Problems in determining the surface density of the Galactic disk

NASA Technical Reports Server (NTRS)

Statler, Thomas S.

1989-01-01

A new method is presented for determining the local surface density of the Galactic disk from distance and velocity measurements of stars toward the Galactic poles. The procedure is fully three-dimensional, approximating the Galactic potential by a potential of Staeckel form and using the analytic third integral to treat the tilt and the change of shape of the velocity ellipsoid consistently. Applying the procedure to artificial data superficially resembling the K dwarf sample of Kuijken and Gilmore (1988, 1989), it is shown that the current best estimates of local disk surface density are uncertain by at least 30 percent. Of this, about 25 percent is due to the size of the velocity sample, about 15 percent comes from uncertainties in the rotation curve and the solar galactocentric distance, and about 10 percent from ignorance of the shape of the velocity distribution above z = 1 kpc, the errors adding in quadrature. Increasing the sample size by a factor of 3 will reduce the error to 20 percent. To achieve 10 percent accuracy, observations will be needed along other lines of sight to constrain the shape of the velocity ellipsoid.

The influence of anode position and structure on cusped field thruster

NASA Astrophysics Data System (ADS)

Niu, Xiang; Liu, Hui; Yang, Chiyu; Jiang, Wenjia; Yu, Daren; Ning, Zhongxi

2018-04-01

A cusped field thruster is a kind of electric propulsion device using multi-stage cusped fields to realize plasma discharges and produce thrust. A previous study showed that plasma discharges in this thruster are non-uniform. In this work, a multi-annulus anode is used to measure the radial distribution of anode current density at different anode positions. The experimental results reveal that some electrons may reach the anode along the axis after they accelerate from the final cusp regardless of the anode positions. To further validate this idea and find out the mechanism of this central path along the axis, the central part of the anode is replaced with ceramics. This results in an increase in the total current with larger contributions at larger radii. The current oscillations also get larger. This brief letter is helpful to further understand the movement of electrons in cusped field thrusters and provide guidance on reducing the non-uniform degree of current density.

Weak annihilation cusp inside the dark matter spike about a black hole.

PubMed

Shapiro, Stuart L; Shelton, Jessie

2016-06-15

We reinvestigate the effect of annihilations on the distribution of collisionless dark matter (DM) in a spherical density spike around a massive black hole. We first construct a very simple, pedagogic, analytic model for an isotropic phase space distribution function that accounts for annihilation and reproduces the "weak cusp" found by Vasiliev for DM deep within the spike and away from its boundaries. The DM density in the cusp varies as r -1/2 for s -wave annihilation, where r is the distance from the central black hole, and is not a flat "plateau" profile. We then extend this model by incorporating a loss cone that accounts for the capture of DM particles by the hole. The loss cone is implemented by a boundary condition that removes capture orbits, resulting in an anisotropic distribution function. Finally, we evolve an initial spike distribution function by integrating the Boltzmann equation to show how the weak cusp grows and its density decreases with time. We treat two cases, one for s -wave and the other for p -wave DM annihilation, adopting parameters characteristic of the Milky Way nuclear core and typical WIMP models for DM. The cusp density profile for p -wave annihilation is weaker, varying like ~ r -0.34 , but is still not a flat plateau.

Climatology of the relationship of cusp-related density anomaly with zonal wind and large-scale FAC based on CHAMP observations: IMF By and solar cycle dependence

NASA Astrophysics Data System (ADS)

Kervalishvili, Guram; Lühr, Hermann

2014-05-01

We present climatology of the relationship of cusp-related density enhancement with the neutral zonal wind velocity, large-scale field-aligned current (FAC), small-scale FAC, and electron temperature using the superposed epoch analysis (SEA) method. The dependence of these variables on the interplanetary magnetic field (IMF) By component orientation and solar cycle are of particular interest. In addition, the obtained results of relative density enhancement (ρrel), zonal wind, electron temperature and FAC are subdivided into three local seasons of 130 days each: local winter (1 January ±65 days), combined equinoxes (1 April ±32 days and 1 October ±32 days), and local summer (1 July ±65 days). Our investigation is based on CHAMP satellite observations and NASA/GSFC's OMNI online data set for solar maximum (Mar/2002-2007) and minimum (Mar/2004-2009) conditions in the Northern Hemisphere. The SEA technique uses the time and location of the thermospheric mass density anomaly peaks as reference parameters. The relative amplitude of cusp-related density enhancement does on average not depend on the IMF By orientation, solar cycle phase, and local season. Also, it is apparent that the IMF By amplitude does not have a big influence on the relative amplitude of the density anomaly. Conversely, there exists a good correlation between ρrel and the negative amplitude of IMF Bz prevailing about half an hour earlier. In the cusp region, both large-scale FAC distribution and thermospheric zonal wind velocity exhibit a clear dependence on the IMF By orientation. In the case of positive (negative) IMF By there is a systematic imbalance between downward (upward) and upward (downward) FACs peaks equatorward and poleward of the reference point, respectively. The zonal wind velocity is directed towards west i.e. towards dawn in a geomagnetic latitude-magnetic local time (MLat-MLT) frame. This is true for all local seasons and solar conditions. The thermospheric density

On the Relation Between Soft Electron Precipitations in the Cusp Region and Solar Wind Coupling Functions

NASA Astrophysics Data System (ADS)

Dang, Tong; Zhang, Binzheng; Wiltberge, Michael; Wang, Wenbin; Varney, Roger; Dou, Xiankang; Wan, Weixing; Lei, Jiuhou

2018-01-01

In this study, the correlations between the fluxes of precipitating soft electrons in the cusp region and solar wind coupling functions are investigated utilizing the Lyon-Fedder-Mobarry global magnetosphere model simulations. We conduct two simulation runs during periods from 20 March 2008 to 16 April 2008 and from 15 to 24 December 2014, which are referred as "Equinox Case" and "Solstice Case," respectively. The simulation results of Equinox Case show that the plasma number density in the high-latitude cusp region scales well with the solar wind number density (ncusp/nsw=0.78), which agrees well with the statistical results from the Polar spacecraft measurements. For the Solstice Case, the plasma number density of high-latitude cusp in both hemispheres increases approximately linearly with upstream solar wind number density with prominent hemispheric asymmetry. Due to the dipole tilt effect, the average number density ratio ncusp/nsw in the Southern (summer) Hemisphere is nearly 3 times that in the Northern (winter) Hemisphere. In addition to the solar wind number density, 20 solar wind coupling functions are tested for the linear correlation with the fluxes of precipitating cusp soft electrons. The statistical results indicate that the solar wind dynamic pressure p exhibits the highest linear correlation with the cusp electron fluxes for both equinox and solstice conditions, with correlation coefficients greater than 0.75. The linear regression relations for equinox and solstice cases may provide an empirical calculation for the fluxes of cusp soft electron precipitation based on the upstream solar wind driving conditions.

Apex Dips of Experimental Flux Ropes: Helix or Cusp?

