulting in signal saturation. The night time signal is relatively free of interference but sometimes local noise is responsible for spike-like signatures. Hence it is concluded that Kolhapur (a low latitude station) is not suitable for the study of cosmic radio noise absorption on 30 MHz with riometer and dual dipole antenna. Proper choice for operating frequency of riometer and antenna gain is suggested for low latitude use of this technique for ionospheric deviative and nondeviative absorption studies.

  12. The 630 nm MIG and the vertical neutral wind in the low latitude nighttime thermosphere

    NASA Technical Reports Server (NTRS)

    Herrero, F. A.; Meriwether, J. W., Jr.

    1994-01-01

    It is shown that large negative divergences (gradients) in the horizontal neutral wind in the equatorial thermosphere can support downward neutral winds in excess of 20 m/s. With attention to the meridional and vertical winds only, the pressure tendency equation is used to derive the expression U(sub z0) approximately equals (Partial derivative U(sub y)/Partial derivative y)H for the vertical wind U(sub z0) at the reference altitude for the pressure tendency equation; H is the atmospheric density scale height, and (Partial derivative U(sub y)/Partial derivative y) is the meridional wind gradient. The velocity gradient associated with the Meridional Intensity Gradient (MIG) of the O((sup 1)D) emission (630 nm) at low latitudes is used to estimate the vertical neutral wind in the MIG region. Velocity gradients derived from MIG data are about 0.5 (m/s)/km) or more, indicating that the MIG region may contain downward neutral winds in excess of 20 m/s. Though direct measurements of the vertical wind are scarce, Fabry-Perot interferometer data of the equatorial F-region above Natal, Brazil, showed downward winds of 30 m/s occurring during a strong meridional wind convergence in 1982. In-situ measurements with the WATS instrument on the DE-2 satellite also show large vertical neutral winds in the equatorial region.

  13. The 630 nm MIG and the vertical neutral wind in the low latitude nighttime thermosphere

    NASA Technical Reports Server (NTRS)

    Herrero, F. A.; Meriwether, J. W., Jr.

    1994-01-01

    It is shown that large negative divergences (gradients) in the horizontal neutral wind in the equatorial thermosphere can support downward neutral winds in excess of 20 m/s. With attention to the meridional and vertical winds only, the pressure tendency equation is used to derive the expression U(sub z0) approximately equals (Partial derivative U(sub y)/Partial derivative y)H for the vertical wind U(sub z0) at the reference altitude for the pressure tendency equation; H is the atmospheric density scale height, and (Partial derivative U(sub y)/Partial derivative y) is the meridional wind gradient. The velocity gradient associated with the Meridional Intensity Gradient (MIG) of the O((sup 1)D) emission (630 nm) at low latitudes is used to estimate the vertical neutral wind in the MIG region. Velocity gradients derived from MIG data are about 0.5 (m/s)/km) or more, indicating that the MIG region may contain downward neutral winds in excess of 20 m/s. Though direct measurements of the vertical wind are scarce, Fabry-Perot interferometer data of the equatorial F-region above Natal, Brazil, showed downward winds of 30 m/s occurring during a strong meridional wind convergence in 1982. In-situ measurements with the WATS instrument on the DE-2 satellite also show large vertical neutral winds in the equatorial region.

  14. Dynamics of turbulent western-boundary currents at low latitude in a shallow-water model

    NASA Astrophysics Data System (ADS)

    Akuetevi, C. Q. C.; Wirth, A.

    2015-06-01

    The dynamics of low latitude turbulent western-boundary currents (WBCs) crossing the Equator are considered using numerical results from integrations of a reduced-gravity shallow-water model. For viscosity values of 1000 m2 s-1 and greater, the boundary layer dynamics compares well to the analytical Munk-layer solution. When the viscosity is reduced, the boundary layer becomes turbulent and coherent structures in the form of anticyclonic eddies, bursts (violent detachments of the viscous sub-layer, VSL) and dipoles appear. Three distinct boundary layers emerge, the VSL, the advective boundary layer and the extended boundary layer. The first is characterized by a dominant vorticity balance between the viscous transport and the advective transport of vorticity; the second by a balance between the advection of planetary vorticity and the advective transport of relative vorticity. The extended boundary layer is the area to which turbulent motion from the boundary extends. The scaling of the three boundary layer thicknesses with viscosity is evaluated. Characteristic scales of the dynamics and dissipation are determined. A pragmatic approach to determine the eddy viscosity diagnostically for coarse-resolution numerical models is proposed.

  15. Interactive Ion-Neutral Dynamics in the Low Latitude Evening Ionosphere

    NASA Astrophysics Data System (ADS)

    Evonosky, W. R.; Richmond, A. D.; Fang, T. W.; Maute, A. I.

    2015-12-01

    Neutral winds in the ionosphere drive global electrodynamic phenomena which alter theupper-atmosphere so significantly that they can affect the orbit of satellites andground-to-spacecraft communications. Understanding these winds and what drives them is centralto prediction and risk management associated with such a dynamic upper atmosphere. This studyexamined the relationship between accelerations acting on neutral winds in the ionosphere and theformation of a vertical shear of those winds in low latitudes (between ±30 magnetic) and earlyevening local times (16-22 LT). Accelerations were calculated using variables output by thethermosphere ionosphere electrodynamics general circulation model (TIEGCM) under differentsolar activity and night-time ionization conditions and visualized both spatially and temporally. Ingeneral, with acceleration values averaged along magnetic latitudes between ±30 degrees(inclusive) and only considering medium solar activity conditions, we found that the ionosphereexhibits distinct layering defined by the dominant accelerations in each layer. We also found hintsthat during different night-time ionization levels, ion drag acceleration tends to remain constantwhile ion and neutral velocities change to conserve the difference between them. When consideringspecific latitudes and solar conditions, previously unreported structures appear which involveinteractions between the ion drag and viscous forces.

  16. Sensitivity of Low-Latitude Ionospheric Convection in the Evening to E-Region Conductivity

    NASA Astrophysics Data System (ADS)

    Richmond, A. D.; Fang, T. W.; Maute, A. I.

    2014-12-01

    Modeling of low-latitude ionospheric electrodynamics reveals a sensitivity of ExB convection in the evening to E-region conductivity. This sensitivity is explained in terms of two related but distinct effects. First, meridional E-region currents associated with Pedersen conductivity partially balance meridional F-region dynamo currents. Since the F-region current density depends more on the pressure-gradient force driving the wind than on the E-region conductivity, changes in the latter provoke an inversely related change in the electric field and plasma convection velocity, even though the relative contribution of the E region to the field-line-integrated conductivity may be small as compared with the F region contribution. The second way in which night-time E-region conductivity affects the evening plasma convection is through regulation of the zonal electric field and vertical/meridional plasma convection. In this case it is the E-region Cowling conductance, rather than the Pedersen conductance, that comes into play. Vertical convection through the E region in the early evening, associated with the pre-reversal enhancement of the vertical drift, is associated with zonal Cowling current that dissipates a relatively large amount of electromagnetic energy, and therefore exerts a drag on the evening plasma convection. This presentation quantifies the sensitivity of the convection to the night-time E-region conductivity, and shows how the convection distribution tends to obey a minimization principle.

  17. Storm-time ionization enhancements at the topside low-latitude ionosphere

    NASA Astrophysics Data System (ADS)

    Dmitriev, A.; Yeh, H.-C.

    2008-05-01

    Ion density enhancements at the topside low-latitude ionosphere during a Bastille storm on 15-16 July 2000 and Halloween storms on 29-31 October 2003 were studied using data from ROCSAT-1/IPEI experiment. Prominent ion density enhancements demonstrate similar temporal dynamics both in the sunlit and in the nightside hemispheres. The ion density increases dramatically (up to two orders of magnitude) during the main phase of the geomagnetic storms and reaches peak values at the storm maximum. The density enhancements are mostly localized in the region of a South Atlantic Anomaly (SAA), which is characterized by very intense fluxes of energetic particles. The dynamics of near-Earth radiation was studied using SAMPEX/LEICA data on >0.6 MeV electrons and >0.8 MeV protons at around 600 km altitude. During the magnetic storms the energetic particle fluxes in the SAA region and in its vicinity increase more than three orders of magnitude. The location of increased fluxes overlaps well with the regions of ion density enhancements. Two mechanisms were considered to be responsible for the generation of storm-time ion density enhancements: prompt penetration of the interplanetary electric field and abundant ionization of the ionosphere by enhanced precipitation of energetic particles from the radiation belt.

  18. Performance of Emcore Third Generation CPV Modules in the Low Latitude Marine Environment of Hawaii

    NASA Astrophysics Data System (ADS)

    Hoffman, Richard; Buie, Damien; King, David; Glesne, Thomas

    2011-12-01

    Emcore third generation concentrating photovoltaic (CPV) modules were evaluated in the low latitude location of Kihei, Hawaii. For comparison, the best available monocrystalline silicon flat panel modules were included in both dual-axis tracked and fixed mount configurations. The daily DC uncorrected efficiency value for the CPV modules averaged over the six-month performance period was 25.9% compared to 16% to 17% for the flat panels. Higher daily energy was obtained from CPV modules than tracked flat panels when daily direct solar insolation was greater than 5 kWh/m2 and more than fixed mount flat panel when direct insolation was greater than 3 kWh/m2. The module energy conversion performance was demonstrated to be predictable using a parametric model developed by Sandia National Laboratory. Soiling accumulation on module entrance surface was surprisingly rapid in the local environment. Measured energy loss rate due to soiling were two to six times larger for CPV compared to flat panel losses.

  19. Combined Impedance Probe and Langmuir Probe Studies of the Low-Latitude E Region

    NASA Technical Reports Server (NTRS)

    Rowland, D. E.; Pfaff, R. F.; Steigies, C. T.

    2008-01-01

    The EQUIS-2 sounding rocket and radar campaign, launched from Kwajalein Atoll in 2004, included a mission to study low-latitude irregularities and electrodynamics, led by NASA GSFC. This mission included two instrumented rockets launched into the nighttime E region (apogee near 120 km), which included comprehensive electrodynamics and neutral density instrumentation. These rockets carried the first of a new generation of impedance probes, that utilize a wide-band drive signal to simultaneously measure the impedance of an antenna in a plasma as a function of frequency from 7 kEIz to 4 MHz. at a rapid cadence. This technique promises to permit true plasma spectroscopy, and resulted in the identification of multiple plasma resonances and accurate measurements of the plasma density, even in the low density nighttime E region. We present analyses of the technique and resulting spectra, and show how these data may be combined with fixed-bias Langmuir Probe data to infer the temperature structure of the E region as well as providing accurate absolute calibrations for the very high time resolution fixed-bias probe data. The data is shown to agree well with data from ionosonde, the ALTAIR radar, and the Peruvian beacon experiment.

  20. Origin and magnitude of low latitude terrestrial precipitation and temperature anomalies during Heinrich events and deglaciation

    NASA Astrophysics Data System (ADS)

    Donders, T. H.; de Boer, H. J.; Finsinger, W.; Grimm, E. C.; Dekker, S. C.; Reichart, G. J.; Wagner-Cremer, F.

    2009-04-01

    Repetitive phases of increased pine at Lake Tulane, Florida have previously been related to strong stadials terminated by so-called Heinrich events. The climatic significance of these pine phases has been interpreted in different ways. Using a pollen-climate inference model, we quantified the climate changes and consistently found mean summer precipitation (PJJA) increases (0.5-0.9 mm/day) and mean November temperature increases (2.0-3.0∘C) that are coeval with Heinrich events and the Younger Dryas. Comparison with marine sea surface temperature records point to a potential source for these heat and moisture anomalies in the Gulf of Mexico or the western tropical Atlantic. A climate model sensitivity analysis indicates that a positive heat anomaly in the Gulf of Mexico and equatorial Atlantic best approximates the pollen-inferred climate reconstructions from Lake Tulane during the Heinrich events and Younger Dryas. We explain the low latitude warming by an increased Loop Current facilitated by the persistence of the Atlantic Warm Pool during summer.

  1. Low energy, low latitude wave-dominated shallow marine depositional systems: examples from northern Borneo

    NASA Astrophysics Data System (ADS)

    Lambiase, Joseph J.; Suraya Tulot

    2013-12-01

    The depositional environments of the wave-dominant successions in the middle to late Miocene Belait and Sandakan Formations in northwestern and northern Borneo, respectively, were determined based on grain size distributions, sedimentary structures and facies successions, as well as trace and microfossil assemblages. Generally, progradational shoreface successions in the Belait Formation were deposited in very low wave energy environments where longshore currents were too weak to generate trough cross-bedding. Shoreface sands are laterally continuous for several km and follow the basin contours, suggesting attached beaches similar to the modern Brunei coastline. In contrast, trough cross-bedding is common in the coarser Sandakan Formation and back-barrier mangrove swamp deposits cap the progradational succession as on the modern northern Dent Peninsula coastline, indicating barrier development and higher wave energy conditions than in the Belait Formation. The Borneo examples indicate that barrier systems that include significant tidal facies form under higher wave energy conditions than attached beaches with virtually no tidal facies. Also, Borneo's low latitude climate promotes back-barrier mangrove which reduces tidal exchange and reduces tidal influence relative to comparable temperate climate systems. The results of the study indicate that depositional systems on low energy, wave-dominated coasts are highly variable, as are the sand bodies and facies associations they generate.

