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Sample records for propagating low-frequency slow

  1. Low frequency propagation in the earth's magnetosphere

    NASA Technical Reports Server (NTRS)

    Dennison, Brian; Ananthakrishnan, S.; Desch, M.; Kaiser, M. L.; Weiler, K. W.

    1990-01-01

    Using a model to simulate wave propagation, estimates were obtained on the effect of the earth's magnetosphere on the imaging potential of the Low-Frequency Space Array mission for observations above the ionosphere at frequencies below about 10 MHz. Results of this simulation show that, for imaging at 1.5 MHz, large orbital radii will be required. It is concluded that successful imaging from within the plasmasphere may depend upon the feasibility of correction schemes.

  2. Low Frequency Guided Plate Waves Propagation in Fiber Reinforced Composites

    NASA Technical Reports Server (NTRS)

    Lih, S-S.; Bar-Cohen, Y.

    1995-01-01

    Conventional destructive techniques for the determination of the elastic stiffness constants of composite materials can be costly and often inaccurate. Reliable nondestructive evaluation methods for monitoring the integrity of composite materials and structures are needed. Guided wave propagation in isotropic plate have been studied. Studies on the low frequency symmetric guide waves are presented.

  3. Long range downwind propagation of low-frequency sound

    NASA Technical Reports Server (NTRS)

    Willshire, W. L., Jr.

    1985-01-01

    The propagation of low-frequency noise outdoors was studied using as the source a large (80-m diameter) 4-megawatt horizontal axis wind turbine. Acoustic measurements were made with low-frequency microphone systems placed on the ground at five downwind sites ranging from 300 m to 10,000 m (6.3 mile) away from the wind turbine. The wind turbine fundamental was 1 Hz and the wind speed was generally 12 - 15 m/s at the hub height (80 m). The harmonic levels, when plotted versus propagation distance, exhibit a 3 dB per doubling of distance divergence. Two plausible explanations identified for this cylindrical spreading behavior were propagation of the low frequency wind turbine noise via a surface wave and downwind refraction. Surface was amplitude predictions were found to be more than 20 dB smaller than the measured levels. Ray-tracing results were used to qualitatively explain measured trends. A normal mode approach was identified as a candidate method for low-frequency acoustic refraction prediction.

  4. Propagation of a low-frequency rectangular pulse in seawater

    NASA Astrophysics Data System (ADS)

    King, Ronold W. P.

    1993-05-01

    As a necessary preliminary to the determination of the electromagnetic field scattered by a metal cylinder submerged in the ocean, the propagation of a low frequency pulse with a rectangular envelope is evaluated numerically as a function of the distance of travel. Graphs of the three component terms and their sum are shown for four distances. The very significant changes in shape and amplitude of the pulse are discussed.

  5. Universality of Slow Earthquakes in the Very Low Frequency Band

    NASA Astrophysics Data System (ADS)

    Ide, S.; Yabe, S.

    2014-12-01

    Deep tectonic tremors have been observed together with signals in the very low frequency (VLF) band from 0.02 to 0.05 Hz, which have been identified as VLF events in limited regions of subduction zones. By stacking broadband seismograms relative to the timing of tremors, we can detect similar signals in all regions where tremors occur in western Japan. These signals are inverted to obtain the moment tensor, and the fault-normal and slip vectors are generally consistent with the geometry of the plate interface and the direction of plate motion. Therefore, these signals are probably radiated by shear slip on the plate interface. The ratio between the seismic energy rate estimated from the tremors and seismic moment rate in the VLF band is almost proportional, with a proportionality constant (i.e., scaled energy) of around 10-10. The spatial distribution of scaled energy may reflect spatial variations in the frequency-dependent characteristics of slow deformation. This method is broadly applicable if abundant data are available. We demonstrate that focal mechanisms can be determined using VLF signals for Cascadia and Mexican subduction zones.

  6. Studies of the propagation of Low Frequency (LF) radio waves

    NASA Astrophysics Data System (ADS)

    Warrington, E. M.; Jones, T. B.

    1993-05-01

    Low frequency (30-300 kHz) radio waves can propagate to great distances with little attenuation in the cavity formed by the earth and the ionosphere. Because of the relatively high frequency at LF, many active propagation modes can occur between the transmitter and receiver. Changes in the ionospheric conductivity or reflection height can influence the phase and amplitude of these modes and, hence, produce mutual interference. Because of these interference effects, the propagation is less stable than at VLF and the received field strength becomes more difficult to predict. In the present investigation, the WAVEHOP program was employed in conjunction with a range of ionospheric models to estimate the receiver field strength over a number of experimental paths. The predicted values were compared with those measured in an attempt to validate the ionospheric models and the method of calculation.

  7. Low-frequency dilatational wave propagation through unsaturated porous media containing two immiscible fluids

    SciTech Connect

    Lo, W.-C.; Sposito, G.; Majer, E.

    2007-02-01

    An analytical theory is presented for the low-frequency behavior of dilatational waves propagating through a homogeneous elastic porous medium containing two immiscible fluids. The theory is based on the Berryman-Thigpen-Chin (BTC) model, in which capillary pressure effects are neglected. We show that the BTC model equations in the frequency domain can be transformed, at sufficiently low frequencies, into a dissipative wave equation (telegraph equation) and a propagating wave equation in the time domain. These partial differential equations describe two independent modes of dilatational wave motion that are analogous to the Biot fast and slow compressional waves in a single-fluid system. The equations can be solved analytically under a variety of initial and boundary conditions. The stipulation of 'low frequency' underlying the derivation of our equations in the time domain is shown to require that the excitation frequency of wave motions be much smaller than a critical frequency. This frequency is shown to be the inverse of an intrinsic time scale that depends on an effective kinematic shear viscosity of the interstitial fluids and the intrinsic permeability of the porous medium. Numerical calculations indicate that the critical frequency in both unconsolidated and consolidated materials containing water and a nonaqueous phase liquid ranges typically from kHz to MHz. Thus engineering problems involving the dynamic response of an unsaturated porous medium to low excitation frequencies (e.g. seismic wave stimulation) should be accurately modeled by our equations after suitable initial and boundary conditions are imposed.

  8. Obliquely propagating low frequency electromagnetic shock waves in two dimensional quantum magnetoplasmas

    SciTech Connect

    Masood, W.

    2009-04-15

    Linear and nonlinear propagation characteristics of low frequency magnetoacoustic waves in quantum magnetoplasmas are studied employing the quantum magnetohydrodynamic model. In this regard, a quantum Kadomtsev-Petviashvili-Burgers (KPB) equation is derived using the small amplitude expansion method. The dissipation is introduced by taking into account the kinematic viscosity among the plasma constituents. Furthermore, the solution of KPB equation is presented using the tangent hyperbolic (tanh) method. The variation in the fast and slow magnetoacoustic shock profiles with the quantum Bohm potential via increasing number density, obliqueness angle {theta}, magnetic field, and the resistivity are also investigated. It is observed that the aforementioned plasma parameters significantly modify the propagation characteristics of nonlinear magnetoacoustic shock waves in quantum magnetoplasmas. The relevance of the present investigation with regard to dense astrophysical environments is also pointed out.

  9. Low frequency acoustic pulse propagation in temperate forests.

    PubMed

    Albert, Donald G; Swearingen, Michelle E; Perron, Frank E; Carbee, David L

    2015-08-01

    Measurements of acoustic pulse propagation for a 30-m path were conducted in an open field and in seven different forest stands in the northeastern United States consisting of deciduous, evergreen, or mixed tree species. The waveforms recorded in forest generally show the pulse elongation characteristic of propagation over a highly porous ground surface, with high frequency scattered arrivals superimposed on the basic waveform shape. Waveform analysis conducted to determine ground properties resulted in acoustically determined layer thicknesses of 4-8 cm in summer, within 2 cm of the directly measured thickness of the litter layers. In winter the acoustic thicknesses correlated with the site-specific snow cover depths. Effective flow resistivity values of 50-88 kN s m(-4) were derived for the forest sites in summer, while lower values typical for snow were found in winter. Reverberation times (T60) were typically around 2 s, but two stands (deciduous and pruned spruce planted on a square grid) had lower values of about 1.2 s. One site with a very rough ground surface had very low summer flow resistivity value and also had the longest reverberation time of about 3 s. These measurements can provide parameters useful for theoretical predictions of acoustic propagation within forests. PMID:26328690

  10. Low-Frequency Acoustic Signals Propagation in Buried Pipelines

    NASA Astrophysics Data System (ADS)

    Ovchinnikov, A. L.; Lapshin, B. M.

    2016-01-01

    The article deals with the issues concerning acoustic signals propagation in the large-diameter oil pipelines caused by mechanical action on the pipe body. Various mechanisms of signals attenuation are discussed. It is shown that the calculation of the attenuation caused only by internal energy loss, i.e, the presence of viscosity, thermal conductivity and liquid pipeline wall friction lead to low results. The results of experimental studies, carried out using the existing pipeline with a diameter of 1200 mm. are shown. It is experimentally proved that the main mechanism of signal attenuation is the energy emission into the environment. The numerical values of attenuation coefficients that are 0,14- 0.18 dB/m for the pipeline of 1200 mm in diameter, in the frequency range from 50 Hz to 500 Hz, are determined.

  11. Using Low Frequency Earthquakes to Examine Slow Slip Beneath the Olympic Peninsula, WA

    NASA Astrophysics Data System (ADS)

    Chestler, S.; Creager, K. C.; Sweet, J. R.

    2014-12-01

    Using data from the Array of Arrays (AofA) and Cascadia Arrays for Earthscope (CAFE) experiments we found 61 families of low frequency earthquakes (LFEs) beneath the Olympic Peninsula, Washington. Detections within each family range from Mw1-2. We cross-correlated 6-second long windows within 99 separate hours of tremor data during the 2010 and 2011 Episodic Tremor and Slip (ETS) events. For each hour, we stacked the autocorrelation functions from a set of 7 three-component base stations chosen for their high signal-to-noise ratios. We extracted windows with correlation coefficients higher than 9 times the median absolute deviation. These time windows contain our preliminary LFE detections. We clustered the time windows and produced template waveforms by stacking the waveforms corresponding to a given cluster. By scanning templates through the entire time period when the AofA network was active (June 2009 - September 2010 and August 2011), we identify bursts of LFE activity lasting 2-10 days during times of known tremor [Wech, 2010], including the August 2010 and 2011 ETS events and three tremor swarms in November 2009, March 2010, and July 2010, each of which is assumed to signify a slow-slip event (SSE). During the larger ETS events, we can identify the passage of the main rupture front, rapid tremor reversals and/or dip-parallel streaks. These alternate propagation modes are less obvious during smaller SSEs. We examine the spatial extent of the SSEs by determining which families were active during each event. While all families were active during the 2010 and 2011 ETS events, fewer families were active during the smaller SSEs. The families active during smaller SSEs are located farther down dip. Finally, we are applying double-difference methods to obtain more robust LFE locations, which will shed light on the patchy structure of the plate interface.

  12. Study of low-frequency-acoustic- and seismic-wave energy propagation on the shelf

    NASA Astrophysics Data System (ADS)

    Rutenko, A. N.; Manul'chev, D. S.; Solov'ev, A. A.

    2013-05-01

    The paper presents the results of field and numerical studies on the features of low-frequency-acoustic- and seismic-wave energy propagation on the shelf of the Sea of Japan. Measurements were conducted with the Mollusk-07 autonomous vertical acousto-hydrophysical measurement system, an electromagnetic low-frequency resonance emitter, and a pulsed pneumoemitter lowered from the ship, as well as a shore-based resonance seismoemitter.

  13. Low frequency sound attenuation in a flow duct using a thin slow sound material.

    PubMed

    Aurégan, Yves; Farooqui, Maaz; Groby, Jean-Philippe

    2016-05-01

    A thin subwavelength material that can be flush mounted in a duct and that gives an attenuation band at low frequencies in air flow channels is presented. To decrease the material thickness, the sound is slowed in the material using folded side branch tubes. The impedance of the material is compared to the optimal value given by the Cremer condition, which can differ greatly from the air characteristic impedance. Grazing flow on this material increases the losses at the interface between the flow and the material. PMID:27250200

  14. Low frequency sound attenuation in a flow duct using a thin slow sound material

    NASA Astrophysics Data System (ADS)

    Aurégan, Yves; Farooqui, Maaz; Groby, Jean-Philippe

    2016-05-01

    We present a thin subwavelength material that can be flush mounted to a duct and which gives a large wide band attenuation at remarkably low frequencies in air flow channels. To decrease the material thickness, the sound is slowed in the material using folded side branch tubes. The impedance of the material is compared to the optimal value, which differs greatly from the characteristic impedance. In particular, the viscous and thermal effects have to be very small to have high transmission losses. Grazing flow on this material increases the losses at the interface between the flow and the material.

  15. Scattering resonance of elastic wave and low-frequency equivalent slow wave

    NASA Astrophysics Data System (ADS)

    Meng, X.; Liu, H.; Hu, T.; Yang, L.

    2015-12-01

    Transmitted wave occurs as fast p-wave and slow p-wave in certain conditions when seismic waves travel through inhomogeneous layers. Energy of slow p-waves is strongest at some frequency band, but rather weak at both high frequency band and low frequency band, called scattering resonance. For practical seismic exploration, the frequency of slow p-wave occurs is below 10Hz, which cannot be explained by Biot's theory which predicts existence of the slow p-wave at ultrasonic band in the porous media. The slow p-wave equation have been derived, but which only adapted to explaining slow p-wave in the ultrasonic band. Experimental observations exhibit that slow p-wave also exists in nonporous media but with enormous low-velocity interbeds. When vertical incidence, elastic wave is simplified as compressing wave, the generation of slow waves is independent on shear wave. In the case of flat interbed and gas bubble, Liu (2006) has studied the transmission of acoustic waves, and found that the slow waves below the 10Hz frequency band can be explained. In the case of general elastic anisotropy medium, the tiheoretical research on the generation of slow waves is insufficient. Aiming at this problem, this paper presents an exponential mapping method based on transmitted wave (Magnus 1954), which can successfully explain the generation of the slow wave transmission in that case. Using the prediction operator (Claerbout 1985) to represent the transmission wave, this can be derived as first order partial differential equation. Using expansions in the frequency domain and the wave number domain, we find that the solutions have different expressions in the case of weak scattering and strong scattering. Besides, the method of combining the prediction operator and the exponential map is needed to extend to the elastic wave equation. Using the equation (Frazer and Fryer 1984, 1987), we derive the exponential mapping solution for the prediction operator of the general elastic medium

  16. Multipath propagation of low-frequency radio waves inferred from high-resolution array analysis

    NASA Astrophysics Data System (ADS)

    Füllekrug, Martin; Smith, Nathan; Mezentsev, Andrew; Watson, Robert; Astin, Ivan; Gaffet, Stéphane; Evans, Adrian; Rycroft, Michael

    2015-11-01

    The low-frequency radio sky shows the locations of electromagnetic radio sources with a characteristic dilution of precision. Here we report a thorough high-resolution analysis of radio waves from low-frequency (˜20-150 kHz) radio communication transmitters which are recorded with a small aperture array of radio receivers during the day. It is found that the observed dilution of precision results from the array geometry of the radio receivers, a birefringent wave propagation, and the correlated multipath propagation of low-frequency radio waves. The influence of the array geometry on the dilution of precision is reduced by taking into account the impulse response of the array. This procedure reveals for the very first time the splitting of one single radio source into two distinct source locations separated by ˜0.2°-1.9° which result from a birefringent wave propagation. The two locations are yet more clearly identified by using the polarity of the modulated wave number vectors of the radio waves. This polarity is also used to quantify the dilution of precision arising from correlated multipath propagation which is discriminated against wave number fluctuations arising from the timing accuracy of the radio receivers. It is found that ˜69% of the wave number variability is of natural origin and ˜31% originates from the timing accuracy of the receivers. The wave number variability from correlated multipath propagation results in a standard deviation ˜2-8% relative to the source location. This compact measurement of correlated multipath propagation is used to characterize the uncertainty of source locations in the radio sky. The identification of correlated multipath propagation strongly suggests the existence of very fast processes acting on time scales <1 ms in the D region ionosphere with physically meaningful effects on low-frequency radio wave propagation. This important result has implications for practical applications in that the observed multipath

  17. Unlocking the Secrets of Slow Slip in Cascadia Using Low-Frequency Earthquakes

    NASA Astrophysics Data System (ADS)

    Sweet, Justin R.

    Recent discoveries in subduction zones worldwide---including here in Cascadia---have illuminated the once shrouded process of plate convergence below the seismogenic zone. Early geodetic [Dragert, et al., 2001] and seismic [Obara, 2002] signals were observed to correlate in space and time, and were associated with periodic episodes of deep slow slip, termed Episodic Tremor and Slip (ETS) [Rogers and Dragert, 2003]. In this dissertation, I present evidence further detailing the process of where, how, and how often deep slow slip occurs using several catalogs of low-frequency earthquakes (LFEs) as slow slip indicators. In the first section I compare four distinct LFE families that span the range of the ETS zone beneath western Washington State. I find that LFE behavior varies systematically with depth: LFE moments, swarm durations, and swarm recurrence intervals are all largest in the updip portion of the ETS zone, and smallest in the downdip portion. I interpret these systematic differences as a result of variation in fault strength on the subduction interface---with the strongest coupling found updip (near the seismogenic zone), and the weakest coupling found downdip. In the second section I look within individual LFE families and perform double-difference event relocations to map out the spatial extent of the LFE patch (or patches) responsible for LFE generation. I determine LFE locking efficiency from estimates of LFE density and released seismic moment. I also track LFE migrations over time in an effort to map the progression of slow slip fronts, rapid tremor reversals (RTRs), and other phenomena.

  18. Suspended Matter in the Ocean: Does It Impact Low Frequency Sound Propagation ?

    NASA Astrophysics Data System (ADS)

    Browning, D. G.; Scheifele, P. M.

    2001-05-01

    In littoral waters the outflow of rivers may play the same role for lidar propagation as fronts and eddys have for sonar propagation. River outflows generally produce a runoff lens, which results in refraction effects similar to fronts and eddys but with associated suspended matter.The key question is whether this suspended matter, which would severely degrade lidar propagation, would also have an impact on the logical alternative - low frequency sound propagation ? Two recent studies in the Kara and Laptev Seas have provided an excellent data base for the distribution, density, and composition of the suspended matter outflow from rivers into the sea. The vertical distribution is more complex than the temperature profile and horizontally can still be significant 500 km from the delta. Both these examples are highly seasonally dependent, peaking during the Spring runoff. This data is used to give predictions from previously reported theories on the scattering and absorption of low frequency sound by particulates and dilute sediments. The diverse predictions indicate that at-sea measurements are needed to resole this question.

  19. Modeling of the propagation of low-frequency electromagnetic radiation in the Earth’s magnetosphere

    SciTech Connect

    Lebedev, N. V. Rudenko, V. V.

    2015-06-15

    A numerical algorithm for solving the set of differential equations describing the propagation of low-frequency electromagnetic radiation in the magnetospheric plasma, including in the presence of geomagnetic waveguides in the form of large-scale plasma density inhomogeneities stretched along the Earth’s magnetic field, has been developed. Calculations of three-dimensional ray trajectories in the magnetosphere and geomagnetic waveguide with allowance for radiation polarization have revealed characteristic tendencies in the behavior of electromagnetic parameters along the ray trajectory. The results of calculations can be used for magnetospheric plasma diagnostics.

  20. Slow-Slip Propagation Speeds

    NASA Astrophysics Data System (ADS)

    Rubin, A. M.; Ampuero, J.

    2007-12-01

    Combined seismic and geodetic data from subduction zones and the Salton Trough have revealed slow slip events with reasonably well-defined propagation speeds. This in turn is suggestive of a more-or-less well- defined front separating nearly locked regions outside the slipping zone from interior regions that slide much more rapidly. Such crack-like nucleation fronts arise naturally in models of rate-and-state friction for lab-like values of a/b, where a and b are the coefficients of the velocity- and state-dependence of the frictional strength (with the surface being velocity-neutral for a/b=1). If the propagating front has a quasi-steady shape, the propagation and slip speeds are kinematically tied via the local slip gradient. Given a sufficiently sharp front, the slip gradient is given dimensionally by Δτp- r/μ', where Δτp-r is the peak-to-residual stress drop at the front and μ' the effective elastic shear modulus. Rate-and-state simulations indicate that Δτp-r is given reasonably accurately by bσ\\ln(Vmaxθi/Dc), where σ is the effective normal stress, Vmax is the maximum slip speed behind the propagating front, θi is the the value of "state" ahead of the propagating front, and Dc is the characteristic slip distance for state evolution. Except for a coefficient of order unity, Δτp-r is independent of the evolution law. This leads to Vprop/Vmax ~μ'/[bσ\\ln(Vmaxθi/Dc)]. For slip speeds a few orders of magnitude above background, \\ln(Vmaxθi/Dc) can with reasonable accuracy be assigned some representative value (~4-5, for example). Subduction zone transients propagate on the order of 10 km/day or 10-1 m/s. Geodetic data constrain the average slip speed to be a few times smaller than 1 cm/day or 10-7 m/s. However, numerical models indicate that the maximum slip speed at the front may be several times larger than the average, over a length scale that is probably too small to resolve geodetically, so a representative value of Vprop/Vmax may be ~106

  1. Low-frequency sound propagation modeling over a locally-reacting boundary using the parabolic approximation

    NASA Technical Reports Server (NTRS)

    Robertson, J. S.; Siegman, W. L.; Jacobson, M. J.

    1989-01-01

    There is substantial interest in the analytical and numerical modeling of low-frequency, long-range atmospheric acoustic propagation. Ray-based models, because of frequency limitations, do not always give an adequate prediction of quantities such as sound pressure or intensity levels. However, the parabolic approximation method, widely used in ocean acoustics, and often more accurate than ray models for lower frequencies of interest, can be applied to acoustic propagation in the atmosphere. Modifications of an existing implicit finite-difference implementation for computing solutions to the parabolic approximation are discussed. A locally-reacting boundary is used together with a one-parameter impedance model. Intensity calculations are performed for a number of flow resistivity values in both quiescent and windy atmospheres. Variations in the value of this parameter are shown to have substantial effects on the spatial variation of the acoustic signal.

  2. Higher order contribution to the propagation characteristics of low frequency transverse waves in a dusty plasma

    NASA Astrophysics Data System (ADS)

    Misra, A. P.; Chowdhury, A. Roy; Paul, S. N.

    2004-09-01

    Characteristic features of low frequency transverse wave propagating in a magnetised dusty plasma have been analysed considering the effect of dust-charge fluctu- ation. The distinctive behaviours of both the left circularly polarised and right circularly polarised waves have been exhibited through the analysis of linear and non-linear disper- sion relations. The phase velocity, group velocity, and group travel time for the waves have been obtained and their propagation characteristics have been shown graphically with the variations of wave frequency, dust density and amplitude of the wave. The change in non-linear wave number shift and Faraday rotation angle have also been exhibited with respect to the plasma parameters. It is observed that the effects of dust particles are significant only when the higher order contributions are considered. This may be referred to as the `dust regime' in plasma.

  3. Comparison of FFP predictions with measurements of a low-frequency signal propagated in the atmosphere

    NASA Technical Reports Server (NTRS)

    Wilson, K. Keith; Thomson, Dennis W.

    1990-01-01

    An experimental study of low-frequency propagation over a distance of 770 m was previously reported (J. Acoust. Soc. Am. Suppl. 1 86, S120 (1989)). For that study, sound speed profiles were reconstructed entirely from surface-layer micrometeorological data. When the acoustic data were compared with theoretical predictions from a fast field program (FFP), it was found that the FFP underpredicted sound levels measured in a shadow zone. Here, the effect on the predictions of including meteorological data for heights greater than the surface layer, i.e., wind profiles measured by a Doppler sodar, is discussed. Vertical structure of turbulence is simulated by stochastically perturbing the mean profiles, and the agreement between the acoustic data and FFP predictions is improved.

  4. MAVEN observation of an obliquely propagating low-frequency wave upstream of Mars

    NASA Astrophysics Data System (ADS)

    Ruhunusiri, Suranga; Halekas, J. S.; Connerney, J. E. P.; Espley, J. R.; McFadden, J. P.; Mazelle, C.; Brain, D.; Collinson, G.; Harada, Y.; Larson, D. E.; Mitchell, D. L.; Livi, R.; Jakosky, B. M.

    2016-03-01

    We report Mars Atmosphere and Volatile EvolutioN mission observations of a large amplitude low-frequency plasma wave that propagated oblique to the ambient magnetic field upstream of Mars along with a non-solar-wind plasma component that had a flow velocity perpendicular to the magnetic field. We consider nine possibilities for this wave that include various combinations of its propagation direction, polarization in the solar wind frame, and ion source responsible for its generation. Using the observed wave parameters and the measured plasma parameters as constraints, we uniquely identify the wave by systematically discarding these possibilities. We determine that the wave is a right-hand polarized wave that propagated upstream in the solar wind frame. We find two possibilities for the ion source that can be responsible for this wave generation. They are either newly born pickup protons or reflected solar wind protons from the bow shock. We determine that the observed non-solar-wind component is not responsible for the wave generation, and it is likely that the non-solar-wind component was merely perturbed by the passage of the wave.

  5. Application of ray theory to propagation of low frequency noise from wind turbines

    NASA Technical Reports Server (NTRS)

    Hawkins, James A.

    1987-01-01

    Ray theory is used to explain data from two experiments (1985 and 1985) on the propagation of low frequency sound generated by the WTS-4 wind turbine. Emphasis is on downwind data, but some upwind measurements taken during the 1985 experiment are also considered. General ray theory for a moving medium is reviewed and ray equations obtained. Restrictions are introduced simplifying the equations and permitting the use of a ray theory program MEDUSA, the computed propagation loss curve of which is compared to the measurements. Good qualitative agreement is obtained with 1984 downwind data. The results indicate that the downwind sound field is that of a near-ground sound channel. Although more scatter is seen in the 1985 data, agreement between theory and data is also good. In particular, the position and magnitude of the jump in the sound levels associated with the beginning of the sound channel is correctly predicted. The theoretical explanation of the upwind data is less successful. Ray theory calculations indicate the formation of a shadow zone that, in fact, does not occur. While no sharp shadow zone is apparent in the data, the general expectation (based on ray theory) that sound levels should be much reduced upwind is confirmed by the data.

  6. A numerical study of transcranial focused ultrasound beam propagation at low frequency

    NASA Astrophysics Data System (ADS)

    Yin, Xiangtao; Hynynen, Kullervo

    2005-04-01

    The feasibility of transcranial ultrasound focusing with a non-moving phased array and without skull-specific aberration correction was investigated using computer simulations. Three cadaver skull CT image data sets were incorporated into an acoustic wave transmission model to simulate transskull ultrasound wave propagation. Using a 0.25 MHz hemispherical array (125 mm radius of curvature, 250 mm diameter, 24 255 elements), the simulated beams could be focused and steered with transducer element driving phases and amplitude adjusted for focal beam steering in water (water-path). A total of 82 foci, spanning wide ranges of distance in the three orthogonal dimensions, were simulated to test the focal beam steering capability inside the three skulls. The acoustic pressure distribution in a volume of 20 × 20 × 20 mm3 centred at each focus was calculated with a 0.5 mm spacing in each axis. Clearly defined foci were retained through the skulls (skull-path) in most cases. The skull-path foci were on average 1.6 ± 0.8 mm shifted from their intended locations. The -3 dB skull-path beam width and length were on average 4.3 ± 1.0 mm and 7.7 ± 1.8 mm, respectively. The skull-path sidelobe levels ranged from 25% to 55% of the peak pressure values. The skull-path peak pressure levels were about 10%-40% of their water-path counterparts. Focusing low-frequency beam through skull without skull-specific aberration correction is possible. This method may be useful for applying ultrasound to disrupt the blood-brain barrier for targeted delivery of therapeutic or diagnostic agents, or to induce microbubbles, or for other uses of ultrasound in brain where the required power levels are low and the sharp focusing is not needed.

  7. Illuminating the Transition Between Steady Sliding and Episodic Tremor and Slow Slip Using Low Frequency Earthquakes at the Downdip Edge

    NASA Astrophysics Data System (ADS)

    Creager, K. C.; Sweet, J.; Vidale, J. E.; Houston, H.