NASA Astrophysics Data System (ADS)

Wongwaitayakornkul, Pakorn; Haw, Magnus A.; Li, Hui; Li, Shengtai; Bellan, Paul M.

2017-10-01

We present a new theory for the presence of apex dips in certain experimental flux ropes. Previously such dips were thought to be projections of a helical loop axis generated by the kink instability. However, new evidence from experiments and simulations suggest that the feature is a 2D cusp rather than a 3D helix. The proposed mechanism for cusp formation is a density pileup region generated by nonlinear interaction of neutral gas cones emitted from fast-gas nozzles. The results indicate that density perturbations can result in large distortions of an erupting flux rope, even in the absence of significant pressure or gravitational forces. The density pileup at the apex also suppresses the m = 1 kink mode by acting as a stationary node. Consequently, more accurate density profiles should be considered when attempting to model the stability and shape of solar and astrophysical flux ropes.

Enhanced Discharge Performance in a Ring Cusp Plasma Source

NASA Technical Reports Server (NTRS)

Foster, John E.; Patterson, Michael J.

2000-01-01

There is a need for a lightweight, low power ion thruster for space science missions. Such an ion thruster is under development at NASA Glenn Research Center. In an effort to better understand the discharge performance of this thruster, a thruster discharge chamber with an anode containing electrically isolated electrodes at the cusps was fabricated and tested. Characteristics of this ring cusp ion discharge were measured without ion beam extraction. Discharge current was measured at collection electrodes located at the magnetic cusps and at the anode body itself. Discharge performance and plasma properties were measured as a function of power, which was varied between 20 and 50 W. It was found that ion production costs decreased by as much as 20 percent when the two most downstream cusp electrodes were allowed to float. Floating the electrodes did not give rise to a significant increase in discharge power even though the plasma density increased markedly. The improved performance is attributed to enhanced electron containment.

Cusp Dynamics-Particle Acceleration by Alfven Waves

NASA Technical Reports Server (NTRS)

Ergun, Robert E.; Parker, Scott A.

2005-01-01

Successful results were obtained from this research project. This investigation answered and/or made progresses on each of the four important questions that were proposed: (1) How do Alfven waves propagate on dayside open field lines? (2) How are precipitating electrons influenced by propagating Alfven waves? (3) How are various cusp electron distributions generated? (4) How are Alfven waves modified by electrons? During the first year of this investigation, the input parameters, such as density and temperature altitude profiles, of the gyrofluid code on the cusp field lines were constructed based on 3-point satellite observations. The initial gyrofluid result was presented at the GEM meeting by Dr. Samuel Jones.

Apex Dips of Experimental Flux Ropes: Helix or Cusp?

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wongwaitayakornkul, Pakorn; Haw, Magnus A.; Bellan, Paul M.

We present a new theory for the presence of apex dips in certain experimental flux ropes. Previously such dips were thought to be projections of a helical loop axis generated by the kink instability. However, new evidence from experiments and simulations suggest that the feature is a 2D cusp rather than a 3D helix. The proposed mechanism for cusp formation is a density pileup region generated by nonlinear interaction of neutral gas cones emitted from fast-gas nozzles. The results indicate that density perturbations can result in large distortions of an erupting flux rope, even in the absence of significant pressuremore » or gravitational forces. The density pileup at the apex also suppresses the m = 1 kink mode by acting as a stationary node. Consequently, more accurate density profiles should be considered when attempting to model the stability and shape of solar and astrophysical flux ropes.« less

Cusped magnetic field mercury ion thruster. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Beattie, J. R.

1976-01-01

The importance of a uniform current density profile in the exhaust beam of an electrostatic ion thruster is discussed in terms of thrust level and accelerator system lifetime. A residence time approach is used to explain the nonuniform beam current density profile of the divergent magnetic field thruster. Mathematical expressions are derived which relate the thruster discharge power loss, propellant utilization, and double to single ion density ratio to the geometry and plasma properties of the discharge chamber. These relationships are applied to a cylindrical discharge chamber model of the thruster. Experimental results are presented for a wide range of the discharge chamber length. The thruster designed for this investigation was operated with a cusped magnetic field as well as a divergent field geometry, and the cusped field geometry is shown to be superior from the standpoint of beam profile uniformity, performance, and double ion population.

STAR COUNT DENSITY PROFILES AND STRUCTURAL PARAMETERS OF 26 GALACTIC GLOBULAR CLUSTERS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Miocchi, P.; Lanzoni, B.; Ferraro, F. R.

We used an appropriate combination of high-resolution Hubble Space Telescope observations and wide-field, ground-based data to derive the radial stellar density profiles of 26 Galactic globular clusters from resolved star counts (which can be all freely downloaded on-line). With respect to surface brightness (SB) profiles (which can be biased by the presence of sparse, bright stars), star counts are considered to be the most robust and reliable tool to derive cluster structural parameters. For each system, a detailed comparison with both King and Wilson models has been performed and the most relevant best-fit parameters have been obtained. This collection ofmore » data represents the largest homogeneous catalog collected so far of star count profiles and structural parameters derived therefrom. The analysis of the data of our catalog has shown that (1) the presence of the central cusps previously detected in the SB profiles of NGC 1851, M13, and M62 is not confirmed; (2) the majority of clusters in our sample are fit equally well by the King and the Wilson models; (3) we confirm the known relationship between cluster size (as measured by the effective radius) and galactocentric distance; (4) the ratio between the core and the effective radii shows a bimodal distribution, with a peak at {approx}0.3 for about 80% of the clusters and a secondary peak at {approx}0.6 for the remaining 20%. Interestingly, the main peak turns out to be in agreement with that expected from simulations of cluster dynamical evolution and the ratio between these two radii correlates well with an empirical dynamical-age indicator recently defined from the observed shape of blue straggler star radial distribution, thus suggesting that no exotic mechanisms of energy generation are needed in the cores of the analyzed clusters.« less

Star Count Density Profiles and Structural Parameters of 26 Galactic Globular Clusters

NASA Astrophysics Data System (ADS)

Miocchi, P.; Lanzoni, B.; Ferraro, F. R.; Dalessandro, E.; Vesperini, E.; Pasquato, M.; Beccari, G.; Pallanca, C.; Sanna, N.

2013-09-01

We used an appropriate combination of high-resolution Hubble Space Telescope observations and wide-field, ground-based data to derive the radial stellar density profiles of 26 Galactic globular clusters from resolved star counts (which can be all freely downloaded on-line). With respect to surface brightness (SB) profiles (which can be biased by the presence of sparse, bright stars), star counts are considered to be the most robust and reliable tool to derive cluster structural parameters. For each system, a detailed comparison with both King and Wilson models has been performed and the most relevant best-fit parameters have been obtained. This collection of data represents the largest homogeneous catalog collected so far of star count profiles and structural parameters derived therefrom. The analysis of the data of our catalog has shown that (1) the presence of the central cusps previously detected in the SB profiles of NGC 1851, M13, and M62 is not confirmed; (2) the majority of clusters in our sample are fit equally well by the King and the Wilson models; (3) we confirm the known relationship between cluster size (as measured by the effective radius) and galactocentric distance; (4) the ratio between the core and the effective radii shows a bimodal distribution, with a peak at ~0.3 for about 80% of the clusters and a secondary peak at ~0.6 for the remaining 20%. Interestingly, the main peak turns out to be in agreement with that expected from simulations of cluster dynamical evolution and the ratio between these two radii correlates well with an empirical dynamical-age indicator recently defined from the observed shape of blue straggler star radial distribution, thus suggesting that no exotic mechanisms of energy generation are needed in the cores of the analyzed clusters.