  2. Characteristics of Low-latitude Coronal Holes near the Maximum of Solar Cycle 24

    NASA Astrophysics Data System (ADS)

    Hofmeister, Stefan J.; Veronig, Astrid; Reiss, Martin A.; Temmer, Manuela; Vennerstrom, Susanne; Vršnak, Bojan; Heber, Bernd

    2017-02-01

    We investigate the statistics of 288 low-latitude coronal holes extracted from SDO/AIA-193 filtergrams over the time range of 2011 January 01–2013 December 31. We analyze the distribution of characteristic coronal hole properties, such as the areas, mean AIA-193 intensities, and mean magnetic field densities, the local distribution of the SDO/AIA-193 intensity and the magnetic field within the coronal holes, and the distribution of magnetic flux tubes in coronal holes. We find that the mean magnetic field density of all coronal holes under study is 3.0 ± 1.6 G, and the percentaged unbalanced magnetic flux is 49 ± 16%. The mean magnetic field density, the mean unsigned magnetic field density, and the percentaged unbalanced magnetic flux of coronal holes depend strongly pairwise on each other, with correlation coefficients cc > 0.92. Furthermore, we find that the unbalanced magnetic flux of the coronal holes is predominantly concentrated in magnetic flux tubes: 38% (81%) of the unbalanced magnetic flux of coronal holes arises from only 1% (10%) of the coronal hole area, clustered in magnetic flux tubes with field strengths >50 G (10 G). The average magnetic field density and the unbalanced magnetic flux derived from the magnetic flux tubes correlate with the mean magnetic field density and the unbalanced magnetic flux of the overall coronal hole (cc > 0.93). These findings give evidence that the overall magnetic characteristics of coronal holes are governed by the characteristics of the magnetic flux tubes.

  3. Reassessment of the thermospheric response to geomagnetic activity at low latitudes

    NASA Astrophysics Data System (ADS)

    Berger, C.; Barlier, F.; Ill, M.

    1988-10-01

    The present study takes advantage of measurements made at low latitudes by the Cactus accelerometer. From such measurements, the response of several thermospheric parameters to geomagnetic activity can be simultaneously and reliably retrieved: total density, density scale height, vertical density scale height gradient, temperature, O/N2 ratio and mean molecular mass. Their behavior exhibits a diurnal variation, some features of which have not been described, especially in the case of strong geomagnetic storms; the night scale height response appears to be stronger than the day one, while its vertical gradients increase by day and slightly decrease at night. The temperature increase is higher by day while the O/N2 ratio decreases by day, and increases at night at constant pressure level as well as at fixed height. By day, significant vertical temperature gradients are also found. These results as well as others are analyzed in the light of existing theories and compared to the predictions of existing thermospheric models. Strong meridional winds at night, heat transport through thermal conductivity, as well as wave dissipation during the day, might be factors helping to account for such behavior.

  4. ICESat profiles of tabular iceberg margins and iceberg breakup at low latitudes

    NASA Astrophysics Data System (ADS)

    Scambos, Ted; Sergienko, Olga; Sargent, Aitbala; MacAyeal, Douglas; Fastook, Jim

    2005-11-01

    ICESat elevation profiles of tabular iceberg margins and the Ronne Ice Shelf edge reveal shapes indicative of two types of bending forces. Icebergs and shelf fronts in sea-ice-covered areas have broad (~1000 m wide), rounded, ~0.6 m high `berms' and outer edges that slope down several meters toward the water. Bergs in warmer water have 2 to 5m `ramparts' with ~1500 m wide edge-parallel `moats' inboard of the edge. This latter pattern was first revealed in images from International Space Station (ISS) showing edge-parallel melt ponds on one iceberg just prior to its disintegration. Model results indicate the patterns are caused by hydrostatic and lithostatic forces acting on the ice face. `Berm' profiles arise from differences between ice and water pressure along the face. `Rampart-moat' profiles result from waterline erosion, creating a submerged bench of ice that lifts the ice edge. We use the results to discuss iceberg breakup at low latitudes.

  5. Spectroscopic confirmation of the low-latitude object FSR 1716 as an old globular cluster

    NASA Astrophysics Data System (ADS)

    Koch, Andreas; Kunder, Andrea; Wojno, Jennifer

    2017-09-01

    Star clusters are invaluable tracers of the Galactic components and the discovery and characterization of low-mass stellar systems can be used to appraise their prevailing disruption mechanisms and time scales. However, owing to significant foreground contamination, high extinction, and still uncharted interfaces of the underlying Milky Way components, objects at low Galactic latitudes are notoriously difficult to characterize. Here, we present the first spectroscopic campaign to identify the chemodynamical properties of the low-latitude star cluster FSR 1716. While its photometric age and distance are far from settled, the presence of RR Lyrae variables indicates a rather old cluster variety. Using medium-resolution (R 10 600) calcium triplet (CaT) spectroscopy obtained with the wide-field, multi-fiber AAOmega instrument, we identified six member candidates with a mean velocity of -30 km s-1 and a velocity dispersion of 2.5 ± 0.9 km s-1. The latter value implies a dynamic mass of 1.3 × 104M⊙, typical of a low-mass globular cluster. Combined with our derived CaT metallicity of -1.38 ± 0.20 dex, this object is finally confirmed as an old, metal-poor globular cluster.

  6. Estimation of E × B drift using a global assimilative ionospheric model: An observation system simulation experiment

    NASA Astrophysics Data System (ADS)

    Pi, Xiaoqing; Wang, Chunming; Hajj, George A.; Rosen, Gary; Wilson, Brian D.; Bailey, Graham J.

    2003-02-01

    A global assimilative ionospheric model (GAIM) has been developed to improve the modeling of ionospheric weather. GAIM adopts a fixed three-dimensional Eulerian grid following a geomagnetic dipole configuration. A four-dimensional variational approach (4DVAR) with the adjoint technique is presented, which attempts to minimize the difference between modeled and measured line-of-sight total electron content (TEC) using nonlinear least squares minimization. The minimization is achieved by solving for corrections to the initial (climatological) model drivers so that the density state becomes consistent with the observations. The 4DVAR approach is exercised with GAIM in an observation system simulation experiment (OSSE) conducted for estimating the weather behavior of E × B drift at low latitudes. The OSSE takes the constellation of global positioning system (GPS) satellites and an existing global GPS receiver network as the observation system. The effectiveness of the 4DVAR technique with such an observation system is assessed in the experiment, which indicates that one can solve for the low-latitude E × B drift and improve the density modeling using ground-based, integrated line-of-sight (TEC) measurements from a relatively small number of stations.

  7. Long-term prediction of the Arctic ionospheric TEC based on time-varying periodograms.

    PubMed

    Liu, Jingbin; Chen, Ruizhi; Wang, Zemin; An, Jiachun; Hyyppä, Juha

    2014-01-01

    Knowledge of the polar ionospheric total electron content (TEC) and its future variations is of scientific and engineering relevance. In this study, a new method is developed to predict Arctic mean TEC on the scale of a solar cycle using previous data covering 14 years. The Arctic TEC is derived from global positioning system measurements using the spherical cap harmonic analysis mapping method. The study indicates that the variability of the Arctic TEC results in highly time-varying periodograms, which are utilized for prediction in the proposed method. The TEC time series is divided into two components of periodic oscillations and the average TEC. The newly developed method of TEC prediction is based on an extrapolation method that requires no input of physical observations of the time interval of prediction, and it is performed in both temporally backward and forward directions by summing the extrapolation of the two components. The backward prediction indicates that the Arctic TEC variability includes a 9 years period for the study duration, in addition to the well-established periods. The long-term prediction has an uncertainty of 4.8-5.6 TECU for different period sets.

  8. A GIS-based assessment of the suitability of SCIAMACHY satellite sensor measurements for estimating reliable CO concentrations in a low-latitude climate.

    PubMed

    Fagbeja, Mofoluso A; Hill, Jennifer L; Chatterton, Tim J; Longhurst, James W S

    2015-02-01

    An assessment of the reliability of the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) satellite sensor measurements to interpolate tropospheric concentrations of carbon monoxide considering the low-latitude climate of the Niger Delta region in Nigeria was conducted. Monthly SCIAMACHY carbon monoxide (CO) column measurements from January 2,003 to December 2005 were interpolated using ordinary kriging technique. The spatio-temporal variations observed in the reliability were based on proximity to the Atlantic Ocean, seasonal variations in the intensities of rainfall and relative humidity, the presence of dust particles from the Sahara desert, industrialization in Southwest Nigeria and biomass burning during the dry season in Northern Nigeria. Spatial reliabilities of 74 and 42 % are observed for the inland and coastal areas, respectively. Temporally, average reliability of 61 and 55 % occur during the dry and wet seasons, respectively. Reliability in the inland and coastal areas was 72 and 38 % during the wet season, and 75 and 46 % during the dry season, respectively. Based on the results, the WFM-DOAS SCIAMACHY CO data product used for this study is therefore relevant in the assessment of CO concentrations in developing countries within the low latitudes that could not afford monitoring infrastructure due to the required high costs. Although the SCIAMACHY sensor is no longer available, it provided cost-effective, reliable and accessible data that could support air quality assessment in developing countries.

  9. Characterizing quasi-periodic disturbances in GPS TEC data and their drivers

    NASA Astrophysics Data System (ADS)

    Wurtz, J.; Coster, A. J.; Goncharenko, L.; Zhang, S.

    2012-12-01

    We examine a large data set (2003-2009) of total electron content (TEC) values derived from a global network of differential GPS receivers that was downloaded from the Madrigal database. We report on TEC oscillations at multiple periods ranging from 5 to ~30 days. A significant portion of oscillations with 9-day and 13.5-day periods is driven by the recurrent geomagnetic activity, as evident from the analysis of geomagnetic indices. The TEC disturbances in response to the recurrent geomagnetic activity are stronger at middle and high latitudes, and are less pronounced at lower latitudes (< 30 degrees). We also observe a correlation between TEC and the 28-day lunar cycle that is more evident at lower (equatorial) latitudes in both northern and southern hemispheres. The TEC disturbances associated with the lunar cycle are well pronounced during the northern hemisphere winter and equinox seasons.

  10. Temporal and spatial distributions of TEC depletions with scintillations and ROTI over south China

    NASA Astrophysics Data System (ADS)

    Deng, Baichang; Huang, Jiang; Kong, Debao; Xu, Jie; Wan, Dehuan; Lin, Guoguo

    2015-01-01

    This study investigated the temporal and spatial distributions of TEC depletions with scintillations and ROTI over south China. Data were collected from two GPS receiver stations in south China from 2011 to 2012. Our results revealed that maxima of distribution of TEC depletions were observed in vernal and autumnal equinox months. During pre-midnight hours and in the region of northern crest of equatorial anomaly, deep TEC depletions (approximately 20 TECU) well coincided with the occurrences of intense scintillations (S4 > 0.5) and large ROTI (approximately 2). But around midnight hours or at the outer edge of northern anomaly crest, only shallow TEC depletions (<10 TECU) were recorded with small ROTI (<1) and no scintillation. Our results suggest that the temporal and spatial variability of TEC depletions during equinox months over south China could be considered as a representative of the evolution phase of the plasma bubbles during post-sunset hours.

  11. TEC variations and IRI-2012 performance at equatorial latitudes over Africa during low solar activity

    NASA Astrophysics Data System (ADS)

    Olawepo, A. O.; Adeniyi, J. O.; Oluwadare, E. J.

    2017-04-01

    GPS-TEC data obtained from Asmara (Geomag. Lat 11.42N, Long. 113.31E), Bahir Dar (Geomag. Lat. 8.04N, Long. 111.19E), and Eldoret (Geomag. Lat. 2.71S, Long. 107.30E), three stations in the equatorial region of the African sector, have been used to study the variations in the vertical total electron content (vTEC) and to investigate the performance of the three topside options in the IRI-2012 during the year 2010, a year of low solar activity (Rz = 16). The results revealed that TEC exhibits diurnal, seasonal and latitudinal characteristics. TEC values over the three stations are minimum during the sunrise hour of 0600LT and maximum around 1200LT-1500LT irrespective of the season. Daytime peak is followed by a steady decay in TEC which continues into the mid-night. TEC values during the equinoxes are observed to be greater than those of the solstices. TEC values during March equinox are higher than those of September equinox for the stations in the northern hemisphere. June solstices exhibit the lowest values for all the stations. For the station in the southern hemisphere, TEC values during December solstice values are slightly higher than those of September equinox. The three topside options in IRI-2012 reproduce the morphology of TEC at the three stations with correlation coefficient ranging between 0.97 and 0.99. The performance of NeQuick option at predicting TEC at these stations is observed to be the best compared to the other two options irrespective of the season.

  12. Ionospheric and magnetic signatures of a high speed solar wind in low latitudes on 13 October 2012

    NASA Astrophysics Data System (ADS)

    Migoya-Orue, Y. O.; Azzouzi, I.; Coisson, P.; Amory Mazaudier, C.; Fleury, R.; Radicella, S. M.