    2012-12-01

    Using data from the Array of Arrays and CAFE experiments, we have identified eight Low-Frequency Earthquake (LFE) families on the subduction plate interface, under the Olympic Peninsula, Washington State. We analyze the time history of each during the time interval 2007-2012. The updip-most family (LFE1) only lights up during the well-known northern Cascadia Episodic Tremor and Slip (ETS) events that recur every 15 months. The recurrence intervals shorten from updip LFE1 to the downdip-most family (LFE4), which repeats every 14 days; 30 times more frequently. This presentation focuses on the downdip family. See the Sweet presentation, this session, for an analysis of the updip-most LFE family. LFEs from family 4 typically have durations of about one hour, with as many as 100 repeats during that time. Unlike their updip counterparts, they occur as discrete events without other LFEs or tremor visible during that time. They are strongly modulated by tidal shear stress. Twice as many LFEs occur during encouraging shear stress as during discouraging times. In contrast, these same LFEs occur when tidal normal stress is compressive which should inhibit slip. To reconcile LFE occurrence with favorable tidal Coulomb stress requires that the friction coefficient be less than 0.2 .This extreme sensitivity to very small shear stresses also suggests near lithostatic pore fluid pressures. We propose that the bursts of LFEs in this family correspond to discrete slow-slip events that occur with remarkable regularity. To add up to plate rates, each burst would correspond to a little more than 1 mm of slip, and each individual LFE to a little less than 0.1 mm, assuming all the slip occurs in the form of LFE activity and each LFE ruptures the same spot. One of these event sequences was captured by our 1-km aperture 80-element Big Skidder Array in 2008. Careful stacked correlation functions from 32 LFEs relative to a reference event showed S-P times varied only up to 0.02s, which

  8. Unraveling the Intricacies of Cascadia Slow-Slip Rupture Using Repeating Low-Frequency Earthquakes

    NASA Astrophysics Data System (ADS)

    Creager, K. C.; Sweet, J. R.; Chestler, S.; Houston, H.; Vidale, J. E.

    2013-12-01

    Using data from the 2-year Array of Arrays and the 6-year CAFE seismic experiments on the Olympic Peninsula, Washington, we have identified and located nine groups (families) of repeating Low-Frequency Earthquakes (LFEs). Each family locates within a few km of the plate interface as estimated by active-source wide-angle reflections (Preston et al., Science, 2003) and repeats 100s to 10,000 times in intriguing temporal patterns. At the down-dip end they light up every 1-2 weeks, each with 10s to 100s of LFE repeats, during typical time scales of about one hour. Double-difference locations indicate the LFEs are constrained to a linear patch 2 km long, on the plate interface elongated in the direction of relative plate motion. In contrast, the up-dip-most LFE family lights up primarily during the big 14-month Episodic Tremor and Slip (ETS) events. It repeatedly exhibits a pattern of being quiet for 14 months, then lighting up with hundreds of repeats during a few hour period, which we interpret as the main ETS rupture front passing the location of the LFE family. This is followed by a five-day period of quiescence punctuated by about a dozen short-duration episodes of frenzied LFE activity. Monitoring LFE families within 20 km of each other, we find their bursts of activity correlate roughly with each other, but with time lags of 10s of minutes to hours. We interpret this activity as the passage of streaks and Rapid Tremor Reversals that occur during the 5 days after the main rupture front passes. After this time, either the stress is relieved, or the fault has healed and the LFE family is quiet for another 14 months. The magnitudes of the down-dip LFEs, based on S-wave amplitudes, range from -1 to 1. In the limited magnitude range over which our LFE catalog is complete (0.6 magnitude units) they appear to follow a Gutenberg-Richter distribution with a very large b-value of 4. There is a systematic pattern of short-duration LFE activity happening often (every 1

  9. Status report on environment, propagation and system modeling for an elf/vlf/lf(extremely low frequency/very low frequency/low frequency) ionospheric dependent propagation code. Technical report

    SciTech Connect

    Rutherford, R.R.; Knapp, W.S.

    1983-06-28

    This report describes the continuing work on WEDCOM during the 1982 contractual period. Environmental models were updated and efficient propagation computational procedures were implemented to reduce execution time. Procedures were developed to provide the WEDCOM user with an automated method of selecting calculation times and ionization profile locations consistent with nuclear environmental sensitivities. VLF/LF signal processing models were adapted for use in WEDCOM and procedures for including atmospheric noise were developed.

  10. Instabilities of low frequency, parallel propagating electromagnetic waves in the earth's foreshock region

    NASA Technical Reports Server (NTRS)

    Sentman, D. D.; Edmiston, J. P.; Frank, L. A.

    1981-01-01

    An instability analysis is presented for parallel and antiparallel propagating electromagnetic waves generated by reflected and diffuse suprathermal ions upstream of the earth's bow shock. Calculations are performed on the basis of upstream particle observations made by the ISEE 1 Quadrispheric Lepedea instrument and low-energy electron measurements made by the ISEE 1 electron spectrometer for a single period. The electromagnetic dispersion relation is computed and the unstable modes and growth times of the fastest growing waves are determined. It is found that the reflected ions destabilize the plasma most strongly at a wave frequency 0.1 that of the ion gyrofrequency by a resonant ion beam instability for waves propagating upstream and by a nonresonant firehose-like instability for waves propagating downstream. The diffuse ions also destabilize the plasma most strongly at the same frequency by means of resonant instabilities of both right- and left-hand polarized waves propagating away from the bow shock.

  11. Acoustical effects of a large ridge on low-frequency sound propagation in stationary and moving atmospheres

    NASA Technical Reports Server (NTRS)

    Robertson, J. S.; Jacobson, M. J.; Siegmann, W. L.; Santandrea, D. P.

    1989-01-01

    The effects of a ridge on a low-frequency acoustic propagation in quiescent and windy atmospheres are investigated using a parabolic approximation. A logarithmic wind-speed profile, commonly employed to model atmospheric wind currents, is modified and used to model two-dimensional atmospheric flow over a triangularly-shaped hill. The parabolic equation is solved using an implicit finite-difference algorithm. Several examples are examined to determine the combined effects of source-ridge distance, ridge dimensions, wind-speed profile, and CW source frequency on the received acoustic field.

  12. Effect of environmental uncertainty on low frequency sonar propagation in a shallow sea

    NASA Astrophysics Data System (ADS)

    Chen, Feng; Shapiro, Georgy; Thain, Richard

    2013-04-01

    Underwater acoustics is widely used in navigational, scientific and military areas. The technique of coupled ocean-acoustic modelling has been of interest for many years. The predictive capability of acoustic propagation modelling is highly dependent on the marine environment and seabed properties. The direction and intensity of sound propagation is determined by the sound speed gradients in the water column, which in turn are dependent upon variations in temperature and salinity. These variations occur on a range of scales - from climatic (tens of years) to the mesoscale (days and weeks) in time and from hundreds of meters to tens of kilometres in space, especially on the continental shelf. In shallow water, extremely dynamic features such as strong density fronts, intense stratifications, eddies, filaments and other mesoscale features exist persistently. These features described above have significant impacts on underwater sound propagation and therefore must be investigated in order to improve the predictive accuracy of acoustic modelling. Uncertainties in the ocean model simulations are transferred to the acoustic field due to the usage of coupled ocean-acoustic system. The area selected for this study is the Celtic Sea, which is typical European continental shelf shallow water. It is filled with mesoscale eddies which contribute to the formation of the residual (tidally averaged) circulation pattern. The sea is strongly stratified from April to November along with bottom fronts, which adds to the formation of density driven currents. In this paper we employ the ocean model POLCOMS which has been validated for different regions of the world ocean and also been used operationally by the UK Met Office for the European Shelf seas to construct the environmental condition for the acoustic model and the sonar performance model HARCAM, which has been validated formally by the U.K. Ministry of Defence over a variety of frequencies, to generate acoustic propagation data

  13. Study of the propagation characteristics of Very Low Frequency Signal as observed from Indian Permanent station Maitri and Bharati

    NASA Astrophysics Data System (ADS)

    Sasmal, Sudipta; Chakrabarti, Sandip Kumar; Pal, Sujay; Palit, Sourav; Chakraborty, Suman

    2016-07-01

    Propagation of Very Low Frequency (VLF) radio signal through the Earth-ionosphere waveguide strongly depends on the plasma properties of the ionospheric D layer. Solar extreme ultraviolet radiation plays the central role in controlling physical and chemical properties of the lower ionospheric layers and hence determining the quality and propagation characteristics of a VLF signal. The nature of interference among different propagating modes varies widely with the length of the propagation path. For a very long path, exposure of solar radiation and thus the degree of ionization vary by a large amount along the path. Study of radio signal characteristics in the Antarctic region during summer period in the Southern Hemisphere gives us a unique opportunity to explore such a possibility. In addition, there is an extra feature in this path - the presence of solar radiation and hence the D region for the whole day during summer in at least some sections of the path. We present long-distance propagation characteristics of VLF signals transmitted from VTX (18.2 kHz) and NWC (19.8 kHz) transmitters recorded simultaneously at Indian permanent stations Maitri (latitude 70 ^{o}45 ^{'}S, longitude 11 ^{o}40 ^{'}E) and Bharati (69 ^{o}24 ^{'}S, 76 ^{o}10 ^{'}E). A very stable diurnal variation of the signal (both amplitude and phase) has been obtained with no signature of nighttime fluctuation due the presence of 24 h of sunlight for both the stations. We present the attenuation rate of the dominant waveguide modes corresponding to those propagation conditions where the effects of the Antarctic polar ice on the attenuation of different propagating waveguide modes are visible. VLF signals show the effects of high energetic solar events. Using ion production and recombination profiles by solar irradiance and incorporating D region ion chemistry processes, we calculate the electron density profile at different heights. Using this profile in the Long Wavelength Propagation Capability

  14. A Probe of Magnetosphere-Ionosphere Coupling using the Propagation Characteristics of Very Low Frequency Signal

    NASA Astrophysics Data System (ADS)

    Nwankwo, V. U. J.; Chakrabarti, S. K.; Ogunmodimu, O. A.

    2015-12-01

    The amplitude and phase of VLF/LF radio signal are sensitive to changes in the electrical conductivity of the lower ionosphere when propagated in the Earth-ionosphere waveguide. This unique characteristic makes it useful in studying sudden ionospheric disturbances and/or anomaly especially those related to prompt X-ray flux output from solar flares and gamma ray bursts (GRBs). However, strong geomagnetic disturbances and/or storm conditions are known to produce large and global ionospheric disturbances, which can significantly affect VLF radio propagation in the D region ionosphere. Other than X-ray flux enhancement of amplitude and phase, diurnal VLF signature may convey other important information especially those related to geomagnetic disturbance/storm induced ionospheric changes. In this paper, using the data of three propagation paths (at latitudes 40-54), we performed detail analysis of the trend of variations of aspects VLF diurnal signal under varying solar and/or geomagnetic space environmental conditions for identification of possible geomagnetic footprint on the ionosphere. We found that trend of variations significantly reflected the prevailing space weather conditions of various time scales. The `dipping' of the signal diurnal amplitude have shown noteworthy consistency with significantly geomagnetic perturbed and/or storm conditions in the time scale of 1-2 days. We also found that dipping of most MDP signal occurred irrespective of the time (of the day), which an event happened, while those of MBSR, MASS, SRT and SST appear to largely depend on event occurrence time and/or duration. Pre-sunset event had more influence on the SST and MASS (dusk signal), while pre-sunrise event had more influence on the SRT and MBSR (dawn signal), and depending on the duration of the event, impact could be extended to the neighbouring point/component in succession. The induced dipping varied with geomagnetic activity/event intensity and/or duration, as well as the

  15. Difference in Simulated Low-Frequency Sound Propagation in the Various Species of Baleen Whale

    NASA Astrophysics Data System (ADS)

    Tsuchiya, Toshio; Naoi, Jun; Futa, Koji; Kikuchi, Toshiaki

    2004-05-01

    Whales found in the north Pacific are known to migrate over several thousand kilometers, from the Alaskan coast where they heartily feed during the summer to low latitude waters where they breed during the winter. Therefore, it is assumed that whales are using the “deep sound channel” for their long-distance communication. The main objective of this study is to clarify the behaviors of baleen whales from the standpoint of acoustical oceanography. Hence, authors investigated the possibility of long distance communication in various species of baleen whales, by simulating the long-distance propagation of their sound transmission, by applying the mode theory to actual sound speed profiles and by simulating their transmission frequencies. As a result, the possibility of long distance communication among blue whales using the deep sound channel was indicated. It was also indicated that communication among fin whales and blue whales can be made possible by coming close to shore slopes such as the Island of Hawaii.

  16. ION HEATING BY A SPECTRUM OF OBLIQUELY PROPAGATING LOW-FREQUENCY ALFVEN WAVES

    SciTech Connect

    Lu Quanming; Chen Liu

    2009-10-10

    Ion stochastic heating by a monochromatic Alfven wave, which propagates obliquely to the background magnetic field, has been studied by Chen et al. It is shown that ions can be resonantly heated at frequencies a fraction of the ion cyclotron frequency when the wave amplitude is sufficiently large. In this paper, the monochromatic wave is extended to a spectrum of left-hand polarized Alfven waves. When the amplitude of the waves is small, the components of the ion velocity have several distinct frequencies, and their motions are quasi-periodic. However, when the amplitude of the waves is sufficiently large, the components of the ion velocity have a spectrum of continuous frequencies near the ion cyclotron frequency due to the nonlinear coupling between the Alfven waves and the ion gyromotion, and the ion motions are stochastic. Compared with the case of a monochromatic Alfven wave, the threshold of the ion stochastic heating by a spectrum of Alfven waves is much lower. Even when their frequencies are only several percent of the ion cyclotron frequency, the ions can also be stochastically heated. The relevance of this heating mechanism to solar corona is also discussed.

  17. Observations and transport theory analysis of low frequency, acoustic mode propagation in the Eastern North Pacific Ocean.

    PubMed

    Chandrayadula, Tarun K; Colosi, John A; Worcester, Peter F; Dzieciuch, Matthew A; Mercer, James A; Andrew, Rex K; Howe, Bruce M

    2013-10-01

    Second order mode statistics as a function of range and source depth are presented from the Long Range Ocean Acoustic Propagation EXperiment (LOAPEX). During LOAPEX, low frequency broadband signals were transmitted from a ship-suspended source to a mode-resolving vertical line array. Over a one-month period, the ship occupied seven stations from 50 km to 3200 km distance from the receiver. At each station broadband transmissions were performed at a near-axial depth of 800 m and an off-axial depth of 350 m. Center frequencies at these two depths were 75 Hz and 68 Hz, respectively. Estimates of observed mean mode energy, cross mode coherence, and temporal coherence are compared with predictions from modal transport theory, utilizing the Garrett-Munk internal wave spectrum. In estimating the acoustic observables, there were challenges including low signal to noise ratio, corrections for source motion, and small sample sizes. The experimental observations agree with theoretical predictions within experimental uncertainty. PMID:24116512

  18. Rupture propagation patterns of deep low-frequency earthquakes depending on source structure and frictional property: numerical analysis based on dynamic model

    NASA Astrophysics Data System (ADS)

    Nakata, R.; Ando, R.; Hori, T.; Ide, S.

    2010-12-01

    Deep nonvolcanic tremor and low-frequency earthquakes (LFEs) occurred along the subduction zones in southwest Japan and Cascadia are explained as shear slips on the plate interface [Ide et al., 2007a], and are considered as ruptures of relatively unstable patches within the region where slow slip event (SSE) occurs [Ito et al., 2007]. The tremor sources migrate with the velocity of about 10 km/day along strike [Obara, 2002], and 100 km/hour along dip [Shelly et al., 2007]. Further, the moment rate spectra of the LFEs have a tendency that decay at the inverse proportional to the frequency [Ide et al., 2007b]. To explain these anisotropies of migration speed and spectral property, Ando et al. [2010] proposed a dynamic model that unstable patches rupture by passing stress pulse of SSE over the patches. The aim of this study is to investigate the frictional properties and detailed structure of LFE source to explain the spectral characteristics and moment rate function based on their model. The LFE source model comprises clustered unstable patches. To model the source process, which radiates seismic waves, we employed the dynamic boundary integral equation method [Ando and Yamashita, 2007] in a 3D full space with a triangular mesh [Tada, 2006]. Within these clusters, some fluctuations are added so that the locations and sizes of their constituent patches follow a Gaussian distribution. We present physical quantities in nondimensionalized form. In this study, we modeled an LFE source with many small patches or large several patches distributed in dense or sparse. For each patch distributions, we calculated rupture propagation with various values of the patch viscosity, background viscosity, and background frictional strength. In the simulations, both passive and spontaneous ruptures appear depending on the viscosity, frictional strength, and patch distribution. The former is caused by the assumed stress pulse and the latter is by interaction between the patches and

  19. ON THE LOW-FREQUENCY BOUNDARY OF SUN-GENERATED MAGNETOHYDRODYNAMIC TURBULENCE IN THE SLOW SOLAR WIND

    SciTech Connect

    Shergelashvili, Bidzina M.; Fichtner, Horst

    2012-06-20

    New aspects of the slow solar wind turbulent heating and acceleration are investigated. A physical meaning of the lower boundary of the Alfven wave turbulent spectra in the solar atmosphere and the solar wind is studied and the significance of this natural parameter is demonstrated. Via an analytical and quantitative treatment of the problem we show that a truncation of the wave spectra from the lower frequency side, which is a consequence of the solar magnetic field structure and its cyclic changes, results in a significant reduction of the heat production and acceleration rates. An appropriate analysis is presented regarding the link of the considered problem with existing observational data and slow solar wind initiation scenarios.

  20. Low-frequency stimulation induces long-term depression and slow onset long-term potentiation at perforant path-dentate gyrus synapses in vivo.

    PubMed

    Gonzalez, Jossina; Morales, Isaiah S; Villarreal, Desiree M; Derrick, Brian E

    2014-03-01

    The expression of homosynaptic long-term depression (LTD) is thought to mediate a crucial role in sustaining memory function. Our in vivo investigations of LTD expression at lateral (LPP) and medial perforant path (MPP) synapses in the dentate gyrus (DG) corroborate prior demonstrations that PP-DG LTD is difficult to induce in intact animals. In freely moving animals, LTD expression occurred inconsistently among LPP-DG and MPP-DG responses. Interestingly, following acute electrode implantation in anesthetized rats, low-frequency stimulation (LFS; 900 pulses, 1 Hz) promotes slow-onset LTP at both MPP-DG and LPP-DG synapses that utilize distinct induction mechanisms. Systemic administration of the N-methyl-d-aspartate (NMDA) receptor antagonist (+/-)-cyclopiperidine-6-piperiperenzine (CPP; 10 mg/kg) 90 min before LFS selectively blocked MPP-DG but not LPP-DG slow onset LTP, suggesting MPP-DG synapses express a NMDA receptor-dependent slow onset LTP whereas LPP-DG slow onset LTP induction is NMDA receptor independent. In experiments where paired-pulse LFS (900 paired pulses, 200-ms paired-pulse interval) was used to induce LTD, paired-pulse LFS of the LPP resulted in rapid onset LTP of DG responses, whereas paired-pulse LFS of the MPP induced slow onset LTP of DG responses. Although LTD observations were very rare following acute electrode implantation in anesthetized rats, LPP-DG LTD was demonstrated in some anesthetized rats with previously implanted electrodes. Together, our data indicate in vivo PP-DG LTD expression is an inconsistent phenomenon that is primarily observed in recovered animals, suggesting perturbation of the dentate through surgery-related tissue trauma influences both LTD incidence and LTP induction at PP-DG synapses in vivo. PMID:24335215

  1. Propagated infra-slow intrinsic brain activity reorganizes across wake and slow wave sleep

    PubMed Central

    Mitra, Anish; Snyder, Abraham Z; Tagliazucchi, Enzo; Laufs, Helmut; Raichle, Marcus E

    2015-01-01

    Propagation of slow intrinsic brain activity has been widely observed in electrophysiogical studies of slow wave sleep (SWS). However, in human resting state fMRI (rs-fMRI), intrinsic activity has been understood predominantly in terms of zero-lag temporal synchrony (functional connectivity) within systems known as resting state networks (RSNs). Prior rs-fMRI studies have found that RSNs are generally preserved across wake and sleep. Here, we use a recently developed analysis technique to study propagation of infra-slow intrinsic blood oxygen level dependent (BOLD) signals in normal adults during wake and SWS. This analysis reveals marked changes in propagation patterns in SWS vs. wake. Broadly, ordered propagation is preserved within traditionally defined RSNs but lost between RSNs. Additionally, propagation between cerebral cortex and subcortical structures reverses directions, and intra-cortical propagation becomes reorganized, especially in visual and sensorimotor cortices. These findings show that propagated rs-fMRI activity informs theoretical accounts of the neural functions of sleep. DOI: http://dx.doi.org/10.7554/eLife.10781.001 PMID:26551562

  2. The propagation characteristics of electromagnetic waves through plasma in the near-field region of low-frequency loop antenna

    SciTech Connect

    Liu, DongLin Li, XiaoPing; Xie, Kai; Liu, ZhiWei

    2015-10-15

    A high-speed vehicle flying through the atmosphere between 100 and 20 km may suffer from a “communication blackout.” In this paper, a low frequency system with an on-board loop antenna to receive signals is presented as a potential blackout mitigation method. Because the plasma sheath is in the near-field region of the loop antenna, the traditional scattering matrix method that is developed for the far-field region may overestimate the electromagnetic (EM) wave's attenuation. To estimate the EM wave's attenuation in the near-field region, EM interference (EMI) shielding theory is introduced. Experiments are conducted, and the results verify the EMI shielding theory's effectiveness. Simulations are also conducted with different plasma parameters, and the results obtained show that the EM wave's attenuation in the near-field region is far below than that in the far-field region. The EM wave's attenuation increases with the increase in electron density and decreases with the increase in collision frequency. The higher the frequency, the larger is the EM wave's attenuation. During the entire re-entry phase of a RAM-C module, the EM wave's attenuations are below 10 dB for EM waves with a frequency of 1 MHz and below 1 dB for EM waves with a frequency of 100 kHz. Therefore, the low frequency systems (e.g., Loran-C) may provide a way to transmit some key information to high-speed vehicles even during the communication “blackout” period.

  3. The propagation characteristics of electromagnetic waves through plasma in the near-field region of low-frequency loop antenna

    NASA Astrophysics Data System (ADS)

    Liu, DongLin; Li, XiaoPing; Xie, Kai; Liu, ZhiWei

    2015-10-01

    A high-speed vehicle flying through the atmosphere between 100 and 20 km may suffer from a "communication blackout." In this paper, a low frequency system with an on-board loop antenna to receive signals is presented as a potential blackout mitigation method. Because the plasma sheath is in the near-field region of the loop antenna, the traditional scattering matrix method that is developed for the far-field region may overestimate the electromagnetic (EM) wave's attenuation. To estimate the EM wave's attenuation in the near-field region, EM interference (EMI) shielding theory is introduced. Experiments are conducted, and the results verify the EMI shielding theory's effectiveness. Simulations are also conducted with different plasma parameters, and the results obtained show that the EM wave's attenuation in the near-field region is far below than that in the far-field region. The EM wave's attenuation increases with the increase in electron density and decreases with the increase in collision frequency. The higher the frequency, the larger is the EM wave's attenuation. During the entire re-entry phase of a RAM-C module, the EM wave's attenuations are below 10 dB for EM waves with a frequency of 1 MHz and below 1 dB for EM waves with a frequency of 100 kHz. Therefore, the low frequency systems (e.g., Loran-C) may provide a way to transmit some key information to high-speed vehicles even during the communication "blackout" period.

  4. The Propagation of Slow Wave Potentials in Pea Epicotyls.

    PubMed Central

    Stahlberg, R.; Cosgrove, D. J.

    1997-01-01

    Slow wave potentials are considered to be electric long-distance signals specific for plants, although there are conflicting ideas about a chemical, electrical, or hydraulic mode of propagation. These ideas were tested by comparing the propagation of hydraulic and electric signals in epicotyls of pea (Pisum sativum L). A hydraulic signal in the form of a defined step increase in xylem pressure (Px) was applied to the root of intact seedlings and propagated nearly instantly through the epicotyl axis while its amplitude decreased with distance from the pressure chamber. This decremental propagation was caused by a leaky xylem and created an axial Px gradient in the epicotyl. Simultaneously along the epicotyl surface, depolarizations appeared with lag times that increased acropetally with distance from the pressure chamber from 5 s to 3 min. When measured at a constant distance, the lag times increased as the size of the applied pressure steps decreased. We conclude that the Px gradient in the epicotyl caused local depolarizations with acropetally increasing lag times, which have the appearance of an electric signal propagating with a rate of 20 to 30 mm min-1. This static description of the slow wave potentials challenges its traditional classification as a propagating electric signal. PMID:12223601

  5. Slow and fast light propagation in nonlinear Kerr media.

    NASA Astrophysics Data System (ADS)

    Yang, Qiguang; Ma, Seongmin; Wang, Huitian; Jung, S. S.

    2005-04-01

    Sub- and superluminal propagation of light pulse in Kerr materials has been investigated. Group velocities as slow as much less than 1 millimeter per second to as fast as negative several hundreds meters per second can be easily obtained in Kerr medium, which possesses large nonlinear refractive index and long relaxation time, such as Cr doped Alexandrite, Ruby, and GdAlO3. The physical mechanism is the strong highly dispersive coupling between different frequency components of the pulse. The new mechanism of slowing down pulses as well as producing superluminal pulses enlarges the very specific materials to all kinds of nonlinear optical materials.

  6. Low-Frequency Type II Radio Detections and Coronagraph Data to Describe and Forecast the Propagation of 71 CMEs/Shocks

    NASA Astrophysics Data System (ADS)

    St Cyr, O. C.; Cremades, H.; Iglesias, F. A.; Xie, H.; Kaiser, M. L.; Gopalswamy, N.

    2015-12-01

    Motivated by improving predictions of arrival times at Earth of shocks driven by coronal mass ejections (CMEs), we have analyzed 71 Earth-directed events in different stages of their propagation. The study is primarily based on approximated locations of interplanetary (IP) shocks derived from Type-II radio emissions detected by the Wind/WAVES experiment during 1997-2007. Distance-time diagrams resulting from the combination of white-light corona, IP Type-II radio, and in situ data lead to the formulation of descriptive profiles of each CME's journey toward Earth. Furthermore, two different methods to track and predict the location of CME-driven IP shocks are presented. The linear method, solely based on Wind/WAVES data, arises after key modifications to a pre-existing technique that linearly projects the drifting low-frequency Type-II emissions to 1 AU. This upgraded method improves forecasts of shock arrival time by almost 50%. The second predictive method is proposed on the basis of information derived from the descriptive profiles, and relies on a single CME height-time point and on low-frequency Type-II radio emissions to obtain an approximate value of the shock arrival time at Earth. In addition, we discuss results on CME-radio emission associations, characteristics of IP propagation, and the relative success of the forecasting methods.

  7. Low-Frequency Type-II Radio Detections and Coronagraph Data Employed to Describe and Forecast the Propagation of 71 CMEs/Shocks

    NASA Astrophysics Data System (ADS)

    Cremades, H.; Iglesias, F. A.; St. Cyr, O. C.; Xie, H.; Kaiser, M. L.; Gopalswamy, N.

    2015-09-01

    Motivated by improving predictions of arrival times at Earth of shocks driven by coronal mass ejections (CMEs), we have analyzed 71 Earth-directed events in different stages of their propagation. The study is primarily based on approximated locations of interplanetary (IP) shocks derived from Type-II radio emissions detected by the Wind/WAVES experiment during 1997 - 2007. Distance-time diagrams resulting from the combination of white-light corona, IP Type-II radio, and in-situ data lead to the formulation of descriptive profiles of each CME's journey toward Earth. Furthermore, two different methods for tracking and predicting the location of CME-driven IP shocks are presented. The linear method, solely based on Wind/WAVES data, arises after key modifications to a pre-existing technique that linearly projects the drifting low-frequency Type-II emissions to 1 AU. This upgraded method improves forecasts of shock-arrival times by almost 50 %. The second predictive method is proposed on the basis of information derived from the descriptive profiles and relies on a single CME height-time point and on low-frequency Type-II radio emissions to obtain an approximate value of the shock arrival time at Earth. In addition, we discuss results on CME-radio emission associations, characteristics of IP propagation, and the relative success of the forecasting methods.

  8. Shift of the interference extrema of low-frequency acoustic propagations near the axis of a deep sound channel

    NASA Astrophysics Data System (ADS)

    Lee, Seongwook; Na, Jungyul; Yoo, Jae Myung; Jurng, Moon-Sub; Oh, Suntaek

    2015-07-01

    Broadband interference patterns measured from acoustic propagations near the axis of a deep sound channel are interpreted. Analyses using mode theory for the waveguide with bilinear sound speed profiles show that the increase in sound speed without gradient variation shifts the positions of intensity maxima to higher frequencies in a fixed range whereas the increase in the gradient shifts the maxima to lower frequencies. Analytic results imply that the frequency shift of intensity extrema appearing in the measurements could be explained by the increase in the sound speed gradient above the axis of the deep sound channel.