Long-term Variability of Beach Cusps

NASA Astrophysics Data System (ADS)

Pianca, C.; Holman, R. A.; Siegle, E.

2016-02-01

The most curious morphological features observed on beaches are the cusps. Due to their rhythmic spacing, beach cusps have attracted many observers and many, often contradictory, theories as to their form. Moreover, most of the research about beach cusps has focused on their formation. Few had available long time series to study such things as the variability of alongshore and cross-shore position and spacing on the cusp field, the presence, longevity and interactions between higher and lower sets of cusps, and the processes by which cusp fields extend, shrink or change length scale. The purpose of this work is to use long-term data sets of video images from two study sites, an intermediate (Duck, USA, 26 years) and a reflective beach (Massaguaçu, Brazil, 3 years), to investigate the temporal and spatial changes of cusps conditions. Time-evolving shoreline data were first extracted using an algorithm called ASLIM (Pianca et al 2015). Cusps were then identified based on the band-passed variability of time exposure image data about this shoreline as a function of elevation relative to MSL. The identified beaches cusps will be analyzed for cusp spacing, positions (upper or lower cusps), alongshore variability, merging events, percentage of cusp events, patterns of the events and time scales of variability. Finally, the relationship of these characteristics to environmental conditions (wave, tides, beach conditions) will be studied.

Apex Dips of Experimental Flux Ropes: Helix or Cusp?

NASA Astrophysics Data System (ADS)

Haw, Magnus; Wongwaitayakornkul, Pakorn; Li, Hui; Li, Shengtai; Bellan, Paul M.

2017-10-01

We present a new theory for the presence of apex dips in certain experimental flux ropes. Previously such dips were thought to be projections of a helical loop axis generated by the kink instability. However, new evidence from experiments and simulations suggest that the feature is a 2D cusp rather than a 3D helix. The proposed mechanism for cusp formation is a density pileup region generated by nonlinear interaction of neutral gas cones emitted from fast-gas nozzles. The results indicate that small density perturbations can result in large distortions of an erupting flux rope, even in the absence of significant pressure or gravity forces. The density pileup at the apex also suppresses the m=1 kink mode by acting as a stationary node. Consequently, more accurate density profiles should be considered when attempting to precisely model the stability and eruption of solar flux ropes such as CME's. This work was supported by NSF under award 1348393, AFOSR under award FA9550-11-1-0184, and DOE under awards DE-FG02-04ER54755 and DE-SC0010471.

Electromagnetic Ion Cyclotron Waves in the High Altitude Cusp: Polar Observations

NASA Technical Reports Server (NTRS)

Le, Guan; Blanco-Cano, X.; Russell, C. T.; Zhou, X.-W.; Mozer, F.; Trattner, K. J.; Fuselier, S. A.; Anderson, B. J.; Vondrak, Richard R. (Technical Monitor)

2001-01-01

High-resolution magnetic field data from the Polar Magnetic Field Experiment (MFE) show that narrow band waves at frequencies approximately 0.2 to 3 Hz are a permanent feature in the vicinity of the polar cusp. The waves have been found in the magnetosphere adjacent to the cusp (both poleward and equatorward of the cusp) and in the cusp itself. The occurrence of waves is coincident with depression of magnetic field strength associated with enhanced plasma density, indicating the entry of magnetosheath plasma into the cusp region. The wave frequencies are generally scaled by the local proton cyclotron frequency, and vary between 0.2 and 1.7 times local proton cyclotron frequency. This suggests that the waves are generated in the cusp region by the precipitating magnetosheath plasma. The properties of the waves are highly variable. The waves exhibit both lefthanded and right-handed polarization in the spacecraft frame. The propagation angles vary from nearly parallel to nearly perpendicular to the magnetic field. We find no correlation among wave frequency, propagation angle and polarization. Combined magnetic field and electric field data for the waves indicate that the energy flux of the waves is guided by the background magnetic field and points downward toward the ionosphere.

Electromagnetic Ion Cyclotron Waves in the High-Altitude Cusp: Polar Observations

NASA Technical Reports Server (NTRS)

Le, G.; Blanco-Cano, X.; Russell, C. T.; Zhou, X.-W.; Mozer, F.; Trattner, K. J.; Fuselier, S. A.; Anderson, B. J.

2005-01-01

High-resolution magnetic field data from the Polar Magnetic Field Experiment (MFE) show that narrow-band waves at frequencies approx. 0.2-3 Hz are a permanent feature in the vicinity of the polar cusp. The waves have been found in the magnetosphere adjacent to the cusp (both poleward and equatorward of the cusp) and in the cusp itself. The occurrence of waves is coincident with depression of magnetic field strength associated with enhanced plasma density, indicating the entry of magnetosheath plasma into the cusp region. The wave frequencies are generally scaled by the local proton cyclotron frequency and vary between 0.2 and 1.7 times local proton cyclotron frequency. This suggests that the waves are generated in the cusp region by the precipitating magnetosheath plasma. The properties of the waves are highly variable. The waves exhibit both left-handed and right-handed polarization in the spacecraft frame. The propagation angles vary from nearly parallel to nearly perpendicular to the magnetic field. We find no correlation among wave frequency, propagation angle, and polarization. Combined magnetic field and electric field data for the waves indicate that the energy flux of the waves is guided by the background magnetic field and points downward toward the ionosphere.

Computational Study of Primary Electrons in the Cusp Region of an Ion Engine's Discharge Chamber

NASA Technical Reports Server (NTRS)

Stueber, Thomas J. (Technical Monitor); Deshpande, Shirin S.; Mahalingam, Sudhakar; Menart, James A.

2004-01-01

In this work a computer code called PRIMA is used to study the motion of primary electrons in the magnetic cusp region of the discharge chamber of an ion engine. Even though the amount of wall area covered by the cusps is very small, the cusp regions are important because prior computational analyses have indicated that most primary electrons leave the discharge chamber through the cusps. The analysis presented here focuses on the cusp region only. The affects of the shape and size of the cusp region on primary electron travel are studied as well as the angle and location at which the electron enters the cusp region. These affects are quantified using the confinement length and the number density distributions of the primary electrons. In addition to these results comparisons of the results from PRIMA are made to experimental results for a cylindrical discharge chamber with two magnetic rings. These comparisons indicate the validity of the computer code called PRIMA.

A statistical survey of heat input parameters into the cusp thermosphere

NASA Astrophysics Data System (ADS)

Moen, J. I.; Skjaeveland, A.; Carlson, H. C.