    2016-03-01

    This paper presents the impact of a fast solar wind on the ionosphere, in low latitudes, on 13 October 2012. On that day, the high speed solar wind reached the Earth around 16:00UT, during the recovery phase of a geomagnetic storm which started around 00:00UT. The solar wind speed was determined to be 580km/s, on the same day, around 17:00UT. Its impact was observed in low and equatorial latitudes, in Africa and in Eastern South America, on the F layer and on the geomagnetic field variations. Through the analysis of magnetic indices, ionosonde characteristics and the horizontal component of the geomagnetic field, we found that the 13 October 2012 event exhibited a local impact, affecting the observatories situated in a longitude sector between 315°E and 45°E. Particularly, the F layer in Africa (observed by the ionosonde at Ascension Island) did not present any lift, and there was a delay for approximately two hours of the ascent of the F layer in America (the ionosonde at Fortaleza). In this case, there was an evident inhibition on the development of spread F at the time of the Pre Reversal Enhancement (PRE) in Africa and Eastern America, while the ionograms of the days before and after presented clear spread F traces. The disturbances of the ionospheric equivalent electric current (Diono) deduced from the variations of the geomagnetic field at M'Bour near Dakar (Africa) and at Kourou (Eastern America) exhibited on the dayside, an anti Sq current which is signature of the influence of the Disturbance Dynamo Electric Field (DDEF).

  13. Nighttime D-region electron density measurements from ELF-VLF tweek radio atmospherics recorded at low latitudes

    NASA Astrophysics Data System (ADS)

    Maurya, A. K.; Veenadhari, B.; Singh, R.; Kumar, S.; Cohen, M.

    2012-12-01

    Dispersive atmospherics (tweeks) observed during 2010 simultaneously at two low latitude stations, Allahabad (geomagnetic lat., 16.79° N) and Nainital (geomagnetic lat. 20.48° N), have been utilized to estimate the nighttime D-region electron density at the ionospheric reflection height under the local nighttime propagation (21:00 - 02:00 LT or 15:30 - 20:30 UT). The analysis of simultaneously recorded tweeks at both the stations on five international quiet days during one month each from summer (June), winter (January) and equinox (March) seasons shows that the D-region electron density varies 21.5-24.5 cm-3 over the ionospheric reflection height of 85-95 km. The average values of Wait lower ionospheric parameters: ionospheric reference height h‧ and sharpness factor β are almost same during winter (86.1-85.9 km, 0.51-0.52 km-1) and equinox (85.6-85.7 km, 0.54 km-1) seasons. The values of h‧ and β during summer season are about 83.5 km and 0.60 km-1 at both stations. Overall, equivalent electron density profile obtained using tweek method shows lower values of electron density by about 5-60% than those obtained using IRI-2007 model and lower/higher by 2-68% than those obtained using rocket technique. The electron density estimated using all three techniques (tweek, IRI 2007, Rocket) is consistent in the altitude range of 82-98 km. The estimated geographic locations of causative lightnings of tweeks were matched with the locations and times of lightnings detected by the World-Wide Lightning Location Network (WWLLN). The WWLLN detected about 27.5% of causative lightnings of tweeks simultaneously observed at both the stations.

  14. Auroral activity associated with Kelvin-Helmholtz instability at the inner edge of the low-latitude boundary layer

    NASA Technical Reports Server (NTRS)

    Farrugia, C. J.; Sandholt, P. E.; Burlaga, L. F.

    1994-01-01

    Auroral activity occurred in the late afternoon sector (approx. 16 MLT) in the northern hemisphere during the passage at Earth of an interplanetary magnetic cloud on January 14, 1988. The auroral activity consisted of a very dynamic display which was preceded and followed by quiet auroral displays. During the quiet displays, discrete rayed arcs aligned along the geomagnetic L shells were observed. In the active stage, rapidly evolving spiral forms centered on magnetic zenith were evident. The activity persisted for many minutes and was characterized by the absence of directed motion. They were strongly suggestive of intense filaments of upward field-aligned currents embedded in the large-scale region 1 current system. Distortions of the flux ropes as they connect from the equatorial magnetosphere to the ionosphere were witnessed. We assess as possible generating mechanisms three nonlocal sources known to be associated with field-aligned currents. Of these, partial compressions of the magnetosphere due to variations of solar wind dynamic pressure seem an unlikely source. The possibility that the auroral forms are due to reconnection is investigated but is excluded because the active aurora were observed on the closed field line region just equatorward of the convection reversal boundary. To support this conclusion further, we apply recent results on the mapping of ionospheric regions to the equatorial plane based on the Tsyganenko 1989 model (Kaufmann et al., 1993). We find that for comparable magnetic activity the aurora map to the equatorial plane at X(sub GSM) = approx. 3 R(sub E) and approx. 2 R(sub E) inward of the magnetopause, that is, the inner edge of the boundary layer close to dusk. Since the auroral forms are manifestly associated with magnetic field shear, a vortical motion at the equatorial end of the flux rope is indicated, making the Kelvin-Helmholtz instability acting at the inner edge of the low-latitude boundary layer the most probable generating

  15. Investigation of Sbas L1/L5 Signals and Their Application to the Ionospheric TEC Studies

    NASA Astrophysics Data System (ADS)

    Padokhin, A. M.; Kunitsyn, V.; Andreeva, E. S.; Nesterov, I. A.; Kurbatov, G. A.

    2013-12-01

    With the development of SBAS systems the dual frequency L1/L5 observations from a number of geostationary satellites are now available. It provides the possibility to retrieve ionospheric TEC from these observations using the same approach as for dual frequency GPS/GLONASS observations. In this work we study the properties of L1/L5 signals of American WAAS and Indian GAGAN geostationary satellites observed with geodetic GNSS receivers located at equatorial and mid-latitudes and estimate corresponding TEC and errors of such estimations. Along with the advantages of geostationary TEC observations, such as almost motionless underionospheric point, there are points that should be taken into account when analyzing geostationary TEC data, including larger amount of plasmaspheric electron content in geostationary TEC compared to GNSS observations, and very low elevation angles of geostationary satellites already at midlatitudes, so the spatial gradients of electron density should be considered. We present long term datasets of geostationary TEC variations during current Solar cycle in various heliogeophysical conditions at equatorial and midlatitude stations and their comparison with the data of nearest ionosondes and high-orbital radiotomography, as well as spectral and wavelet analysis of geostationary TEC variations data, providing typical periods of observed TEC variations at different time scales (e.g. diurnal variations, disturbances associated with sunset and sunrise terminator etc.). We also study the possibility to include the geostationary TEC data to the ionospheric tomography procedures. The authors acknowledge the support of the Russian Foundation for Basic Research (12-05-33065, 12-05-31231, 13-05-01122, 13-05-10080, 14-05-31445), grant of the President of Russian Federation MK-2544.2012.5 and Lomonosov Moscow State University Program of Development.

  16. Atmospheric density remote sensing of mesosphere and thermosphere to be used for spacecraft design by adopting VHF radar and HF Doppler sounder at low latitude west Pacific site during winter time

    NASA Technical Reports Server (NTRS)

    Hung, R. J.; Tsao, Y. D.; Johnson, D. L.; Chen, A. J.; Lee, C. C.

    1989-01-01

    Simultaneous observations of VHF radar and HF Doppler array systems located at Chung Li (Taiwan) are used to observe three-dimensional wind speeds and gravity waves. The density perturbations are determined at different altitudes of the mesosphere and thermosphere during weak convective motions of the cold front in the winter. The present observations are believed to be valuable for space projects dealing with the low-latitude atmosphere.

  17. Investigation of the morphology and Wait's parameter variations of the low-latitude D region ionosphere using the multiple harmonics of tweeks

    NASA Astrophysics Data System (ADS)

    Tan, Le Minh

    2016-06-01

    Recording the tweeks with a maximum up to eight harmonics using the receiver installed at Tay Nguyen University (12.65° N, 108.02° E) during 2013-2014, we investigated the morphology of the nighttime D-region ionosphere. Tweeks were recorded on 5 quiet nights per month. The results show that the mean reflection height in 2014 (Rz = 79.3) is lower by 3.3 km than that in 2013 (Rz = 64.9). The reflection height at low latitudes is higher than that at high latitudes. The mean reference height h‧ in 2013 is higher about 0.9 km than that in 2014 and the mean sharpness factor β in 2013 is higher by 0.07 km-1 than that in 2014. The short-term variation of reflection heights for tweeks with harmonics m = 1-3 and sunspot number have the negative correlation coefficients. However, the correlations between them are not clear. On some nights, from 19:00-21:00 LT, the reflection height temporal variability shows a moderate to strong negative correlation with the tweek occurrence. This suggests that the reflection height variation may be caused by QE fields generated by lightning discharges. The variations of tweek reflection heights observed during 2013-2014, at low latitudes could be significantly caused by the ionization effect by Lyman- α and Lyman- β coming from geocorona, variation of neutral density, particle precipitations, and by direct energy coupling between lightning and lower ionosphere.

  18. Ionospheric TEC Estimations with the Signals of Various Geostationary Navigational Satellites

    NASA Astrophysics Data System (ADS)

    Kurbatov, G. A.; Padokhin, A. M.; Kunitsyn, V.; Yasyukevich, Y.

    2015-12-01

    The development of GNSS and SBAS systems provides the possibility to retrieve ionospheric TEC from the dual frequency observations from a number of geostationary satellites using the same approach as for dual frequency GPS/GLONASS observations. In this connection, the quality of geostationary data, first of all the level of noise in TEC estimations is of great interest and importance. In this work we present the results of the comparison of the noise patterns in TEC estimations using signals of geostationary satellites of augumentation systems - indian GAGAN, european EGNOS and american WAAS, as well as the signals of chinees COMPASS/Beidou navigational system. We show that among above mentioned systems geostationary COMPASS/Beidou satellites provide best noise level in TEC estimations (RMS~0.1TECU), which corresponds to those of GPS/GLONASS, while GAGAN and WAAS TEC RMS could reach up to 1.5 TECU with typical values of 0.25-0.5 TECU which is up to one order greater than for common GPS/GLONASS observations. EGNOS TEC estimations being even more noisy (TEC RMS up to 10TECU) than WAAS and GAGAN ones at present time are not suitable for ionospheric studies. We also present geostationary TEC response to increasing solar X-Ray and EUV ionizing radiation during several recent X-class flares. Good correlation was found between TEC and EUV flux for the stations at the sunlit hemisphere. We also present geostationary TEC response to geomagnetic field variations during strong and moderate geomagnetic storms (including G4 St. Patricks Day Storm of 2015) showing examples of both positive and negative TEC anomalies of order of tens of TECU during main storm phase. Our results show the capability of geostationary GNSS and SBAS observations for continuous monitoring of ionospheric TEC. Intensively growing networks of dedicated receivers (for example MGEX network) and increasing number of dual-frequency geostationary satellites in SBAS and GNSS constellations potentially make it a

  19. The 630 nm MIG and the vertical neutral wind in the low latitude nighttime thermosphere

    SciTech Connect

    Herrero, F.A. ); Meriwether, J.W. Jr. )

    1994-01-15

    It is shown that large negative divergences (gradients) in the horizontal neutral wind in the equatorial thermosphere can support downward neutral winds in excess of 20 m/s. With attention to the meridional and vertical winds only, the pressure tendency equation is used to derive the expression U[sub zO] [approx] [l angle][partial derivative]U[sub y]/[partial derivative]y[r angle]H for the vertical wind U[sub zO] at the reference altitude for the pressure tendency equation; H is the atmospheric density scale height, and [l angle][partial derivative]U[sub y]/[partial derivative]y[r angle] is the meridional wind gradient. The velocity gradient associated with the Meridional Intensity Gradient (MIG) of the O([sup 1]D) emission (630 nm) at low latitudes is used to estimate the vertical neutral wind in the MIG region. Velocity gradients derived from MIG data are about 0.5 (m/s)/km or more, indicating that the MIG region may contain downward neutral winds in excess of 20 m/s. Though direct measurements of the vertical wind are scarce, Fabry-Perot interferometer data of the equatorial F-region above Natal, Brazil, showed downward winds of 30 m/s occurring during a strong meridional wind convergence in 1982. In-situ measurements with the WATS instrument on the DE-2 satellite also show large vertical neutral winds in the equatorial region. 12 refs., 21 figs.

  20. Day-to-day variability of geomagnetic hourly amplitudes at low latitudes

    NASA Astrophysics Data System (ADS)

    Okeke, F. N.; Agodi Onwumechili, C.; Rabiu, Babatunde A.

    1998-08-01

    A study of the variability of the amplitude of Sq at a fixed hour from one day to the next at nine stations from the dip equator to about 22° north of it has produced interesting results. The amplitude and sign of the variability change virtually randomly, making the mean practically zero. The variability occurs at all hours of the day. Its magnitudes in the components D, H and Z have the same diurnal variation, which peaks in the noon period like Sq(H) in low latitudes, and a weak seasonal variation that peaks at the June solstice (local summer). It is demonstrated that changes in the current intensities of the equatorial electrojet (EEJ) and the worldwide part of the Sq (WSq) current layers have contrasting phases and can sometimes be in antiphase. Indeed, the changes are mostly independent. Inclusion of the magnetic element D revealed that the EEJ current system has not only an east-west but also a north-south component. The study shows that the meridional component of the EEJ current intensity evidenced at the Kodaikanal and Annamalainagar stations is an integral part of the zonal component at Trivandrum. This confirms the results of Rastogi (1996) and validates those of Onwumechili (1997). The results suggest that ionospheric conductivity mainly controls the magnitude, while the electric field and ultimately winds mainly control the phase and randomness of the day-to-day variability of the hourly amplitudes of Sq. The random component is attributed to local and/or regional atmospheric winds, probably of gravity wave origin.