  9. Elephant low-frequency vocalizations propagate in the ground and seismic playbacks of these vocalizations are detectable by wild African elephants (Loxodonta africana)

    NASA Astrophysics Data System (ADS)

    O'Connell-Rodwell, Caitlin E.; Wood, Jason D.; Gunther, Roland; Klemperer, Simon; Rodwell, Timothy C.; Puria, Sunil; Sapolsky, Robert; Kinzley, Colleen; Arnason, Byron T.; Hart, Lynette A.

    2001-05-01

    Seismic correlates of low-frequency vocalizations in African and Asian elephants propagate in the ground at different velocities, with the potential of traveling farther than their airborne counterparts. A semblance technique applied to linear moveouts on narrow-bandpass-filtered data, coupled with forward modeling, demonstrates that the complex waves observed are the interference of an air wave and a Rayleigh wave traveling at the appropriate velocities. The Rayleigh wave appears to be generated at or close to the elephant, either by coupling through the elephant's body or through the air near the body to the ground. Low-frequency elephant vocalizations were reproduced seismically and played back to both a captive elephant and to elephant breeding herds in the wild, monitoring the elephants' behavioral responses, spacing between herd members and time spent at the water hole as an index of heightened vigilance. Breeding herds detected and responded appropriately to seismically transmitted elephant warning calls. The captive studies promise to elucidate a vibrotactile threshold of sensitivity for the elephant foot. Elephants may benefit from the exploitation of seismic cues as an additional communication modality, thus expanding their signaling repertoire and extending their range of potential communication and eavesdropping beyond that possible with airborne sound.

  10. Investigation into the propagation of Omega very low frequency signals and techniques for improvement of navigation accuracy including differential and composite omega

    NASA Technical Reports Server (NTRS)

    1973-01-01

    An analysis of Very Low Frequency propagation in the atmosphere in the 10-14 kHz range leads to a discussion of some of the more significant causes of phase perturbation. The method of generating sky-wave corrections to predict the Omega phase is discussed. Composite Omega is considered as a means of lane identification and of reducing Omega navigation error. A simple technique for generating trapezoidal model (T-model) phase prediction is presented and compared with the Navy predictions and actual phase measurements. The T-model prediction analysis illustrates the ability to account for the major phase shift created by the diurnal effects on the lower ionosphere. An analysis of the Navy sky-wave correction table is used to provide information about spatial and temporal correlation of phase correction relative to the differential mode of operation.

  11. Low-Frequency Radioastronomy Basics

    NASA Astrophysics Data System (ADS)

    Zarka, P.

    2011-04-01

    With the many large instruments in construction or in project, the present epoch corresponds to a renewal of low-frequency radioastronomy. The field will attract new researchers and students not expert of the radioastronomy techniques. With this audience in mind, we present here a very brief introduction to radioastronomy basics, including propagation and polarization of low-frequency radio waves as well as instrumental aspects. Basic formulas are given. The references and internet links will allow the interested reader to go further.

  12. On whether azimuthal isotropy and alongshelf translational invariance are present in low-frequency acoustic propagation along the New Jersey shelfbreak.

    PubMed

    Lynch, James F; Emerson, Chris; Abbot, Philip A; Gawarkiewicz, Glen G; Newhall, Arthur E; Lin, Ying-Tsong; Duda, Timothy F

    2012-02-01

    To understand the issues associated with the presence (or lack) of azimuthal isotropy and horizontal (along isobath) invariance of low-frequency (center frequencies of 600 Hz and 900 Hz) acoustic propagation in a shelfbreak environment, a series of experiments were conducted under the Autonomous Wide-Aperture Cluster for Surveillance component of the Shallow Water 2006 experiment. Transmission loss data reported here were from two mobile acoustic sources executing (nearly) circular tracks transmitting to sonobuoy receivers in the circle centers, and from one 12.5 km alongshelf acoustic track. The circle radii were 7.5 km. Data are from September 8, 2006. Details of the acoustic and environmental measurements are presented. Simple analytic and computer models are used to assess the variability expected due to the ocean and seabed conditions encountered. A comparison of model results and data is made, which shows preliminary consistency between the data and the models, but also points towards further work that should be undertaken specifically in enlarging the range and frequency parameter space, and in looking at integrated transmission loss. PMID:22352604

  13. Slow crack propagation in glass and creep prediction

    NASA Astrophysics Data System (ADS)

    Mallet, Celine; Fortin, Jerome; Gueguen, Yves

    2013-04-01

    The context of our study is the observation of the time-dependent deformation of cracked glass. The aim of our study is to observe the slow crack propagation, to quantify it and to predict finally the creep behavior. We performed creep experiments in compaction conditions in a triaxial cell, on cracked boro-silicate glass samples. The chemical composition of the investigated glass is very close to the composition of waste vitrified packages. The matrix of the original glass (OG) is perfectly amorphous, without porosity. A few isolated air bubbles are trapped during the glass flow. Cracks are introduced in the OG through thermal shocks. Strain and acoustic emission (AE) are recorded. Several experiments are performed at different confining pressures (15 or 25 MPa), different pore fluid conditions (with argon gas, considered as the dry case, with tap water saturated porosity, or with distilled water) and different temperatures (ambiant temperature, 50oC or 80oC). Linear increase of the volumetric strain is first observed. A dilatancy increase is recorded. Note that dilatancy does not appear in constant strain rate tests. Constant stress tests show that dilatancy develops during a time interval that depends on the stress level. In addition AE rate are recorded. A non zero AE rate is an evidence of crack propagation. We use a micro-mechanical model that gives the stress intensity factor at the crack tips. This factor depends on stress and geometrical parameters (all known). An exponential law describe the rate of crack propagation, as a function of temperature, environment and applied stresses. This model allows us to predict the creep rate in glass. Assuming a constant crack aspect ratio, crack length and volumetric strain are related. The volumetric strain rate is calculated from model and compared to the data.

  14. Modeling of slow crack propagation in heterogeneous rocks

    NASA Astrophysics Data System (ADS)

    Lengliné, Olivier; Stormo, Arne; Hansen, Alex; Schmittbuhl, Jean

    2015-04-01

    Crack propagation in heterogeneous media is a rich problem which involves the interplay of various physical processes. The problem has been intensively investigated theoretically, numerically, and experimentally, but a unifying model capturing all the experimental features has not been entirely achieved despite its broad range of implications in Earth sciences problems. The slow propagation of a crack front where long range elastic interactions are dominant, is of crucial importance to fill the gap between experiments and models. Several theoretical and numerical works have been devoted to quasi-static models. Such models give rise to an intermittent local activity characterized by a depinning transition and can be viewed as a critical phenomenon. However these models fail to reproduce all experimental conditions, notably the front morphology does not display any cross-over length with two different roughness exponents above and below the cross-over as observed experimentally. Here, we compare experimental observations of a slow interfacial crack propagation along an heterogeneous interface to numerical simulations from a cantilever fiber bundle model. The model consists of a planar set of brittle fibers between an elastic half-space and a rigid square root shaped plate which loads the system in a cantilever configuration. The latter is shown to provide an improved opening and stress field in the process zone around the crack tip. The model shares a similar scale invariant roughening of the crack front both at small and large scales and a similar power law distribution of the local velocity of the crack front to experiments. Implications for induced seismicity at the brittle-creep transition are discussed. We show that a creep route for induced seismicity is possible when heterogeneities exist along the fault. Indeed, seismic event occurrences in time and space are in strong relation with the development of the aseismic motion recorded during the experiment and the

  15. Crowdsourcing a Spatial Temporal Study of Low Frequency (LF) Propagation Effects Due to a Total Solar Eclipse: Engaging Students and Citizens in STEM

    NASA Astrophysics Data System (ADS)

    Lumsden, N. A.; Lukes, L.; Nelson, J.; Liles, W. C.; Kerby, K. C.; Crowov, F.; Rockway, J.

    2015-12-01

    The first experiments to study the effects of a solar eclipse on radio wave propagation were done in 1912 utilizing Low Frequency (LF; 30 - 300 kHz) radio waves at a handful of sites across Europe before any theory of the ionosphere had been confirmed and even before the word "ionosphere" existed. In the 1920s, a large cooperative experiment was promoted in the U.S. by Scientific American magazine. They collected over 2000 reports of AM broadcast stations from throughout the U.S. Unfortunately, many of the submissions were unusable because they lacked critical information such as date, time or location. We propose to use the 2017 solar eclipse over the continental U.S. to conduct the first wide-area LF propagation study. To perform this study, we plan to crowdsource the collection of the data by engaging student groups, citizens, and the scientific community. The tools for the different collection stations will consist of a simple homemade antenna, a simple receiver to convert the radio frequency (RF) signals to audio frequencies and a smart phone app. By using the time, date and location features of the smart phone, the problems experienced in the Scientific American experiment will be minimized. By crowdsourcing the observation sites, a number of different short, medium and long-paths studies can be obtained as the total eclipse crosses the continental U.S. The transmitter for this experiment will be WWVB located near Fort Collins, Colorado on 60.000 kHz. This is a U.S. frequency standard that is operated by NIST and transmits time codes. A second frequency, 55.500 kHz transmitted by a LF station in Dixon, CA is also being considered for this experiment. We will present an overall strategy for recruiting participants/crowdsourcing the RF collections during the 2017 total solar eclipse. Preliminary coverage calculations will be presented for WWVB and Dixon, as well as path loss calculations that can be expected during the solar eclipse condition. We will also

  16. Propagation and damping of slow MHD waves in a flowing viscous coronal plasma

    NASA Astrophysics Data System (ADS)

    Kumar, Nagendra; Kumar, Anil; Murawski, K.

    2016-04-01

    We investigate the propagation of slow MHD waves in a flowing viscous solar coronal plasma. The compressive viscosity and steady flow along and opposite to the wave propagation are taken into account to study the damping of slow waves. We numerically solve the MHD equations by MacCormack method to examine the effect of steady flow on the damping of slow MHD waves in viscous solar coronal plasma. Amplitude of velocity perturbation and damping time of slow waves decrease with the increase in the value of Mach number. Flow causes a phase shift in the perturbed velocity amplitude and an increase in wave period. The damping of slow waves in flowing viscous plasma is stronger than the damping of waves in viscous plasma. Slow wave in backward flow damps earlier than the wave in forward flow.

  17. Superluminal and Ultra-Slow Light Propagation in Room-Temperature Solids

    NASA Astrophysics Data System (ADS)

    Boyd, Robert W.; Bigelow, Matthew S.; Lepeshkin, Nick N.

    2004-12-01

    We have observed ultra-slow light propagation (57 m s-1) in ruby and superluminal (-800 m s-1) light propagation in alexandrite at room temperature. The modified light speed results from the rapid variation in refractive index associated with spectral holes and antiholes produced by the process of coherent population oscillations.

  18. Correlations between Kilohertz Quasi-periodic Oscillations and Low-Frequency Features Attributed to Radial Oscillations and Diffusive Propagation in the Viscous Boundary Layer around a Neutron Star

    NASA Astrophysics Data System (ADS)

    Titarchuk, Lev; Osherovich, Vladimir

    1999-06-01

    We present a dimensional analysis of two characteristic timescales in the boundary layer where the disk adjusts to the rotating neutron star (NS). The boundary layer is treated as a transition region between the NS surface and the first Keplerian orbit. The radial transport of the angular momentum in this layer is controlled by a viscous force defined by the Reynolds number, which in turn is related to the mass accretion rate. We show that the observed low-Lorentzian frequency is associated with radial oscillations in the boundary layer, where the observed break frequency is determined by the characteristic diffusion time of the inward motion of the matter in the accretion flow. Predictions of our model regarding relations between those two frequencies and the frequencies of kilohertz quasi-periodic oscillations (kHz QPOs) compare favorably with recent observations of the source 4U 1728-34. This Letter contains a theoretical classification of kHz QPOs in NS binaries and the related low-frequency features. Thus, results concerning the relationship between the low-Lorentzian frequency of viscous oscillations and the break frequency are presented in the framework of our model of kHz QPOs viewed as Keplerian oscillations in a rotating frame of reference.

  19. Influence of finite bandwidth on the propagation of information in fast- and slow-light media

    NASA Astrophysics Data System (ADS)

    Amano, Heisuke; Tomita, Makoto

    2016-06-01

    We examined the propagation of information encoded as nonanalytical points on temporally Gaussian-shaped optical pulses in fast- and slow-light systems. The bandwidth of the input pulses determined the sharpness of the nonanalytical points. A sharp bending nonanalytical point propagated with luminal velocity in both fast- and slow-light systems, in good agreement with relativistic causality. As the bandwidth was reduced, the bending point became broad and propagated with the relevant group velocities. This transition was, however, qualitatively different in the fast- and slow-light systems. We also examined the predictability of the future pulse shape beyond the practical nonanalytical point on the basis of the expansion. When the bandwidth was reduced below a critical value, the expansion well predicted the future pulse shape.

  20. Probing geomagnetic storm-driven magnetosphere-ionosphere dynamics in D-region via propagation characteristics of very low frequency radio signals

    NASA Astrophysics Data System (ADS)

    Nwankwo, Victor U. J.; Chakrabarti, Sandip K.; Ogunmodimu, Olugbenga

    2016-07-01

    The amplitude and phase of VLF/LF radio signals are sensitive to changes in electrical conductivity of the lower ionosphere which imprints its signature on the Earth-ionosphere waveguide. This characteristic makes it useful in studying sudden ionospheric disturbances, especially those related to prompt X-ray flux output from solar flares and gamma ray bursts (GRBs). However, strong geomagnetic disturbance and storm conditions are known to produce large and global ionospheric disturbances, which can significantly affect VLF radio propagation in the D region of the ionosphere. In this paper, using the data of three propagation paths at mid-latitudes (40-54°), we analyse the trend in variation of aspects of VLF diurnal signal under varying solar and geomagnetic space environmental conditions in order to identify possible geomagnetic footprints on the D region characteristics. We found that the trend of variations generally reflected the prevailing space weather conditions in various time scales. In particular, the 'dipping' of mid-day signal amplitude peak (MDP) occurs after significant geomagnetic perturbed or storm conditions in the time scale of 1-2 days. The mean signal amplitude before sunrise (MBSR) and mean signal amplitude after sunset (MASS) also exhibit storm-induced dipping, but they appear to be influenced by event's exact occurrence time and the highly variable conditions of dusk-to-dawn ionosphere. We also observed few cases of the signals rise (e.g., MDP, MBSR or MASS) following a significant geomagnetic event. This effect may be related to storms associated phenomena or effects arising from sources other than solar origin. The magnitude of induced dipping (or rise) significantly depends on the intensity and duration of event(s), as well as the propagation path of the signal. The post-storm day signal (following a main event, with lesser or significantly reduced geomagnetic activity) exhibited a tendency of recovery to pre-storm day level. In the

  1. Low frequency cultural noise

    NASA Astrophysics Data System (ADS)

    Sheen, Dong-Hoon; Shin, Jin Soo; Kang, Tae-Seob; Baag, Chang-Eob

    2009-09-01

    Abnormal cultural seismic noise is observed in the frequency range of 0.01-0.05 Hz. Cultural noise generated by human activities is generally observed in frequencies above 1 Hz, and is greater in the daytime than at night. The low-frequency noise presented in this paper exhibits a characteristic amplitude variation and can be easily identified from time domain seismograms in the frequency range of interest. The amplitude variation is predominantly in the vertical component, but the horizontal components also show variations. Low-frequency noise is markedly periodic, which reinforces its interpretation as cultural noise. Such noise is observed world-wide, but is limited to areas in the vicinity of railways. The amplitude variation in seismograms correlates strongly with railway timetables, and the waveform shows a wavelength shift associated with the Doppler effect, which indicates that the origin of seismic background noise in the frequency range 0.01-0.05 Hz is railways.

  2. Rate and rhythm dependency of propagation from normal myocardium to a Ba++, K+-induced slow response zone in rabbit left atrium.

    PubMed

    Masuda, M O; de Carvalho, A P

    1982-03-01

    Strips of rabbit atrium 2-3 mm wide and 10-12 mm long were used to study how normal propagating action potentials excite a region where slow responses are the only form of electrical activity. One end of the preparation was bathed in normal Tyrode's solution. The rest was exposed to Tyrode's solution with high K+ (12.7 mM) and Ba++ (1 mM) (TKBa solution). The normal end was electrically stimulated and activity was monitored extra- and intracellularly (3 M KCl microelectrodes) as it propagated into the TKBa-treated region. We observed that the slow response could be elicited optimally by the normal action potential only within a limited range of stimulation frequencies (from around 0.3 to 1 Hz). At higher frequencies, progressive "fatigue" of slow response was observed. At frequencies lower than this range, normal action potentials were unable to stimulate slow response. To clarify the mechanism under this behavior, slow responses were directly elicited by electrical stimulation of strips wholly bathed in TKBa. Rate dependency of slow response excitability was again observed. Conclusions are: slow response excitability is enhanced transiently at each stimulation; the enhanced excitability state subsides very slowly and may take as long as 10 seconds to disappear; the enhanced state is cumulative from cycle to cycle so that excitability increases with increasing frequencies of stimulation above 0.1 Hz; and high frequency block due to fatigue and block due to excitability depression at low frequency delimit an optimal frequency window for slow response excitation and conduction. PMID:7060236

  3. Superluminal and slow light propagation in a room-temperature solid.

    PubMed

    Bigelow, Matthew S; Lepeshkin, Nick N; Boyd, Robert W

    2003-07-11

    We have observed both superluminal and ultraslow light propagation in an alexandrite crystal at room temperature. Group velocities as slow as 91 meters per second to as fast as -800 meters per second were measured and attributed to the influence of coherent population oscillations involving chromium ions in either mirror or inversion sites within the crystal lattice. Namely, ions in mirror sites are inversely saturable and cause superluminal light propagation, whereas ions in inversion sites experience conventional saturable absorption and produce slow light. This technique for producing large group indices is considerably easier than the existing methods to implement and is therefore suitable for diverse applications. PMID:12855803

  4. Low-frequency pulse propagation over 510 km in the Philippine Sea: A comparison of observed and theoretical pulse spreading.

    PubMed

    Andrew, Rex K; Ganse, Andrew; White, Andrew W; Mercer, James A; Dzieciuch, Matthew A; Worcester, Peter F; Colosi, John A

    2016-07-01

    Observations of the spread of wander-corrected averaged pulses propagated over 510 km for 54 h in the Philippine Sea are compared to Monte Carlo predictions using a parabolic equation and path-integral predictions. Two simultaneous m-sequence signals are used, one centered at 200 Hz, the other at 300 Hz; both have a bandwidth of 50 Hz. The internal wave field is estimated at slightly less than unity Garrett-Munk strength. The observed spreads in all the early ray-like arrivals are very small, <1 ms (for pulse widths of 17 and 14 ms), which are on the order of the sampling period. Monte Carlo predictions show similar very small spreads. Pulse spread is one consequence of scattering, which is assumed to occur primarily at upper ocean depths where scattering processes are strongest and upward propagating rays refract downward. If scattering effects in early ray-like arrivals accumulate with increasing upper turning points, spread might show a similar dependence. Real and simulation results show no such dependence. Path-integral theory prediction of spread is accurate for the earliest ray-like arrivals, but appears to be increasingly biased high for later ray-like arrivals, which have more upper turning points. PMID:27475148

  5. Energetic Particle Propagation in the Inner Heliosphere as Deduced from Low Frequency (less than 100 kHz) Observations of Type III Radio Bursts

    NASA Technical Reports Server (NTRS)

    Cane, H. V.; Erickson, W. C.

    2003-01-01

    Solar energetic particle (SEP) events are well-associated with solar flares. It is observed that the delay between the time of the flare and the first-arriving particles at a spacecraft increases with increasing difference between the flare longitude and the footpoint of the field line on which the spacecraft is located. This difference we call the "connection angle" and can be as large as approximately 120 deg. Recently it has been found that all SEP events are preceded by type III radio bursts. These bursts are plasma emission caused by the propagation of 2-50 keV flare electrons through the solar corona and into the solar wind. The drift of these type III radio bursts to lower and lower frequencies enables the propagation of the flare electrons to be traced from the Sun to about 1 AU. We have made an extensive analysis of the type III bursts associated with greater than 20 MeV proton events and find that, in most cases, the radio emission extends to the local plasma frequency when the energetic particles arrive within a few hours of the flare. We conclude that this emission at the lowest possible frequency is generated close to the spacecraft. We then use the time from when the burst started at the Sun to when it reached the local plasma frequency to infer the time it took the radio producing electrons to travel to the spacecraft. We find that these delay times are organized by the connection angle and correlate with the proton delay times. We also find that the differences between the radio delays at Wind and Ulysses are matched by differences in the relative arrival times of the energetic particles at the two spacecraft. The consistent timing between the relative arrival times of energetic electrons and protons and the start of the lowest frequency radio emissions suggests that the first arriving particles of both species are accelerated as part of the flare process and that they propagate to the spacecraft along trajectories similar to those of the lower

  6. Low Frequency Sky Surveys

    NASA Astrophysics Data System (ADS)

    Lubin, Philip M.

    2015-08-01

    We propose to survey the sky from 10-100 GHz covering greater than 50% of the sky in intensity and polarizatiton. This will allow us to mep out the synchrotron and free - free background as well as the spinning dust component to sufficient sensitivity to allow detailed modeling and removal of the galactic foregrounds allowing for deeper polarization surveys searching for signatures of inflation. While most measurements have concentrated on the region above 100 GHz this reggion is more complex in dust contmination that originally thought. Dust is best measured at high frequencies but the atmosphere greatly hinders extremely deep dust surveys due to water vapor. Surveys ar low frequency will be complimentary to the higher frequency measurements.

  7. Modeling of Reflective Propagating Slow-mode Wave in a Flaring Loop

    NASA Astrophysics Data System (ADS)

    Fang, X.; Yuan, D.; Van Doorsselaere, T.; Keppens, R.; Xia, C.

    2015-11-01

    Quasi-periodic propagating intensity disturbances have been observed in large coronal loops in extreme ultraviolet images over a decade, and are widely accepted to be slow magnetosonic waves. However, spectroscopic observations from Hinode/EIS revealed their association with persistent coronal upflows, making this interpretation debatable. We perform a 2.5D magnetohydrodynamic simulation to imitate the chromospheric evaporation and the following reflected patterns in a flare loop. Our model encompasses the corona, transition region, and chromosphere. We demonstrate that the quasi periodic propagating intensity variations captured by the synthesized Solar Dynamics Observatory/Atmospheric Imaging Assembly 131, 94 Å emission images match the previous observations well. With particle tracers in the simulation, we confirm that these quasi periodic propagating intensity variations consist of reflected slow mode waves and mass flows with an average speed of 310 km s‑1 in an 80 Mm length loop with an average temperature of 9 MK. With the synthesized Doppler shift velocity and intensity maps of the Solar and Heliospheric Observatory/Solar Ultraviolet Measurement of Emitted Radiation Fe xix line emission, we confirm that these reflected slow mode waves are propagating waves.

  8. Simulating Reflective Propagating Slow-wave/flow in a Flaring Loop

    NASA Astrophysics Data System (ADS)

    Fang, X.

    2015-12-01

    Quasi-periodic propagating intensity disturbances have been observed in large coronal loops in EUV images over a decade, and are widely accepted to be slow magnetosonic waves. However, spectroscopic observations from Hinode/EIS revealed their association with persistent coronal upflows, making this interpretation debatable. We perform a 2.5D magnetohydrodynamic simulation to imitate the chromospheric evaporation and the following reflected patterns in a post flare loop. Our model encompasses the corona, transition region, and chromosphere. We demonstrate that the quasi periodic propagating intensity variations captured by our synthesized AIA 131, 94~Å~emission images match the previous observations well. With particle tracers in the simulation, we confirm that these quasi periodic propagating intensity variations consist of reflected slow mode waves and mass flows with an average speed of 310 km/s in an 80 Mm length loop with an average temperature of 9 MK. With the synthesized Doppler shift velocity and intensity maps in SUMER Fe XIX line emission, we confirm that these reflected slow mode waves are propagating waves.

  9. Rapid high-amplitude circumferential slow wave propagation during normal gastric pacemaking and dysrhythmias

    PubMed Central

    O'Grady, Gregory; Du, Peng; Paskaranandavadivel, Nira; Angeli, Timothy R.; Lammers, Wim JEP; Asirvatham, Samuel J.; Windsor, John A.; Farrugia, Gianrico; Pullan, Andrew J.; Cheng, Leo K.

    2012-01-01

    Background Gastric slow waves propagate aborally as rings of excitation. Circumferential propagation does not normally occur, except at the pacemaker region. We hypothesized that: i) the unexplained high-velocity, high-amplitude activity associated with the pacemaker region is a consequence of circumferential propagation; ii) rapid, high-amplitude circumferential propagation emerges during gastric dysrhythmias; iii) the driving network conductance might switch between ICC-MP and circular ICC-IM during circumferential propagation; iv) extracellular amplitudes and velocities are correlated. Methods An experimental-theoretical study was performed. HR gastric mapping was performed in pigs during normal activation, pacing and dysrhythmia. Activation profiles, velocities and amplitudes were quantified. ICC pathways were theoretically evaluated in a bidomain model. Extracellular potentials were modelled as a function of membrane potentials. Key Results High-velocity, high-amplitude activation was only recorded in the pacemaker region when circumferential conduction occurred. Circumferential propagation accompanied dysrhythmia in 8/8 experiments, was faster than longitudinal propagation (8.9 vs 6.9 mm/s; p=0.004), and of higher amplitude (739 vs 528 μV; p=0.007). Simulations predicted that ICC-MP could be the driving network during longitudinal propagation, whereas during ectopic pacemaking, ICC-IM could outpace and activate ICC-MP in the circumferential axis. Experimental and modeling data demonstrated a linear relationship between velocities and amplitudes (p<0.001). Conclusions & Inferences The high-velocity and high-amplitude profile of the normal pacemaker region is due to localized circumferential propagation. Rapid circumferential propagation also emerges during a range of gastric dysrhythmias, elevating extracellular amplitudes and organizing transverse wavefronts. One possible explanation for these findings is bidirectional coupling between ICC-MP and circular

  10. Measurement of the information velocity in fast- and slow-light optical pulse propagation

    NASA Astrophysics Data System (ADS)

    Stenner, Michael David

    This thesis describes a study of the velocity of information on optical pulses propagating through fast- and slow-light media. In fast- and slow-light media, the group velocity vg is faster than the speed of light in vacuum c (vg > c or vg < 0) or slower than c (0 < vg < c) respectively. While it is largely accepted that optical pulses can travel at these extreme group velocities, the velocity of information encoded on them is still the subject of considerable debate. There are many contradictory theories describing the velocity of information on optical pulses, but no accepted techniques for its experimental measurement. The velocity of information has broad implications for the principle of relativistic causality (which requires that information travels no faster than c) and for modern communications and computation. In this thesis, a new technique for measuring the information velocity vi is described and implemented for fast- and slow-light media. The fast- and slow-light media are generated using modern dispersion-tailoring techniques that use large atomic coherences to generate strong normal and anomalous dispersion. The information velocity in these media can then be measured using information-theoretic concepts by creating an alphabet of two distinct pulse symbols and transmitting the symbols through the media. By performing a detailed statistical analysis of the received information as a function of time, it is possible to calculate vi. This new technique makes it possible for the first time to measure the velocity of information on optical pulses. Applying this technique to fast-light pulses, where vg/c = -0.051 +/- 0.002, it is found that vi /c = 0.4(+0.7--0.2). In the slow-light case, where vg/c = 0.0097 +/- 0.0003, information is found to propagate at vi/c = 0.6. In the slow-light case, the error bars are slightly more complicated. The fast bound is -0.5c (which is faster than positive values) and the slow bound is 0.2c . These results represent the

  11. Propagation of Long-Wavelength Nonlinear Slow Sausage Waves in Stratified Magnetic Flux Tubes

    NASA Astrophysics Data System (ADS)

    Barbulescu, M.; Erdélyi, R.

    2016-05-01

    The propagation of nonlinear, long-wavelength, slow sausage waves in an expanding magnetic flux tube, embedded in a non-magnetic stratified environment, is discussed. The governing equation for surface waves, which is akin to the Leibovich-Roberts equation, is derived using the method of multiple scales. The solitary wave solution of the equation is obtained numerically. The results obtained are illustrative of a solitary wave whose properties are highly dependent on the degree of stratification.