2017-12-01

Based on three winters of observational data, we present those ionosphere parameters deemed most critical to realistic space weather ionosphere and thermosphere representation and prediction, in regions impacted by variability in the cusp. The CHAMP spacecraft revealed large variability in cusp thermosphere densities, measuring frequent satellite drag enhancements, up to doublings. The community recognizes a clear need for more realistic representation of plasma flows and electron densities near the cusp. Existing average-value models produce order of magnitude errors in these parameters, resulting in large under estimations of predicted drag. We fill this knowledge gap with statistics-based specification of these key parameters over their range of observed values. The EISCAT Svalbard Radar (ESR) tracks plasma flow Vi , electron density Ne, and electron, ion temperatures Te, Ti , with consecutive 2-3 minute windshield-wipe scans of 1000x500 km areas. This allows mapping the maximum Ti of a large area within or near the cusp with high temporal resolution. In magnetic field-aligned mode the radar can measure high-resolution profiles of these plasma parameters. By deriving statistics for Ne and Ti , we enable derivation of thermosphere heating deposition under background and frictional-drag-dominated magnetic reconnection conditions. We separate our Ne and Ti profiles into quiescent and enhanced states, which are not closely correlated due to the spatial structure of the reconnection foot point. Use of our data-based parameter inputs can make order of magnitude corrections to input data driving thermosphere models, enabling removal of previous two fold drag errors.

UV Observations of Atomic Oxygen in the Cusp Region

NASA Astrophysics Data System (ADS)

Fritz, B.; Lessard, M.; Dymond, K.; Kenward, D. R.; Lynch, K. A.; Clemmons, J. H.; Hecht, J. H.; Hysell, D. L.; Crowley, G.

2017-12-01

The Rocket Experiment for Neutral Upwelling (RENU) 2 launched into the dayside cusp on 13 December, 2015. The sounding rocket payload carried a comprehensive suite of particle, field, and remote sensing instruments to characterize the thermosphere in a region where pockets of enhanced neutral density have been detected [Lühr et al, 2004]. An ultraviolet photomultiplier tube (UV PMT) was oriented to look along the magnetic field line and remotely detect neutral atomic oxygen (OI) above the payload. The UV PMT measured a clear enhancement as the payload descended through a poleward moving auroral form, an indicator of structure in both altitude and latitude. Context for the UV PMT measurement is provided by the Special Sensor Ultraviolet Imager (SSULI) instrument on the Defense Meteorological Space Program (DMSP) satellite, which also measured OI as it passed through the cusp. UV tomography of SSULI observations produces a two-dimensional cross-section of volumetric emission rates in the high-latitude thermosphere prior to the RENU 2 flight. The volume emission rate may then be inverted to produce a profile of neutral density in the thermosphere. A similar technique is used to interpret the UV PMT measurement and determine structure in the thermosphere as RENU 2 descended through the cusp.

Circularly symmetric cusped random beams in free space and atmospheric turbulence.

PubMed

Wang, Fei; Korotkova, Olga

2017-03-06

A class of random stationary, scalar sources producing cusped average intensity profiles (i.e. profiles with concave curvature) in the far field is introduced by modeling the source degree of coherence as a Fractional Multi-Gaussian-correlated Schell-Model (FMGSM) function with rotational symmetry. The average intensity (spectral density) generated by such sources is investigated on propagation in free space and isotropic and homogeneous atmospheric turbulence. It is found that the FMGSM beam can retain the cusped shape on propagation at least in weak or moderate turbulence regimes; however, strong turbulence completely suppresses the cusped intensity profile. Under the same atmospheric conditions the spectral density of the FMGSM beam at the receiver is found to be much higher than that of the conventional Gaussian Schell-model (GSM) beam within the narrow central area, implying that for relatively small collecting apertures the power-in-bucket of the FMGSM beam is higher than that of the GSM beam. Our results are of importance to energy delivery, Free-Space Optical communications and imaging in the atmosphere.

Beach-cusp formation

USGS Publications Warehouse

Sallenger, A.H.

1979-01-01

Field experiments on beach-cusp formation were undertaken to document how the cuspate form develops and to test the edge-wave hypothesis on the uniform spacing of cusps. These involved observations of cusps forming from an initially plane foreshore. The cuspate form was observed to be a product of swash modification of an intertidal beach ridge as follows. A ridge, cut by a series of channels quasi-equally spaced along its length, was deposited onto the lower foreshore. The ridge migrated shoreward with flood tide, while the longshore positions of the channels remained fixed. On ebb tide, changes in swash circulation over the ridge allowed the upwash to flow shoreward through the channels and the channel mouths were eroded progressively wider until adjacent mouths met, effecting a cuspate shape. Measured spacings of cusps, ranging in size from less than 1 m to more than 12 m, agree well with computed spacings due to either zero-mode subharmonic or zero-mode synchronous edge waves. Edge-wave-induced longshore variations in run up will cause water ponded behind a ridge to converge at points of low swash and flow seaward as relatively narrow currents eroding channels spaced at one edge-wave wavelength for synchronous edge waves or one half wavelength for subharmonic edge waves. The channels are subsequently modified into cusp troughs as described above.

The dependence of gamma-ray burst X-ray column densities on the model for Galactic hydrogen

NASA Astrophysics Data System (ADS)

Arcodia, R.; Campana, S.; Salvaterra, R.

2016-05-01

We study the X-ray absorption of a complete sample of 99 bright Swift gamma-ray bursts (GRBs). In recent years, a strong correlation has been found between the intrinsic X-ray absorbing column density (NH(z)) and the redshift. This absorption excess in high-z GRBs is now thought to be due to the overlooked contribution of the absorption along the intergalactic medium (IGM), by means of both intervening objects and the diffuse warm-hot intergalactic medium along the line of sight. In this work we neglect the absorption along the IGM, because our purpose is to study the eventual effect of a radical change in the Galactic absorption model on the NH(z) distribution. Therefore, we derive the intrinsic absorbing column densities using two different Galactic absorption models: the Leiden Argentine Bonn HI survey and the more recent model that includes molecular hydrogen. We find that if, on the one hand, the new Galactic model considerably affects the single column density values, on the other hand, there is no drastic change in the distribution as a whole. It becomes clear that the contribution of Galactic column densities alone, no matter how improved, is not sufficient to change the observed general trend and it has to be considered as a second order correction. The cosmological increase of NH(z) as a function of redshift persists and, to explain the observed distribution, it is necessary to include the contribution of both the diffuse intergalactic medium and the intervening systems along the line of sight of the GRBs.

Inter-cusp Ion and Electron Transport in a Nstar-derivative Ion Thruster

NASA Technical Reports Server (NTRS)

Foster, John E.

2001-01-01

Diffusion of electrons and ions to anode surfaces between the magnetic cusps of a NASA Solar Electric Propulsion Technology Application Readiness ion thruster has been characterized. Ion flux measurements were made at the anode and at the screen grid electrode. The measurements indicated that the average ion current density at the anode and at the screen grid were approximately equal. Additionally, it was found that the electron flux to the anode between cusps is best described by the classical cross-field diffusion coefficient.