  1. Isotopic evolution of saline lakes in the low-latitude and polar regions

    SciTech Connect

    Horita, Juske

    2009-01-01

    Isotopic fractionations associated with two primary processes (evaporation and freezing of water) are discussed, which are responsible for the formation and evolution of saline lakes in deserts from both low-latitude and the Polar regions. In an evaporative system, atmospheric parameters (humidity and isotopic composition of water vapor) have strong influence on the isotopic behavior of saline lakes, and in a freezing system, salinity build-up largely controls the extent of freezing and associated isotope fractionation. In both systems, salinity has a direct impact on the isotopic evolution of saline lakes. It is proposed that a steady-state terminal lake model with short-term hydrologic and environmental perturbations can serve as a useful framework for investigating both evaporative and freezing processes of perennial saline lakes. Through re-assessment of own work and literature data for saline lakes, it was demonstrated that effective uses of the isotope activity compositions of brines and salinity-chemistry data could reveal dynamic changes and evolution in the isotopic compositions of saline lakes in response to hydrologic and environmental changes. The residence time of isotopic water molecules in lakes determines the nature of responses in the isotopic compositions following perturbations in the water and isotope balances (e.g., dilution by inflow, water deficit by increased evaporation, and/ or reduction in inflow). The isotopic profiles of some saline lakes from the Polar regions show that they switched the two contrasting modes of operation between evaporative and freezing systems, in response to climate and hydrological changes in the past.

  2. Isotopic Evolution of Saline Lakes in the Low-Latitude and Polar Regions

    SciTech Connect

    Horita, Juske

    2009-01-01

    Isotopic fractionations associated with two primary processes (evaporation and freezing of water) are discussed, which are responsible for the formation and evolution of saline lakes in deserts from both low-latitude and the Polar regions. In an evaporative system, atmospheric parameters (humidity and isotopic composition of water vapor) have strong influence on the isotopic behavior of saline lakes, and in a freezing system, salinity build-up largely controls the extent of freezing and associated isotope fractionation. In both systems, salinity has a direct impact on the isotopic evolution of saline lakes. It is proposed that a steady-state 'terminal lake' model with short-term hydrologic and environmental perturbations can serve as a useful framework for investigating both evaporative and freezing processes of perennial saline lakes. Through re-assessment of own work and literature data for saline lakes, it was demonstrated that effective uses of the isotope activity compositions of brines and salinity-chemistry data could reveal dynamic changes and evolution in the isotopic compositions of saline lakes in response to hydrologic and environmental changes. The residence time of isotopic water molecules in lakes determines the nature of responses in the isotopic compositions following perturbations in the water and isotope balances (e.g., dilution by inflow, water deficit by increased evaporation, and/or reduction in inflow). The isotopic profiles of some saline lakes from the Polar regions show that they switched the two contrasting modes of operation between evaporative and freezing systems, in response to climate and hydrological changes in the past.

  3. Interannual modulation of the Pacific Decadal Oscillation (PDO) on the low-latitude western North Pacific

    NASA Astrophysics Data System (ADS)

    Wu, Chau-Ron

    2013-03-01

    To investigate the interannual variability in the northwestern Pacific, an empirical mode decomposition (EMD) was applied to 17-year Absolute Dynamic Topography (ADT) data west of Luzon Island, the Philippines. The mean sea surface height in this area is an appropriate index for the Kuroshio intrusion into the South China Sea (SCS). Significant interannual fluctuations were extracted by the EMD. The interannual variability was strongly correlated with the Pacific Decadal Oscillation (PDO) index, but not the El Niño-Southern Oscillation (ENSO). This indicated the potential impact of the PDO on the circulation in the area. In the warm phase of the PDO (positive index), a southerly anomalous wind off the Philippines causes a northward shift of the North Equatorial Current Bifurcation Latitude (NECBL). This leads to a weakened Kuroshio off Luzon, favoring Kuroshio intrusion into the SCS. The northward migration of the NECBL also results in a weakened Kuroshio off southeast Taiwan and a larger Kuroshio transport off northeast Taiwan. The abundant westward propagating eddies impinging on the Kuroshio in the Subtropical Countercurrent region increases this transport. Although the ENSO has little effect on monsoonal winds during the warm PDO phase, it has a strong impact on the monsoon and meridional migration of the NECBL during the cold phase of the PDO. Therefore, NECBL variations only show a close correspondence with the ENSO during the cold PDO phase. Because the influence of the ENSO is not stationary, the impact of the PDO should be taken into account when examining interannual variability in the low-latitude western North Pacific.

  4. Ionospheric effects at low latitudes during the March 22, 1979, geomagnetic storm

    SciTech Connect

    Fesen, C.G. ); Crowley, G.; Roble, R.G. )

    1989-05-01

    This paper investigates the response of the equatorial ionosphere to the neutral atmosphere perturbations produced by the magnetic storm of March 22, 1979. A numerical model of the equatorial ionosphere is used to calculate the maximum electron densities and F layer heights associated with a storm-perturbed neutral atmosphere and circulation model. Possible electric field perturbations due to the storm are ignored. The neutral atmosphere and dynamics are simulated by the National Center for Atmospheric Research thermospheric general circulation model (TGCM) for the storm day of March 22, 1979, and the preceding quiet day. The most striking feature of the TGCM storm day simulations is the presence of waves in the neutral composition, wind, and temperature fields which propagate from high latitudes to the equator. The TGCM-calculated fields for the two days are input into a low-latitude ionosphere model which calculates n{sub max} and h{sub max} between {plus minus}20{degree}dip latitude. The calculated nighttime 6300-{angstrom} airglow emission and the altitude profiles of electron concentration are also highly perturbed by the storm. Examination of ionosonde data for March 22, 1979, shows remarkable agreement between the measured and predicted changes in f{sub 0}F{sub 2} and h{sub max} near 140{degree}W. Poorer agreement near 70{degree}W may be due to the neglect of electric field perturbations and the approximations inherent in the modeling. The results of these simulations indicate that the major factor influencing the storm time ionospheric behavior in this case is the neutral wind.

  5. Low-latitude ionosphere dynamics as deduced from meridional ionosonde chain: Ionospheric ceiling

    NASA Astrophysics Data System (ADS)

    Maruyama, Takashi; Uemoto, Junpei; Tsugawa, Takuya; Supnithi, Pornchai; Ishii, Mamoru; Komolmis, Tharadol

    Interest in the equatorial anomaly in the ionosphere has been focused mostly on f_oF_2, and not much attention was paid to h_mF_2 except for the time rate of change of it in connection with the vertical plasma drift velocity. There have been few climatological studies on h_mF_2 variations associated with development of the equatorial anomaly. In this paper, we revisit the equatorial anomaly in terms of height variations. For this purpose, we analyzed scaled ionogram parameters from three stations located along the magnetic meridian that is a primary component of Southeast Asia low-latitude ionospheric network (SEALION); one at the magnetic equator and the others at conjugate off-equatorial latitudes near 10 degrees magnetic latitude. The daytime h_mF_2 was investigated for each season during the solar minimum period, 2006-2007 and 2009. The peak height increased for approximately 3 hr after sunrise at all locations, as expected from the daytime upward E×B drift. The apparent upward drift ceased before noon at the magnetic equator, while the layer continued to increase at the off-equatorial latitudes, reaching altitudes higher than the equatorial height around noon. The noon time restricted layer height at the magnetic equator did not depend much on the season, while the maximum peak height at the off-equatorial latitudes largely varied with season. The daytime specific limiting height of the equatorial ionosphere was termed ionospheric ceiling. Numerical modeling using the SAMI2 code reproduced the features of the ionospheric ceiling quite well. Dynamic parameters provided by the SAMI2 modeling were investigated and it was shown that the ionospheric ceiling is another aspect of the fountain effect, in which increased diffusion of plasma at higher altitudes has a leading role.

  6. A comprehensive analysis of the occurrence and characteristics of midperiod ULF waves at low latitude

    NASA Astrophysics Data System (ADS)

    Villante, Umberto; Tiberi, Pietro

    2015-03-01

    A long-term analysis (1985-2012) allowed to investigate the occurrence and characteristics of clearly defined ULF events (f ≈ 10-100 mHz) detected during daytime at low-latitude (L'Aquila, Italy; L ≈ 1.6) during quiet and moderately perturbed magnetospheric conditions. Selected events (≈30,000 on each component) typically have much greater amplitude on the H component with respect to D. They often reveal two prominent spectral peaks, one in band A (f < ≈ 45 mHz), mostly related to the transmission of upstream waves, and the other one in band B (f > ≈ 55 mHz), including resonances of local field lines and higher-frequency upstream waves. The occurrence of resonant phenomena on the D component is also clearly evidenced. The solar wind speed is confirmed as the key element for the manifestation of events; by contrast, their appearance is inhibited during extremely low solar wind densities. The events mostly manifest between dawn and early afternoon, with highest occurrence at ≈08:00-10:00 LT, as might be expected for the usual distribution of the interplanetary magnetic field orientation determining more or less favorable conditions for a foreshock region on the morning flank of the bow shock. No evidence is found for a favorite occurrence for low cone angles (θXB < ≈ 10°) rather, they preferentially manifest for θXB ≈ 25°-40°. The polarization pattern, much more definite in the afternoon, is consistent with the expected antisunward propagation; in the morning sector, it also suggests the possible occurrence of sunward propagating modes, mostly at f < ≈ 22 mHz. The tilt angle of the major axis of the polarization ellipses during daytime hours is oriented in the NW/SE quadrant and experiences remarkable changes at sunrise and sunset; it also shows a seasonal modulation with larger angles in the winter.

  7. Holocene biomass burning recorded in polar and low-latitude ice cores

    NASA Astrophysics Data System (ADS)

    Kehrwald, N. M.; Zennaro, P.; Zangrando, R.; Gabrielli, P.; Thompson, L. G.; Gambaro, A.; Barbante, C.

    2011-12-01

    Ice cores contain specific molecular markers including levoglucosan (1,6-anhydro-β-D-glucopyranose) and other pyrochemical evidence that provides much-needed information on the role of fire in regions with no existing data of past fire activity. Levoglucosan is a cellulose combustion product produced at burning temperatures of 300°C or greater. We first trace fire emissions from a boreal forest source in the Canadian Shield through transport and deposition at Summit, Greenland. Atmospheric and surface samples suggest that levoglucosan in snow can record biomass burning events up to 1000s of kilometers away. Levoglucosan does degrade by interacting with hydroxyl radicals in the atmosphere, but it is emitted in large quantities, allowing the use as a biomass burning tracer. These quantified atmospheric biomass burning emissions and associated parallel oxalate and levoglucosan peaks in snow pit samples validates levoglucosan as a proxy for past biomass burning in snow records and by extension in ice cores. The temporal and spatial resolution of chemical markers in ice cores matches the core in which they are measured. The longest temporal resolution extends back approximately eight glacial cycles in the EPICA Dome C ice core, but many ice cores provide high-resolution Holocene records. The spatial resolution of chemical markers in ice cores depends on the core location where low-latitude ice cores primarily reflect regional climate parameters, and polar ice cores integrate hemispheric signals. Here, we compare levoglucosan flux measured during the late Holocene in the Kilimanjaro (3°04.6'S; 37°21.2'E, 5893 masl) and NEEM, Greenland (77°27' N; 51°3'W, 2454 masl) ice cores. We contrast the Holocene results with levoglucosan flux across the past 600,000 years in the EPICA Dome C (75°06'S, 123°21'E, 3233 masl) ice core.

  8. The leading role of atomic oxygen in the collocation of elves and hydroxyl nightglow in the low-latitude mesosphere

    NASA Astrophysics Data System (ADS)

    Wu, Y. J.; Williams, E.; Chang, S. C.; Chou, J. K.; Hsu, R. R.; Friedrich, M.; Kuo, C. L.; Chen, A. B.; Peng, K. M.; Su, H. T.; Frey, H. U.; Mende, S. B.; Takahashi, Y.; Lee, L. C.