  12. Multi-channel wireless mapping of gastrointestinal serosal slow wave propagation

    PubMed Central

    Paskaranandavadivel, Niranchan; Wang, Rui; Sathar, Shameer; O’Grady, Gregory; Cheng, Leo K; Farajidavar, Aydin

    2015-01-01

    Background High-resolution (HR) extracellular mapping allows accurate profiling of normal and dysrhythmic slow wave patterns. A current limitation is that cables traverse the abdominal wall or a natural orifice, risking discomfort, dislodgement or infection. Wireless approaches offer advantages, but a multi-channel system is required, capable of recording slow waves and mapping propagation with high fidelity. Methods A novel multi-channel (n=7) wireless mapping system was developed and compared to a wired commercial system. Slow wave signals were recorded from the porcine gastric and intestinal serosa in-vivo. Signals were simultaneously acquired using both systems, and were filtered and processed to map activation wavefronts. For validation, the frequency and amplitude of detected events were compared, together with the speed and direction of mapped wavefronts. Key Results The wireless device achieved comparable signal quality to the reference device, and slow wave frequencies were identical. Amplitudes of the acquired gastric and intestinal slow wave signals were consistent between the devices. During normal propagation, spatiotemporal mapping remained accurate in the wireless system, however, during ectopic dysrhythmic pacemaking, the lower sampling resolution of the wireless device led to reduced accuracy in spatiotemporal mapping. Conclusions and Inferences A novel multichannel wireless device is presented for mapping slow wave activity. The device achieved high quality signals, and has the potential to facilitate chronic monitoring studies and clinical translation of spatiotemporal mapping. The current implementation may be applied to detect normal patterns and dysrhythmia onset, but HR mapping with finely spaced arrays currently remains necessary to accurately define dysrhythmic patterns. PMID:25599978

  13. Propagation of quasisolitons in a fiber Bragg grating written in a slow saturable fiber amplifier

    SciTech Connect

    Shapira, Yuval P.; Horowitz, Moshe

    2011-05-15

    We show, by using numerical simulations, that quasisolitons can propagate over a long distance in a fiber Bragg grating that is written in a slow saturable fiber amplifier, such as an erbium-doped fiber amplifier. During the pulse propagation, the front end of the pulse experiences a net gain while the rear end of pulse is attenuated due to the combination of gain saturation and loss. However, the pulse profile almost does not change after propagating over a length of 5 m that is approximately 2500 times larger than the spatial pulse width. The pulse amplitude has an approximately hyperbolic secant profile. We develop a reduced model by using a multiscale analysis to study solitary-wave propagation when nonlinearity and gain are small. When gain saturation also becomes small we find analytically a new family of solitary-wave hyperbolic-secant solutions that approximately solve the reduced model. The solitary waves propagate slightly faster than Bragg solitons that propagate in fiber Bragg gratings without gain and loss.

  14. Low Frequency Radio Experiment (LORE)

    NASA Astrophysics Data System (ADS)

    Manoharan, P. K.; Naidu, Arun; Joshi, B. C.; Roy, Jayashree; Kate, G.; Pethe, Kaiwalya; Galande, Shridhar; Jamadar, Sachin; Mahajan, S. P.; Patil, R. A.

    2016-03-01

    In this paper, we present a case study of Low Frequency Radio Experiment (LORE) payload to probe the corona and the solar disturbances at solar offsets greater than 2 solar radii, i.e., at frequencies below 30 MHz. The LORE can be complimentary to the planned Indian solar mission, “Aditya-L1” and its other payloads as well as synergistic to ground-based interplanetary scintillation (IPS) observations, which are routinely carried out by the Ooty Radio Telescope. We discuss the baseline design and technical details of the proposed LORE and its particular suitability for providing measurements on the detailed time and frequency structure of fast drifting type-III and slow drifting type-II radio bursts with unprecedented time and frequency resolutions. We also brief the gonio-polarimetry, which is possible with better-designed antennas and state-of-the-art electronics, employing FPGAs and an intelligent data management system. These would enable us to make a wide range of studies, such as nonlinear plasma processes in the Sun-Earth distance, in-situ radio emission from coronal mass ejections (CMEs), interplanetary CME driven shocks, nature of ICMEs driving decelerating IP shocks and space weather effects of solar wind interaction regions.

  15. Oscillatory instability in slow crack propagation in rubber under large deformation.

    PubMed

    Endo, Daiki; Sato, Katsuhiko; Hayakawa, Yoshinori

    2012-07-01

    We performed experiments to investigate slow fracture in thin rubber films under uniaxial tension using high-viscosity oils. In this system we observed an oscillating instability in slowly propagating cracks for small applied strains. The transition between oscillatory and straight patterns occurred near the characteristic strain at which rubber exhibits a nonlinear stress-strain relation. This suggests that nonlinear elasticity plays an important role in the formation of the observed pattern. This was confirmed by numerical simulation for neo-Hookean and linear elasticity models. PMID:23005490

  16. Oscillatory instability in slow crack propagation in rubber under large deformation

    NASA Astrophysics Data System (ADS)

    Endo, Daiki; Sato, Katsuhiko; Hayakawa, Yoshinori

    2012-07-01

    We performed experiments to investigate slow fracture in thin rubber films under uniaxial tension using high-viscosity oils. In this system we observed an oscillating instability in slowly propagating cracks for small applied strains. The transition between oscillatory and straight patterns occurred near the characteristic strain at which rubber exhibits a nonlinear stress-strain relation. This suggests that nonlinear elasticity plays an important role in the formation of the observed pattern. This was confirmed by numerical simulation for neo-Hookean and linear elasticity models.

  17. Low Frequency Radio Experiment (LORE)

    NASA Astrophysics Data System (ADS)

    Manoharan, Periasamy K.; Joshi, Bhal Chandra; Naidu, Arun Kumar

    High temporal and frequency resolution observations of solar generated disturbances below 15 MHz in the near-Sun region and at Sun-Earth distances in conjunction with optical and high energy observations of Sun are essential to understand the structure and evolution of eruptions, such as, flares, coronal mass ejections (CMEs), and their associated solar wind disturbances at heights above the photosphere and their consequences in the interplanetary medium. This talk presents a case study of Low Frequency Radio Experiment (LORE) payload to probe the corona and the solar disturbances at solar offsets greater than 2 solar radii below 30 MHz. The LORE, although not part of Aditya-L1 mission, can be complimentary to planned Aditya-L1 coronagraph and its other on-board payloads as well as synergistic to ground based observations, which are routinely carried out by Ooty Radio Telescope. We discuss the baseline design and technical details of the proposed LORE and it is particularly suitable for providing data on the detailed time and frequency structure of fast drifting Type-III and slow drifting Type-II radio bursts with unprecedented time and frequency resolution as well as goniopolarimetry, made possible with better designed antennas and state-of-art electronics, employing FPGAs and an intelligent data management system. This would enable wide ranging studies such as studies of nonlinear plasma processes, CME in-situ radio emission, CME driven phenomena, interplanetary CME driven shocks, ICMEs driven by decelerating IP shocks and space weather effects of Solar Wind interaction regions. The talk will highlight the science objectives as well as the proposed technical design features.

  18. 'Burst-Like' Slow Slip Propagation on Frictional Faults in the Laboratory

    NASA Astrophysics Data System (ADS)

    Parker, J.; Selvadurai, P. A.; Glaser, S. D.

    2015-12-01

    We present laboratory findings on burst-like premonitory slip propagation that leads to fault rupture. The experiments take place on a PMMA-PMMA interface in a direct shear configuration, where the effective strength heterogeneity is controlled by the non-uniform distribution of asperities throughout the fault. A pressure sensitive film was used to locate, size and measure normal stress on individual asperities. Prior to rapid sliding, we observed slow premonitory slip which accumulated non-uniformly along the fault. Slow displacement was measured using slip sensors placed at seven locations along the fault strike and showed intermittent, 'burst-like' increases in spectral power between the frequencies of 60 to 150 Hz. Each burst event lasted between 5 to 12 seconds, and a local increase in the extended fault slip rate was observed after its cessation. The 'burst-like' features migrated along the fault at speeds between Vprop ~ 1.3 mm/s to 9.3 mm/s. Propagation speed of the 'burst-like' front Vprop was dependent on the normal stress confining the fault σn- increased normal stress caused the rupture to move slower by increasing 'effective fault strength'. Finally, foreshocks were recorded using absolutely calibrated acoustic emission sensors and occurred at the later stages of the slow slip phase. The source radii of the foreshocks [Brune, 1970] ranged from 0.21 to 1.09 mm and their ruptures occurred over timescales ~5-7 orders of magnitude faster than the 'burst-like' slow slip signals. Observations of similar variations in time scales have been made between regular earthquakes and aseismic transients in the field [Ide, 2007]. These results will help develop a mechanistic understanding of the effective fault strength heterogeneity necessary for the development of slow earthquakes and tremor-like shaking. References: Ide, S., D. R. Beroza, G. C. Shelly & T. Uchide (2007), 'A scaling law for slow earthquakes', Nature 447, 76-79. Brune, J. N. (1970), 'Tectonic stress

  19. Local propagation speed constrained estimation of the slowness vector from non-planar array observations.

    PubMed

    Nouvellet, Adrien; Roueff, François; Le Pichon, Alexis; Charbit, Maurice; Vergoz, Julien; Kallel, Mohamed; Mejri, Chourouq

    2016-01-01

    The estimation of the slowness vector of infrasound waves propagating across an array is a critical process leading to the determination of parameters of interest such as the direction of arrival. The sensors of an array are often considered to be located in a horizontal plane. However, due to topography, the altitudes of the sensors are not identical and introduce a bias on the estimate if neglected. However, the unbiased 3D estimation procedure, while suppressing the bias, leads to an increase of the variance. Accounting for an a priori constraint on the slowness vector significantly reduces the variance and could therefore improve the performance of the estimation if the introduced bias by incorrect a priori information remains negligible. This study focuses on measuring the benefits of this approach with a thorough investigation of the bias and variance of the constrained 3D estimator, which is not available in the existing literature. This contribution provides such computations based on an asymptotic Gaussian approximation. Simulations are carried out to assess the theoretical results both with synthetic and real data. Thus, a constrained 3D estimator is proposed yielding the best bias/variance compromise if good knowledge of the propagation wave speed is accessible. PMID:26827049

  20. Analysis of Jovian low frequency radio emissions

    NASA Technical Reports Server (NTRS)

    Gurnett, D. A.

    1985-01-01

    The density of ions in the Io plasma torus and the scattering of these ions by low frequency electromagnetic emissions detected by Voyager 1 were studied. The ion density profile was investigated using whistler dispersion measurements provided by the Voyager plasma instrument. The scale height and absolute density of H+ ions in the vicinity of the plasma torus were determined by combining the measured plasma densities with the whistler dispersion measurements. A theoretical analysis of the modes of propagation of low frequency electromagnetic emissions in the torus was undertaken. Polarization reversal effects and rough estimates of the ion diffusion coefficient were utilized. Numerical evaluation of the ion diffusion coefficients in the torus were made using the observed Voyager 1 wave intensities. Results show that the observed wave intensities produce significant ion diffusion effects in the ion torus.

  1. Frequency-dependent damping in propagating slow magneto-acoustic waves

    SciTech Connect

    Prasad, S. Krishna; Banerjee, D.; Van Doorsselaere, T.

    2014-07-10

    Propagating slow magneto-acoustic waves are often observed in polar plumes and active region fan loops. The observed periodicities of these waves range from a few minutes to a few tens of minutes and their amplitudes were found to decay rapidly as they travel along the supporting structure. Previously, thermal conduction, compressive viscosity, radiation, density stratification, and area divergence were identified to be some of the causes for change in the slow wave amplitude. Our recent studies indicate that the observed damping in these waves is frequency-dependent. We used imaging data from the Solar Dynamics Observatory/Atmospheric Imaging Assembly to study this dependence in detail and for the first time via observations we attempted to deduce a quantitative relation between the damping length and frequency of these oscillations. We developed a new analysis method to obtain this relation. The observed frequency dependence does not seem to agree with the current linear wave theory and it was found that the waves observed in the polar regions show a different dependence from those observed in the on-disk loop structures despite the similarity in their properties.

  2. Compared propagation characteristics of superluminal and slow light in SOA and EDFA based on rectangle signals

    NASA Astrophysics Data System (ADS)

    Wang, Fu; Wang, Zhi; Wu, Chongqing; Sun, Zhenchao; Mao, Yaya; Liu, Lanlan; Li, Qiang

    2015-10-01

    Based on the general mechanism of the coherent population oscillations (CPO) in the Semiconductor optical amplifiers (SOA) and Erbium doped fiber amplifiers (EDFA), the group time delay of rectangle signal propagating in the active media is deduced. Compared with the sinusoidal signal, the time delay difference between the fundamental harmonics (FHFD: fundamental harmonic fractional delay) is first investigated in detail for the rectangle signal which is more popularly used in the digital signal systems. The plenty of simulations based on the propagation equations and some experiments for the sinusoidal and rectangle signals are used to analyze the differences and evaluate the slow and superluminal light effects. Furthermore, the time delay/advance always takes place accompanying with the signal distortion, which is evaluated by the total harmonic distortion (THD). The distortion caused by the SOA is smaller than that by the EDFA. A factor Q which is defined to evaluate the trade-off between the FHFD and the THD, shows that higher input power or higher optical gain is better for optical signal processing and optical telecommunications, and the SOA is more suitable for the higher modulation frequency (>10 GHz).

  3. Superconducting microwave parametric amplifier based on a quasi-fractal slow propagation line

    NASA Astrophysics Data System (ADS)

    Adamyan, A. A.; de Graaf, S. E.; Kubatkin, S. E.; Danilov, A. V.

    2016-02-01

    Quantum limited amplifiers are sought after for a wide range of applications within quantum technologies and sensing. One promising candidate is the travelling wave parametric amplifier which exploits the non-linear kinetic inductance of a superconducting transmission line. This type of microwave amplifier promises to deliver a high gain, a quantum limited noise performance over several GHz bandwidth, and a high dynamic range. However, practical realizations of this type of device have so far been limited by fabrication defects, since the length of the superconducting transmission line required for achieving substantial parametric gain is on the order of ˜1 m. Here, we report on a design for a microwave traveling wave amplifier based on a slow propagation line comprising a central strip with high kinetic inductance and quasi-fractal line-to-ground capacitors. Due to an enhanced per unit length inductance (73 nH cm-1) and capacitance (15 pF cm-1), the line has a microwave propagation velocity as low as 9.8 × 108 cm s-1. This translates into parametric gain up to 0.5 dB cm-1 and a total gain of 6 dB for just a ˜10 cm long transmission line. Moreover, the flexibility of the presented design allows balancing the line inductance and capacitance in order to keep the characteristic impedance close to 50 Ω and to suppress standing waves, both factors being essential in order to implement a practical parametric amplifier in the microwave domain.

  4. Identification of low-frequency fluctuations in the terrestrial magnetosheath

    NASA Technical Reports Server (NTRS)

    Song, P.; Russell, C. T.; Gary, S. P.

    1994-01-01

    On the basis of magnetohydrodynamic (MHD) theory we develop a scheme for distinguishing among the four low-frequency modes which may propagate in a high-beta anisotropic plasma such as the magnetosheath: the fast and slow magnetosonic, the Alfven, and mirror modes. We use four parameters: the ratio of transverse to compressional powers in the magnetic field, the ratio of the wave powers in the thermal pressure and in the magnetic field, the ratio of the perturbations in the thermal and magnetic pressures, and the ratio of the wave powers in the velocity and in the magnetic field. In the test case of an Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) magnetosheath pass near the Sun-Earth line downstream of a quasi-perpendicular shock, the four modes can be clearly distinguished both spatially and spectrally. Near the bow shock, the waves are Alfvenic in a large frequency range, 1 to 100 mHz. In the middle and inner magnetosheath, the waves below 10 mHz are Alfvenic. The fast mode waves occur in the higher-frequency end of the enhanced spectrum, 80 mHz for the middle magnetosheath and 55 mHz for the inner sheath. The wave enhancement in the intermediate frequencies is slow modes in the inner sheath and mirror modes in the middle sheath. This confirms the earlier report of the existence of the slow mode waves near the magnetopause. These slow waves provide evidence that the magnetopause is an active source of the waves in the sheath. We also show that the measured frequency of a wave is close to an invariant if the magnetosheath flow is in a steady state. Therefore changes in the frequencies of enhanced waves indicate emergence, or damping, or mode conversion of the waves.

  5. Low-frequency VLBI in space and interstellar refraction

    SciTech Connect

    Dennison, B.; Booth, R.S.

    1986-08-01

    The proposed orbiting Quasat antenna, equipped with a low-frequency capability (e.g. 327 MHz), would be uniquely suited for studying refractive focusing (slow scintillation) in the interstellar medium, which is suspected of being responsible for at least some apparent low-frequency variability of extragalactic sources. The authors consider in some detail various technical considerations, including the decorrelating effects of the ionosphere and interplanetary medium, and conclude that low-frequency VLBI observations involving Quasat and Earth-based antennas would be feasible, particularly if sources are observed when they are in the anti-solar hemisphere.

  6. Extremely Low Frequency Electromagnetic Investigation on Mars

    NASA Astrophysics Data System (ADS)

    Kozakiewicz, Joanna; Kulak, Andrzej; Kubisz, Jerzy; Zietara, Krzysztof

    2016-07-01

    Natural electromagnetic (EM) signals of extremely low frequencies (ELF, 3 Hz-3 kHz) can be used to study many of the electromagnetic processes and properties occurring in the Martian environment. Sources of these signals, related to electrical activity in the atmosphere, are very significant since they can influence radio wave propagation on the planet, the atmospheric composition, and the ionospheric structure. In addition, such EM signals can be employed in many purposes such as: surveying the subsurface of Mars or studying the impact of the space weather on the Martian ionosphere. As ELF waves propagate on very long distances, it is possible to explore properties of the entire planet using single-station recordings. In this study, we propose an experiment that allows measuring ELF signals from the Martian surface. Such measurements can be used for detection of electric discharges in the atmosphere and water reservoirs in the planetary subsurface.

  7. Experimental evidence of dynamical propagation for solitary waves in ultra slow stochastic non-local Kerr medium.

    PubMed

    Louis, H; Tlidi, M; Louvergneaux, E

    2016-07-11

    We perform a statistical analysis of the optical solitary wave propagation in an ultra-slow stochastic non-local focusing Kerr medium such as liquid crystals. Our experimental results show that the localized beam trajectory presents a dynamical random walk whose beam position versus the propagation distance z depicts two different kind of evolutions A power law is found for the beam position standard deviation during the first stage of propagation. It obeys approximately z3/2 up to ten times the power threshold for solitary wave generation. PMID:27410886

  8. Experimental evidence of dynamical propagation for solitary waves in ultra slow stochastic non-local Kerr medium.

    PubMed

    Louis, H; Tlidi, M; Louvergneaux, E

    2016-07-11

    We perform a statistical analysis of the optical solitary wave propagation in an ultra-slow stochastic non-local focusing Kerr medium such as liquid crystals. Our experimental results show that the localized beam trajectory presents a dynamical random walk whose beam position versus the propagation distance z depicts two different kind of evolutions A power law is found for the beam position standard deviation during the first stage of propagation. It obeys approximately z3/2 up to ten times the power threshold for solitary wave generation. PMID:27410887

  9. LOFAR, the low frequency array

    NASA Astrophysics Data System (ADS)

    Vermeulen, R. C.

    2012-09-01

    LOFAR, the Low Frequency Array, is a next-generation radio telescope designed by ASTRON, with antenna stations concentrated in the north of the Netherlands and currently spread into Germany, France, Sweden and the United Kingdom; plans for more LOFAR stations exist in several other countries. Utilizing a novel, phased-array design, LOFAR is optimized for the largely unexplored low frequency range between 30 and 240 MHz. Digital beam-forming techniques make the LOFAR system agile and allow for rapid re-pointing of the telescopes as well as the potential for multiple simultaneous observations. Processing (e.g. cross-correlation) takes place in the LOFAR BlueGene/P supercomputer, and associated post-processing facilities. With its dense core (inner few km) array and long (more than 1000 km) interferometric baselines, LOFAR reaches unparalleled sensitivity and resolution in the low frequency radio regime. The International LOFAR Telescope (ILT) is now issuing its first call for observing projects that will be peer reviewed and selected for observing starting in December. Part of the allocations will be made on the basis of a fully Open Skies policy; there are also reserved fractions assigned by national consortia in return for contributions from their country to the ILT. In this invited talk, the gradually expanding complement of operationally verified observing modes and capabilities are reviewed, and some of the exciting first astronomical results are presented.

  10. PROPAGATING SLOW MAGNETOACOUSTIC WAVES IN CORONAL LOOPS OBSERVED BY HINODE/EIS

    SciTech Connect

    Wang, T. J.; Ofman, L.; Davila, J. M.

    2009-05-10

    We present the first Hinode/EUV Imaging Spectrometer observations of 5 minute quasi-periodic oscillations detected in a transition-region line (He II) and five coronal lines (Fe X, Fe XII, Fe XIII, Fe XIV, and Fe XV) at the footpoint of a coronal loop. The oscillations exist throughout the whole observation, characterized by a series of wave packets with nearly constant period, typically persisting for 4-6 cycles with a lifetime of 20-30 minutes. There is an approximate in-phase relation between Doppler shift and intensity oscillations. This provides evidence for slow magnetoacoustic waves propagating upward from the transition region into the corona. We find that the oscillations detected in the five coronal lines are highly correlated, and the amplitude decreases with increasing temperature. The amplitude of Doppler shift oscillations decrease by a factor of about 3, while that of relative intensity decreases by a factor of about 4 from Fe X to Fe XV. These oscillations may be caused by the leakage of the photospheric p-modes through the chromosphere and transition region into the corona, which has been suggested as the source for intensity oscillations previously observed by Transition Region and Coronal Explorer. The temperature dependence of the oscillation amplitudes can be explained by damping of the waves traveling along the loop with multithread structure near the footpoint. Thus, this property may have potential value for coronal seismology in diagnostic of temperature structure in a coronal loop.

  11. On Sun-to-Earth Propagation of Coronal Mass Ejections: II. Slow Events and Comparison with Others

    NASA Astrophysics Data System (ADS)

    Liu, Ying D.; Hu, Huidong; Wang, Chi; Luhmann, Janet G.; Richardson, John D.; Yang, Zhongwei; Wang, Rui

    2016-02-01

    As a follow-up study on Sun-to-Earth propagation of fast coronal mass ejections (CMEs), we examine the Sun-to-Earth characteristics of slow CMEs combining heliospheric imaging and in situ observations. Three events of particular interest, the 2010 June 16, 2011 March 25, and 2012 September 25 CMEs, are selected for this study. We compare slow CMEs with fast and intermediate-speed events, and obtain key results complementing the attempt of Liu et al. to create a general picture of CME Sun-to-Earth propagation: (1) the Sun-to-Earth propagation of a typical slow CME can be approximately described by two phases, a gradual acceleration out to about 20-30 solar radii, followed by a nearly invariant speed around the average solar wind level; (2) comparison between different types of CMEs indicates that faster CMEs tend to accelerate and decelerate more rapidly and have shorter cessation distances for the acceleration and deceleration; (3) both intermediate-speed and slow CMEs would have speeds comparable to the average solar wind level before reaching 1 au; (4) slow CMEs have a high potential to interact with other solar wind structures in the Sun-Earth space due to their slow motion, providing critical ingredients to enhance space weather; and (5) the slow CMEs studied here lack strong magnetic fields at the Earth but tend to preserve a flux-rope structure with an axis generally perpendicular to the radial direction from the Sun. We also suggest a “best” strategy for the application of a triangulation concept in determining CME Sun-to-Earth kinematics, which helps to clarify confusions about CME geometry assumptions in the triangulation and to improve CME analysis and observations.

  12. Spectroscopic observations of propagating disturbances in a polar coronal hole: evidence of slow magneto-acoustic waves

    NASA Astrophysics Data System (ADS)

    Gupta, G. R.; Teriaca, L.; Marsch, E.; Solanki, S. K.; Banerjee, D.

    2012-10-01

    Aims: We focus on detecting and studying quasi-periodic propagating features that have been interpreted in terms of both slow magneto-acoustic waves and of high-speed upflows. Methods: We analyzed long-duration spectroscopic observations of the on-disk part of the south polar coronal hole taken on 1997 February 25 by the SUMER spectrometer onboard SOHO. We calibrated the velocity with respect to the off-limb region and obtained time-distance maps in intensity, Doppler velocity, and line width. We also performed a cross-correlation analysis on different time series curves at different latitudes. We studied average spectral line profiles at the roots of propagating disturbances and along the propagating ridges, and performed a red-blue asymmetry analysis. Results: We clearly find propagating disturbances in intensity and Doppler velocity with a projected propagation speed of about 60 ± 4.8 km s-1 and a periodicity of ≈14.5 min. To our knowledge, this is the first simultaneous detection of propagating disturbances in intensity as well as in Doppler velocity in a coronal hole. During the propagation, an intensity enhancement is associated with a blueshifted Doppler velocity. These disturbances are clearly seen in intensity also at higher latitudes (i.e., closer to the limb), while disturbances in Doppler velocity become faint there. The spectral line profiles averaged along the propagating ridges are found to be symmetric, to be well fitted by a single Gaussian, and have no noticeable red-blue asymmetry. Conclusions: Based on our analysis, we interpret these disturbances in terms of propagating slow magneto-acoustic waves.

  13. Generation of unusually low frequency plasmaspheric hiss

    NASA Astrophysics Data System (ADS)

    Chen, Lunjin; Thorne, Richard M.; Bortnik, Jacob; Li, Wen; Horne, Richard B.; Reeves, G. D.; Kletzing, C. A.; Kurth, W. S.; Hospodarsky, G. B.; Spence, H. E.; Blake, J. B.; Fennell, J. F.

    2014-08-01

    It has been reported from Van Allen Probe observations that plasmaspheric hiss intensification in the outer plasmasphere, associated with a substorm injection on 30 September 2012, occurred with a peak frequency near 100 Hz, well below the typical plasmaspheric hiss frequency range, extending down to ˜20 Hz. We examine this event of unusually low frequency plasmaspheric hiss to understand its generation mechanism. Quantitative analysis is performed by simulating wave raypaths via the HOTRAY ray tracing code with measured plasma density and calculating raypath-integrated wave gain evaluated using the measured energetic electron distribution. We demonstrate that the growth rate due to substorm-injected electrons is positive but rather weak, leading to small wave gain (˜10 dB) during a single equatorial crossing. Propagation characteristics aided by the sharp density gradient associated with the plasmapause, however, can enable these low-frequency waves to undergo cyclic raypaths, which return to the unstable region leading to repeated amplification to yield sufficient net wave gain (>40 dB) to allow waves to grow from the thermal noise.

  14. Low Frequency Interstellar Scattering and Pulsar Observations

    NASA Technical Reports Server (NTRS)

    Cordes, James M.

    1992-01-01

    Radio astronomy at frequencies from 2 to 30 MHz challenges time tested methods for extracting usable information from observations. One fundamental reason for this is that propagation effects due to the magnetoionic ionosphere, interplanetary medium, and interstellar matter (ISM) increase strongly with wavelength. The problems associated with interstellar scattering off of small scale irregularities in the electron density are addressed. What is known about interstellar scattering is summarized on the basis of high frequency observations, including scintillation and temporal broadening of pulsars and angular broadening of various galactic and extragalactic radio sources. Then those high frequency phenomena are addressed that are important or detectable at low frequencies. The radio sky becomes much simpler at low frequencies, most pulsars will not be seen as time varying sources, intensity variations will be quenched or will occur on time scales much longer than a human lifetime, and many sources will be angularly broadened and/or absorbed into the noise. Angular broadening measurements will help delineate the galactic distribution and power spectrum of small scale electron density irregularities.

  15. Low-Frequency Waves in Space Plasmas

    NASA Astrophysics Data System (ADS)

    Keiling, Andreas; Lee, Dong-Hun; Nakariakov, Valery

    2016-02-01

    Low-frequency waves in space plasmas have been studied for several decades, and our knowledge gain has been incremental with several paradigm-changing leaps forward. In our solar system, such waves occur in the ionospheres and magnetospheres of planets, and around our Moon. They occur in the solar wind, and more recently, they have been confirmed in the Sun's atmosphere as well. The goal of wave research is to understand their generation, their propagation, and their interaction with the surrounding plasma. Low-frequency Waves in Space Plasmas presents a concise and authoritative up-to-date look on where wave research stands: What have we learned in the last decade? What are unanswered questions? While in the past waves in different astrophysical plasmas have been largely treated in separate books, the unique feature of this monograph is that it covers waves in many plasma regions, including: Waves in geospace, including ionosphere and magnetosphere Waves in planetary magnetospheres Waves at the Moon Waves in the solar wind Waves in the solar atmosphere Because of the breadth of topics covered, this volume should appeal to a broad community of space scientists and students, and it should also be of interest to astronomers/astrophysicists who are studying space plasmas beyond our Solar System.

  16. Earthquake triggering by slow earthquake propagation: the case of the large 2014 slow slip event in Guerrero, Mexico.

    NASA Astrophysics Data System (ADS)

    Radiguet, M.; Perfettini, H.; Cotte, N.; Gualandi, A.; Kostoglodov, V.; Lhomme, T.; Walpersdorf, A.; Campillo, M.; Valette, B.