Galactic gamma-ray observations and galactic structure

NASA Technical Reports Server (NTRS)

Stecker, F. W.

1975-01-01

Recent observations of gamma-rays originating in the galactic disk together with radio observations, support an emerging picture of the overall structure of our galaxy with higher interstellar gas densities and star formation rates in a region which corresponds to that of the inner arms. The emerging picture is one where molecular clouds make up the dominant constituent of the interstellar gas in the inner galaxy and play a key role in accounting for the gamma-rays and phenomena associated with the production of young stars and other population 1 objects. In this picture, cosmic rays are associated with supernovae and are primarily of galactic origin. These newly observed phenomena can be understood as consequences of the density wave theories of spiral structure. Based on these new developments, the suggestion is made that a new galactic population class, Population O, be added to the standard Populations 1 and 2 in order to recognize important differences in dynamics and distribution between diffuse galactic H1 and interstellar molecular clouds.

The polar cusp: Cluster observations and simulations

NASA Astrophysics Data System (ADS)

Escoubet, C. Philippe; Berchem, Jean; Pitout, Frederic; Richard, Robert; Trattner, Karlheinz; Grison, Benjamin; Taylor, Matthew; Laakso, Harri; Masson, Arnaud; Dunlop, Malcolm; Dandouras, Iannis; Reme, Henri; Fazakerley, Andrew N.

The polar cusp, together with the magnetopause, are the magnetospheric regions in direct contact with the shocked solar wind flowing continuously from the Sun. Therefore any changes in the solar wind plasma reaching the magnetopause induce changes in the polar cusp with a delay of a few minutes to a few tens of minutes. For instance a change of the interplanetary magnetic field (IMF) direction from South to North will displace the polar cusp poleward and at the same time will change the injection of ions from the subsolar magnetopause to the magnetotail lobes. In the mid and low-altitude cusp a spacecraft will then observe a reversal of the dispersion in energy of the ions. We will use Cluster string of pearl configuration in the mid-altitude polar cusp to investigate the temporal variations of ion injections in the polar cusp. In the period from July to September, the Cluster spacecraft follow each other in the mid-altitude cusp with a delay of few minutes up to one hour. A few examples of cusp crossings will be presented to illustrate the influence of solar wind changes in the polar cusp. We will show that a sudden change in the IMF direction from South to North produces a double cusp crossing. By opposition, a change of the IMF from North to South produces a temporal injection on the equatorward side of the cusp and an erosion of the magnetosphere. Finally, we will show that when the interplanetary conditions are stable with the IMF pointing Northward or Southward for more than 10 min the polar cusp ion dispersion stays constant. MHD and large-scale particle simulations will also be used to complement the Cluster data.

ON GALACTIC DENSITY MODELING IN THE PRESENCE OF DUST EXTINCTION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bovy, Jo; Rix, Hans-Walter; Schlafly, Edward F.

Inferences about the spatial density or phase-space structure of stellar populations in the Milky Way require a precise determination of the effective survey volume. The volume observed by surveys such as Gaia or near-infrared spectroscopic surveys, which have good coverage of the Galactic midplane region, is highly complex because of the abundant small-scale structure in the three-dimensional interstellar dust extinction. We introduce a novel framework for analyzing the importance of small-scale structure in the extinction. This formalism demonstrates that the spatially complex effect of extinction on the selection function of a pencil-beam or contiguous sky survey is equivalent to amore » low-pass filtering of the extinction-affected selection function with the smooth density field. We find that the angular resolution of current 3D extinction maps is sufficient for analyzing Gaia sub-samples of millions of stars. However, the current distance resolution is inadequate and needs to be improved by an order of magnitude, especially in the inner Galaxy. We also present a practical and efficient method for properly taking the effect of extinction into account in analyses of Galactic structure through an effective selection function. We illustrate its use with the selection function of red-clump stars in APOGEE using and comparing a variety of current 3D extinction maps.« less

On Galactic Density Modeling in the Presence of Dust Extinction

NASA Astrophysics Data System (ADS)

Bovy, Jo; Rix, Hans-Walter; Green, Gregory M.; Schlafly, Edward F.; Finkbeiner, Douglas P.

2016-02-01

Inferences about the spatial density or phase-space structure of stellar populations in the Milky Way require a precise determination of the effective survey volume. The volume observed by surveys such as Gaia or near-infrared spectroscopic surveys, which have good coverage of the Galactic midplane region, is highly complex because of the abundant small-scale structure in the three-dimensional interstellar dust extinction. We introduce a novel framework for analyzing the importance of small-scale structure in the extinction. This formalism demonstrates that the spatially complex effect of extinction on the selection function of a pencil-beam or contiguous sky survey is equivalent to a low-pass filtering of the extinction-affected selection function with the smooth density field. We find that the angular resolution of current 3D extinction maps is sufficient for analyzing Gaia sub-samples of millions of stars. However, the current distance resolution is inadequate and needs to be improved by an order of magnitude, especially in the inner Galaxy. We also present a practical and efficient method for properly taking the effect of extinction into account in analyses of Galactic structure through an effective selection function. We illustrate its use with the selection function of red-clump stars in APOGEE using and comparing a variety of current 3D extinction maps.

Cluster Observations of Ion Dispersions near the Exterior Cusp

NASA Astrophysics Data System (ADS)

Escoubet, C.; Grison, B.; Berchem, J.; Trattner, K. J.; Pitout, F.; Richard, R. L.; Taylor, M. G.; Laakso, H. E.; Masson, A.; Dunlop, M. W.; Dandouras, I. S.; Reme, H.; Fazakerley, A. N.; Daly, P. W.

2013-12-01

The cusps are the places where the Earth's magnetic field lines, connected to the inner side of the magnetopause, converge. It is therefore the place where signatures of processes occurring near the subsolar point, in the tail lobes, as well as near the dawn and dusk flanks are observed. The main process that injects solar wind plasma into the polar cusp is now generally accepted to be magnetic reconnection. Depending on the IMF direction, this process will take place equatorward (for IMF southward), poleward (for IMF northward) or on the side (for IMF azimuthal) of the cusp. We report a Cluster crossing on 5 January 2002 near the exterior cusp on the southern dusk side. The IMF was mainly azimuthal (IMF-By around -5 nT), the solar wind speed around 280 km/s and the density around 5 cm-3. The four Cluster spacecraft were still in the "magnetotail" configuration with two perfect tetrahedra of 2000 km around apogee and turning into an elongated configuration near the magnetopause. C4 was the first spacecraft to enter the cusp around 19:52:04 UT, followed by C2 at 19:52:35 UT, C1 at 19:54:24 UT and C3 at 20:13:15 UT. C4 and C1 observed two ion energy dispersions at 20:10 UT and 20:40 UT and C3 at 20:35 UT and 21:15 UT. We will investigate the origin of the injections forming the dispersions and if these can be explained by the reconnection between the interplanetary magnetic field and the Earth's magnetic field.