    2017-05-01

    The elve is the dominant type of transient luminous event (TLE) at the base of the E region ionosphere. The hydroxyl nightglow (OH* nightglow hereafter) has been reported at a similar altitude. Statistical studies show that the mean heights of elve and OH* nightglow are 87 km and 87.5 km, respectively, and that 91% of the 291 limb elves are located within ±5 km of the altitude of the brightest OH* nightglow emission and both elves and OH* nightglow show the semiannual oscillation (SAO) at low latitude. The physical reasons for the collocation of elves and OH* nightglow are the main focus of attention. A model for elve emission with the environmental-adapted electron density profile is tested with three density profiles of atomic oxygen (O): the original profile from the NRLMSISE-00 model and two other profiles which are shifted 5 km upward/downward from the original. For higher altitudes of the given O density profile, the peak altitudes of the elve emission layers are also increased. This result reveals the leading role of atomic oxygen in the collocation. Furthermore, the altitude variation of elves is compared with the VLF reflection height observed by DEMETER. The latter height not only shows the SAO similarity to the elve/OH* heights but also exhibits a difference between land and ocean. The possible relationship between VLF reflection height and elve/OH* height is also discussed.Plain Language SummaryThe collocation of elves and OH Meinel band nightglow implies that OH* nightglow is the visible upper boundary of the VLF waveguide.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://eric.ed.gov/?q=Science+AND+Definition+AND+Team&pg=5&id=ED314615','ERIC'); return false;" href="https://eric.ed.gov/?q=Science+AND+Definition+AND+Team&pg=5&id=ED314615"><span>V-<span class="hlt">TECS</span> Guide for Computer Operator.</span></a></p> <p><a target="_blank" href="http://www.eric.ed.gov/ERICWebPortal/search/extended.jsp?_pageLabel=advanced">ERIC Educational Resources Information Center</a></p> <p>South Carolina State Dept. of Education, Columbia. Office of Vocational Education.</p> <p></p> <p>This V-<span class="hlt">TECS</span> (Vocational-Technical Consortium of States) Guide is an extension or continuation of the V-<span class="hlt">TECS</span> catalog for the occupation of computer operator. The guide is designed to help South Carolina teachers to promote the art of learning while teaching subject matter. The guide addresses the three domains of learning: psychomotor, cognitive,…</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://files.eric.ed.gov/fulltext/ED464059.pdf','ERIC'); return false;" href="http://files.eric.ed.gov/fulltext/ED464059.pdf"><span>The Az<span class="hlt">TEC</span> Mathematics Project (AMP).</span></a></p> <p><a target="_blank" href="http://www.eric.ed.gov/ERICWebPortal/search/extended.jsp?_pageLabel=advanced">ERIC Educational Resources Information Center</a></p> <p>Johnson, Gae R.</p> <p></p> <p>The Az<span class="hlt">TEC</span> Mathematics Project (AMP) is a statewide partnership among Arizona's Regents universities and state community colleges, partner school districts, and economic communities. Az<span class="hlt">Tec</span> is committed to preparing highly qualified K-12 mathematics and science teachers. AMP targeted Native American teachers and teachers of Native American students…</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014JASTP.108...77P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014JASTP.108...77P"><span>Threshold magnitude for Ionospheric <span class="hlt">TEC</span> response to earthquakes</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Perevalova, N. P.; Sankov, V. A.; Astafyeva, E. I.; Zhupityaeva, A. S.</p> <p>2014-02-01</p> <p>We have analyzed ionospheric response to earthquakes with magnitudes of 4.1-8.8 which occurred under quiet geomagnetic conditions in different regions of the world (the Baikal region, Kuril Islands, Japan, Greece, Indonesia, China, New Zealand, Salvador, and Chile). This investigation relied on measurements of total electron content (<span class="hlt">TEC</span>) variations made by ground-based dual-frequency GPS receivers. To perform the analysis, we selected earthquakes with permanent GPS stations installed close by. Data processing has revealed that after 4.1-6.3-magnitude earthquakes wave disturbances in <span class="hlt">TEC</span> variations are undetectable. We have thoroughly analyzed publications over the period of 1965-2013 which reported on registration of wave TIDs after earthquakes. This analysis demonstrated that the magnitude of the earthquakes having a wave response in the ionosphere was no less than 6.5. Based on our results and on the data from other researchers, we can conclude that there is a threshold magnitude (near 6.5) below which there are no pronounced earthquake-induced wave <span class="hlt">TEC</span> disturbances. The probability of detection of post-earthquake TIDs with a magnitude close to the threshold depends strongly on geophysical conditions. In addition, reliable identification of the source of such TIDs generally requires many GPS stations in an earthquake zone. At low magnitudes, seismic energy is likely to be insufficient to generate waves in the neutral atmosphere which are able to induce <span class="hlt">TEC</span> disturbances <span class="hlt">observable</span> at the level of background fluctuations.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2006cosp...36.2408W','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2006cosp...36.2408W"><span>Modeling and mapping the ionospheric <span class="hlt">TEC</span> based on eigen mode analyses</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Wan, W.; Ning, B.; Liu, L.; Mao, T.; Xiong, B.</p> <p></p> <p>Eigen mode analyses such as Empirical Orthogonal Function EOF and Canonical Correlation Analysis CCA have been applied to the historical data of the ionospheric total electron content <span class="hlt">TEC</span> <span class="hlt">observed</span> from a GPS network in Asia-Australia Sector The obtained results i e the empirical eigen functions Ek Lat LT which depicts both the latitudinal Lat and local time LT variation of <span class="hlt">TEC</span> and coefficient Ak day which represent the long term auunal solar cycle variation of <span class="hlt">TEC</span> are very useful in the investigation of different scale ionospheric variations The present work is devoted to the further use of the eigen functions Ek Lat LT and the corresponding coefficient Ak day First the eigen functions Ek Lat LT were used as base functions to expand <span class="hlt">TEC</span> This expanding provide a new technique to map the distribution <span class="hlt">TEC</span> Lat LT by fitting the model coefficients from GPS <span class="hlt">observation</span> A system was then set up for nowcasting the ionospheric <span class="hlt">TEC</span> over China based on real time <span class="hlt">observation</span> of 4 GPS stations along 120E meridian chain Secondly the coefficients Ak day were used to construct an empirical <span class="hlt">TEC</span> model in Asia-Australia Sector For this purpose we fit each coefficients Ak day with nonlinear functions of solar activity index F107 and the day of a year The obtained empirical <span class="hlt">TEC</span> model is available in prediction of precious <span class="hlt">TEC</span> distribution It is conclude that the application of eigen mode analyses in the present work together with the GPS network <span class="hlt">observation</span> is valuable in the monitoring the weather and predicting the climatology in the ionosphere</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014JGRA..119.6954C','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014JGRA..119.6954C"><span>Propagation of gravity waves and spread F in the <span class="hlt">low-latitude</span> ionosphere over Tucumán, Argentina, by continuous Doppler sounding: First results</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Chum, J.; Bonomi, F. A. M.; Fišer, J.; Cabrera, M. A.; Ezquer, R. G.; Burešová, D.; Laštovička, J.; Baše, J.; Hruška, F.; Molina, M. G.; Ise, J. E.; Cangemi, J. I.; Å indelářová, T.</p> <p>2014-08-01</p> <p>Results of systematic analysis of propagation directions and horizontal velocities of gravity waves (GWs) and spread F structures in <span class="hlt">low-latitude</span> ionosphere (magnetic inclination ~27°) in Tucumán region, Argentina, are presented. Measurements were carried out by multipoint continuous Doppler system during 1 year from December 2012 to November 2013. It was found that meridian propagation of GWs dominated and that southward propagation prevailed in the local summer. Oblique spread structures <span class="hlt">observed</span> in Doppler shift spectrograms and associated with spread F propagated roughly eastward at velocities from ~70 to ~180 m/s and were <span class="hlt">observed</span> at night from ~ September to ~ March. The velocities were computed for 182 events and the azimuths for 64 events. Continuous Doppler sounding makes it possible to analyze more events compared to optical <span class="hlt">observations</span> often used for propagation studies since the measurements do not depend on weather.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014cosp...40E2584P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014cosp...40E2584P"><span><span class="hlt">TEC</span> disturbances during major Sudden Stratospheric Warmings in the mid-latitude ionosphere.</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Polyakova, Anna; Voeykov, Sergey; Chernigovskaya, Marina; Perevalova, Natalia</p> <p></p> <p>Using total electron content (<span class="hlt">TEC</span>) global ionospheric maps, dual-frequency GPS receivers <span class="hlt">TEC</span> data and MLS (Microwave Limb Sounder, EOS Aura) atmospheric temperature data the ionospheric disturbances during the strong sudden stratospheric warmings (SSWs) of 2008/2009 and 2012/2013 winters are investigated in Russia's Asia region. It is established that during the SSW maximum the midday <span class="hlt">TEC</span> decrease and the night/morning <span class="hlt">TEC</span> increase compared to quiet days are <span class="hlt">observed</span> in the mid-latitude ionosphere. As a result it caused the decrease of the diurnal <span class="hlt">TEC</span> variations amplitude of about two times in comparison with the undisturbed level. The analysis of <span class="hlt">TEC</span> deviations from the background level during the SSWs has shown that deviations dynamics vary depending on the <span class="hlt">observation</span> point position. Negative deviations of <span class="hlt">TEC</span> are registered in the ionosphere above the region of maximum stratosphere heating (the region of the stratospheric circulation change) as well as above the anticyclone. On the contrary, <span class="hlt">TEC</span> values increase compared to the quiet day's values above the stratosphere cyclone. It is shown that during maximum phase of a warming, and within several days after it the amplification of wave <span class="hlt">TEC</span> variations intensity with periods of up to 60 min is registered in ionosphere. The indicated effects may be attributed to the vertical transfer of molecular gas from a stratospheric heating region to the thermosphere as well as to the increase in activity of planetary and gravity waves which is usually <span class="hlt">observed</span> during strong SSWs. The study is supported by the RF President Grant of Public Support for RF Leading Scientific Schools (NSh-2942.2014.5), the RF President Grant No. MK-3771.2012.5 and RFBR Grant No. 12-05-00865_а.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013AnGeo..31..153P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013AnGeo..31..153P"><span><span class="hlt">Low-latitude</span> equinoctial spread-F occurrence at different longitude sectors under low solar activity</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Pezzopane, M.; Zuccheretti, E.; Abadi, P.; de Abreu, A. J.; de Jesus, R.; Fagundes, P. R.; Supnithi, P.; Rungraengwajiake, S.; Nagatsuma, T.; Tsugawa, T.; Cabrera, M. A.; Ezquer, R. G.</p> <p>2013-02-01</p> <p>We present the results of a comparative study of spread-F signatures over five <span class="hlt">low-latitude</span> sites: Chiangmai (CGM; 18.8° N, 98.9° E, mag. Lat. 8.8° N), Thailand; Tanjungsari (TNJ; 6.9° S, 107.6° E, mag. Lat. 16.9° S), Indonesia; Palmas (PAL; 10.2° S, 311.8° E, mag. Lat. 0.9° S) and São José Dos Campos (SJC; 23.2° S, 314.1° E, mag. Lat. 14.0° S), Brazil; and Tucumán (TUC; 26.9° S, 294.6° E, mag. Lat. 16.8° S), Argentina. The investigation was based on simultaneous ionograms recorded by an FMCW (frequency-modulated continuous-wave) at CGM, an IPS-71 (digital ionosonde from KEL aerospace) at TNJ, a CADI (Canadian Advanced Digital Ionosonde) at PAL and SJC, and an AIS-INGV (Advanced Ionospheric Sounder - Istituto Nazionale di Geofisica e Vulcanologia) at TUC, during the equinoctial periods March-April (R12 = 2.0 and R12 = 2.2) and September-October (R12 = 6.1 and R12 = 7.0) 2009, for very low solar activity. Spread-F signatures were categorized into two types: the range spread-F (RSF) and the frequency spread-F (FSF). The study confirms that the dynamics and the physical processes responsible for these phenomena are actually complicated. In fact, the features that arise from the investigation are different, depending on both the longitude sector and on the hemisphere. For instance, TUC, under the southern crest of the ionospheric equatorial ionization anomaly (EIA), shows a predominance of RSF signatures, while both SJC, under the southern crest of EIA but in a different longitude sector, and CGM, under the northern crest of EIA, show a predominance of FSF signatures. Moreover, the spread-F occurrence over the longitude sector that includes CGM and TNJ is significantly lower than the spread-F occurrence over the longitude sector of PAL, SJC, and TUC.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/19930006385','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/19930006385"><span>Energetic and magnetosheath energy particle signatures of the <span class="hlt">low-latitude</span> boundary layer at low altitudes near noon</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Roeder, J. L.; Lyons, L. R.</p> <p>1992-01-01</p> <p>The <span class="hlt">low-latitude</span> boundary layer (LBL) and its separation from the cusp have previously been identified using <span class="hlt">observations</span> of particle precipitation at magnetosheath energies. Using S3-3 satellite <span class="hlt">observations</span>, we have determined that these identifications can also be made from energetic particle <span class="hlt">observations</span> on polar-orbiting satellites. It is found that the equatorward boundary of the LBL is identifiable as an approximately discontinuous decrease in 33-keV electron fluxes from low to high latitudes. Both the energetic ion and electron fluxes decrease discontinuously at the boundary between the LBL and the cusp or polar cap. A distinct LBL is nearly always identifiable in energetic particle measurements in the 10-14 MLT region when counting rates are statistically significant. The identifications obtained using the energetic particle measurements have been compared to those obtained using criteria developed by Newell and Meng (1988, 1989) for magnetosheath energy particle precipitation. In this way, we have evaluated the accuracy of both techniques and used the energetic particle measurements to supplement the identifications obtained using the Newell and Meng criteria. We propose that the Newell and Meng threshold on ion energy flux can be reduced by a factor of 6. This modification provides identification of the LBL for lower ion intensity levels than has previously been thought possible. Source, acceleration, and scattering processes have also been studied within and in the vicinity of the LBL. <span class="hlt">Observed</span> trapped pitch angle distributions of energetic electrons imply that the LBL is at least partially on closed field lines. Strong scattering of energetic protons is found within and equatorward of the LBL and thus must occur at least partially along closed field lines. Field-aligned electron acceleration by parallel electric fields can be discerned within and poleward of the LBL, but a more detailed analysis is necessary for a statistical study. Conical ion</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/27250195','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/27250195"><span>A simple expression for sound attenuation due to surface duct energy leakage in <span class="hlt">low-latitude</span> oceans.