    2015-12-01

    Since their discovery nearly two decades ago, the importance of slow slip events (SSEs) in the processes of strain accommodation in subduction zones has been revealed. Nevertheless, the influence of slow aseismic slip on the nucleation of large earthquakes remains unclear. In this study, we focus on the Guerrero region of the Central American subduction zone in Mexico, where large SSEs have been observed since 1998, with a recurrence period of about 4 years, and produce aseismic slip in the Guerrero seismic gap. We investigate the large 2014 SSE (equivalent Mw=7.7), which initiated in early 2014 and lasted until the end of October 2014. During this time period, the 18 April Papanoa earthquake (Mw7.2) occurred on the western limit of the Guerrero gap. We invert the continuous GPS time series using the PCAIM (Principal Component Analysis Inversion Method) to assess the space and time evolution of slip on the subduction. To focus on the aseismic processes, we correct the cGPS time series from the co-seismic offsets. Our results show that the slow slip event initiated in the Guerrero gap region, as already observed during the previous SSEs. The Mw7.2 Papanoa earthquake occurred on the western limit of the region that was slipping aseismically before the earthquake. After the Papanoa earthquake, the aseismic slip rate increases. This geodetic signal consists of both the ongoing SSE and the postseismic (afterslip) response due to the Papanoa earthquake. The majority of the post-earthquake aseismic slip is concentrated downdip from the main earthquake asperity, but significant slip is also observed in the Guerrero gap region. Compared to previous SSEs in that region, the 2014 SSE produced a larger aseismic slip and the maximum slip is located downdip from the main brittle asperity corresponding to the Papanoa earthquake, a region that was not identified as active during the previous SSEs. Since the Mw 7.2 Papanoa earthquake occurred about 2 months after the onset of the

  17. Characterization and Impact of Low Frequency Wind Turbine Noise Emissions

    NASA Astrophysics Data System (ADS)

    Finch, James

    Wind turbine noise is a complex issue that requires due diligence to minimize any potential impact on quality of life. This study enhances existing knowledge of wind turbine noise through focused analyses of downwind sound propagation, directionality, and the low frequency component of the noise. Measurements were conducted at four wind speeds according to a design of experiments at incremental distances and angles. Wind turbine noise is shown to be highly directional, while downwind sound propagation is spherical with limited ground absorption. The noise is found to have a significant low frequency component that is largely independent of wind speed over the 20-250 Hz range. The generated low frequency noise is shown to be audible above 40 Hz at the MOE setback distance of 550 m. Infrasound levels exhibit higher dependency on wind speed, but remain below audible levels up to 15 m/s.

  18. [Low-Frequency Flow Oscillation

    NASA Technical Reports Server (NTRS)

    Bragg, Michael B.

    1997-01-01

    The results of the research conducted under this grant are presented in detail in three Master theses, by Heinrich, Balow, and Broeren. Additional analysis of the experimental data can be found in two AIAA Journal articles and two conference papers. Citations for all of the studies' publications can be found in the bibliography which is attached. The objective of Heinrich's study was to document the low-frequency flow oscillation on the LRN-1007 airfoil, which had been previously observed at low Reynolds number, to determine its origin, and explore the phenomenon at higher Reynolds number. Heinrich performed detailed flow visualization on the airfoil using surface fluorescent oil and laser-sheet off-body visualization. A large leading-edge separation bubble and trailing-edge separation was identified on the airfoil just prior to the onset of the unsteady stall flow oscillation. From the laser-sheet data, the unsteady flow appeared as a massive boundary-layer separation followed by flow reattachment. Hot-wire data were taken in the wake to identify the presence of the flow oscillation and the dominant frequency. The oscillation was found in the flow from a Reynolds number of 0.3 to 1.3 x 10 exp 6. The Strouhal number based on airfoil projected height was nominally 0.02 and increased slightly with increasing Reynolds number and significantly with increasing airfoil angle of attack. Balow focused his research on the leading-edge separation bubble which was hypothesized to be the origin of the low-frequency oscillation. Initially, experimental measurements in the bubble at the onset of the low-frequency oscillation were attempted to study the characteristics of the bubble and explain possible relationships to the shear-layer-flapping phenomena. Unfortunately, the bubble proved to be extremely sensitive to the probe interference and it drastically reduced the size of the bubble. These detailed measurements were then abandoned by Balow. However, this led to a series of

  19. Low frequency AC waveform generator

    DOEpatents

    Bilharz, Oscar W.

    1986-01-01

    Low frequency sine, cosine, triangle and square waves are synthesized in circuitry which allows variation in the waveform amplitude and frequency while exhibiting good stability and without requiring significant stabilization time. A triangle waveform is formed by a ramped integration process controlled by a saturation amplifier circuit which produces the necessary hysteresis for the triangle waveform. The output of the saturation circuit is tapped to produce the square waveform. The sine waveform is synthesized by taking the absolute value of the triangular waveform, raising this absolute value to a predetermined power, multiplying the raised absolute value of the triangle wave with the triangle wave itself and properly scaling the resultant waveform and subtracting it from the triangular waveform itself. The cosine is synthesized by squaring the triangular waveform, raising the triangular waveform to a predetermined power and adding the squared waveform raised to the predetermined power with a DC reference and subtracting the squared waveform therefrom, with all waveforms properly scaled. The resultant waveform is then multiplied with a square wave in order to correct the polarity and produce the resultant cosine waveform.

  20. Low frequency ac waveform generator

    DOEpatents

    Bilharz, O.W.

    1983-11-22

    Low frequency sine, cosine, triangle and square waves are synthesized in circuitry which allows variation in the waveform amplitude and frequency while exhibiting good stability and without requiring significant stablization time. A triangle waveform is formed by a ramped integration process controlled by a saturation amplifier circuit which produces the necessary hysteresis for the triangle waveform. The output of the saturation circuit is tapped to produce the square waveform. The sine waveform is synthesized by taking the absolute value of the triangular waveform, raising this absolute value to a predetermined power, multiplying the raised absolute value of the triangle wave with the triangle wave itself and properly scaling the resultant waveform and subtracting it from the triangular waveform to a predetermined power and adding the squared waveform raised to the predetermined power with a DC reference and subtracting the squared waveform therefrom, with all waveforms properly scaled. The resultant waveform is then multiplied with a square wave in order to correct the polarity and produce the resultant cosine waveform.

  1. Reflection of Propagating Slow Magneto-acoustic Waves in Hot Coronal Loops: Multi-instrument Observations and Numerical Modeling

    NASA Astrophysics Data System (ADS)

    Mandal, Sudip; Yuan, Ding; Fang, Xia; Banerjee, Dipankar; Pant, Vaibhav; Van Doorsselaere, Tom

    2016-09-01

    Slow MHD waves are important tools for understanding coronal structures and dynamics. In this paper, we report a number of observations from the X-Ray Telescope (XRT) on board HINODE and Solar Dynamic Observatory/Atmospheric Imaging Assembly (AIA) of reflecting longitudinal waves in hot coronal loops. To our knowledge, this is the first report of this kind as seen from the XRT and simultaneously with the AIA. The wave appears after a micro-flare occurs at one of the footpoints. We estimate the density and temperature of the loop plasma by performing differential emission measure (DEM) analysis on the AIA image sequence. The estimated speed of propagation is comparable to or lower than the local sound speed, suggesting it to be a propagating slow wave. The intensity perturbation amplitude, in every case, falls very rapidly as the perturbation moves along the loop and eventually vanishes after one or more reflections. To check the consistency of such reflection signatures with the obtained loop parameters, we perform a 2.5D MHD simulation, which uses the parameters obtained from our observation as inputs, and perform forward modeling to synthesize AIA 94 Å images. Analyzing the synthesized images, we obtain the same properties of the observables as for the real observation. From the analysis we conclude that a footpoint heating can generate a slow wave which then reflects back and forth in the coronal loop before fading. Our analysis of the simulated data shows that the main agent for this damping is anisotropic thermal conduction.

  2. Ionospheric very low frequency transmitter

    SciTech Connect

    Kuo, Spencer P.

    2015-02-15

    The theme of this paper is to establish a reliable ionospheric very low frequency (VLF) transmitter, which is also broad band. Two approaches are studied that generate VLF waves in the ionosphere. The first, classic approach employs a ground-based HF heater to directly modulate the high latitude ionospheric, or auroral electrojet. In the classic approach, the intensity-modulated HF heater induces an alternating current in the electrojet, which serves as a virtual antenna to transmit VLF waves. The spatial and temporal variations of the electrojet impact the reliability of the classic approach. The second, beat-wave approach also employs a ground-based HF heater; however, in this approach, the heater operates in a continuous wave mode at two HF frequencies separated by the desired VLF frequency. Theories for both approaches are formulated, calculations performed with numerical model simulations, and the calculations are compared to experimental results. Theory for the classic approach shows that an HF heater wave, intensity-modulated at VLF, modulates the electron temperature dependent electrical conductivity of the ionospheric electrojet, which, in turn, induces an ac electrojet current. Thus, the electrojet becomes a virtual VLF antenna. The numerical results show that the radiation intensity of the modulated electrojet decreases with an increase in VLF radiation frequency. Theory for the beat wave approach shows that the VLF radiation intensity depends upon the HF heater intensity rather than the electrojet strength, and yet this approach can also modulate the electrojet when present. HF heater experiments were conducted for both the intensity modulated and beat wave approaches. VLF radiations were generated and the experimental results confirm the numerical simulations. Theory and experimental results both show that in the absence of the electrojet, VLF radiation from the F-region is generated via the beat wave approach. Additionally, the beat wave approach

  3. Tunable phase control of slow and fast light propagation in a slab doped by four-level quantum dot nanostructure

    NASA Astrophysics Data System (ADS)

    Jafarzadeh, Hossein; Sangachin, Elnaz Ahmadi; Asadpour, Seyyed Hossein

    2015-12-01

    Tunable phase control of the slow and fast light propagation through a defect slab medium doped by four-level InGaN/GaN quantum dot structure is demonstrated. By solving the Schrödinger and Poisson’s equations self-consistently, a spherical InGaN quantum dot with GaN barrier shell which can interact by terahertz (THz) signal field is designed numerically. It is found that the phase variation of THz signal field imparts the tunability in the group velocity of the transmitted and reflected pulses through a dielectric slab.

  4. Theory for accelerated slow crack propagation in polyethylene fuel pipes. Annual report, 1987-1988

    SciTech Connect

    Moet, A.; Chudnovsky, A.; Chaoui, K.; Strebel, J.

    1988-06-01

    The report describes a test for assessing the resistance of polyethylene fuel gas pipe materials to brittle crack propagation. The test employs fatigue loading to a notched specimen. Pipe specimens prepared from 2306-IIC and 2306-IA exhibit an initial stage of brittle crack propagation which becomes progressively ductile as it approaches ultimate failure by tearing. The complete test duration is extremely short in comparison to others currently employed, yet it similarly ranks both materials tested. Further, crack layer analysis is employed to evaluate the specific energy of fracture, gamma, a fundamental parameter characteristic of the material's resistance to brittle-crack propagation. It is also found from microscopic examinations that brittle fatigue involves a crazing mechanism known to occur under creep condition.

  5. Spatial structure of low-frequency wind noise.

    PubMed

    Wilson, D Keith; Greenfield, Roy J; White, Michael J

    2007-12-01

    The distinguishing spatial properties of low-frequency microphone wind noise (turbulent pressure disturbances) are examined with a planar, 49-element array. Individual, propagating transient pressure disturbances are imaged by wavelet processing to the array data. Within a given frequency range, the wind disturbances are much smaller and less spatially coherent than sound waves. Conventional array processing techniques are particularly sensitive to wind noise when sensor separations are small compared to the acoustic wavelengths of interest. PMID:18247645

  6. Anomalous sound propagation and slow kinetics in dynamically compressed amorphous carbon

    NASA Astrophysics Data System (ADS)

    Reed, Evan J.; Maiti, Amitesh; Fried, Laurence E.

    2010-01-01

    We have performed molecular-dynamics simulations of dynamic compression waves propagating through amorphous carbon using the Tersoff potential and find that a variety of dynamic compression features appear for two different initial densities. These features include steady elastic shocks, steady chemically reactive shocks, unsteady elastic waves, and unsteady chemically reactive waves. We show how these features can be distinguished by analyzing time-dependent propagation speeds, time-dependent sound speeds, and comparison to multiscale shock technique (MSST) simulations. Understanding such features is a key challenge in quasi-isentropic experiments involving phase transformations. In addition to direct simulations of dynamic compression, we employ the MSST and find agreement with the direct method for this system for the shocks observed. We show how the MSST can be extended to include explicit material viscosity and demonstrate on an amorphous Lennard-Jones system.

  7. Slow-light propagation using mode locking of spin precession in quantum dots

    SciTech Connect

    Shabaev, A.; Dutton, Z.; Kennedy, T. A.; Efros, Al. L.

    2010-11-15

    We propose using mode locking to enable coherent nonlinear optical effects in inhomogenously broadened spin ensembles. We carry out detailed calculations for quantum dot systems in which increased spin coherence via mode locking has been recently observed [A. Greilich et al., Science 313, 341 (2006); 317, 1896 (2007)]. We show how, in the presence of spin locking, a strong pulse-matching effect occurs, providing a powerful tool for high-bandwidth linear optical processing. We then go on to study 'slow light' in this system and show that high-bandwidth pulses can be controllably delayed by a time comparable to the pulse width.

  8. Low-frequency oscillations in Hall thrusters

    NASA Astrophysics Data System (ADS)

    Wei, Li-Qiu; Han, Liang; Yu, Da-Ren; Guo, Ning

    2015-05-01

    In this paper, we summarize the research development of low-frequency oscillations in the last few decades. The findings of physical mechanism, characteristics and stabilizing methods of low-frequency oscillations are discussed. It shows that it is unreasonable and incomplete to model an ionization region separately to analyze the physical mechanism of low-frequency oscillations. Electro-dynamics as well as the formation conditions of ionization distribution play an important role in characteristics and stabilizing of low-frequency oscillations. Understanding the physical mechanism and characteristics of low- frequency oscillations thoroughly and developing a feasible method stabilizing this instability are still important research subjects. Project supported by the National Natural Science Foundation of China (Grant No. 51477035), the Fundamental Research Funds for the Central Universities, China (Grant No. HIT.NSRIF 2015064), and the Open Research Fund Program of State Key Laboratory of Cryogenic Vacuum Technology and Physics, China (Grant No. ZDK201304).

  9. Low frequency hybrid instability in quantum magneto semiconductor plasmas

    NASA Astrophysics Data System (ADS)

    Rasheed, A.; Jamil, M.; Areeb, F.; Siddique, M.; Salimullah, M.

    2016-05-01

    The excitation of electrostatic, comparatively low frequency, lower-hybrid waves (LHWs) induced by electron beam in semiconductor plasma is examined using a quantum hydrodynamic model. Various quantum effects are taken into account including the recoil effect, Fermi degenerate pressure, and exchange-correlation potential. The effects of different parameters like the electron-to-hole number density ratio, scaled electron beam temperature and streaming speed, propagation angle and cyclotron frequency over the growth, and phase speed of LHWs are investigated. It is noticed that an increase in the electron number density and streaming speed enhance the instability. Similar effects are observed on decreasing the propagation angle with magnetic field.

  10. Achilles tendon loading patterns during barefoot walking and slow running on a treadmill: An ultrasonic propagation study.

    PubMed

    Wulf, M; Wearing, S C; Hooper, S L; Smeathers, J E; Horstmann, T; Brauner, T

    2015-12-01

    Measurement of tendon loading patterns during gait is important for understanding the pathogenesis of tendon "overuse" injury. Given that the speed of propagation of ultrasound in tendon is proportional to the applied load, this study used a noninvasive ultrasonic transmission technique to measure axial ultrasonic velocity in the right Achilles tendon of 27 healthy adults (11 females and 16 males; age, 26 ± 9 years; height, 1.73 ± 0.07 m; weight, 70.6 ± 21.2 kg), walking at self-selected speed (1.1 ± 0.1 m/s), and running at fixed slow speed (2 m/s) on a treadmill. Synchronous measures of ankle kinematics, spatiotemporal gait parameters, and vertical ground reaction forces were simultaneously measured. Slow running was associated with significantly higher cadence, shorter step length, but greater range of ankle movement, higher magnitude and rate of vertical ground reaction force, and higher ultrasonic velocity in the tendon than walking (P < 0.05). Ultrasonic velocity in the Achilles tendon was highly reproducible during walking and slow running (mean within-subject coefficient of variation < 2%). Ultrasonic maxima (P1, P2) and minima (M1, M2) were significantly higher and occurred earlier in the gait cycle (P1, M1, and M2) during running than walking (P < 0.05). Slow running was associated with higher and earlier peaks in loading of the Achilles tendon than walking. PMID:25913324

  11. Dispersion relations and polarizations of low-frequency waves in two-fluid plasmas

    SciTech Connect

    Zhao, Jinsong

    2015-04-15

    Analytical expressions for the dispersion relations and polarizations of low-frequency waves in magnetized plasmas based on two-fluid model are obtained. The properties of waves propagating at different angles (to the ambient magnetic field B{sub 0}) and β (the ratio of the plasma to magnetic pressures) values are investigated. It is shown that two linearly polarized waves—namely, the fast and Alfvén modes in the low-β (β≪1) plasmas, the fast and slow modes in the β∼1 plasmas, and the Alfvén and slow modes in the high-β (β≫1) plasmas—become circularly polarized at the near-parallel (to B{sub 0}) propagation. The negative magnetic-helicity of the Alfvén mode occurs only at small or moderate angles in the low-β plasmas, and the ion cross-helicity of the slow mode is nearly the same as that of the Alfvén mode in the high-β plasmas. It is also shown that the electric polarization δE{sub z}/δE{sub y} decreases with the temperature ratio T{sub e}/T{sub i} for the long-wavelength waves, and the transition between left- and right-hand polarizations of the Alfvén mode in T{sub e}/T{sub i}≠0 plasmas can disappear when T{sub e}/T{sub i}=0. The approximate dispersion relations in the near-perpendicular propagation, low-β, and high-β limits can quite accurately describe the three modes.

  12. Investigating Low-Frequency Earthquake Properties

    NASA Astrophysics Data System (ADS)

    Sweet, J. R.; Creager, K. C.

    2013-12-01

    Low-frequency earthquakes (LFEs) have been associated with tectonic tremor and slow slip on the deep extension of subduction zones faults and major strike-slip faults. These tiny earthquakes are thought to represent small amounts of slip on the plate interface in places with high pore fluid pressure and low effective stress. Some tectonic tremor has been shown to consist of the superposition of many LFEs occurring at nearly the same time [Shelly et al., 2007]. In northern Cascadia, we use data from the Array of Arrays and CAFE experiments to detect and study properties of a curious LFE family on the downdip extension of the transition zone. This family--the deepest we've yet discovered in Cascadia--occurs in small swarms of ~1 hour duration every 1 to 2 weeks. Over our 6-year dataset we have identified ~9000 individual repeats of this LFE. Using double-difference techniques on one swarm, we have found that the LFEs lie on a dipping plane (<300m thick) that locates on the inferred plate interface [McCrory et al., 2012; Preston et al., 2003]. Individual LFE locations have errors of order 100m. Interestingly, we also find that over the course of an hour the LFEs migrate updip over a distance of ~2km in a narrow channel parallel to the plate convergence direction. We suggest this migration represents the movement of a slow slip front, and may be similar to previously described tremor streaks, also seen in Cascadia [Ghosh et al., 2010]; however our migration velocity of 2km/hr would be intermediate between the slower reported velocities of rapid tremor reversals [Houston et al., 2010] and the much faster tremor streaks. We find that amplitudes for this LFE family occupy a relatively narrow range, with louder events equally likely to occur at the beginning, middle, or end of a swarm of LFEs. We estimate LFE magnitudes of -0.8 to +0.8 by comparing peak-to-peak amplitudes and PNSN assigned magnitudes for 8 small earthquakes within 10km of the LFEs. Amplitudes below about

  13. Low-frequency cosmology from the moon

    NASA Astrophysics Data System (ADS)

    Klein Wolt, M.; Aminaei, A.; Pourshaghaghi, H.; Koopmans, L.; Falcke, H.

    2013-09-01

    From a low-frequency point of view, the moon provides excess to the virtually unexplored radio frequency domain below 30 MHz that is not accessible from Earth due to the atmospheric cutoff and interference from man-made RFI. We show that with a single low-frequency radio antenna the detection of the 21-cm Dark Ages signal is possible within integration times of months, and address the size and integration times required for a future low-frequency array to perform detailed tomography and power spectral analysis of the Dark Ages signal.

  14. Implementation of dispersion-free slow acoustic wave propagation and phase engineering with helical-structured metamaterials

    PubMed Central

    Zhu, Xuefeng; Li, Kun; Zhang, Peng; Zhu, Jie; Zhang, Jintao; Tian, Chao; Liu, Shengchun

    2016-01-01

    The ability to slow down wave propagation in materials has attracted significant research interest. A successful solution will give rise to manageable enhanced wave–matter interaction, freewheeling phase engineering and spatial compression of wave signals. The existing methods are typically associated with constructing dispersive materials or structures with local resonators, thus resulting in unavoidable distortion of waveforms. Here we show that, with helical-structured acoustic metamaterials, it is now possible to implement dispersion-free sound deceleration. The helical-structured metamaterials present a non-dispersive high effective refractive index that is tunable through adjusting the helicity of structures, while the wavefront revolution plays a dominant role in reducing the group velocity. Finally, we numerically and experimentally demonstrate that the helical-structured metamaterials with designed inhomogeneous unit cells can turn a normally incident plane wave into a self-accelerating beam on the prescribed parabolic trajectory. The helical-structured metamaterials will have profound impact to applications in explorations of slow wave physics. PMID:27198887

  15. Implementation of dispersion-free slow acoustic wave propagation and phase engineering with helical-structured metamaterials

    NASA Astrophysics Data System (ADS)

    Zhu, Xuefeng; Li, Kun; Zhang, Peng; Zhu, Jie; Zhang, Jintao; Tian, Chao; Liu, Shengchun

    2016-05-01

    The ability to slow down wave propagation in materials has attracted significant research interest. A successful solution will give rise to manageable enhanced wave-matter interaction, freewheeling phase engineering and spatial compression of wave signals. The existing methods are typically associated with constructing dispersive materials or structures with local resonators, thus resulting in unavoidable distortion of waveforms. Here we show that, with helical-structured acoustic metamaterials, it is now possible to implement dispersion-free sound deceleration. The helical-structured metamaterials present a non-dispersive high effective refractive index that is tunable through adjusting the helicity of structures, while the wavefront revolution plays a dominant role in reducing the group velocity. Finally, we numerically and experimentally demonstrate that the helical-structured metamaterials with designed inhomogeneous unit cells can turn a normally incident plane wave into a self-accelerating beam on the prescribed parabolic trajectory. The helical-structured metamaterials will have profound impact to applications in explorations of slow wave physics.

  16. Implementation of dispersion-free slow acoustic wave propagation and phase engineering with helical-structured metamaterials.

    PubMed

    Zhu, Xuefeng; Li, Kun; Zhang, Peng; Zhu, Jie; Zhang, Jintao; Tian, Chao; Liu, Shengchun

    2016-01-01

    The ability to slow down wave propagation in materials has attracted significant research interest. A successful solution will give rise to manageable enhanced wave-matter interaction, freewheeling phase engineering and spatial compression of wave signals. The existing methods are typically associated with constructing dispersive materials or structures with local resonators, thus resulting in unavoidable distortion of waveforms. Here we show that, with helical-structured acoustic metamaterials, it is now possible to implement dispersion-free sound deceleration. The helical-structured metamaterials present a non-dispersive high effective refractive index that is tunable through adjusting the helicity of structures, while the wavefront revolution plays a dominant role in reducing the group velocity. Finally, we numerically and experimentally demonstrate that the helical-structured metamaterials with designed inhomogeneous unit cells can turn a normally incident plane wave into a self-accelerating beam on the prescribed parabolic trajectory. The helical-structured metamaterials will have profound impact to applications in explorations of slow wave physics. PMID:27198887

  17. Comparison of optical and acoustical monitoring during a crack propagation, implication for slow earthquake dynamics

    NASA Astrophysics Data System (ADS)

    Lengliné, Olivier; Schmittbuhl, Jean; Elkhoury, Jean; Toussaint, Renaud; Daniel, Guillaume; Maloy, Knut Jurgen

    2010-05-01

    Observations of aseismic transients in several tectonic context suggest that they might be linked to seismicity. However a clear observation and description of these phenomena and their interaction is lacking. This owes to the difficulty of characterizing with a sufficient resolution processes taking place at depth. Here we aim to study these interactions between aseismic and seismic slip taking advantage of an unique experimental setup. We conducted a series of mode I crack propagation experiments on transparent materials (PMMA). The crack advance is trapped in a weakness plane which is the interface between two previously sandblasted and annealed plexiglass plates. A fast video camera taking up to 500 frames per second ensures the tracking of the front rupture. The acoustic system is composed of a maximum of 44 channels continuously recording at 5 MHz for a few tens of seconds. Piezo-electric sensors are composed of a 32 elements linear array and individual sensors surrounding the crack front. An automatic detection and localization procedure allows us to obtain the position of acoustic emission (A.E.) that occurred during the crack advance. Crack front image processing reveals an intermittent opening which might be linked to the time and space clustering of the AE. An analogy between the mode I (opening) and the mode III (antiplane slip) allows us to interpret our results in term of slip on faults. Our experiment thus helps to reveal the interplay between seismic and aseismic slip on faults.

  18. The influence of cochlear shape on low-frequency hearing

    PubMed Central

    Manoussaki, Daphne; Chadwick, Richard S.; Ketten, Darlene R.; Arruda, Julie; Dimitriadis, Emilios K.; O'Malley, Jen T.

    2008-01-01

    The conventional theory about the snail shell shape of the mammalian cochlea is that it evolved essentially and perhaps solely to conserve space inside the skull. Recently, a theory proposed that the spiral's graded curvature enhances the cochlea's mechanical response to low frequencies. This article provides a multispecies analysis of cochlear shape to test this theory and demonstrates that the ratio of the radii of curvature from the outermost and innermost turns of the cochlear spiral is a significant cochlear feature that correlates strongly with low-frequency hearing limits. The ratio, which is a measure of curvature gradient, is a reflection of the ability of cochlear curvature to focus acoustic energy at the outer wall of the cochlear canal as the wave propagates toward the apex of the cochlea. PMID:18413615

  19. Low-Frequency Waves in HF Heating of the Ionosphere

    NASA Astrophysics Data System (ADS)

    Sharma, A. S.; Eliasson, B.; Milikh, G. M.; Najmi, A.; Papadopoulos, K.; Shao, X.; Vartanyan, A.

    2016-02-01

    Ionospheric heating experiments have enabled an exploration of the ionosphere as a large-scale natural laboratory for the study of many plasma processes. These experiments inject high-frequency (HF) radio waves using high-power transmitters and an array of ground- and space-based diagnostics. This chapter discusses the excitation and propagation of low-frequency waves in HF heating of the ionosphere. The theoretical aspects and the associated models and simulations, and the results from experiments, mostly from the HAARP facility, are presented together to provide a comprehensive interpretation of the relevant plasma processes. The chapter presents the plasma model of the ionosphere for describing the physical processes during HF heating, the numerical code, and the simulations of the excitation of low-frequency waves by HF heating. It then gives the simulations of the high-latitude ionosphere and mid-latitude ionosphere. The chapter also briefly discusses the role of kinetic processes associated with wave generation.

  20. External potassium and action potential propagation in rat fast and slow twitch muscles.

    PubMed

    Kössler, F; Lange, F; Caffier, G; Küchler, G

    1991-10-01

    The role of extracellular K+ concentration in the propagation velocity of action potential was tested in isolated rat skeletal muscles. Different K+ concentrations were produced by KCl additions to extracellular solution. Action potentials were measured extracellularly by means of two annular platinum electrodes. Fibre bundles of m. soleus (SOL), m. extensor digitorum longus (EDL), red (SMR) and white (SMW) part of m. sternomastoideus were maximum stimulated. The conduction velocity (c.v.) was calculated from the distance between the electrodes and the time delay of the potentials measured at 22 degrees C. In Tyrode solution containing 5 mmol/l K+, the c.v. was close to 1 m.s-1. Bundles of the fast muscle type seemed to have a somewhat higher c.v. The differences observed in these studies were not significant. At higher temperatures, the c.v. increased (Q10 of approx. 2) and a dissociation between SMR and SMW muscles appeared. An elevation of K+ concentration to 10 mmol/l induced a drop of the c.v. by approx. 25% and 15% in EDL and SOL muscles, respectively. After return to normal solution, the recovery was not complete within 30 min. In K+ free solution the c.v. of EDL and SM muscles rose by a factor of 1.5, but less in SOL muscles. The weaker response of SOL to K+ modification was related to the higher resistance of this muscle to fatigue. This suggestion was supported by experiments on fatigued fibre bundles. Immediately after a tetanic stimulation producing fatigue, the c.v. of EDL and SOL muscles dropped similarly as in 10 mmol/l K+; again, the drop was less for SOL muscles. Adrenaline (0.5-10.0 mumol/l) enhanced both the c.v. and the twitch amplitude. The results support the suggestion that extracellular K+ accumulation during activity is an essential factor of muscle fatigue. PMID:1816028

  1. Low-frequency oscillations in radiative-convective systems

    NASA Technical Reports Server (NTRS)

    Hu, QI; Randall, David A.