Criticality of the electron-nucleus cusp condition to local effective potential-energy theories

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pan Xiaoyin; Sahni, Viraht; Graduate School of the City University of New York, 360 Fifth Avenue, New York, New York 10016

2003-01-01

Local(multiplicative) effective potential energy-theories of electronic structure comprise the transformation of the Schroedinger equation for interacting Fermi systems to model noninteracting Fermi or Bose systems whereby the equivalent density and energy are obtained. By employing the integrated form of the Kato electron-nucleus cusp condition, we prove that the effective electron-interaction potential energy of these model fermions or bosons is finite at a nucleus. The proof is general and valid for arbitrary system whether it be atomic, molecular, or solid state, and for arbitrary state and symmetry. This then provides justification for all prior work in the literature based on themore » assumption of finiteness of this potential energy at a nucleus. We further demonstrate the criticality of the electron-nucleus cusp condition to such theories by an example of the hydrogen molecule. We show thereby that both model system effective electron-interaction potential energies, as determined from densities derived from accurate wave functions, will be singular at the nucleus unless the wave function satisfies the electron-nucleus cusp condition.« less

Cluster Observations of Particle Injections in the Exterior Cusp

NASA Astrophysics Data System (ADS)

Escoubet, C. P.; Grison, B.; Berchem, J.; Trattner, K. J.; Lavraud, B.; Pitout, F.; Soucek, J.; Richard, R. L.; Laakso, H. E.; Masson, A.; Dunlop, M. W.; Dandouras, I. S.; Reme, H.; Fazakerley, A. N.; Daly, P. W.

2014-12-01

The main process that injects solar wind plasma into the polar cusp is now generally accepted to be magnetic reconnection. Depending on the IMF direction, this process takes place equatorward (for IMF southward), poleward (for IMF northward) or on the dusk or dawn sides (for IMF azimuthal) of the cusp. We report a Cluster crossing on 5 January 2002 near the exterior cusp on the southern dusk side. The IMF was mainly azimuthal (IMF-By around -5 nT), the solar wind speed lower than usual around 280 km/s with the density of order 5 cm-3. The four Cluster spacecraft had an elongated configuration near the magnetopause. C4 was the first spacecraft to enter the cusp around 19:52:04 UT, followed by C2 at 19:52:35 UT, C1 at 19:54:24 UT and C3 at 20:13:15 UT. C4 and C1 observed two ion energy dispersions at 20:10 UT and 20:40 UT and C3 at 20:35 UT and 21:15 UT. Using the time of flight technique on the upgoing and downgoing ions, which leads to energy dispersions, we obtain distances of the ion sources between 14 and 20 RE from the spacecraft. Using Tsyganenko model, we find that these sources are located on the dusk flank, past the terminator. The first injection by C3 is seen at approximately the same time as the 2nd injection on C1 but their sources at the magnetopause were separated by more than 7 RE. This would imply that two distinct sources were active at the same time on the dusk flank of the magnetosphere. In addition, a flow reversal was observed at the magnetopause on C4 which would be an indication that reconnection is taking place near the exterior cusp.

WOOLLY CUSPS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Nauenberg, M.; Pais, A.

1962-04-01

A study is made of the elastic scattering 1 + 2 yields 1 + 2 in the energy region where the inelastic process 1 + 2 yields 3 + 4 sets in, for the case that particle 3 is unstable. By woolly cusp'' is meant the phenomenon that corresponds to the sharp cusp in the stable case. The procedure followed is to consider the inelastic channel to be of the three-body type, where the three- body states are parametrized by a Breit-Wigner formula around a mean mass m of particle 3. The connection between a woolly and a sharp cuspmore » is made evident. The problem is studied in terms of a twochannel S-wave K matrix. In the two- channel approximation the woolly cusp necessarily shows a decrease in the elastic cross section sigma above a characteristic energy. As a function of energy, sigma must either show a maximum or an inflection point. In either case, the energy at which this happens may lie above or below the inelastic threshold for the fictitious case that particle 3 has a sharp mass m. The sign and magnitude of the elastic scattering phase shift at this m point'' approximately determines which case is actually realized. (auth)« less

A new multi-line cusp magnetic field plasma device (MPD) with variable magnetic field.

PubMed

Patel, A D; Sharma, M; Ramasubramanian, N; Ganesh, R; Chattopadhyay, P K

2018-04-01

A new multi-line cusp magnetic field plasma device consisting of electromagnets with core material has been constructed with a capability to experimentally control the relative volume fractions of magnetized to unmagnetized plasma volume as well as accurate control on the gradient length scales of mean density and temperature profiles. Argon plasma has been produced using a hot tungsten cathode over a wide range of pressures 5 × 10 -5 -1 × 10 -3 mbar, achieving plasma densities ranging from 10 9 to 10 11 cm -3 and the electron temperature in the range 1-8 eV. The radial profiles of plasma parameters measured along the non-cusp region (in between two consecutive magnets) show a finite region with uniform and quiescent plasma, where the magnetic field is very low such that the ions are unmagnetized. Beyond that region, both plasma species are magnetized and the profiles show gradients both in temperature and density. The electrostatic fluctuation measured using a Langmuir probe radially along the non-cusp region shows less than 1% (δI isat /I isat < 1%). The plasma thus produced will be used to study new and hitherto unexplored physics parameter space relevant to both laboratory multi-scale plasmas and astrophysical plasmas.

A new multi-line cusp magnetic field plasma device (MPD) with variable magnetic field

NASA Astrophysics Data System (ADS)

Patel, A. D.; Sharma, M.; Ramasubramanian, N.; Ganesh, R.; Chattopadhyay, P. K.

2018-04-01

A new multi-line cusp magnetic field plasma device consisting of electromagnets with core material has been constructed with a capability to experimentally control the relative volume fractions of magnetized to unmagnetized plasma volume as well as accurate control on the gradient length scales of mean density and temperature profiles. Argon plasma has been produced using a hot tungsten cathode over a wide range of pressures 5 × 10-5 -1 × 10-3 mbar, achieving plasma densities ranging from 109 to 1011 cm-3 and the electron temperature in the range 1-8 eV. The radial profiles of plasma parameters measured along the non-cusp region (in between two consecutive magnets) show a finite region with uniform and quiescent plasma, where the magnetic field is very low such that the ions are unmagnetized. Beyond that region, both plasma species are magnetized and the profiles show gradients both in temperature and density. The electrostatic fluctuation measured using a Langmuir probe radially along the non-cusp region shows less than 1% (δIisat/Iisat < 1%). The plasma thus produced will be used to study new and hitherto unexplored physics parameter space relevant to both laboratory multi-scale plasmas and astrophysical plasmas.

Studies of small-scale plasma inhomogeneities in the cusp ionosphere using sounding rocket data

NASA Astrophysics Data System (ADS)

Chernyshov, Alexander A.; Spicher, Andres; Ilyasov, Askar A.; Miloch, Wojciech J.; Clausen, Lasse B. N.; Saito, Yoshifumi; Jin, Yaqi; Moen, Jøran I.