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Duan, Rui; Yang, Kunde; Ma, Yuanliang; Chapman, N Ross</p> <p>2016-05-01</p> <p>This paper presents an expression for the attenuation of sound energy in an ocean surface duct due to energy leakage outside the duct. Dominant parameters determining the attenuation are the sound frequency and the surface duct thickness. The attenuation is found to be exponentially dependent on a scaled frequency that combines the two parameters. Data from experiments in <span class="hlt">low-latitude</span> oceans with three different surface duct thicknesses are used to verify the exponential expression derived for the attenuation.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://ntrs.nasa.gov/search.jsp?R=19950045561&hterms=crossed+fields&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D40%26Ntt%3Dcrossed%2Bfields','NASA-TRS'); return false;" href="https://ntrs.nasa.gov/search.jsp?R=19950045561&hterms=crossed+fields&qs=Ntx%3Dmode%2Bmatchall%26Ntk%3DAll%26N%3D0%26No%3D40%26Ntt%3Dcrossed%2Bfields"><span>The morningside <span class="hlt">low-latitude</span> boundary layer as determined from electric and magnetic field measurements on Geotail</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Mozer, F. S.; Hayakawa, H.; Kokubun, S.; Nakamura, M.; Okada, T.; Yamamoto, T.; Tsuruda, K.</p> <p>1994-01-01</p> <p>On October 17, 1992, the Geotail satellite crossed the dawnside magnetopause approximately 41 times. At the majority of these crossings, the magnetic field and the normal component of the electric field were larger and the plasma density was smaller in a <span class="hlt">low-latitude</span> boundary layer than they were in either the nearby magnetosheath or in the magnetosphere. These results are intepreted in terms of the Kelvin-Helmholtz instability associated with velocity shear at the magnetopause. Consistent with this interpretation, it is shown that the <span class="hlt">low-latitude</span> boundary layer was the region where the flow decreased from its magnetosheath to magnetospheric value. Evidence is presented that the magnetopause was locally oriented within less than 20 deg of its nominal geometry on these crossings and that it moved with an amplitude of 1500-6000 kilometers. The thickness of the <span class="hlt">low-latitude</span> boundary layer averaged 800 kilometers and the electric potential across it averaged 4 kilovolts with a spread of a factor of 2. Thus, the viscous interaction and similar processes are not significant contributors to magnetospheric convection.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016JGRA..121.9111K','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016JGRA..121.9111K"><span>Effects of disturbed electric fields in the <span class="hlt">low-latitude</span> and equatorial ionosphere during the 2015 St. Patrick's Day storm</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Kuai, Jiawei; Liu, Libo; Liu, Jing; Sripathi, S.; Zhao, Biqiang; Chen, Yiding; Le, Huijun; Hu, Lianhuan</p> <p>2016-09-01</p> <p>The 2015 St. Patrick's Day geomagnetic storm with SYM-H value of -233 nT is an extreme space weather event in the current 24th solar cycle. In this work, we investigated the main mechanisms of the profound ionospheric disturbances over equatorial and <span class="hlt">low</span> <span class="hlt">latitudes</span> in the Asian-Australian sector and the American sector during this super storm event. The results reveal that the disturbed electric fields, which comprise penetration electric fields (PEFs) and disturbance dynamo electric fields (DDEFs), play a decisive role in the ionospheric storm effects in <span class="hlt">low</span> <span class="hlt">latitude</span> and equatorial regions. PEFs occur on 17 March in both the American sector and the Asian-Australian sector. The effects of DDEFs are also remarkable in the two longitudinal sectors. Both the DDEFs and PEFs show the notable local time dependence, which causes the sector differences in the characteristics of the disturbed electric fields. This differences would further lead to the sector differences in the <span class="hlt">low-latitude</span> ionospheric response during this storm. The negative storm effects caused by the long-duration DDEFs are intense over the Asian-Australian sector, while the repeated elevations of hmF2 and the equatorial ionization anomaly intensifications caused by the multiple strong PEFs are more distinctive over the American sector. Especially, the storm time F3 layer features are caught on 17 March in the American equatorial region, proving the effects of the multiple strong eastward PEFs.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://hdl.handle.net/2060/19810011392','NASA-TRS'); return false;" href="http://hdl.handle.net/2060/19810011392"><span>Thermionic Energy Conversion (<span class="hlt">TEC</span>) topping thermoelectrics</span></a></p> <p><a target="_blank" href="http://ntrs.nasa.gov/search.jsp">NASA Technical Reports Server (NTRS)</a></p> <p>Morris, J. F.</p> <p>1981-01-01</p> <p>Performance expectations for thermionic and thermoelectric energy conversion systems are reviewed. It is noted that internal radiation effects diminish thermoelectric figures of merit significantly at 1000 K and substantially at 2000 K; the effective thermal conductivity contribution of intrathermoelectric radiative dissipation increases with the third power of temperature. It is argued that a consideration of thermoelectric power generation with high temperature heat sources should include utilization of thermionic energy conversion (<span class="hlt">TEC</span>) topping thermoelectrics. However <span class="hlt">TEC</span> alone or <span class="hlt">TEC</span> topping more efficient conversion systems like steam or gas turbines, combined cycles, or Stirling engines would be more desirable generally.</p> </li> </ol> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_19");'>19</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li class="active"><span>21</span></li> <li><a href="#" onclick='return showDiv("page_22");'>22</a></li> <li><a href="#" onclick='return showDiv("page_23");'>23</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div><!-- col-sm-12 --> </div><!-- row --> </div><!-- page_21 --> <div id="page_22" class="hiddenDiv"> <div class="row"> <div class="col-sm-12"> <div class="pull-right"> <ul class="pagination"> <li><a href="#" onclick='return showDiv("page_1");'>«</a></li> <li><a href="#" onclick='return showDiv("page_20");'>20</a></li> <li><a href="#" onclick='return showDiv("page_21");'>21</a></li> <li class="active"><span>22</span></li> <li><a href="#" onclick='return showDiv("page_23");'>23</a></li> <li><a href="#" onclick='return showDiv("page_24");'>24</a></li> <li><a href="#" onclick='return showDiv("page_25");'>»</a></li> </ul> </div> </div> </div> <div class="row"> <div class="col-sm-12"> <ol class="result-class" start="421"> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015AGUFMEP53A1002S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015AGUFMEP53A1002S"><span>On the Origin of the Crestone Crater: <span class="hlt">Low-Latitude</span> Periglacial Features in San Luis Valley, Colorado</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Schwans, E.; Meng, T. M.; Prudhomme, K.; Morgan, M. L.</p> <p>2015-12-01</p> <p> this claim. Results indicate that the Crestone Crater and nearby similar structures are relic collapsed hydraulic pingos, formed during Pleistocene periglacial activity. This conclusion provides further insight into periglacial landforms at <span class="hlt">low</span> <span class="hlt">latitudes</span> while demonstrating the value of LiDAR analysis of small geologic features on a regional scale.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2017EGUGA..19.5157L','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2017EGUGA..19.5157L"><span>High vs <span class="hlt">low</span> <span class="hlt">latitude</span> sequence of events over the last deglaciation using ice core isotopic proxies and an isotope-enabled climate model</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Landais, Amaelle; Roche, Didier; Prié, Frédéric; Minster, Bénédicte; Masson-Delmotte, Valérie; Vinther, Bo; Capron, Emilie; Popp, Trevor; Rhodes, Rachael</p> <p>2017-04-01</p> <p>. Similarities are also <span class="hlt">observed</span> in other marine and continental records despite dating limitations when compared to ice core records. Finally, we use the iLOVECLIM model to test possible scenarios leading to this two phase sequence of Heinrich 1 in mid-<span class="hlt">low</span> <span class="hlt">latitudes</span> with a muted signature in Greenland ice d18O record.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016JGRA..121.3476J','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016JGRA..121.3476J"><span>Latitudinal variation of the specific local time of postmidnight enhancement peaks in F layer electron density at <span class="hlt">low</span> <span class="hlt">latitudes</span>: A case study</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Jiang, Chunhua; Deng, Chi; Yang, Guobin; Liu, Jing; Zhu, Peng; Yokoyama, Tatsuhiro; Song, Huan; Lan, Ting; Zhou, Chen; Wu, Xiongbin; Zhang, Yuannong; Zhao, Zhengyu; Komolmis, Tharadol; Supnithi, Pornchai; Yatini, Clara Y.</p> <p>2016-04-01</p> <p>Ionospheric nighttime enhancements are manifested in an increase of the electron density at nighttime. This paper studies the latitudinal variation of the specific local time of postmidnight enhancement peaks using ionosondes distributed at <span class="hlt">low</span> <span class="hlt">latitudes</span>. To obtain the parameters of the ionosphere, we manually extracted ionograms recorded by ionosondes. Cases show that there are significant latitudinal variations in the <span class="hlt">observed</span> local time of the postmidnight enhancement peaks. Results show that the lower the geomagnetic latitude, the earlier the enhancement peak occurred in the geomagnetic northern hemisphere. Additionally, the enhancement peaks occurred earlier in the geomagnetic southern hemisphere than that in the geomagnetic northern hemisphere for these present cases. We suggest that the combined effect of the geomagnetic inclination and transequatorial meridional wind might be the main driving force for latitudinal variation of the local time of the occurrence.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012cosp...39.1905R','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012cosp...39.1905R"><span>Study of zonal large scale wave structure (LSWS) and equatorial scintillation with <span class="hlt">low-latitude</span> GRBR network over Southeast Asia and African sectors</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Ram Sudarsanam, Tulasi; Yamamoto, Mamoru; Gurubaran, Subramanian; Tsunoda, Roland</p> <p>2012-07-01</p> <p>The day-to-day variability of Equatorial Spread-F, when and where the equatorial plasma bubbles (EPBs) may initiate, were the challenging problems that puzzling the space weather researchers for several decades. The zonal large scale wave structure (LSWS) at the base of F-layer is the earliest manifestation of seed perturbation for the evolution of EPBs by R-T instability processes, hence, found to play deterministic role on the development of ESF. Yet, only a little is known about LSWS with lack of sufficient <span class="hlt">observations</span>, primarily because of inability to detect the LSWS with the currently existing instruments except with steerable incoherent scatter radar such as ALTAIR radar. This situation, however, was recently changed with launch of C/NOFS in a unique low-inclination (13 ^{o}) orbit. With the availability of CERTO beacon transmissions from C/NOFS in a near equatorial orbit, it is now possible to detect and resolve the roles by LSWS on a regular basis. A ground based <span class="hlt">low-latitude</span> GNU Radio Beacon Receiver (GRBR) Network has been recently established that provide coverage of Southeast Asia, Pacific and African <span class="hlt">low-latitude</span> regions. Recent <span class="hlt">observations</span> suggest that these wave structures with zonal wave lengths varying between 200 and 800 km can be earliest detected even before E-region sunset and found to grow significantly after sunset, probably, aided by the polarization electric fields. Further, these zonal structures consistently found to be aligned with field lines for several hundreds of kilometers and EPBs were found to grow from the westward walls of upwellings. The characteristic differences on the strength of LSWS between the Asian and African longitudes were identified during the recent increasing solar activity and discussed in this paper.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2015yCat..74403462U','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2015yCat..74403462U"><span>VizieR Online Data Catalog: Az<span class="hlt">TEC</span>/ASTE 1.1mm survey of SSA22 (Umehata+, 2014)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Umehata, H.; Tamura, Y.; Kohno, K.; Hatsukade, B.; Scott, K. S.; Kubo, M.; Yamada, T.; Ivison, R. J.; Cybulski, R.; Aretxaga, I.; Austermann, J.; Hughes, D. H.; Ezawa, H.; Hayashino, T.; Ikarashi, S.; Iono, D.; Kawabe, R.; Matsuda, Y.; Matsuo, H.; Nakanishi, K.; Oshima, T.; Perera, T.; Takata, T.; Wilson, G. W.; Yun, M. S.</p> <p>2015-01-01</p> <p>We used the Az<span class="hlt">TEC</span> camera operating at 1.1mm mounted on the ASTE 10-m submillimetre telescope located at Pampa la Bola, near Cerro Chajnantor in northern Chile. All of the Az<span class="hlt">TEC</span>/ASTE <span class="hlt">observations</span> of SSA22 were carried out at night during 2007 August-September and 2008 August-September. (4 data files).</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2012EGUGA..14.6924V','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2012EGUGA..14.6924V"><span>Photochemical and microbial transformation of terrestrial dissolved organic matter - Lena River vs. rivers in mid and <span class="hlt">low</span> <span class="hlt">latitudes</span></span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Vähätalo, A. V.; Aarnos, H.; Paolucci, E. M.; Musibono, D. E.; Khan, S. R.; Gelinas, Y.; Shantz, A.; Huang, Q.; Schneider, W.; Rezende, C. E.; Petrescu, E.; Reader, H. E.</p> <p>2012-04-01</p> <p> biodegradability model based on a reactivity continuum to the <span class="hlt">observed</span> biodegradation of DOC. In the dark control samples, the biodegradation of DOC was moderate, but the irradiation speeded up the decomposition of DOC a lot. The potential photodecomposition of DOM can be realized with a few hundred kilometers from the mouth of those rivers discharging to the mid- or <span class="hlt">low-latitude</span> ocean. For the DOM of Lena River, the potential photodegradation is realized only partially in the receiving Laptev Sea having seasons with no or little solar radiation and additionally covered by sea ice over most of year.