    1994-01-01

    Although eastward propagation has long been considered one of the essential features of the Madden-Julian waves, recent observations have revealed a stationary or quasi-stationary component in the oscillations, particularly in measures of the diabatic heating rate. Wave-CISK theories of the low-frequency oscillations have struggled to explain the observed period and vertical structure of the waves. On the other hand, theoretical and numerical studies have shown that low-frequency waves strongly resembling the observed oscillations can be excited by specified low-frequency oscillations of the convective heating. A problem with the latter set of theories is that the cause of the oscillatory heating has not been satisfactorily explained. It is proposed here that the observed low-frequency wave motions are the response to forcing by an essentially stationary, self-excited oscillating heat source that is produced by nonlinear interactions among radiation, cumulus convection, and the surface fluxes of sensible heat and moisture. Feedback of the large-scale motions on the latent heating is not required. Results from two very different one-dimensional models are presented to support this hypothesis. The physical processes included in the models are essentially the same, that is, radiation, cumulus convection, and the surface fluxes of sensible heat and moisture; the first model is highly simplified, however, while the second includes relatively sophisticated parameterizations of all the relevant physical processes. Results from both models show low-frequency oscillations of the latent heating, temperature, and moisture. Experiments show that the oscillations are favored by a warm sea surface and weak surface wind speeds, consistent with the observed conditions over the Indian Ocean and the tropical western Pacific Ocean.

  2. Kerr Non-linerity in Slow Light Propagation for Quantum Teleportation

    NASA Astrophysics Data System (ADS)

    Tombesi, Paolo

    2001-05-01

    -linearity can be obtained using the recently demonstrated ultraslow light propagation,(hau)L.V. Hau et al., Nature (London) 397, 594 (1999). achieved via electromagnetically induced transparency (E. Arimondo, in Progress in Optics) XXXV, ed. by E. Wolf, (Elsevier, Amsterdam, 1996);S.E. Harris, Phys. Today 50, 36 (1997); M.O. Scully and M.S. Zubairy, Quantum Optics (Cambridge University Press, Cambridge, UK, 1997). in ensembles of cold atoms.

  3. Very low frequency earthquakes in Cascadia migrate with tremor

    NASA Astrophysics Data System (ADS)

    Ghosh, Abhijit; Huesca-Pérez, Eduardo; Brodsky, Emily; Ito, Yoshihiro

    2015-05-01

    We find very low frequency earthquakes (VLFEs) in Cascadia under northern Washington during 2011 episodic tremor and slip event. VLFEs are rich in low-frequency energy (20-50 s) and depleted in higher frequencies (higher than 1 Hz) compared to local earthquakes. Based on a grid search centroid moment tensor inversion, we find that VLFEs are located near the plate interface in the zone where tremor and slow slip are observed. In addition, they migrate along strike with tremor activity. Their moment tensor solutions show double-couple sources with shallow thrust mechanisms, consistent with shear slip at the plate interface. Their magnitude ranges between Mw 3.3 and 3.7. Seismic moment released by a single VLFE is comparable to the total cumulative moment released by tremor activity during an entire episodic tremor and slip event. The VLFEs contribute more seismic moment to this episodic tremor and slip event than cumulative tremor activity and indicate a higher seismic efficiency of slow earthquakes in Cascadia than previously thought. Spatiotemporal correlation of VLFE and tremor activity suggests that they are the results of the same physical processes governing slow earthquakes.

  4. On apparent temperature in low-frequency Alfvenic turbulence

    SciTech Connect

    Nariyuki, Yasuhiro

    2012-08-15

    Low-frequency, parallel propagating Alfvenic turbulence in collisionless plasmas is theoretically studied. Alfvenic turbulence is derived as an equilibrium state (Beltrami field) in the magnetohydrodynamic equations with the pressure anisotropy and multi-species of ions. It is shown that the conservation of the total 'apparent temperature' corresponds to the Bernoulli law. A simple model of the radially expanding solar wind including Alfvenic turbulence is also discussed. The conversion of the wave energy in the 'apparent temperature' into the 'real temperature' is facilitated with increasing radial distance.

  5. Improved low frequency stability of bolometric detectors

    NASA Technical Reports Server (NTRS)

    Wilbanks, T.; Devlin, M.; Lange, A. E.; Beeman, J. W.; Sato, S.

    1990-01-01

    An ac bridge readout system has been developed that greatly improves the low-frequency stability of bolometric detectors. The readout can be implemented with a simple circuit appropriate for use in space applications. A matched pair of detectors was used in the readout to achieve system noise within a factor of two of the fundamental noise limit of the detectors at frequencies as low as 10 mHz. The low-frequency stability of the readout system allows slower, more sensitive detectors to be used in many applications, and it facilitates observing strategies that are well suited to spaceborne observations.

  6. Low frequency acoustic and electromagnetic scattering

    NASA Technical Reports Server (NTRS)

    Hariharan, S. I.; Maccamy, R. C.

    1986-01-01

    This paper deals with two classes of problems arising from acoustics and electromagnetics scattering in the low frequency stations. The first class of problem is solving Helmholtz equation with Dirichlet boundary conditions on an arbitrary two dimensional body while the second one is an interior-exterior interface problem with Helmholtz equation in the exterior. Low frequency analysis show that there are two intermediate problems which solve the above problems accurate to 0(k/2/ log k) where k is the frequency. These solutions greatly differ from the zero frequency approximations. For the Dirichlet problem numerical examples are shown to verify the theoretical estimates.

  7. Low frequency acoustic and electromagnetic scattering

    NASA Technical Reports Server (NTRS)

    Hariharan, S. I.; Maccamy, R. C.

    1983-01-01

    This paper deals with two classes of problems arising from acoustics and electromagnetics scattering in the low frequency stations. The first class of problem is solving Helmholtz equation with Dirichlet boundary conditions on an arbitrary two dimensional body while the second one is an interior-exterior interface problem with Helmholtz equation in the exterior. Low frequency analysis show that there are two intermediate problems which solve the above problems accurate to 0(k(2) log k) where k is the frequency. These solutions greatly differ from the zero frequency approximations. For the Dirichlet problem numerical examples are shown to verify the theoretical estimates.

  8. Low-frequency sound exposure causes reversible long-term changes of cochlear transfer characteristics.

    PubMed

    Drexl, Markus; Otto, Larissa; Wiegrebe, Lutz; Marquardt, Torsten; Gürkov, Robert; Krause, Eike

    2016-02-01

    Intense, low-frequency sound presented to the mammalian cochlea induces temporary changes of cochlear sensitivity, for which the term 'Bounce' phenomenon has been coined. Typical manifestations are slow oscillations of hearing thresholds or the level of otoacoustic emissions. It has been suggested that these alterations are caused by changes of the mechano-electrical transducer transfer function of outer hair cells (OHCs). Shape estimates of this transfer function can be derived from low-frequency-biased distortion product otoacoustic emissions (DPOAE). Here, we tracked the transfer function estimates before and after triggering a cochlear Bounce. Specifically, cubic DPOAEs, modulated by a low-frequency biasing tone, were followed over time before and after induction of the cochlear Bounce. Most subjects showed slow, biphasic changes of the transfer function estimates after low-frequency sound exposure relative to the preceding control period. Our data show that the operating point changes biphasically on the transfer function with an initial shift away from the inflection point followed by a shift towards the inflection point before returning to baseline values. Changes in transfer function and operating point lasted for about 180 s. Our results are consistent with the hypothesis that intense, low-frequency sound disturbs regulatory mechanisms in OHCs. The homeostatic readjustment of these mechanisms after low-frequency offset is reflected in slow oscillations of the estimated transfer functions. PMID:26706707

  9. Hearing Foreign Languages through Low Frequencies.

    ERIC Educational Resources Information Center

    Roberge, Claude

    A study to assess the feasibility of the use of low frequencies for teaching foreign language to the hearing impaired is described. The subjects were unimpaired Japanese students, aged 18 and 19, in beginning French language study. Recorded sentences translated into English, French, and Mandarin Chinese were combined in various ways and presented…

  10. Is low frequency ocean sound increasing globally?

    PubMed

    Miksis-Olds, Jennifer L; Nichols, Stephen M

    2016-01-01

    Low frequency sound has increased in the Northeast Pacific Ocean over the past 60 yr [Ross (1993) Acoust. Bull. 18, 5-8; (2005) IEEE J. Ocean. Eng. 30, 257-261; Andrew, Howe, Mercer, and Dzieciuch (2002) J. Acoust. Soc. Am. 129, 642-651; McDonald, Hildebrand, and Wiggins (2006) J. Acoust. Soc. Am. 120, 711-717; Chapman and Price (2011) J. Acoust. Soc. Am. 129, EL161-EL165] and in the Indian Ocean over the past decade, [Miksis-Olds, Bradley, and Niu (2013) J. Acoust. Soc. Am. 134, 3464-3475]. More recently, Andrew, Howe, and Mercer's [(2011) J. Acoust. Soc. Am. 129, 642-651] observations in the Northeast Pacific show a level or slightly decreasing trend in low frequency noise. It remains unclear what the low frequency trends are in other regions of the world. In this work, data from the Comprehensive Nuclear-Test Ban Treaty Organization International Monitoring System was used to examine the rate and magnitude of change in low frequency sound (5-115 Hz) over the past decade in the South Atlantic and Equatorial Pacific Oceans. The dominant source observed in the South Atlantic was seismic air gun signals, while shipping and biologic sources contributed more to the acoustic environment at the Equatorial Pacific location. Sound levels over the past 5-6 yr in the Equatorial Pacific have decreased. Decreases were also observed in the ambient sound floor in the South Atlantic Ocean. Based on these observations, it does not appear that low frequency sound levels are increasing globally. PMID:26827043

  11. Slow earthquakes coincident with episodic tremors and slow slip events.

    PubMed

    Ito, Yoshihiro; Obara, Kazushige; Shiomi, Katsuhiko; Sekine, Shutaro; Hirose, Hitoshi

    2007-01-26

    We report on the very-low-frequency earthquakes occurring in the transition zone of the subducting plate interface along the Nankai subduction zone in southwest Japan. Seismic waves generated by very-low-frequency earthquakes with seismic moment magnitudes of 3.1 to 3.5 predominantly show a long period of about 20 seconds. The seismicity of very-low-frequency earthquakes accompanies and migrates with the activity of deep low-frequency tremors and slow slip events. The coincidence of these three phenomena improves the detection and characterization of slow earthquakes, which are thought to increase the stress on updip megathrust earthquake rupture zones. PMID:17138867

  12. Extreme low frequency acoustic measurement system

    NASA Technical Reports Server (NTRS)

    Shams, Qamar A. (Inventor); Zuckerwar, Allan J. (Inventor)

    2013-01-01

    The present invention is an extremely low frequency (ELF) microphone and acoustic measurement system capable of infrasound detection in a portable and easily deployable form factor. In one embodiment of the invention, an extremely low frequency electret microphone comprises a membrane, a backplate, and a backchamber. The backchamber is sealed to allow substantially no air exchange between the backchamber and outside the microphone. Compliance of the membrane may be less than ambient air compliance. The backplate may define a plurality of holes and a slot may be defined between an outer diameter of the backplate and an inner wall of the microphone. The locations and sizes of the holes, the size of the slot, and the volume of the backchamber may be selected such that membrane motion is substantially critically damped.

  13. The AKR emission cone at low frequencies

    NASA Technical Reports Server (NTRS)

    Calvert, W.

    1981-01-01

    It is noted that certain of the ISEE-1 observations between the plasmasphere and the auroral zone have revealed the emission cone of auroral kilometric radiation (AKR) unaffected by plasmaspheric refraction. At some distance from the source, the cone produced a sharp low-frequency boundary in the AKR signals, which was displaced above the cyclotron frequency. The variation of this boundary, together with other aspects of the AKR signals, suggested that the AKR emission cone closed toward a hollow, roughly 45 deg limit cone with decreasing frequency, duplicating the behavior previously found with ISIS-1 at the opposite end of the AKR spectrum. It is pointed out that the hollow limit cone at low frequencies is a new feature, not previously reported.

  14. Low-Frequency Electromagnetic Backscattering from Tunnels

    SciTech Connect

    Casey, K; Pao, H

    2007-01-16

    Low-frequency electromagnetic scattering from one or more tunnels in a lossy dielectric half-space is considered. The tunnel radii are assumed small compared to the wavelength of the electromagnetic field in the surrounding medium; a tunnel can thus be modeled as a thin scatterer, described by an equivalent impedance per unit length. We examine the normalized backscattering width for cases in which the air-ground interface is either smooth or rough.

  15. LOFAR: The LOw-Frequency ARray

    NASA Astrophysics Data System (ADS)

    van Haarlem, M. P.; Wise, M. W.; Gunst, A. W.; Heald, G.; McKean, J. P.; Hessels, J. W. T.; de Bruyn, A. G.; Nijboer, R.; Swinbank, J.; Fallows, R.; Brentjens, M.; Nelles, A.; Beck, R.; Falcke, H.; Fender, R.; Hörandel, J.; Koopmans, L. V. E.; Mann, G.; Miley, G.; Röttgering, H.; Stappers, B. W.; Wijers, R. A. M. J.; Zaroubi, S.; van den Akker, M.; Alexov, A.; Anderson, J.; Anderson, K.; van Ardenne, A.; Arts, M.; Asgekar, A.; Avruch, I. M.; Batejat, F.; Bähren, L.; Bell, M. E.; Bell, M. R.; van Bemmel, I.; Bennema, P.; Bentum, M. J.; Bernardi, G.; Best, P.; Bîrzan, L.; Bonafede, A.; Boonstra, A.-J.; Braun, R.; Bregman, J.; Breitling, F.; van de Brink, R. H.; Broderick, J.; Broekema, P. C.; Brouw, W. N.; Brüggen, M.; Butcher, H. R.; van Cappellen, W.; Ciardi, B.; Coenen, T.; Conway, J.; Coolen, A.; Corstanje, A.; Damstra, S.; Davies, O.; Deller, A. T.; Dettmar, R.-J.; van Diepen, G.; Dijkstra, K.; Donker, P.; Doorduin, A.; Dromer, J.; Drost, M.; van Duin, A.; Eislöffel, J.; van Enst, J.; Ferrari, C.; Frieswijk, W.; Gankema, H.; Garrett, M. A.; de Gasperin, F.; Gerbers, M.; de Geus, E.; Grießmeier, J.-M.; Grit, T.; Gruppen, P.; Hamaker, J. P.; Hassall, T.; Hoeft, M.; Holties, H. A.; Horneffer, A.; van der Horst, A.; van Houwelingen, A.; Huijgen, A.; Iacobelli, M.; Intema, H.; Jackson, N.; Jelic, V.; de Jong, A.; Juette, E.; Kant, D.; Karastergiou, A.; Koers, A.; Kollen, H.; Kondratiev, V. I.; Kooistra, E.; Koopman, Y.; Koster, A.; Kuniyoshi, M.; Kramer, M.; Kuper, G.; Lambropoulos, P.; Law, C.; van Leeuwen, J.; Lemaitre, J.; Loose, M.; Maat, P.; Macario, G.; Markoff, S.; Masters, J.; McFadden, R. A.; McKay-Bukowski, D.; Meijering, H.; Meulman, H.; Mevius, M.; Middelberg, E.; Millenaar, R.; Miller-Jones, J. C. A.; Mohan, R. N.; Mol, J. D.; Morawietz, J.; Morganti, R.; Mulcahy, D. D.; Mulder, E.; Munk, H.; Nieuwenhuis, L.; van Nieuwpoort, R.; Noordam, J. E.; Norden, M.; Noutsos, A.; Offringa, A. R.; Olofsson, H.; Omar, A.; Orrú, E.; Overeem, R.; Paas, H.; Pandey-Pommier, M.; Pandey, V. N.; Pizzo, R.; Polatidis, A.; Rafferty, D.; Rawlings, S.; Reich, W.; de Reijer, J.-P.; Reitsma, J.; Renting, G. A.; Riemers, P.; Rol, E.; Romein, J. W.; Roosjen, J.; Ruiter, M.; Scaife, A.; van der Schaaf, K.; Scheers, B.; Schellart, P.; Schoenmakers, A.; Schoonderbeek, G.; Serylak, M.; Shulevski, A.; Sluman, J.; Smirnov, O.; Sobey, C.; Spreeuw, H.; Steinmetz, M.; Sterks, C. G. M.; Stiepel, H.-J.; Stuurwold, K.; Tagger, M.; Tang, Y.; Tasse, C.; Thomas, I.; Thoudam, S.; Toribio, M. C.; van der Tol, B.; Usov, O.; van Veelen, M.; van der Veen, A.-J.; ter Veen, S.; Verbiest, J. P. W.; Vermeulen, R.; Vermaas, N.; Vocks, C.; Vogt, C.; de Vos, M.; van der Wal, E.; van Weeren, R.; Weggemans, H.; Weltevrede, P.; White, S.; Wijnholds, S. J.; Wilhelmsson, T.; Wucknitz, O.; Yatawatta, S.; Zarka, P.; Zensus, A.; van Zwieten, J.

    2013-08-01

    LOFAR, the LOw-Frequency ARray, is a new-generation radio interferometer constructed in the north of the Netherlands and across europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. Spreading out from a core located near the village of Exloo in the northeast of the Netherlands, a total of 40 LOFAR stations are nearing completion. A further five stations have been deployed throughout Germany, and one station has been built in each of France, Sweden, and the UK. Digital beam-forming techniques make the LOFAR system agile and allow for rapid repointing of the telescope as well as the potential for multiple simultaneous observations. With its dense core array and long interferometric baselines, LOFAR achieves unparalleled sensitivity and angular resolution in the low-frequency radio regime. The LOFAR facilities are jointly operated by the International LOFAR Telescope (ILT) foundation, as an observatory open to the global astronomical community. LOFAR is one of the first radio observatories to feature automated processing pipelines to deliver fully calibrated science products to its user community. LOFAR's new capabilities, techniques and modus operandi make it an important pathfinder for the Square Kilometre Array (SKA). We give an overview of the LOFAR instrument, its major hardware and software components, and the core science objectives that have driven its design. In addition, we present a selection of new results from the commissioning phase of this new radio observatory.

  16. Low Frequency Seiche in a Large Bay

    NASA Astrophysics Data System (ADS)

    MacMahan, J. H.; Thornton, E. B.; Gallagher, E. L.; Reniers, A.

    2014-12-01

    Short-term observations of sea surface elevations (η) along the 10m isobath, and long-term observations inside and outside of a large bay (Monterey Bay, CA) were obtained to describe the nodal structure of the mode 0-3 seiches within the bay and the low frequency (<346 cpd) seiche forcing mechanism. The measured nodal pattern validates previous numerical estimates associated with a northern amplitude bias, though variability exists across the modal frequency band, particularly for mode 0 and 1. Low frequency oceanic η white noise within seiche frequency bands (24-69 cpd) provides a continuous forcing of the bay seiche with a η2 (variance) amplification of 16-40 for the different modes. The temporal variation of the oceanic η white noise is significantly correlated (R2=0.86) at the 95% confidence interval with the bay seiche η that varies seasonally. The oceanic η white noise is hypothesized as being from low frequency, free, infragravity waves that are forced by short waves. This work was funded by NPS, ONR, & NSF.

  17. Rectification of low-frequency thermal waves in graded SicGe1 - c

    NASA Astrophysics Data System (ADS)

    Jou, D.; Carlomagno, I.; Cimmelli, V. A.

    2016-05-01

    We study propagation of low-frequency thermal waves in graded SicGe1-c. We show that in some ranges of stoichiometry variation one obtains rectification of thermal waves, i.e., the system allows the propagation of waves in one direction but not in the opposite one through the graded system, or through a thin graded layer separating two different systems. The rectification of low-frequency thermal waves could be switched by imposing on the graded layer a suitable temperature gradient.

  18. Ultra low frequency electromagnetic fire alarm system for underground mines

    SciTech Connect

    Not Available

    1991-01-01

    During an underground mine fire, air can be rapidly depleted of oxygen and contaminated with smoke and toxic fire gases. Any delay in warning miners could have disastrous consequences. Unfortunately, present mine fire alarm systems, such as stench, audible or visual alarms, telephones, and messengers, are often slow, unreliable, and limited in mine area coverage. Recent research by the U.S. Bureau of Mines has demonstrated that ultra-low-frequency electromagnetic signaling can be used for an underground mine fire alarm. In field tests of prototype equipment at five mines, electromagnetic signals from 630 to 2,000 Hz were transmitted through mine rock for distances as great as 1,645 m to an intrinsically safe receiver. The prototype system uses off-the-shelf components and state-of-the-art technology to ensure high reliability and low cost. When utilized, this technology would enable simultaneous and instantaneous warning of all underground personnel, regardless of their location or work activity, thereby increasing the likelihood of their successfully escaping a mine disaster. This paper presents the theoretical basis for through-the-rock ultra-low-frequency electromagnetic transmission, design of the prototype transmitter and receiver, and the results of in-mine tests of the prototype system.

  19. Low frequency electric and magnetic fields

    NASA Technical Reports Server (NTRS)

    Spaniol, Craig

    1989-01-01

    Following preliminary investigations of the low frequency electric and magnetic fields that may exists in the Earth-ionospheric cavity, measurements were taken with state-of-the art spectrum analyzers. As a follow up to this activity, an investigation was initiated to determine sources and values for possible low frequency signal that would appear in the cavity. The lowest cavity resonance is estimated at about 8 Hz, but lower frequencies may be an important component of our electromagnetic environment. The potential field frequencies produced by the electron were investigated by a classical model that included possible cross coupling of the electric and gravitation fields. During this work, an interesting relationship was found that related the high frequency charge field with the extremely low frequency of the gravitation field. The results of numerical calculations were surprisingly accurate and this area of investigation is continuing. The work toward continued development of a standardized monitoring facility is continuing with the potential of installing the prototype at West Virginia State College early in 1990. This installation would be capable of real time monitoring of ELF signals in the Earth-ionoshpere cavity and would provide some directional information. A high gain, low noise, 1/f frequency corrected preamplifier was designed and tested for the ferrite core magnetic sensor. The potential application of a super conducting sensor for the ELF magnetic field detection is under investigation. It is hoped that a fully operational monitoring network could pinpoint the location of ELF signal sources and provide new information on where these signals originate and what causes them, assuming that they are natural in origin.

  20. Analyzing low frequency waves associated with plasma sheet flow channels

    NASA Astrophysics Data System (ADS)

    Xing, X.; Liang, J.; Wang, C. P.; Lyons, L. R.; Angelopoulos, V.

    2014-12-01

    Low frequency (0.006~0.02 Hz) magnetic oscillations are frequently observed to be associated with the substorm-related dipolarization in the near-Earth plasma sheet. It has been suggested that these oscillations are possibly triggered by ballooning instability in the transition region. However, our multi-point observations using THEMIS spacecraft have shown that similar oscillations are observed to be associated with the earthward moving flow channels as they penetrate from middle tail to the transition region. Linear MHD wave analysis suggested that these oscillations ahead of the dipolarization front are magnetosonic waves. For most of the cases, the thermal pressure and magnetic pressures variations are anti-phase, indicating slow mode waves. However, by taking advantage of the spacecraft located very close in X-Y plane and slightly away from the central plasma sheet, we found that for many events the phase relation between the thermal and magnetic pressure variations is Z-dependent, which suggests that the observational evidence for slow mode may not be applicable. In order to further examine these waves, we performed a MHD analysis in inhomogeneous plasma sheet. The calculation shows that for Harris Sheet configuration, the thermal and magnetic pressures variations can be anti-phase for any wave other than slow mode waves where the vertical velocity disturbance reaches its maximum, thus this phase relation may not be used as an identifier of magnetosonic wave modes. We will show the dispersion relation and wave generated disturbances obtained from the numerical calculations.

  1. Experimental study of low frequency drift instability

    SciTech Connect

    Ioffe, M.S.; Kanaev, B.I.; Pastukhov, V.P.

    1994-05-01

    Experimental studies of nondissipative low frequency drift instability are reported; the plasma of a long mirror trap with edge casp anchors was investigated. The instability growth was found to take place only in a limited number of operation modes even in the case of all the growth requirements being satisfied. Furthermore, the instability development is rather moderate, and the associated anomalous losses appear to be small compared to the classical Coulomb losses. Possible factors accounting for the {open_quotes}soft{close_quotes} instability evolution are discussed. 13 refs., 6 figs.

  2. Low-frequency vibrational modes of glutamine

    NASA Astrophysics Data System (ADS)

    Wang, Wei-Ning; Wang, Guo; Zhang, Yan

    2011-12-01

    High-resolution terahertz absorption and Raman spectra of glutamine in the frequency region 0.2 THz-2.8 THz are obtained by using THz time domain spectroscopy and low-frequency Raman spectroscopy. Based on the experimental and the computational results, the vibration modes corresponding to the terahertz absorption and Raman scatting peaks are assigned and further verified by the theoretical calculations. Spectral investigation of the periodic structure of glutamine based on the sophisticated hybrid density functional B3LYP indicates that the vibrational modes come mainly from the inter-molecular hydrogen bond in this frequency region.

  3. Minimization of nanosatellite low frequency magnetic fields.

    PubMed

    Belyayev, S M; Dudkin, F L

    2016-03-01

    Small weight and dimensions of the micro- and nanosatellites constrain researchers to place electromagnetic sensors on short booms or on the satellite body. Therefore the electromagnetic cleanliness of such satellites becomes a central question. This paper describes the theoretical base and practical techniques for determining the parameters of DC and very low frequency magnetic interference sources. One of such sources is satellite magnetization, the reduction of which improves the accuracy and stability of the attitude control system. We present design solutions for magnetically clean spacecraft, testing equipment, and technology for magnetic moment measurements, which are more convenient, efficient, and accurate than the conventional ones. PMID:27036801

  4. Minimization of nanosatellite low frequency magnetic fields

    NASA Astrophysics Data System (ADS)

    Belyayev, S. M.; Dudkin, F. L.

    2016-03-01

    Small weight and dimensions of the micro- and nanosatellites constrain researchers to place electromagnetic sensors on short booms or on the satellite body. Therefore the electromagnetic cleanliness of such satellites becomes a central question. This paper describes the theoretical base and practical techniques for determining the parameters of DC and very low frequency magnetic interference sources. One of such sources is satellite magnetization, the reduction of which improves the accuracy and stability of the attitude control system. We present design solutions for magnetically clean spacecraft, testing equipment, and technology for magnetic moment measurements, which are more convenient, efficient, and accurate than the conventional ones.

  5. Gauribidanur Low-Frequency Solar Spectrograph

    NASA Astrophysics Data System (ADS)

    Kishore, P.; Kathiravan, C.; Ramesh, R.; Rajalingam, M.; Barve, Indrajit V.

    2014-10-01

    A new radio spectrograph, dedicated to observe the Sun, has been recently commissioned by the Indian Institute of Astrophysics (IIA) at the Gauribidanur Radio Observatory, about 100 km North of Bangalore. The instrument, called the Gauribidanur Low-frequency Solar Spectrograph (GLOSS), operates in the frequency range≈40 - 440 MHz. Radio emission in this frequency range originates close to the Sun, typically in the radial distance range r≈1.1 - 2.0 R⊙. This article describes the characteristics of the GLOSS and the first results.

  6. A new low-frequency backward mode in inhomogeneous plasmas

    SciTech Connect

    Vranjes, J.

    2014-07-15

    When an electromagnetic transverse wave propagates through an inhomogeneous plasma so that its electric field has a component in the direction of the background density gradient, there appears a disbalance of charge in every plasma layer, caused by the density gradient. Due to this, some additional longitudinal electric field component appears in the direction of the wave vector. This longitudinal field may couple with the usual electrostatic longitudinal perturbations like the ion acoustic, electron Langmuir, and ion plasma waves. As a result, these standard electrostatic waves are modified and in addition to this a completely new low-frequency mode appears. Some basic features of the coupling and modification of the ion acoustic wave, and properties of the new mode are discussed here, in ordinary electron-ion and in pair plasmas.

  7. Resonant interactions between cometary ions and low frequency electromagnetic waves

    NASA Technical Reports Server (NTRS)

    Thorne, Richard M.; Tsurutani, Bruce T.