2018-04-01

Microprocesses associated with plasma inhomogeneities are studied on the basis of data from the Investigation of Cusp Irregularities (ICI-3) sounding rocket. The ICI-3 rocket is devoted to investigating a reverse flow event in the cusp F region ionosphere. By numerical stability analysis, it is demonstrated that inhomogeneous-energy-density-driven (IEDD) instability can be a mechanism for the excitation of small-scale plasma inhomogeneities. The Local Intermittency Measure (LIM) method also applied the rocket data to analyze irregular structures of the electric field during rocket flight in the cusp. A qualitative agreement between high values of the growth rates of the IEDD instability and the regions with enhanced LIM is observed. This suggests that IEDD instability is connected to turbulent non-Gaussian processes.

On the evolutionary advantage of multi-cusped teeth

PubMed Central

Bush, Mark B.; Barani, Amir; Lawn, Brian R.

2016-01-01

A hallmark of mammalian evolution is a progressive complexity in postcanine tooth morphology. However, the driving force for this complexity remains unclear: whether to expand the versatility in diet source, or to bolster tooth structural integrity. In this study, we take a quantitative approach to this question by examining the roles of number, position and height of multiple cusps in determining sustainable bite forces. Our approach is to use an extended finite-element methodology with due provision for step-by-step growth of an embedded crack to determine how fracture progresses with increasing occlusal load. We argue that multi-cusp postcanine teeth are well configured to withstand high bite forces provided that multiple cusps are contacted simultaneously to share the load. However, contact on a single near-wall cusp diminishes the strength. Location of the load points and cusp height, rather than cusp number or radius, are principal governing factors. Given these findings, we conclude that while complex tooth structures can enhance durability, increases in cusp number are more likely to be driven by the demands of food manipulation. Structural integrity of complex teeth is maintained when individual cusps remain sufficiently distant from the side walls and do not become excessively tall relative to tooth width. PMID:27558851

Statistical Study in the mid-altitude cusp region: wave and particle data comparison using a normalized cusp crossing duration

NASA Astrophysics Data System (ADS)

Grison, B.; Escoubet, C. P.; Pitout, F.; Cornilleau-Wehrlin, N.; Dandouras, I.; Lucek, E.

2009-04-01

In the mid altitude cusp region the DC magnetic field presents a diamagnetic cavity due to intense ion earthward flux coming from the magnetosheath. A strong ultra low frequency (ULF) magnetic activity is also commonly observed in this region. Most of the mid altitude cusp statistical studies have focused on the location of the cusp and its dependence and response to solar wind, interplanetary magnetic field, dipole tilt angle parameters. In our study we use the database build by Pitout et al. (2006) in order to study the link of wave power in the ULF range (0.35-10Hz) measured by STAFF SC instrument with the ion plasma properties as measured by CIS (and CODIF) instrument as well as the diamagnetic cavity in the mid-altitude cusp region with FGM data. To compare the different crossings we don`t use the cusp position and dynamics but we use a normalized cusp crossing duration that permits to easily average the properties over a large number of crossings. As usual in the cusp, it is particularly relevant to sort the crossings by the corresponding interplanetary magnetic field (IMF) orientation in order to analyse the results. In particular we try to find out what is the most relevant parameter to link the strong wave activity with. The global statistic confirms previous single case observations that have noticed a simultaneity between ion injections and wave activity enhancements. We will also present results concerning other ion parameters and the diamagnetic cavity observed in the mid altitude cusp region.

Structure of the Outer Cusp and Sources of the Cusp Precipitation during Intervals of a Horizontal IMF

NASA Technical Reports Server (NTRS)

Berchem, Jean; Nemecek, Z.; Safrankova, J.; Prech, L.; Simunek, J.; Sauvaud, J.-A.; Fedorov, A.; Stenuit, H.; Fuselier, S. A.; Savin, S.;

The density of dark matter in the Galactic bulge and implications for indirect detection

DOE PAGES

Hooper, Dan

2016-11-29

A recent study, making use of the number of horizontal branch stars observed in infrared photometric surveys and kinematic measurements of M-giant stars from the BRAVA survey, combined with N-body simulations of stellar populations, has presented a new determination of the dark matter mass within the bulge-bar region of the Milky Way. That study constrains the total mass within themore » $$\\pm 2.2 \\times \\pm 1.4 \\times \\pm 1.2$$ kpc volume of the bulge-bar region to be ($$1.84 \\pm 0.07) \\times 10^{10} \\, M_{\\odot}$$, of which 9-30% is made up of dark matter. Here, we use this result to constrain the the Milky Way's dark matter density profile, and discuss the implications for indirect dark matter searches. Furthermore uncertainties remain significant, these results favor dark matter distributions with a cusped density profile. For example, for a scale radius of 20 kpc and a local dark matter density of 0.4 GeV/cm$^3$, density profiles with an inner slope of 0.69 to 1.40 are favored, approximately centered around the standard NFW value. In contrast, profiles with large flat-density cores are disfavored by this information.« less

Multi-point observations of Ion Dispersions near the Exterior Cusp with Cluster

NASA Astrophysics Data System (ADS)

Escoubet, C.-Philippe; Grison, Benjamin; Berchem, Jean; Trattner, Kralheinz; Pitout, Frederic; Richard, Robert; Taylor, Matt; Soucek, Jan; Laakso, Harri; Masson, Arnaud; Dunlop, Malcolm; Dandouras, Iannis; Reme, Henri; Fazakerley, Andrew; Daly, Patrick

2014-05-01

The exterior cusp is the most external region of the polar magnetosphere in direct contact with the plasma and the magnetic field from the solar wind. Unlike the rest of the magnetopause surface, the exterior cusp is a singular region with small and turbulent magnetic field and where large entry of plasma from solar origin takes place. The main process that injects solar wind plasma into the polar cusp is now generally accepted to be magnetic reconnection. Depending on the IMF direction, this process will take place equatorward (for IMF southward), poleward (for IMF northward) or on the dusk or dawn sides (for IMF azimuthal) of the cusp. We report a Cluster crossing on 5 January 2002 near the exterior cusp on the southern dusk side. The IMF was mainly azimuthal (IMF-By around -5 nT), the solar wind speed lower than usual around 280 km/s and the density around 5 cm-3. The four Cluster spacecraft were still in the "magnetotail" configuration with two perfect tetrahedra of 2000 km around apogee and turning into an elongated configuration near the magnetopause. C4 was the first spacecraft to enter the cusp around 19:52:04 UT, followed by C2 at 19:52:35 UT, C1 at 19:54:24 UT and C3 at 20:13:15 UT. C4 and C1 observed two ion energy dispersions at 20:10 UT and 20:40 UT and C3 at 20:35 UT and 21:15 UT. Using the time of flight technique on the upgoing and downgoing ions in the dispersions, we obtain an altitude of the sources of these ions between 14 and 20 RE. Using Tsyganenko model, these sources are located on the dusk flank, past the terminator. In addition, before entering the cusp, the magnetopause crossing was characterized by a large shear in By and bipolar plasma flows, suggesting that reconnection was taking place near the exterior cusp. We will discuss the extent of the reconnection line along the flank of the magnetopause based on these observations.