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/11123316','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/11123316"><span><span class="hlt">Tec</span> kinase signaling in T cells is regulated by phosphatidylinositol 3-kinase and the <span class="hlt">Tec</span> pleckstrin homology domain.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Yang, W C; Ching, K A; Tsoukas, C D; Berg, L J</p> <p>2001-01-01</p> <p><span class="hlt">Tec</span>, the prototypical member of the <span class="hlt">Tec</span> family of tyrosine kinases, is abundantly expressed in T cells and other hemopoietic cell types. Although the functions of Itk and Txk have recently been investigated, little is known about the role of <span class="hlt">Tec</span> in T cells. Using antisense oligonucleotide treatment to deplete <span class="hlt">Tec</span> protein from primary T cells, we demonstrate that <span class="hlt">Tec</span> plays a role in TCR signaling leading to IL-2 gene induction. Interestingly, <span class="hlt">Tec</span> kinases are the only known family of tyrosine kinases containing a pleckstrin homology (PH) domain. Using several PH domain mutants overexpressed in Jurkat T cells, we show that the <span class="hlt">Tec</span> PH domain is required for <span class="hlt">Tec</span>-mediated IL-2 gene induction and TCR-mediated <span class="hlt">Tec</span> tyrosine phosphorylation. Furthermore, we show that <span class="hlt">Tec</span> colocalizes with the TCR after TCR cross-linking, and that both the <span class="hlt">Tec</span> PH and Src homology (SH) 2 domains play a role in this association. Wortmannin, a phosphatidylinositol 3-kinase inhibitor, abolishes <span class="hlt">Tec</span>-mediated IL-2 gene induction and <span class="hlt">Tec</span> tyrosine phosphorylation, and partially suppresses <span class="hlt">Tec</span> colocalization with the activated TCR. Thus, our data implicate the <span class="hlt">Tec</span> kinase PH domain and phosphatidylinositol 3-kinase in <span class="hlt">Tec</span> signaling downstream of the TCR.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2017AdSpR..59.1223C','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2017AdSpR..59.1223C"><span>Variation of <span class="hlt">TEC</span> and related parameters over the Indian EIA region from ground and space based GPS <span class="hlt">observations</span> during the low solar activity period of May 2007-April 2008</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Chakravarty, S. C.; Nagaraja, Kamsali; Jakowski, N.</p> <p>2017-03-01</p> <p>The annual variations of ionospheric Total Electron Content (<span class="hlt">TEC</span>), F-region peak ionisation (NmF2) and the ionospheric slab thickness (τ) over the Indian region during the low solar activity period of May 2007-April 2008 have been studied. For this purpose the ground based <span class="hlt">TEC</span> data obtained from GAGAN measurements and the space based data from GPS radio occultation technique using CHAMP have been utilised. The results of these independent measurements are combined to derive additional parameters such as the equivalent slab thickness of the total and the bottom-side ionospheric regions (τT and τB). The one year hourly average values of all these parameters over the ionospheric anomaly latitude region (10-26°N) are presented here along with the statistical error estimates. It is expected that these results are potentially suited to be used as base level values during geomagnetically quiet and undisturbed solar conditions.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.ncbi.nlm.nih.gov/pubmed/28249036','PUBMED'); return false;" href="https://www.ncbi.nlm.nih.gov/pubmed/28249036"><span>Humpback whale "super-groups" - A novel <span class="hlt">low-latitude</span> feeding behaviour of Southern Hemisphere humpback whales (Megaptera novaeangliae) in the Benguela Upwelling System.</span></a></p> <p><a target="_blank" href="https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pubmed">PubMed</a></p> <p>Findlay, Ken P; Seakamela, S Mduduzi; Meÿer, Michael A; Kirkman, Stephen P; Barendse, Jaco; Cade, David E; Hurwitz, David; Kennedy, Amy S; Kotze, Pieter G H; McCue, Steven A; Thornton, Meredith; Vargas-Fonseca, O Alejandra; Wilke, Christopher G</p> <p>2017-01-01</p> <p>Southern Hemisphere humpback whales (Megaptera novaeangliae) generally undertake annual migrations from polar summer feeding grounds to winter calving and nursery grounds in subtropical and tropical coastal waters. Evidence for such migrations arises from seasonality of historic whaling catches by latitude, Discovery and natural mark returns, and results of satellite tagging studies. Feeding is generally believed to be limited to the southern polar region, where Antarctic krill (Euphausia superba) has been identified as the primary prey item. Non-migrations and / or suspended migrations to the polar feeding grounds have previously been reported from a summer presence of whales in the Benguela System, where feeding on euphausiids (E. lucens), hyperiid amphipods (Themisto gaudichaudii), mantis shrimp (Pterygosquilla armata capensis) and clupeid fish has been described. Three recent research cruises (in October/November 2011, October/November 2014 and October/November 2015) identified large tightly-spaced groups (20 to 200 individuals) of feeding humpback whales aggregated over at least a one-month period across a 220 nautical mile region of the southern Benguela System. Feeding behaviour was identified by lunges, strong milling and repetitive and consecutive diving behaviours, associated bird and seal feeding, defecations and the pungent "fishy" smell of whale blows. Although no dedicated prey sampling could be carried out within the tightly spaced feeding aggregations, <span class="hlt">observations</span> of E. lucens in the region of groups and the full stomach contents of mantis shrimp from both a co-occurring predatory fish species (Thyrsites atun) and one entangled humpback whale mortality suggest these may be the primary prey items of at least some of the feeding aggregations. Reasons for this recent novel behaviour pattern remain speculative, but may relate to increasing summer humpback whale abundance in the region. These novel, predictable, inter-annual, <span class="hlt">low</span> <span class="hlt">latitude</span> feeding events</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=5332018','PMC'); return false;" href="https://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=5332018"><span>Humpback whale “super-groups” – A novel <span class="hlt">low-latitude</span> feeding behaviour of Southern Hemisphere humpback whales (Megaptera novaeangliae) in the Benguela Upwelling System</span></a></p> <p><a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?DB=pmc">PubMed Central</a></p> <p>Seakamela, S. Mduduzi; Meÿer, Michael A.; Kirkman, Stephen P.; Barendse, Jaco; Cade, David E.; Hurwitz, David; Kennedy, Amy S.; Kotze, Pieter G. H.; McCue, Steven A.; Thornton, Meredith; Vargas-Fonseca, O. Alejandra; Wilke, Christopher G.</p> <p>2017-01-01</p> <p>Southern Hemisphere humpback whales (Megaptera novaeangliae) generally undertake annual migrations from polar summer feeding grounds to winter calving and nursery grounds in subtropical and tropical coastal waters. Evidence for such migrations arises from seasonality of historic whaling catches by latitude, Discovery and natural mark returns, and results of satellite tagging studies. Feeding is generally believed to be limited to the southern polar region, where Antarctic krill (Euphausia superba) has been identified as the primary prey item. Non-migrations and / or suspended migrations to the polar feeding grounds have previously been reported from a summer presence of whales in the Benguela System, where feeding on euphausiids (E. lucens), hyperiid amphipods (Themisto gaudichaudii), mantis shrimp (Pterygosquilla armata capensis) and clupeid fish has been described. Three recent research cruises (in October/November 2011, October/November 2014 and October/November 2015) identified large tightly-spaced groups (20 to 200 individuals) of feeding humpback whales aggregated over at least a one-month period across a 220 nautical mile region of the southern Benguela System. Feeding behaviour was identified by lunges, strong milling and repetitive and consecutive diving behaviours, associated bird and seal feeding, defecations and the pungent “fishy” smell of whale blows. Although no dedicated prey sampling could be carried out within the tightly spaced feeding aggregations, <span class="hlt">observations</span> of E. lucens in the region of groups and the full stomach contents of mantis shrimp from both a co-occurring predatory fish species (Thyrsites atun) and one entangled humpback whale mortality suggest these may be the primary prey items of at least some of the feeding aggregations. Reasons for this recent novel behaviour pattern remain speculative, but may relate to increasing summer humpback whale abundance in the region. These novel, predictable, inter-annual, <span class="hlt">low</span> <span class="hlt">latitude</span> feeding</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2003AGUFMSA12B1100P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2003AGUFMSA12B1100P"><span>L-Band <span class="hlt">TEC</span> Measurements and Lower Frequency Scintillation</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Pedersen, T. R.; Beach, T. L.</p> <p>2003-12-01</p> <p>Signal amplitude measurements from the GPS satellites are currently limited to L-band frequencies above 1 GHz, which often remain unaffected by conditions causing even severe scintillation at more sensitive lower frequencies. Use of differential carrier phase data from dual frequency receivers to drive phase screen models and estimate scintillation at other frequencies is one potential means of monitoring scintillation over a wider range of frequencies. However, this process is complicated by the presence of a diffractive component in the L-band signal phase which can obscure the true structure in total electron content (<span class="hlt">TEC</span>) needed as input to phase screen models. Signal amplitudes and phases at L1 and L2 frequencies (1.57 and 1.23 GHz, respectively) are calculated after propagation through one-dimensional power-law phase screens and then the resulting differential carrier phase compared with the initial phase values in the screen. Scintillation at a variety of frequencies is then computed from both the original screen and the simulated differential carrier phase, and the two results compared to examine the effects of the unobservable diffractive phase component contained in <span class="hlt">observational</span> <span class="hlt">TEC</span> data. Initial results show an increase of ~10% in S4 index computed at 250 MHz from simulated differential carrier phase compared to the direct phase screen computation. These results suggest that under many conditions L-band <span class="hlt">TEC</span> <span class="hlt">observations</span> can be used effectively to estimate VHF and UHF scintillation over a wide range of scintillation levels, and that the differences resulting from use of <span class="hlt">observed</span> <span class="hlt">TEC</span> instead of true ionospheric phase can be accounted for by a relatively simple correction factor.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016JSWSC...6A..29H','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016JSWSC...6A..29H"><span>An empirical model of ionospheric total electron content (<span class="hlt">TEC</span>) near the crest of the equatorial ionization anomaly (EIA)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Hajra, Rajkumar; Chakraborty, Shyamal Kumar; Tsurutani, Bruce T.; DasGupta, Ashish; Echer, Ezequiel; Brum, Christiano G. M.; Gonzalez, Walter D.; Sobral, José Humberto Andrade</p> <p>2016-07-01</p> <p>We present a geomagnetic quiet time (Dst > -50 nT) empirical model of ionospheric total electron content (<span class="hlt">TEC</span>) for the northern equatorial ionization anomaly (EIA) crest over Calcutta, India. The model is based on the 1980-1990 <span class="hlt">TEC</span> measurements from the geostationary Engineering Test Satellite-2 (ETS-2) at the Haringhata (University of Calcutta, India: 22.58° N, 88.38° E geographic; 12.09° N, 160.46° E geomagnetic) ionospheric field station using the technique of Faraday rotation of plane polarized VHF (136.11 MHz) signals. The ground station is situated virtually underneath the northern EIA crest. The monthly mean <span class="hlt">TEC</span> increases linearly with F10.7 solar ionizing flux, with a significantly high correlation coefficient (r = 0.89-0.99) between the two. For the same solar flux level, the <span class="hlt">TEC</span> values are found to be significantly different between the descending and ascending phases of the solar cycle. This ionospheric hysteresis effect depends on the local time as well as on the solar flux level. On an annual scale, <span class="hlt">TEC</span> exhibits semiannual variations with maximum <span class="hlt">TEC</span> values occurring during the two equinoxes and minimum at summer solstice. The semiannual variation is strongest during local noon with a summer-to-equinox variability of ~50-100 <span class="hlt">TEC</span> units. The diurnal pattern of <span class="hlt">TEC</span> is characterized by a pre-sunrise (0400-0500 LT) minimum and near-noon (1300-1400 LT) maximum. Equatorial electrodynamics is dominated by the equatorial electrojet which in turn controls the daytime <span class="hlt">TEC</span> variation and its maximum. We combine these long-term analyses to develop an empirical model of monthly mean <span class="hlt">TEC</span>. The model is validated using both ETS-2 measurements and recent GNSS measurements. It is found that the present model efficiently estimates the <span class="hlt">TEC</span> values within a 1-σ range from the <span class="hlt">observed</span> mean values.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2014OcDyn..64..557S','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2014OcDyn..64..557S"><span>On the dynamics of <span class="hlt">low</span> <span class="hlt">latitude</span>, wide and shallow coastal system: numerical simulations of the Upper Gulf of Thailand</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Saramul, Suriyan; Ezer, Tal</p> <p>2014-04-01</p> <p>A high-resolution (˜1 km horizontal grid and 21 vertical layers) numerical model based on the Princeton Ocean Model (POM) has been used to study the 3D dynamics of the Upper Gulf of Thailand (UGOT). While influenced by tides and rivers like other estuarine systems, the UGOT is unique because it is wide (˜100 km × 100 km), it is shallow (average depth of only ˜15 m), it is located in <span class="hlt">low</span> <span class="hlt">latitudes</span> (˜12.5°N-13.5°N), and it is influenced by the seasonal monsoon. Sensitivity studies were thus conducted to evaluate the impact that surface heat fluxes, monsoonal winds, river runoffs, and the <span class="hlt">low</span> <span class="hlt">latitude</span> may have on the dynamics; the latter has been evaluated by modifying the Coriolis parameter and comparing simulations representing low and mid latitudes. The circulation in the UGOT changes seasonally from counter-clockwise during the northeast monsoon (dry season) to clockwise during the southwest monsoon (wet season). River discharges generate coastal jets, whereas river plumes tend to be more symmetric near the river mouth and remain closer to the coast in <span class="hlt">low</span> <span class="hlt">latitudes</span>, compared with mid-latitude simulations. River plumes are also dispersed along the coast in different directions during different stages of the monsoonal winds. The model results are compared favorably with a simple wind-driven analytical estuarine model. Comparisons between an El Niño year (1998) and a La Niña year (2000) suggest that water temperatures, warmer by as much as 2 °C in 1998 relative to 2000, are largely driven by decrease cloudiness during the El Niño year. The developed model of the UGOT could be used in the future to address various environmental problems affecting the region.