    1987-01-01

    The conditions for resonant wave amplification in a plasma with a ring-beam distribution which is intended to model pick-up ions in a cometary environment are investigated. The inclination between the interplanetary field and the solar wind is found to play a crucial role in governing both the resonant frequency and the growth rate of any unstable mode. It is suggested that the low-frequency MHD mode should experience the most rapid amplification for intermediate inclination. In the frame of the solar wind, such waves should propagate along the field in the direction upstream toward the sun with a phase speed lower than the beaming velocity of the pick-up ions. This mechanism may account for the presence of the interior MHD waves noted by satellites over a region surrounding comets Giacobini-Zinner and Halley.

  8. Pulsar timing sensitivity to very-low-frequency gravitational waves

    SciTech Connect

    Jenet, Fredrick A.; Armstrong, J. W.; Tinto, Massimo

    2011-04-15

    We compute the sensitivity, constrained by instrumental, propagation, and other fundamental noises, of pulsar timing to very-low-frequency gravitational waves (GWs). Reaching predicted GW signal strengths will require suppression of time-of-arrival fluctuations caused by interstellar plasma turbulence and a reduction of white rms timing noise to < or approx. 100 ns. Assuming negligible intrinsic pulsar rotational noise, perfect time transfer from time standard to observatory, and stable pulse profiles, the resulting single-pulsar signal-to-noise ratio=1 sensitivity is limited by terrestrial time standards at h{sub rms}{approx}2x10{sup -16} [f/ (1 cycle/year)]-1/2 for f<3x10{sup -8} Hz, where f is the Fourier frequency and a bandwidth of 1 cycle/(10 years) is assumed. Since this sensitivity is comparable to predicted GW signal levels, a reliable detection will require substantial signal-to-noise ratio improvement via pulsar timing array.

  9. Low-Frequency Radio Bursts and Space Weather

    NASA Technical Reports Server (NTRS)

    Gopalswamy, N.

    2016-01-01

    Low-frequency radio phenomena are due to the presence of nonthermal electrons in the interplanetary (IP) medium. Understanding these phenomena is important in characterizing the space environment near Earth and other destinations in the solar system. Substantial progress has been made in the past two decades, because of the continuous and uniform data sets available from space-based radio and white-light instrumentation. This paper highlights some recent results obtained on IP radio phenomena. In particular, the source of type IV radio bursts, the behavior of type III storms, shock propagation in the IP medium, and the solar-cycle variation of type II radio bursts are considered. All these phenomena are closely related to solar eruptions and active region evolution. The results presented were obtained by combining data from the Wind and SOHO missions.

  10. Pulsar timing sensitivity to very-low-frequency gravitational waves

    NASA Astrophysics Data System (ADS)

    Jenet, Fredrick A.; Armstrong, J. W.; Tinto, Massimo

    2011-04-01

    We compute the sensitivity, constrained by instrumental, propagation, and other fundamental noises, of pulsar timing to very-low-frequency gravitational waves (GWs). Reaching predicted GW signal strengths will require suppression of time-of-arrival fluctuations caused by interstellar plasma turbulence and a reduction of white rms timing noise to ≲100ns. Assuming negligible intrinsic pulsar rotational noise, perfect time transfer from time standard to observatory, and stable pulse profiles, the resulting single-pulsar signal-to-noiseratio=1 sensitivity is limited by terrestrial time standards at hrms˜2×10-16[f/(1cycle/year)]-1/2 for f<3×10-8Hz, where f is the Fourier frequency and a bandwidth of 1 cycle/(10 years) is assumed. Since this sensitivity is comparable to predicted GW signal levels, a reliable detection will require substantial signal-to-noise ratio improvement via pulsar timing array.

  11. Low-frequency earthquakes at the southern Cascadia margin

    NASA Astrophysics Data System (ADS)

    Plourde, Alexandre P.; Bostock, Michael G.; Audet, Pascal; Thomas, Amanda M.

    2015-06-01

    We use seismic waveform data from the Mendocino Experiment to detect low-frequency earthquakes (LFEs) beneath Northern California during the April 2008 tremor-and-slip episode. In southern Cascadia, 59 templates were generated using iterative network cross correlation and stacking and grouped into 34 distinct LFE families. The main front of tremor epicenters migrates along strike at 9 km d-1; we also find one instance of rapid tremor reversal, observed to propagate in the opposite direction at 10-20 km h-1. As in other regions of Cascadia, LFE hypocenters from this study lie several kilometers above a recent plate interface model. South of Cascadia, LFEs were discovered on the Maacama and Bucknell Creek faults. The Bucknell Creek Fault may be the youngest fault yet observed to host LFEs. These fault zones also host shallow earthquake swarms with repeating events that are distinct from LFEs in their spectral and recurrence characteristics.

  12. Low-frequency electromagnetic plasma waves at comet P/Grigg-Skjellerup: Analysis and interpretation

    NASA Technical Reports Server (NTRS)

    Neubauer, Fritz M.; Glassmeier, Karl-Heinz; Coates, A. J.; Johnstone, A. D.

    1993-01-01

    The propagation and polarization characteristic of low-frequency electromagnetic wave fields near comet P/Grigg-Skjellerup (P/GS) are analyzed using magnetic field and plasma observations obtained by the Giotto magnetometer experiment and the Johnstone plasma analyzer during the encounter at the comet on July 10, 1992. The results have been physically interpreted.

  13. Low-frequency electrostatic waves in the solar wind

    NASA Astrophysics Data System (ADS)

    Sreeraj, T.; Singh, Satyavir; Singh Lakhina, Gurbax

    2016-07-01

    Electrostatic ion cyclotron waves are one of the ubiquitous features in space and laboratory plasmas. Here we present a linear study of electrostatic ion cyclotron waves in the solar wind. We model the solar wind by three-component magnetised plasma consisting of hot electrons with kappa distribution and fluid cold protons and doubly charged Helium ions. A numerical analysis of the linear electrostatic dispersion relation has been carried out for slow solar wind parameters and for -oblique wave propagation. The system supports four different modes i.e., fast and slow acoustic modes, and proton and Helium cyclotron modes. It has to be emphasised that for parallel propagation, physically acceptable solution to the dispersion relation are those of fast and slow acoustic modes. For oblique propagation, the coupling between various modes can be seen. Moreover, when the angle of propagation is increased the separation between acoustic modes and cyclotron modes increases and at perpendicular propagation, only proton and Helium-cyclotron modes can exist. The effect of various parameters like number density and temperature of Helium ions and kappa index on the dispersive properties has also been investigated. As the number density of helium ions increases, frequency of proton cyclotron mode decreases and frequency of Helium cyclotron mode increases at a fixed wave number. When the value of kappa increases, the frequency of the proton cyclotron mode increases but it does not have significant effect on the frequency of the Helium cyclotron mode. Likewise, when the temperature of Helium ions increases, the frequency of Helium cyclotron mode increases, however, the frequency of proton cyclotron mode remains more or less unchanged.

  14. Model helicopter rotor low frequency broadband noise

    NASA Technical Reports Server (NTRS)

    Humbad, N. G.; Harris, W. L.

    1982-01-01

    The results of an experimental investigation of low frequency broadband noise (LFBN) radiated from model helicopter rotors are presented. The results up to tip Mach number of 0.50 suggest that the peak sound pressure level (SPL) of LFBN appears to follow tip Mach number to a fourth power law and rms velocity of turbulence to a second power law. The experimental results on the effect of tip speed and advance ratio on the peak SPL of LFBN can be explained on the basis of a simple scaling law. However, the experimental results on the effect of blade loading on the peak SPL of LFBN is still not clearly understood. A simple peak SPL scaling law for noise from a helicopter in forward flight encountering a sinusoidal gust is also developed. The trends predicted by the scaling law with the experimental results are found satisfactory for the cases of variation of the peak SPL of LFBN with tip speed and advance ratio.

  15. On helicopter rotor low frequency broadband noise

    NASA Technical Reports Server (NTRS)

    Williams, Morgan; Harris, Wesley L.

    1985-01-01

    The effect of shear-layer-type inflow turbulence on the low-frequency broadband noise of a model helicopter rotor is experimentally studied. The measurements and the one-dimensional energy spectral density indicate that the upstream airfoil wake turbulence is nonisotropic, but approaches isotropy at high wavenumbers. Turbulence measurements also indicate that the wake turbulence is weak. The effect of the inflow turbulence intensity on the peak sound pressure level follows an intensity-velocity squared scaling law. A number of length scales and turbulence intensities exist which can be measured in the airfoil wake depending on the position at which the measurements are taken. Comparison of experimental and theoretical sound pressure power spectral densities indicates that the initial anisotropy of the inflow turbulence does not invalidate the isotropic turbulence assumption made in noise prediction models as long as measured turbulence intensities and length scales are used.

  16. Low-frequency radio navigation system

    NASA Technical Reports Server (NTRS)

    Wallis, D. E. (Inventor)

    1983-01-01

    A method of continuous wave navigation using four transmitters operating at sufficiently low frequencies to assure essentially pure groundwave operation is described. The transmitters are keyed to transmit constant bursts (1/4 sec) in a time-multiplexed pattern with phase modulation of at least one transmitter for identification of the transmitters and with the ability to identify the absolute phase of the modulated transmitter and the ability to modulate low rate data for transmission. The transmitters are optimally positioned to provide groundwave coverage over a service region of about 50 by 50 km for the frequencies selected in the range of 200 to 500 kHz, but their locations are not critical because of the beneficial effect of overdetermination of position of a receiver made possible by the fourth transmitter. Four frequencies are used, at least two of which are selected to provide optimal resolution. All transmitters are synchronized to an average phase as received by a monitor receiver.

  17. Low Frequency Electromagnetic Pulse and Explosions

    SciTech Connect

    Sweeney, J J

    2011-02-01

    This paper reviews and summarizes prior work related to low frequency (< 100 Hz) EMP (ElectroMagnetic Pulse) observed from explosions. It focuses on how EMP signals might, or might not, be useful in monitoring underground nuclear tests, based on the limits of detection, and physical understanding of these signals. In summary: (1) Both chemical and nuclear explosions produce an EMP. (2) The amplitude of the EMP from underground explosions is at least two orders of magnitude lower than from above ground explosions and higher frequency components of the signal are rapidly attenuated due to ground conductivity. (3) In general, in the near field, that is distances (r) of less than 10s of kilometers from the source, the amplitude of the EMP decays approximately as 1/r{sup 3}, which practically limits EMP applications to very close (<{approx}1km) distances. (4) One computational model suggests that the EMP from a decoupled nuclear explosion may be enhanced over the fully coupled case. This has not been validated with laboratory or field data. (5) The magnitude of the EMP from an underground nuclear explosion is about two orders of magnitude larger than that from a chemical explosion, and has a larger component of higher frequencies. In principle these differences might be used to discriminate a nuclear from a chemical explosion using sensors at very close (<{approx}1 km) distances. (6) Arming and firing systems (e.g. detonators, exploding bridge wires) can also produce an EMP from any type of explosion. (7) To develop the understanding needed to apply low frequency EMP to nuclear explosion monitoring, it is recommended to carry out a series of controlled underground chemical explosions with a variety of sizes, emplacements (e.g. fully coupled and decoupled), and arming and firing systems.

  18. Low-frequency Stoneley energy for stratigraphic evaluation

    SciTech Connect

    Hlaing, K.K.; Lemoy, C.; Maret, J.P.; Kremer, Y.; Borland, W.H.; Maw, M.

    1994-07-01

    Conventional sonic measurements of shear and compressional slowness are body waves that travel within the formation and are commonly used for petrophysical analysis of a well. Low-frequency Stoneley waves travel within the well bore and are traditionally used to interpret fractures and formation permeability, usually by analyzing the energy losses and, to a lesser extent, the slowness. The authors have found that Stoneley energy has been very useful in the identification of vuggy carbonate facies linked to paleokarstic surfaces in the Upper Burman limestone reservoir of Miocene age, in the YADANA gas deposit, offshore Myanmar. One good example is seen in well YAD-1 where the carbonate reservoir has been cored, allowing precise facies and porosity type determination. Matching Stoneley energy and core description show a striking correlation between loss of energy and vuggy carbonate facies due to karstic diagenetic processes, always in relation with reefal or near reefal facies. Accordingly, facies interpretation has tentatively been done in the deeper, noncored reservoir zone, where losses of energy are important and considered as indicating karstic influence and the specific environment.

  19. Interim prediction method for low frequency core engine noise

    NASA Technical Reports Server (NTRS)

    Huff, R. G.; Clark, B. J.; Dorsch, R. G.

    1974-01-01

    A literature survey on low-frequency core engine noise is presented. Possible sources of low frequency internally generated noise in core engines are discussed with emphasis on combustion and component scrubbing noise. An interim method is recommended for predicting low frequency core engine noise that is dominant when jet velocities are low. Suggestions are made for future research on low frequency core engine noise that will aid in improving the prediction method and help define possible additional internal noise sources.

  20. Imaging Jupiter Radiation Belts At Low Frequencies

    NASA Astrophysics Data System (ADS)

    Girard, J. N.; de Pater, I.; Zarka, P.; Santos-Costa, D.; Sault, R.; Hess, S.; Cecconi, B.; Fender, R.; Pewg, Lofar

    2014-04-01

    , at different epochs only provided, each time, glimpses of the spectral content in different observational configurations. As the synchrotron emission frequency peaks at Vmax / E2B (with Vmax in MHz, E, the electron energy in MeV and B, the magnetic field in Gauss), the low frequency content of this emission is associated with low energy electron populations inside the inner belt and the energetic electrons located in regions of weaker magnetic field (at few jovian radii). Therefore, there is much interest in extending and completing the current knowledge of the synchrotron emission from the belts, with low frequency resolved observations. LOFAR, the LOw Frequency ARray (LOFAR) [6], is a giant flexible and digital ground-based radio interferometer operating in the 30-250 MHz band. It brings very high time (~ μs), frequency (~ kHz) and angular resolutions (~1") and huge sensitivity (mJy). In November 2011, a single 10-hour track enabled to cover an entire planetary rotation and led to the first resolved image of the radiation belts between 127- 172 MHz [7,8]. In Feb 2013, an 2×5h30 joint LOFAR/ WSRT observing campaign seized the state of the radiation belts from 45 MHz up to 5 GHz. We will present the current state of the study (imaging, reconstruction method and modeling) of the radiation belts dynamic with this current set of observations. LOFAR can contribute to the understanding of the physics taking place in the inner belt as well as possibly providing a fast and a systematic "diagnostic" of the state of the belts. The latter represents an opportunity to give context and ground-based support for the arrival of JUNO (NASA) scheduled in July 2016 and also for future missions, such as JUICE (ESA), at the vicinity of Jupiter by the exploration of its icy satellites.

  1. Low-frequency oscillations in radiative-convective models

    SciTech Connect

    Hu, Qi; Randall, D.A.

    1991-12-31

    Although eastward propagation is usually regarded as an essential feature of the low-frequency ``Madden-Julian oscillation`` observed in the tropical atmosphere, many observations indicate that there is an important stationary or quasi-stationary component of the oscillation. Yasunari (1979), for example, investigated the stationary 30--60 day variation in upper tropospheric cloudiness in the Asian summer monsoon region. In a case study of the 30--60 day oscillation. Hsu et al. (1990) found a strong stationary oscillation of the divergence, outgoing longwave mdiadon and other fields. A recent observational study by Weickmann and Khalsa (1990) offers further evidence that the Madden-Julian oscillation has an important stationary component. In this paper, we present evidence that intraseasonal oscillations can be produced by local radiative and convective processes. This suggests that the observed propagating Madden-Julian wave is produced by interactions between these local processes and the large scale motion field, and is not essential for the existence of the observed oscillation.

  2. Low-frequency oscillations in radiative-convective models

    SciTech Connect

    Hu, Qi; Randall, D.A.

    1991-01-01

    Although eastward propagation is usually regarded as an essential feature of the low-frequency Madden-Julian oscillation'' observed in the tropical atmosphere, many observations indicate that there is an important stationary or quasi-stationary component of the oscillation. Yasunari (1979), for example, investigated the stationary 30--60 day variation in upper tropospheric cloudiness in the Asian summer monsoon region. In a case study of the 30--60 day oscillation. Hsu et al. (1990) found a strong stationary oscillation of the divergence, outgoing longwave mdiadon and other fields. A recent observational study by Weickmann and Khalsa (1990) offers further evidence that the Madden-Julian oscillation has an important stationary component. In this paper, we present evidence that intraseasonal oscillations can be produced by local radiative and convective processes. This suggests that the observed propagating Madden-Julian wave is produced by interactions between these local processes and the large scale motion field, and is not essential for the existence of the observed oscillation.

  3. The LWA1 Low Frequency Sky Survey

    NASA Astrophysics Data System (ADS)

    Dowell, Jayce; Taylor, Gregory B.; LWA Collaboration

    2015-01-01

    The LWA1 Low Frequency Sky Survey is a survey of the sky visible from the first station of the Long Wavelength Array (LWA1) across the frequency range of 35 to 80 MHz. The primary motivation behind this effort is to improve our understanding of the sky at these frequencies. In particular, an understanding of the low frequency foreground emission is necessary for work on detecting the epoch of reionization and the cosmic dark ages where the foreground signal dwarfs the expected redshifted HI signal by many orders of magnitude (Pritchard & Loeb 2012, Rep. Prog. Phys., 75, 086901). The leading model for the sky in the frequency range of 20 to 200 MHz is the Global Sky Model (GSM) by de Oliveria-Costas et al. (2008, MNRAS, 288, 247). This model is based upon a principle component analysis of 11 sky maps ranging in frequency from 10 MHz to 94 GHz. Of these 11 maps, only four are below 1 GHz; 10 MHz from Caswell (1976, MNRAS, 177, 601), 22 MHz from Roger et al. (1999, A&AS, 137, 7), 45 MHz from Alvarez et al. (1997, A&AS, 124, 315) and Maeda et al. (1999, A&AS, 140, 145), and 408 MHz from Haslam et al. (1982, A&AS, 47, 1). Thus, within this model, the region of interest to both cosmic dawn and the epoch of reionization is largely unconstrained based on the available survey data, and are also limited in terms of the spatial coverage and calibration. A self-consistent collection of maps is necessary for both our understanding of the sky and the removal of the foregrounds that mask the redshifted 21-cm signal.We present the current state of the survey and discuss the imaging and calibration challenges faced by dipole arrays that are capable of imaging nearly 2π steradians of sky simultaneously over a large fractional bandwidth.Construction of the LWA has been supported by the Office of Naval Research under Contract N00014-07-C-0147. Support for operations and continuing development of the LWA1 is provided by the National Science Foundation under grants AST-1139963 and AST

  4. Energy scavenging from low frequency vibrations

    NASA Astrophysics Data System (ADS)

    Galchev, Tzeno V.

    The development of three energy conversion devices that are able to transform vibrations in their surroundings to electrical energy is discussed in this thesis. These energy harvesters are based upon a newly invented architecture called the Parametric Frequency Increased Generator (PFIG). The PFIG structure is designed to efficiently convert low frequency and non-periodic vibrations into electrical power. The three PFIG devices have a combined operating range covering two orders of magnitude in acceleration (0.54--19.6m/s 2) and a frequency range spanning up to 60Hz; making them some of the most versatile generators in existence. The PFIG utilizes a bi-stable mechanical structure to initiate high-frequency mechanical oscillations in an electromechanical scavenger. By up-converting the ambient vibration frequency to a higher internal operation frequency, the PFIG achieves better electromechanical coupling. The fixed internal displacement and dynamics of the PFIG allow it to operate more efficiently than resonant generators when the ambient vibration amplitude is higher than the internal displacement limit of the device. The PFIG structure is capable of efficiently converting mechanical vibrations with variable characteristics including amplitude and frequency, into electrical power. The first electromagnetic harvester can generate a peak power of 163microW and an average power of 13.6microW from an input acceleration of 9.8m/s 2 at 10Hz, and it can operate up to 60Hz. The internal volume of the generator is 2.12cm3 (3.75 including casing). It sets the state-of-the-art in efficiency in the <20Hz range. The volume figure of merit is 0.068%, which is a 10x improvement over other published works. It has a record high bandwidth figure of merit (0.375%). A second piezoelectric implementation generates 3.25microW of average power under the same excitation conditions, while the volume of the generator is halved (1.2cm3). A third PFIG was developed for critical

  5. An autocorrelation method to detect low frequency earthquakes within tremor

    USGS Publications Warehouse

    Brown, J.R.; Beroza, G.C.; Shelly, D.R.

    2008-01-01

    Recent studies have shown that deep tremor in the Nankai Trough under western Shikoku consists of a swarm of low frequency earthquakes (LFEs) that occur as slow shear slip on the down-dip extension of the primary seismogenic zone of the plate interface. The similarity of tremor in other locations suggests a similar mechanism, but the absence of cataloged low frequency earthquakes prevents a similar analysis. In this study, we develop a method for identifying LFEs within tremor. The method employs a matched-filter algorithm, similar to the technique used to infer that tremor in parts of Shikoku is comprised of LFEs; however, in this case we do not assume the origin times or locations of any LFEs a priori. We search for LFEs using the running autocorrelation of tremor waveforms for 6 Hi-Net stations in the vicinity of the tremor source. Time lags showing strong similarity in the autocorrelation represent either repeats, or near repeats, of LFEs within the tremor. We test the method on an hour of Hi-Net recordings of tremor and demonstrates that it extracts both known and previously unidentified LFEs. Once identified, we cross correlate waveforms to measure relative arrival times and locate the LFEs. The results are able to explain most of the tremor as a swarm of LFEs and the locations of newly identified events appear to fill a gap in the spatial distribution of known LFEs. This method should allow us to extend the analysis of Shelly et al. (2007a) to parts of the Nankai Trough in Shikoku that have sparse LFE coverage, and may also allow us to extend our analysis to other regions that experience deep tremor, but where LFEs have not yet been identified. Copyright 2008 by the American Geophysical Union.

  6. Observations of a low-frequency cutoff in magnetospheric radio noise received on Imp 6

    NASA Technical Reports Server (NTRS)

    Vesecky, J. F.; Frankel, M. S.

    1975-01-01

    The quasi-continuous component of the magnetospheric noise observed by Imp 6, lying between 30 and 110 kHz, often exhibits a low-frequency cutoff when the spacecraft is in the interplanetary medium or the magnetosheath. A hypothesis is considered in which this low-frequency cutoff, f-co, is caused by overdense plasma situated somewhere along the noise-source-to-satellite path. The plasma is assumed to have a plasma frequency approximately equal to f-co, thus cutting off propagation below f-co.

  7. Low frequency dynamical stabilisation in optical tweezers

    NASA Astrophysics Data System (ADS)

    Richards, Christopher J.; Smart, Thomas J.; Jones, Philip H.; Cubero, David

    2015-08-01

    It is well known that a rigid pendulum with minimal friction will occupy a stable equilibrium position vertically upwards when its suspension point is oscillated at high frequency. The phenomenon of the inverted pendulum was explained by Kapitza by invoking a separation of timescales between the high frequency modulation and the much lower frequency pendulum motion, resulting in an effective potential with a minimum in the inverted position. We present here a study of a microscopic optical analogue of Kapitza's pendulum that operates in different regimes of both friction and driving frequency. The pendulum is realized using a microscopic particle held in a scanning optical tweezers and subject to a viscous drag force. The motion of the optical pendulum is recorded and analyzed by digital video microscopy and particle tracking to extract the trajectory and stable orientation of the particle. In these experiments we enter the regime of low driving frequency, where the period of driving is comparable to the characteristic relaxation time of the radial motion of the pendulum with finite stiffness. In this regime we find stabilization of the pendulum at angles other than the vertical (downwards) is possible for modulation amplitudes exceeding a threshold value where, unlike the truly high frequency case studied previously, both the threshold amplitude and equilibrium position are found to be functions of friction. Experimental results are complemented by an analytical theory for induced stability in the low frequency driving regime with friction.

  8. Very low frequency earthquakes spatiotemporally asynchronous with strong tremor during the 2014 episodic tremor and slip event in Cascadia

    NASA Astrophysics Data System (ADS)

    Hutchison, Alexandra A.; Ghosh, Abhijit

    2016-07-01

    We find very low frequency earthquakes (VLFEs) in Cascadia during the 2014 episodic tremor and slip event under Washington and Vancouver Island using a grid search centroid moment tensor inversion method. The very low frequency earthquakes occur when and where there is no strong tremor detected, contrasting with previous findings where tremor and very low frequency earthquakes have a clear spatiotemporal relationship. This challenges our current understanding of the dynamic relationship between the different types of slow earthquakes and their relationship to slow slip. We interpret these findings to suggest that VLFEs and tremor may have their own "seismic cycles," resulting from asperities of different sizes or characteristics that respond pseudo-independently to stress loading. We propose that very low frequency earthquakes and tremor are subject to asynchronous seismic cycles.

  9. Human Hippocampal Increases in Low-Frequency Power during Associative Prediction Violations

    PubMed Central

    Chen, Janice; Dastjerdi, Mohammad; Foster, Brett L.; LaRocque, Karen F.; Rauschecker, Andreas M.; Parvizi, Josef; Wagner, Anthony D.

    2013-01-01

    Environmental cues often trigger memories of past events (associative retrieval), and these memories are a form of prediction about imminent experience. Learning is driven by the detection of prediction violations, when the past and present diverge. Using intracranial electroencephalography (iEEG), we show that associative prediction violations elicit increased low-frequency power (in the slow-theta range) in human hippocampus, that this low-frequency power increase is modulated by whether conditions allow predictions to be generated, that the increase rapidly onsets after the moment of violation, and that changes in low-frequency power are not present in adjacent perirhinal cortex. These data suggest that associative mismatch is computed within hippocampus when cues trigger predictions that are violated by imminent experience. PMID:23571081

  10. Low-Frequency Earthquakes in Cascadia

    NASA Astrophysics Data System (ADS)

    Sweet, J. R.; Creager, K. C.; Ghosh, A.; Vidale, J. E.

    2009-12-01

    Low-frequency earthquakes (LFEs) are a recently identified class of earthquakes that have been observed to occur coincidentally with non-volcanic tremor in time and space. These LFEs also have a frequency spectra that is nearly identical to that of tremor—implying a common source for these two phenomena. Indeed, it has been proposed that tremor may simply be a superposition of many individual LFEs (Shelly et al., 2006, 2007, Nature). As such, LFEs have been used to constrain the location of tremor. We first reported LFEs in Cascadia last year, following the deployment of an 80-station, 1-km aperture seismic array on the Olympic Peninsula of western Washington State. This past year we have deployed 8 small aperture, 3-component seismic arrays across the northern Olympic Peninsula in the hopes of recording and locating additional tremor and LFEs. These arrays are composed of 10 3-component and 10 vertical component EarthScope seismometers. We use a combination of methods to identify and locate LFEs in our new, expanded dataset. Potential LFEs are first flagged by searching for peaks in the cross correlation of vertical and horizontal components that correspond to S minus P times of arriving energy (La Rocca, 2009, Science). These targets are then used as template events and are cross correlated with several hours of continuous data to find matching events. Using stacking and correlation we obtain accurate S minus P times for some arrays, and differential S and P times between arrays. We use these times to obtain robust estimates of LFE hypocenters. Unfortunately none of the 2009 data from the array of arrays covers a period of Episodic Tremor and Slip (ETS), but several smaller tremor bursts were recorded.

  11. Mode properties of low-frequency waves: Kinetic theory versus Hall-MHD

    NASA Technical Reports Server (NTRS)

    Krauss-Varban, D.; Omidi, N.; Quest, K. B.

    1994-01-01

    In fluid theory, the ordering of low-frequency modes in a homogeneous plasma is based on the phase velocity, since modes do not intersect each other in dispersion diagrams as a function of wavenumber or other parameters. In linear kinetic theory, modes cross each other. Thus a consistent and useful classification should be based on the physical properties of the modes instead. This paper attempts such a classification by documeting the dispersion and general mode properties of the low-frequency waves (omega much less than (OMEGA(sub ci) OMEGA(sub ce) (exp 1/2)), where OMEGA(sub ci), OMEGA(sub ce) are the cyclotron frequencies of the ions and electrons, respectively) in kinetic theory, and by comparing them to the results of two-fluid theory. Kinetic theory gives a seperate Alfven/ion-cyclotron (A/IC) wave with phase speed Omega/k approximately = v(sub A) cos theta for omega much less than OMEGA(sub ci), where v(sub A) is the Alfven velocity and theta the angle of propagation between wave vector k and background magnetic field B(sub o). For a given wavenumber, the magnetosonic mode is a double-valued solution with a singular point in theta, beta parameter space, where beta is the ratio of thermal pressure to magnetic pressure. It is shown that a branch cut starting at the singular point theta approximately 30 deg, beta approximately 3 and leading to larger beta gives a practical and consitent seperation of this double-valued magnetosonic solution. Selection of this branch cut results in a moderately damped fast/magnetos onic and a heavily damped slow/sound wave. A comprehensive review of the polarization, compressibility and other mode properties is given and shown to be consistent with the selected branch cut. At small wavenumbers, the kinetic mode properties typically start to deviate significantly from their fluid counterparts at beta approximately 0.5. At larger beta, there is no longer a consistent correspondence between the fluid and kinetic modes. Kinetic

  12. Ray tracing of Jovian low frequency radiation

    NASA Technical Reports Server (NTRS)

    Six, N. Frank

    1987-01-01

    The radio emission from Jupiter in the decametric range (2 to 40 MHz) has been studied for three decades. It is known that this emission is correlated with particular longitude regions on the planet and with the position of the satellite Io. The Planetary Radio Astronomy experiment on the Voyager 1 and 2 spacecraft observed this emission, obtaining valuable new information. The objectives of this study are as follows: (1) calculate raypaths for decametric wavelength radiation in Jupiter's magnetosphere; (2) compare the model-dependent raypaths with the Voyager observations; and (3) deduce characteristics of the source regions and the influence of propagation effects.