Source of the dayside cusp aurora.

PubMed

Mende, S B; Frey, H U; Angelopoulos, V

2016-08-01

Monochromatic all-sky imagers at South Pole and other Antarctic stations of the Automatic Geophysical Observatory chain recorded the aurora in the region where the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites crossed the dayside magnetopause. In several cases the magnetic field lines threading the satellites when mapped to the atmosphere were inside the imagers' field of view. From the THEMIS magnetic field and the plasma density measurements, we were able to locate the position of the magnetopause crossings and map it to the ionosphere using the Tsyganenko-96 field model. Field line mapping is reasonably accurate on the dayside subsolar region where the field is strong, almost dipolar even though compressed. From these coordinated observations, we were able to prove that the dayside cusp aurora of high 630 nm brightness is on open field lines, and it is therefore direct precipitation from the magnetosheath. The cusp aurora contained significant highly structured N 2 + 427.8 nm emission. The THEMIS measurements of the magnetosheath particle energy and density taken just outside the magnetopause compared to the intensity of the structured N 2 + 427.8 nm emissions showed that the precipitating magnetosheath particles had to be accelerated. The most likely electron acceleration mechanism is by dispersive Alfvén waves propagating along the field line. Wave-accelerated suprathermal electrons were seen by FAST and DMSP. The 427.8 nm wavelength channel also shows the presence of a lower latitude hard-electron precipitation zone originating inside the magnetosphere.

Source of the dayside cusp aurora

PubMed Central

Frey, H. U.; Angelopoulos, V.

2016-01-01

Abstract Monochromatic all‐sky imagers at South Pole and other Antarctic stations of the Automatic Geophysical Observatory chain recorded the aurora in the region where the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites crossed the dayside magnetopause. In several cases the magnetic field lines threading the satellites when mapped to the atmosphere were inside the imagers' field of view. From the THEMIS magnetic field and the plasma density measurements, we were able to locate the position of the magnetopause crossings and map it to the ionosphere using the Tsyganenko‐96 field model. Field line mapping is reasonably accurate on the dayside subsolar region where the field is strong, almost dipolar even though compressed. From these coordinated observations, we were able to prove that the dayside cusp aurora of high 630 nm brightness is on open field lines, and it is therefore direct precipitation from the magnetosheath. The cusp aurora contained significant highly structured N2 + 427.8 nm emission. The THEMIS measurements of the magnetosheath particle energy and density taken just outside the magnetopause compared to the intensity of the structured N2 + 427.8 nm emissions showed that the precipitating magnetosheath particles had to be accelerated. The most likely electron acceleration mechanism is by dispersive Alfvén waves propagating along the field line. Wave‐accelerated suprathermal electrons were seen by FAST and DMSP. The 427.8 nm wavelength channel also shows the presence of a lower latitude hard‐electron precipitation zone originating inside the magnetosphere. PMID:27867797

New structures of power density spectra for four Kepler active galactic nuclei

NASA Astrophysics Data System (ADS)

Dobrotka, A.; Antonuccio-Delogu, V.; Bajčičáková, I.

2017-09-01

Many nearby active galactic nuclei display a significant short-term variability. In this work, we reanalyse photometric data of four active galactic nuclei observed by Kepler in order to study the flickering activity, with our main goal to search for multiple components in the power density spectra. We find that all four objects have similar characteristics, with two break frequencies at approximately log( f /Hz) = -5.2 and -4.7. We consider some physical phenomena whose characteristic time-scales are consistent with those observed, in particular mass accretion fluctuations in the inner geometrically thick disc (hot X-ray corona) and unstable relativistic Rayleigh-Taylor modes. The former is supported by detection of the same break frequencies in the Swift X-ray data of ZW229-15. We also discuss rms-flux relations, and we detect a possible typical linear trend at lower flux levels. Our findings support the hypothesis of a multiplicative character of variability, in agreement with the propagating accretion fluctuation model.

Cusp-related Pc3-5 Wave Activity

NASA Astrophysics Data System (ADS)

Pilipenko, V.; Engebretson, M. J.; Kozlovsky, A.; Belakhovsky, V.; Lessard, M.; Yeoman, T. K.

2009-12-01

Pc3-5 pulsations were found to be an ubiquitous element of dayside ULF wave activity at the cusp region. We examine observations of Pc3-5 wave activity by search coil and flux-gate magnetometers at three locations on Svalbard, covering geomagnetic latitudes 74o-76o. To identify the ionospheric projections of the cusp, we use the width of the return signal from the SuperDARN Finland radar covering the Svalbard archipelago. The ULF meridional spatial structure is examined using the amplitude-phase gradient technique. This analysis shows no specific mode conversion pattern near the cusp region. The amplitude gradient mainly has the same direction at all frequencies, and only during periods when the cusp is shifted to very high latitudes, the gradient may change sign. The phase delay is chaotic and does not show any consistent pattern. This behavior corresponds to the occurrence of a localized peak in the latitudinal distribution of Pc3-5 power, but not under the cusp proper as was previously thought, but about several degrees southward from the equatorward cusp boundary. We suppose that compressional Pc3 fluctuations leaking from the magnetosheath into the entry layer of the magnetosphere can modulate the precipitating electron fluxes, which produce the ground response.

Active galactic nuclei

PubMed Central

Fabian, Andrew C.

1999-01-01

Active galactic nuclei are the most powerful, long-lived objects in the Universe. Recent data confirm the theoretical idea that the power source is accretion into a massive black hole. The common occurrence of obscuration and outflows probably means that the contribution of active galactic nuclei to the power density of the Universe has been generally underestimated. PMID:10220363

Solar wind controls on Mercury's magnetospheric cusp

NASA Astrophysics Data System (ADS)

He, Maosheng; Vogt, Joachim; Heyner, Daniel; Zhong, Jun

2017-06-01

This study assesses the response of the cusp to solar wind changes comprehensively, using 2848 orbits of MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) observation. The assessment entails four steps: (1) propose and validate an approach to estimate the solar wind magnetic field (interplanetary magnetic field (IMF)) for MESSENGER's cusp transit; (2) define an index σ measuring the intensity of the magnetic disturbance which significantly peaks within the cusp and serves as an indicator of the cusp activity level; (3) construct an empirical model of σ as a function of IMF and Mercury's heliocentric distance rsun, through linear regression; and (4) use the model to estimate and compare the polar distribution of the disturbance σ under different conditions for a systematic comparison. The comparison illustrates that the disturbance peak over the cusp is strongest and widest extending in local time for negative IMF Bx and negative IMF Bz, and when Mercury is around the perihelion. Azimuthal shifts are associated with both IMF By and rsun: the cusp moves toward dawn when IMF By or rsun decrease. These dependences are explained in terms of the IMF Bx-controlled dayside magnetospheric topology, the component reconnection model applied to IMF By and Bz, and the variability of solar wind ram pressure associated with heliocentric distance rsun. The applicability of the component reconnection model on IMF By indicates that at Mercury reconnection occurs at lower shear angles than at Earth.