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2009GeCoA..73.4077Y','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2009GeCoA..73.4077Y"><span>Oxygen and carbon stable isotopes of modern land snail shells as environmental indicators from a <span class="hlt">low-latitude</span> oceanic island</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Yanes, Yurena; Romanek, Christopher S.; Delgado, Antonio; Brant, Heather A.; Noakes, John E.; Alonso, María R.; Ibáñez, Miguel</p> <p>2009-07-01</p> <p>Land snails provide a unique opportunity to study terrestrial paleoenvironments because their shells, which are generally highly abundant and well-preserved in the fossil record, contain a temporal record of environmental change in the form of isotope codes. To evaluate the utility of this approach for a <span class="hlt">low-latitude</span> oceanic setting, 207 modern shells of 18 species of land snail were analyzed for their oxygen and carbon isotope composition along a north and south facing altitudinal gradient (10-2160 m a.s.l.) in Tenerife Island (˜28°N) of the Canary Archipelago. Shells collected at each locality showed a relatively large range in isotope composition which was greater along the south facing transect (drier and hotter), suggesting that the variance in shell isotope values may be related to water-stress. Although pooled isotope values did not generally show strong relationships with environmental variables (i.e., altitude, temperature and precipitation), mean isotope values were strongly associated with some climatic factors when grouped by site. The mean δ 18O value of the shell (δ 18O shell) by site displayed a negative correlation with elevation, which is consistent with the positive relationship <span class="hlt">observed</span> between temperature and the δ 18O value of rain (δ 18O rain). Calculated δ 18O values of the snail body water (δ 18O body) derived from <span class="hlt">observed</span> temperatures and δ 18O shell values (using the equation of Grossman and Ku [Grossman E. L. and Ku T. L. (1986) Oxygen and carbon isotope fractionation in biogenic aragonite. Chem. Geol. (Isotope Geosci. Sec.)59, 59-74]) displayed a trend with respect to altitude that was similar to measured and hypothetical δ 18O values for local rain water. The calculated δ 18O body values from the shell declined 0.17‰ (VSMOW) per 100 m, which is consistent with the "altitude effect" <span class="hlt">observed</span> for tropical rains in Western Africa, and it correlated negatively with rainfall amount. Accordingly, lower δ 18O shell values</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2013AGUFMSA24A..03B','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2013AGUFMSA24A..03B"><span>Spatial Structure and Dynamics of Intermediate -scale Irregularities in the Post-sunset <span class="hlt">Low-latitude</span> Ionosphere (Invited)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Bhattacharyya, A.</p> <p>2013-12-01</p> <p>Efforts to forecast the strength and latitudinal extent of scintillations on VHF and higher frequency trans-ionospheric radio signals recorded in <span class="hlt">low-latitude</span> regions require a knowledge of the evolution of the spectrum of ionospheric irregularities in the intermediate scale range (~ 100m - few km), and dynamics of the irregularities. At present, the 3-D models that have been developed to simulate the evolution of equatorial plasma bubbles (EPBs), capture the dynamics of EPBs but do not have adequate spatial resolution to yield the intermediate scale irregularity spectrum. Power spectra of weak scintillations recorded on a VHF, UHF, or L-band signal provide direct information about the intermediate scale irregularity spectrum. However, for strong scintillations, it is not possible to derive the irregularity spectral slopes from scintillation spectra, and information about the irregularity spectrum is deduced from other characteristics, such as the coherence scale of the ground scintillation pattern of intensity, which depends on the irregularity spectrum as seen from model calculation results presented here. Information about the spatio-temporal evolution of intermediate scale irregularities is derived from scintillation data on the basis of these theoretical results. For this study, spaced receiver measurements of intensity scintillations on a VHF signal transmitted from a geo-stationary satellite and recorded at an equatorial station are used. Generally, in the initial phase of EPB development after sunset, there is a large de-correlation between the spaced receiver signals, that may be attributed to fluctuations in the velocity of the irregularities due to the perturbation electric fields associated with the interchange instability, which gives rise to the irregularities. The 'random velocity' parameter, computed from spaced receiver intensity scintillation measurements, is a measure of the de-correlation. The coherence scale and the random velocity, together</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016NHESS..16..543Y','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016NHESS..16..543Y"><span>Review of variations in Mw < 7 earthquake motions on position and <span class="hlt">TEC</span> (Mw = 6.5 Aegean Sea earthquake sample)</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Yildirim, Omer; Inyurt, Samed; Mekik, Cetin</p> <p>2016-02-01</p> <p>Turkey is a country located in the middle latitude zone, where tectonic activity is intensive. Recently, an earthquake of magnitude 6.5 Mw occurred offshore in the Aegean Sea on 24 May 2014 at 09:25 UTC, which lasted about 40 s. The earthquake was also felt in Greece, Romania, and Bulgaria in addition to Turkey. In recent years, ionospheric anomaly detection studies have been carried out because of seismicity with total electron content (<span class="hlt">TEC</span>) computed from the global navigation satellite system's (GNSS) signal delays and several interesting findings have been published. In this study, both <span class="hlt">TEC</span> and positional variations have been examined separately following a moderate size earthquake in the Aegean Sea. The correlation of the aforementioned ionospheric variation with the positional variation has also been investigated. For this purpose, a total of 15 stations was used, including four continuously operating reference stations in Turkey (CORS-TR) and stations in the seismic zone (AYVL, CANA, IPSA, and YENC), as well as international GNSS service (IGS) and European reference frame permanent network (EPN) stations. The ionospheric and positional variations of the AYVL, CANA, IPSA, and YENC stations were examined using Bernese v5.0 software. When the precise point positioning <span class="hlt">TEC</span> (PPP-<span class="hlt">TEC</span>) values were examined, it was <span class="hlt">observed</span> that the <span class="hlt">TEC</span> values were approximately 4 TECU (total electron content unit) above the upper-limit <span class="hlt">TEC</span> value at four stations located in Turkey, 3 days before the earthquake at 08:00 and 10:00 UTC. At the same stations, on the day before the earthquake at 06:00, 08:00, and 10:00 UTC, the <span class="hlt">TEC</span> values were approximately 5 TECU below the lower-limit <span class="hlt">TEC</span> value. The global ionosphere model <span class="hlt">TEC</span> (GIM-<span class="hlt">TEC</span>) values published by the Centre for Orbit Determination in Europe (CODE) were also examined. Three days before the earthquake, at all stations, it was <span class="hlt">observed</span> that the <span class="hlt">TEC</span> values in the time period between 08:00 and 10:00 UTC were approximately 2 TECU</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016JGRA..121.9030W','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016JGRA..121.9030W"><span>GPS <span class="hlt">TEC</span> variations in the polar cap ionosphere: Solar wind and IMF dependence</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Watson, Chris; Jayachandran, P. T.; MacDougall, John W.</p> <p>2016-09-01</p> <p>This statistical study examines the solar wind dependence of total electron content (<span class="hlt">TEC</span>) variations arising from mesoscale (tens to hundreds of kilometers) structuring of the polar cap ionosphere. Six years of <span class="hlt">TEC</span> measurements were collected from five high-data rate Global Positioning System (GPS) receivers of the Canadian High Arctic Ionospheric Network (CHAIN), from which high-resolution magnetic local time-latitude maps of <span class="hlt">TEC</span> variation occurrence rate and amplitude were created. Ionosonde radars were used to identify <span class="hlt">TEC</span> variations arising from ionization of the E and F region ionospheres. Statistical <span class="hlt">TEC</span> maps were examined as a function of solar wind and interplanetary magnetic field (IMF) measurements. Statistical results showed that occurrence rate of <span class="hlt">TEC</span> variations was highest in localized dayside regions, with exact local time and latitude of peak occurrence depending primarily on the dayside coupling rate of the solar wind and magnetosphere, as well as IMF orientation and magnitude in the Y-Z plane. Occurrence of <span class="hlt">TEC</span> variations throughout the polar cap increased with solar wind-magnetosphere coupling rate and IMF magnitude. The solar wind dependence of occurrence rate largely reflected the location and rate of dayside magnetic reconnection and subsequent particle precipitation and polar cap convection. Amplitudes of <span class="hlt">TEC</span> variations were largest around noon and increased throughout the polar cap with increased solar wind-magnetosphere coupling rate. These statistical results improve upon the existing <span class="hlt">observational</span> picture of the polar ionosphere and will potentially facilitate development of models and techniques for mitigating impacts of the polar ionosphere on navigation signals and communication links.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016JASTP.145..170P','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016JASTP.145..170P"><span>Global structure of ionospheric <span class="hlt">TEC</span> anomalies driven by geomagnetic storms</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Pancheva, D.; Mukhtarov, P.; Andonov, B.</p> <p>2016-07-01</p> <p>This study examines the structure and variability of the ionospheric <span class="hlt">TEC</span> anomalies driven by geomagnetic storms. For this purpose the CODE global ionospheric <span class="hlt">TEC</span> data from four geomagnetically disturbed periods (29 October-1 November 2003, 7-10 November 2004, 14-15 December 2006, and 5-6 August 2011) have been considered. By applying the tidal analysis to the geomagnetically forced <span class="hlt">TEC</span> anomalies we made an attempt to identify the tidal or stationary planetary wave (SPW) signatures that may contribute to the generation of these anomalies. It has been found that three types of positive anomalies with different origin and different latitudinal appearance are <span class="hlt">observed</span>. These are: (i) anomalies located near latitudes of ±40° and related to the enhancement and poleward moving of the equatorial ionization anomaly (EIA) crests; (ii) anomalies located near latitudes of ±60° and seen predominantly in the night-side ionosphere, and (iii) very high latitude anomalies having mainly zonally symmetric structure and related to the auroral heating and thermospheric expansion. The decomposition analysis revealed that these anomalies can be reconstructed as a result of superposition of the following components: zonal mean (ZM), diurnal migrating (DW1), zonally symmetric diurnal (D0), and stationary planetary wave 1 (SPW1).</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016cosp...41E.109A','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016cosp...41E.109A"><span>Investigation of <span class="hlt">TEC</span> Variations over Mid-Latitude during Quit and Disturbed Days of March 2015</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Atıcı, Ramazan; Saǧır, Selçuk; Güzel, Esat</p> <p>2016-07-01</p> <p>The variations during 09-14-March-2015 quit days and 15-20 March 2015 disturbed days of Total Electron Content (<span class="hlt">TEC</span>) values (provided by IONOLAB group) obtained by analysis the data from Ankara Global Position System (GPS) station of Turkey located at mid-latitude, IRI -2012 model the and IRI-PLUS model are investigated. Also, the variations of the geomagnetic, interplanetary and solar wind parameters are examined. As a result of investigations, <span class="hlt">TEC</span> values from all three models are not change too much at quit days. Unlike, at the disturbed days, although IRI-2012 and IRI-PLUS <span class="hlt">TEC</span> values are not change too much, a noticeable change in GPS-<span class="hlt">TEC</span> values is occurred. GPS-<span class="hlt">TEC</span> values are rapidly increased on 17-March 2015 to be severe magnetic storm (Dst = -124 nT). Then, on following days it was <span class="hlt">observed</span> to significantly decrease. Thus, it is said that GPS-<span class="hlt">TEC</span> values are more sensitive than IRI-2012 and IRI-PLUS models to variations occurred on disturbed days.</p> </li> <li> <p><a target="_blank" onclick="trackOutboundLink('http://adsabs.harvard.edu/abs/2016SpWea..14..300L','NASAADS'); return false;" href="http://adsabs.harvard.edu/abs/2016SpWea..14..300L"><span>The response of local power grid at <span class="hlt">low-latitude</span> to geomagnetic storm: An application of the Hilbert Huang transform</span></a></p> <p><a target="_blank" href="http://adsabs.harvard.edu/abstract_service.html">NASA Astrophysics Data System (ADS)</a></p> <p>Liu, Jin; Wang, Chuan-Bing; Liu, Lu; Sun, Wei-Huai</p> <p>2016-04-01</p> <p>The Hilbert-Huang transform (HHT) is an adaptive data analysis method that can accommodate the variety of data generated by nonlinear and nonstationary processes in nature. In this paper, we focus on the small geomagnetically induced current (GIC) at the local substations in <span class="hlt">low-latitude</span> power grid of China, responding to a moderate storm on 14-18 July 2012. The HHT is applied to analyze the neutral point currents (NPCs) of transformers measured at different substations, and the GIC indices converted from local geomagnetic field measurements. The original data are decomposed into intrinsic mode functions (IMFs) using the ensemble empirical mode decomposition. After removal of the quasi-diurnal components related with the solar quiet variation, the IMFs representing storm disturbances are transformed into Hilbert energy spectra. The results show that some transformers have more or less responses to the moderate storm in the form of Hilbert energy spectra with the frequency around 2-3 mHz. A comparison on the amplitude changes of the spectra total energy of NPCs' perturbation during storm time intervals at different sites suggests that a shell type of three-phase single transformer group seems to be more vulnerable in the storm. Although the <span class="hlt">low-latitude</span> power grids usually show very small GIC, these can be used to investigate the potential risk of space weather to the system.</p> </li>