  13. Undulations from amplified low frequency surface waves

    SciTech Connect

    Coutant, Antonin; Parentani, Renaud

    2014-04-15

    We study the linear scattering of gravity waves in longitudinal inhomogeneous stationary flows. When the flow becomes supercritical, it is known that counterflow propagating shallow waves are blocked and converted into deep waves. Here we show that in the zero-frequency limit, the reflected waves are amplified in such a way that the free surface develops an undulation, i.e., a zero-frequency wave of large amplitude with nodes located at specific places. This amplification involves negative energy waves and implies that flat surfaces are unstable against incoming perturbations of arbitrary small amplitude. The relation between this instability and black hole radiation (the Hawking effect) is established.

  14. Constraints on Subduction Zone Processes from Low Frequency Earthquakes

    NASA Astrophysics Data System (ADS)

    Bostock, M. G.

    2015-12-01

    The discovery of tectonic tremor and constituent low-frequency earthquakes (LFEs) offers seismologists new opportunities to study both deformational processes and structure within the subduction zone forearc. This assertion is especially true for northern Cascadia where i) regular seismicity is sparse, and ii) a relatively transparent overriding plate inflicts minimal distortion upon direct P and S wave arrivals from LFEs. Despite low signal-to-noise ratios, LFEs are highly repetitive and signal can be enhanced through construction of stacked templates. Studies in both Cascadia and Nankai reveal an association between LFE hypocenters and a high Vp/Vs, low-velocity zone (LVZ) that is inferred to represent overpressured upper oceanic crust. Scattered signals within Vancouver Island templates, interpreted to originate at boundaries of the LVZ, place LFEs within the LVZ and suggest that this structure may define a distributed (several km) zone of deformation. A recent analysis of LFE magnitudes indicates that LFEs exhibit scaling relations distinct from both regular earthquakes and longer period (10's of seconds to days) phenomena associated with slow slip. Regular earthquakes generally obey a scaling of moment proportional to duration cubed consistent with self similarity, whereas long period slow slip phenomena exhibit a linear scaling between moment and duration that can be accommodated through constant slip or constant stress drop models. In contrast, LFE durations are nearly constant suggesting that moment is governed by slip alone and that asperity size remains approximately constant. The implied dimensions (~1 km2), the persistance of LFEs in time and their stationarity in space point to structural heterogeneity, perhaps related to pockets of upper oceanic crust impervious to hydrothermal circulation, as a fundamental control.

  15. Low-frequency fluctuations in the magnetosheath near the magnetopause

    NASA Technical Reports Server (NTRS)

    Denton, Richard E.; Gary, S. Peter; Li, Xinlin; Anderson, Brian J.; Labelle, James W.; Lessard, Marc

    1995-01-01

    There are four low-frequency modes which may propagate in a high-beta nearly bi-Maxwellian plasma. These are the magnetosonic, Alfven, ion acoustic, and mirror modes. This manuscript defines a procedure based on linear Vlasov theory for the unique identification of these modes by use of transport ratios, dimensionless ratios of the fluctuating field and plasma quantities. A single parameter, the mode deviation is calculated using the plasma and magnetic field data gathered by the Active Magnetospheric Particle Tracer Explorers/Ion Release Module (AMPTE/IRM) spacecraft to identify the modes observed in the terrestial magnetosheath near the magnetopause. As well as determining the mode which best describes the observed fluctuations, it gives us a measure of whether or not the resulting identification is unique. Using 17 time periods temporally close to a magnetopause crossing, and confining our study to the frequency range from 0.01 to 0.04 Hz, we find that the only clearly identified mode in this frequency range is the mirror mode. Most commonly, the quasi-perpendicular mirror mode (with wave vector k roughly perpendicular to the background magnetic field B(sub zero) is observed. In two events the quasi-parallel mirror mode k parallel B(sub zero) was identified.

  16. New observations of the low frequency interplanetary radio emissions

    NASA Technical Reports Server (NTRS)

    Kurth, W. S.; Gurnett, D. A.

    1991-01-01

    Recent Voyager 1 observations reveal reoccurrences of the low frequency interplanetary radio emissions. Three of the new events are weak transient events which rise in frequency from the range of 2-2.5 kHz to about 3 kHz with drift rates of approximately 1.5 kHz/year. The first of the transient events begins in mid-1989 and the more recent pair of events both were first detected in late 1991. In addition, there is an apparent onset of a 2-kHz component of the emission beginning near day 70 of 1991. The new transient emissions are barely detectable on Voyager 1 and are below the threshold of detectability on Voyager 2, which is less sensitive than Voyager 1. The new activity provides new opportunities to test various theories of the triggering, generation, and propagation of the outer heliospheric radio emissions and may signal a response of the source of the radio emissions to the increased solar activity associated with the recent peak in the solar cycle.

  17. Low frequency radioastronomy of the inner heliosphere: the way forward.

    NASA Astrophysics Data System (ADS)

    Cecconi, Baptiste; Zarka, Philippe; Bergman, Jan; Falcke, Heino; Boonstra, Albert-Jan; Briand, Carine; Girard, Julien; Klein Wolt, Marc; Baan, Willem; Segret, Boris; Maksimovic, Milan

    Low frequency radioastronomy observatories for the heliosphere have been using similar instrumentation for decades. The Cassini, STEREO, and the future Solar Orbiter mission are embarking goniopolarmetric radio receiver connected to 3 electric antennas. Such instrument provides the spectral matrix (or part of it) from which the wave parameters can be derived. With a point source assumption (plane wave), we derive the direction of arrival of the wave, the polarization and the flux density. In case of a spatially extended source (disk shaped, with a given radial profile), the source centroid direction and the apparent source size is provided. This type of instrumentation cannot provide much more parameters, as there is a maximum of to 9 independent measurements for each time-frequency step. We propose a concept of radioastronomy instrumentation using a swarm of small satellites (possibly cubesats) with sensitive radio receivers measuring the wave front and phase of the radio waves on each spacecraft. This instrument will also provide 3-dimensional interferometric measurement. Such resolved imaging capabilities of the inner heliosphere would be a real step forward to better understand the radio emissions mechanisms and the propagation processes. We will present the various existing projects and the roadmap to reach the goal.

  18. The Low-Frequency Variability of the Tropical Atlantic Ocean

    NASA Technical Reports Server (NTRS)

    Haekkinen, Sirpa; Mo, Kingtse C.; Koblinsky, Chester J. (Technical Monitor)

    2001-01-01

    Upper ocean temperature variability in the tropical Atlantic is examined from the Comprehensive Ocean Atmosphere Data Set (COADS) as well as from an ocean model simulation forced by COADS anomalies appended to a monthly climatology. Our findings are as follows: Only the sea surface temperatures (SST) in the northern tropics are driven by heat fluxes, while the southern tropical variability arises from wind driven ocean circulation changes. The subsurface temperatures in the northern and southern tropics are found to have a strong linkage to buoyancy forcing changes in the northern North Atlantic. Evidence for Kelvin-like boundary wave propagation from the high latitudes is presented from the model simulation. This extratropical influence is associated with wintertime North Atlantic Oscillation (NAO) forcing and manifests itself in the northern and southern tropical temperature anomalies of the same sign at depth of 100-200 meters as result of a Rossby wave propagation away from the eastern boundary in the wake of the boundary wave passage. The most apparent association of the southern tropical sea surface temperature anomalies (STA) arises with the anomalous cross-equatorial winds which can be related to both NAO and the remote influence from the Pacific equatorial region. These teleconnections are seasonal so that the NAO impact on the tropical SST is the largest it mid-winter but in spring and early summer the Pacific remote influence competes with NAO. However, NAO appears to have a more substantial role than the Pacific influence at low frequencies during the last 50 years. The dynamic origin of STA is indirectly confirmed from the SST-heat flux relationship using ocean model experiments which remove either anomalous wind stress forcing or atmospheric forcing anomalies contributing to heat exchange.

  19. New families of low frequency earthquakes beneath the Olympic Peninsula, Washington

    NASA Astrophysics Data System (ADS)

    Chestler, S.; Creager, K. C.; Sweet, J. R.

    2013-12-01

    Using data from the Array of Arrays (AofA) and Cascadia Arrays for Earthscope (CAFÉ) experiments we search for new families of low frequency earthquakes (LFEs) beneath the Olympic Peninsula, Washington. LFE families are clusters of repeating LFEs that occur in approximately the same location. Following methodology similar to Bostock et al. [2012, G3], we cross correlate 6-second long windows within an hour of data during the 2010 and 2011 ETS events. We apply this to 99 hours of tremor data. For each hour, we stack the autocorrelation functions from a set of 7 3-component base stations chosen for their high signal-to-noise ratios (SNRs). We extract a maximum of 10 windows per hour with correlation coefficients higher than 9 times the median absolute deviation (MAD). These time windows contain our preliminary LFE detections. We then cross correlate these data and group them using a hierarchical clustering algorithm. We produce template waveforms by stacking the waveforms corresponding to a given cluster. To strengthen the templates we scan them through on day of tremor and stack all waveforms that correlate with the original template. Our efforts have yielded dozens of new families scattered beneath the AofA stations. These additional LFE families add to the 9 known families beneath the Olympic Peninsula [Sweet et al., AGU fall meeting, 2012]. The detection of more LFE families will allow us to (1) interpolate the pattern of stress transfer through the transition zone [Wech et al., Nature Geoscie., 2011], (2) gain insight into the distribution of asperities, or sticky spots, on the plate interface [Ghosh et al., JGR, 2012], and (3) track slow slip rupture propagation with unprecedented spatial and temporal accuracy.

  20. Low-frequency earthquakes in Shikoku, Japan, and their relationship to episodic tremor and slip.

    PubMed

    Shelly, David R; Beroza, Gregory C; Ide, Satoshi; Nakamula, Sho

    2006-07-13

    Non-volcanic seismic tremor was discovered in the Nankai trough subduction zone in southwest Japan and subsequently identified in the Cascadia subduction zone. In both locations, tremor is observed to coincide temporally with large, slow slip events on the plate interface downdip of the seismogenic zone. The relationship between tremor and aseismic slip remains uncertain, however, largely owing to difficulty in constraining the source depth of tremor. In southwest Japan, a high quality borehole seismic network allows identification of coherent S-wave (and sometimes P-wave) arrivals within the tremor, whose sources are classified as low-frequency earthquakes. As low-frequency earthquakes comprise at least a portion of tremor, understanding their mechanism is critical to understanding tremor as a whole. Here, we provide strong evidence that these earthquakes occur on the plate interface, coincident with the inferred zone of slow slip. The locations and characteristics of these events suggest that they are generated by shear slip during otherwise aseismic transients, rather than by fluid flow. High pore-fluid pressure in the immediate vicinity, as implied by our estimates of seismic P- and S-wave speeds, may act to promote this transient mode of failure. Low-frequency earthquakes could potentially contribute to seismic hazard forecasting by providing a new means to monitor slow slip at depth. PMID:16838019

  1. Sausage Mode Propagation in a Thick Magnetic Flux Tube

    NASA Astrophysics Data System (ADS)

    Pardi, A.; Ballai, I.; Marcu, A.; Orza, B.

    2014-04-01

    The aim of this paper is to model the propagation of slow magnetohydrodynamic (MHD) sausage waves in a thick expanding magnetic flux tube in the context of the quiescent (VAL-C) solar atmosphere. The propagation of these waves is found to be described by the Klein-Gordon equation. Using the governing MHD equations and the VAL-C atmosphere model we study the variation of the cut-off frequency along and across the magnetic tube guiding the waves. Due to the radial variation of the cut-off frequency the flux tubes act as low frequency filters for the waves.

  2. High energy, low frequency, ultrasonic transducer

    DOEpatents

    Brown, Albert E.

    2000-01-01

    A wide bandwidth, ultrasonic transducer to generate nondispersive, extensional, pulsed acoustic pressure waves into concrete reinforced rods and tendons. The wave propagation distance is limited to double the length of the rod. The transducer acoustic impedance is matched to the rod impedance for maximum transfer of acoustic energy. The efficiency of the transducer is approximately 60 percent, depending upon the type of active elements used in the transducer. The transducer input energy is, for example, approximately 1 mJ. Ultrasonic reflections will occur at points along the rod where there are changes of one percent of a wavelength in the rod diameter. A reduction in the rod diameter will reflect a phase reversed echo, as compared with the reflection from an incremental increase in diameter. Echo signal processing of the stored waveform permits a reconstruction of those echoes into an image of the rod. The ultrasonic transducer has use in the acoustic inspection of long (40+foot) architectural reinforcements and structural supporting members, such as in bridges and dams.

  3. Subsurface Ice Detection via Low Frequency Surface Electromagnetic Method

    NASA Astrophysics Data System (ADS)

    Stillman, D. E.; Grimm, R. E.; Mcginnis, R. N.

    2014-12-01

    The geophysical detection of ice in the Cryosphere is typically conducted by measuring the absence of water. These interpretations can become non-unique in dry soils or in clay- and silt-rich soils that contain significant quantities of unfrozen water. Extensive laboratory measurements of electrical properties were made on permafrost samples as a function of frequency, temperature, and water content. These laboratory measurements show that the amount of ice can be uniquely obtained by measuring a frequency dependence of the electrical properties over a large frequency range (20 kHz - 10 Hz). In addition, the electrical properties of permafrost are temperature dependent, which can allow for an estimate of subsurface temperature. In order to test this approach in the field, we performed field surveys at four locations in Alaska. We used three low frequency electromagnetic methods: Spectral Induced Polarization (SIP: 20 kHz - 10 Hz), Capacively Coupled Resistivity (CCR: OhmMapper - 16.5 kHz), and DC Resistivity (Syscal ~ 8 Hz). At the Cold Regions Research and Engineering Laboratory permafrost tunnel near Fox, AK, we used SIP to measure the average ice concentration of 80 v% and determined the temperature to be -3±1°C by matching survey results to lab data. SIP data acquisition is very slow; therefore, at three sites near Tok, AK, we used CCR to perform reconnaissance of the area. Then SIP and DC resistivity were performed at anomalous areas. The three survey types give very similar absolute resistivity values. We found that while SIP gives the most quantitative results, the frequency dependence from the CCR and DC resistivity surveys is all that are needed to determine ice content in permafrost.

  4. Low-frequency otolith and semicircular canal interactions after canal inactivation

    NASA Technical Reports Server (NTRS)

    Angelaki, D. E.; Merfeld, D. M.; Hess, B. J.

    2000-01-01

    During sustained constant velocity and low-frequency off-vertical axis rotations (OVAR), otolith signals contribute significantly to slow-phase eye velocity. The adaptive plasticity of these responses was investigated here after semicircular canal plugging. Inactivation of semicircular canals results in a highly compromised and deficient vestibulo-ocular reflex (VOR). Based on the VOR enhancement hypothesis, one could expect an adaptive increase of otolith-borne angular velocity signals due to combined otolith/canal inputs after inactivation of the semicircular canals. Contrary to expectations, however, the steady-state slow-phase velocity during constant velocity OVAR decreased in amplitude over time. A similar progressive decrease in VOR gain was also observed during low-frequency off-vertical axis oscillations. This response deterioration was present in animals with either lateral or vertical semicircular canals inactivated and was limited to the plane(s) of the plugged canals. The results are consistent with the idea that the low-frequency otolith signals do not simply enhance VOR responses. Rather, the nervous system appears to correlate vestibular sensory information from the otoliths and the semicircular canals to generate an integral response to head motion.

  5. Geophysics. Migrating tremor off southern Kyushu as evidence for slow slip of a shallow subduction interface.

    PubMed

    Yamashita, Y; Yakiwara, H; Asano, Y; Shimizu, H; Uchida, K; Hirano, S; Umakoshi, K; Miyamachi, H; Nakamoto, M; Fukui, M; Kamizono, M; Kanehara, H; Yamada, T; Shinohara, M; Obara, K

    2015-05-01

    Detection of shallow slow earthquakes offers insight into the near-trench part of the subduction interface, an important region in the development of great earthquake ruptures and tsunami generation. Ocean-bottom monitoring of offshore seismicity off southern Kyushu, Japan, recorded a complete episode of low-frequency tremor, lasting for 1 month, that was associated with very-low-frequency earthquake (VLFE) activity in the shallow plate interface. The shallow tremor episode exhibited two migration modes reminiscent of deep tremor down-dip of the seismogenic zone in some other subduction zones: a large-scale slower propagation mode and a rapid reversal mode. These similarities in migration properties and the association with VLFEs strongly suggest that both the shallow and deep tremor and VLFE may be triggered by the migration of episodic slow slip events. PMID:25954006

  6. Low-frequency radio observations of SN 2011dh and the evolution of its post-shock plasma properties

    NASA Astrophysics Data System (ADS)

    Yadav, Naveen; Ray, Alak; Chakraborti, Sayan

    2016-06-01

    We present late time, low-frequency observations of SN 2011dh made using the Giant Metrewave Radio Telescope (GMRT). Our observations at 325, 610 and 1280 MHz conducted between 93 and 421 d after the explosion supplement the millimeter and centimetre wave observations conducted between 4 and 15 d after explosion using the Combined Array for Research in Millimeter-wave Astronomy (CARMA) and extensive radio observations (1.0-36.5 GHz) conducted between 16 and 93 d after explosion using Jansky Very Large Array (JVLA). We fit a synchrotron self absorption model (SSA) to the 610 and 1280 MHz radio light curves. We use it to determine the radius (Rp) and magnetic field (Bp) at 173 and 323 d after the explosion. A comparison of the peak radio luminosity Lop with the product of the peak frequency νp and time to peak tp shows that the supernova evolves between the epochs of CARMA, JVLA and GMRT observations. It shows a general slowing down of the expansion speed of the radio emitting region on a time-scale of several hundred days during which the shock is propagating through a circumstellar medium set up by a wind with a constant mass-loss parameter, dot{M}/{v}_w. We derive the mass-loss parameter (A⋆) based on 610 and 1280 MHz radio light curves, which are found to be consistent with each other within error limits.

  7. Frequency-specific alternations in the amplitude of low-frequency fluctuations in chronic tinnitus

    PubMed Central

    Chen, Yu-Chen; Xia, Wenqing; Luo, Bin; Muthaiah, Vijaya P. K.; Xiong, Zhenyu; Zhang, Jian; Wang, Jian; Salvi, Richard; Teng, Gao-Jun

    2015-01-01

    Tinnitus, a phantom ringing, buzzing, or hissing sensation with potentially debilitating consequences, is thought to arise from aberrant spontaneous neural activity at one or more sites within the central nervous system; however, the location and specific features of these oscillations are poorly understood with respect to specific tinnitus features. Recent resting-state functional magnetic resonance imaging (fMRI) studies suggest that aberrant fluctuations in spontaneous low-frequency oscillations (LFO) of the blood oxygen level-dependent (BOLD) signal may be an important factor in chronic tinnitus; however, the role that frequency-specific components of LFO play in subjective tinnitus remains unclear. A total of 39 chronic tinnitus patients and 41 well-matched healthy controls participated in the resting-state fMRI scans. The LFO amplitudes were investigated using the amplitude of low-frequency fluctuation (ALFF) and fractional ALFF (fALFF) in two different frequency bands (slow-4: 0.027–0.073 Hz and slow-5: 0.01–0.027 Hz). We observed significant differences between tinnitus patients and normal controls in ALFF/fALFF in the two bands (slow-4 and slow-5) in several brain regions including the superior frontal gyrus (SFG), inferior frontal gyrus, middle temporal gyrus, angular gyrus, supramarginal gyrus, and middle occipital gyrus. Across the entire subject pool, significant differences in ALFF/fALFF between the two bands were found in the midbrain, basal ganglia, hippocampus and cerebellum (Slow 4 > Slow 5), and in the middle frontal gyrus, supramarginal gyrus, posterior cingulate cortex, and precuneus (Slow 5 > Slow 4). We also observed significant interaction between frequency bands and patient groups in the orbitofrontal gyrus. Furthermore, tinnitus distress was positively correlated with the magnitude of ALFF in right SFG and the magnitude of fALFF slow-4 band in left SFG, whereas tinnitus duration was positively correlated with the magnitude of ALFF in

  8. Long term estimations of low frequency noise levels over water from an off-shore wind farm.

    PubMed

    Bolin, Karl; Almgren, Martin; Ohlsson, Esbjörn; Karasalo, Ilkka

    2014-03-01

    This article focuses on computations of low frequency sound propagation from an off-shore wind farm. Two different methods for sound propagation calculations are combined with meteorological data for every 3 hours in the year 2010 to examine the varying noise levels at a reception point at 13 km distance. It is shown that sound propagation conditions play a vital role in the noise impact from the off-shore wind farm and ordinary assessment methods can become inaccurate at longer propagation distances over water. Therefore, this paper suggests that methodologies to calculate noise immission with realistic sound speed profiles need to be combined with meteorological data over extended time periods to evaluate the impact of low frequency noise from modern off-shore wind farms. PMID:24606254

  9. 7. Survivable low frequency communication system pathway, looking east ...

    Library of Congress Historic Buildings Survey, Historic Engineering Record, Historic Landscapes Survey

    7. Survivable low frequency communication system pathway, looking east - Ellsworth Air Force Base, Delta Flight, Launch Control Facility, County Road CS23A, North of Exit 127, Interior, Jackson County, SD

  10. Low-frequency broadband noise generated by a model rotor

    NASA Technical Reports Server (NTRS)

    Aravamudan, K. S.; Harris, W. L.

    1979-01-01

    Low-frequency broadband noise generated by model rotors is attributed to the interaction of ingested turbulence with the rotor blades. The influence of free-stream turbulence in the low-frequency broadband noise radiation from model rotors has been experimentally investigated. The turbulence was generated in the M.I.T. anechoic wind tunnel facility with the aid of bipolar grids of various sizes. The spectra and the intensity of the low-frequency broadband noise have been studied as a function of parameters which characterize the turbulence and of helicopter performance parameters. The location of the peak intensity was observed to be strongly dependent on the rotor-tip velocity and on the longitudinal integral scale of turbulence. The size scale of turbulence had negligible effect on the intensity of low-frequency broadband noise. The experimental data show good agreement with an ad hoc model based on unsteady aerodynamics.

  11. Searching for Low-Frequency Radio Transients from Supernovae

    NASA Astrophysics Data System (ADS)

    Tsai-Wei, Jr.; Cutchin, Sean; Kothari, Manthan; Schmitt, Christian; Kavic, Michael; Simonetti, John

    2011-10-01

    Supernovae events may be accompanied by prompt emission of a low-frequency electromagnetic transient. These transient events are created by the interaction of a shock wave of charged particles created by SN core-collapse with a stars ambient magnetic field. Such events can be detected in low-frequency radio array. Here we discuss an ongoing search for such events using two radio arrays: the Long Wavelength Array (LWA) and Eight-meter-wavelength Transient Array (ETA).

  12. Compensation Low-Frequency Errors in TH-1 Satellite

    NASA Astrophysics Data System (ADS)

    Wang, Jianrong; Wang, Renxiang; Hu, Xin

    2016-06-01

    The topographic mapping products at 1:50,000 scale can be realized using satellite photogrammetry without ground control points (GCPs), which requires the high accuracy of exterior orientation elements. Usually, the attitudes of exterior orientation elements are obtained from the attitude determination system on the satellite. Based on the theoretical analysis and practice, the attitude determination system exists not only the high-frequency errors, but also the low-frequency errors related to the latitude of satellite orbit and the time. The low-frequency errors would affect the location accuracy without GCPs, especially to the horizontal accuracy. In SPOT5 satellite, the latitudinal model was proposed to correct attitudes using approximately 20 calibration sites data, and the location accuracy was improved. The low-frequency errors are also found in Tian Hui 1 (TH-1) satellite. Then, the method of compensation low-frequency errors is proposed in ground image processing of TH-1, which can detect and compensate the low-frequency errors automatically without using GCPs. This paper deal with the low-frequency errors in TH-1: First, the analysis about low-frequency errors of the attitude determination system is performed. Second, the compensation models are proposed in bundle adjustment. Finally, the verification is tested using data of TH-1. The testing results show: the low-frequency errors of attitude determination system can be compensated during bundle adjustment, which can improve the location accuracy without GCPs and has played an important role in the consistency of global location accuracy.

  13. Structure of the tsunamigenic plate boundary and low-frequency earthquakes in the southern Ryukyu Trench

    PubMed Central

    Arai, Ryuta; Takahashi, Tsutomu; Kodaira, Shuichi; Kaiho, Yuka; Nakanishi, Ayako; Fujie, Gou; Nakamura, Yasuyuki; Yamamoto, Yojiro; Ishihara, Yasushi; Miura, Seiichi; Kaneda, Yoshiyuki

    2016-01-01

    It has been recognized that even weakly coupled subduction zones may cause large interplate earthquakes leading to destructive tsunamis. The Ryukyu Trench is one of the best fields to study this phenomenon, since various slow earthquakes and tsunamis have occurred; yet the fault structure and seismic activity there are poorly constrained. Here we present seismological evidence from marine observation for megathrust faults and low-frequency earthquakes (LFEs). On the basis of passive observation we find LFEs occur at 15–18 km depths along the plate interface and their distribution seems to bridge the gap between the shallow tsunamigenic zone and the deep slow slip region. This suggests that the southern Ryukyu Trench is dominated by slow earthquakes at any depths and lacks a typical locked zone. The plate interface is overlaid by a low-velocity wedge and is accompanied by polarity reversals of seismic reflections, indicating fluids exist at various depths along the plate interface. PMID:27447546

  14. Structure of the tsunamigenic plate boundary and low-frequency earthquakes in the southern Ryukyu Trench.

    PubMed

    Arai, Ryuta; Takahashi, Tsutomu; Kodaira, Shuichi; Kaiho, Yuka; Nakanishi, Ayako; Fujie, Gou; Nakamura, Yasuyuki; Yamamoto, Yojiro; Ishihara, Yasushi; Miura, Seiichi; Kaneda, Yoshiyuki

    2016-01-01

    It has been recognized that even weakly coupled subduction zones may cause large interplate earthquakes leading to destructive tsunamis. The Ryukyu Trench is one of the best fields to study this phenomenon, since various slow earthquakes and tsunamis have occurred; yet the fault structure and seismic activity there are poorly constrained. Here we present seismological evidence from marine observation for megathrust faults and low-frequency earthquakes (LFEs). On the basis of passive observation we find LFEs occur at 15-18 km depths along the plate interface and their distribution seems to bridge the gap between the shallow tsunamigenic zone and the deep slow slip region. This suggests that the southern Ryukyu Trench is dominated by slow earthquakes at any depths and lacks a typical locked zone. The plate interface is overlaid by a low-velocity wedge and is accompanied by polarity reversals of seismic reflections, indicating fluids exist at various depths along the plate interface. PMID:27447546

  15. Structure of the tsunamigenic plate boundary and low-frequency earthquakes in the southern Ryukyu Trench

    NASA Astrophysics Data System (ADS)

    Arai, Ryuta; Takahashi, Tsutomu; Kodaira, Shuichi; Kaiho, Yuka; Nakanishi, Ayako; Fujie, Gou; Nakamura, Yasuyuki; Yamamoto, Yojiro; Ishihara, Yasushi; Miura, Seiichi; Kaneda, Yoshiyuki

    2016-07-01

    It has been recognized that even weakly coupled subduction zones may cause large interplate earthquakes leading to destructive tsunamis. The Ryukyu Trench is one of the best fields to study this phenomenon, since various slow earthquakes and tsunamis have occurred; yet the fault structure and seismic activity there are poorly constrained. Here we present seismological evidence from marine observation for megathrust faults and low-frequency earthquakes (LFEs). On the basis of passive observation we find LFEs occur at 15-18 km depths along the plate interface and their distribution seems to bridge the gap between the shallow tsunamigenic zone and the deep slow slip region. This suggests that the southern Ryukyu Trench is dominated by slow earthquakes at any depths and lacks a typical locked zone. The plate interface is overlaid by a low-velocity wedge and is accompanied by polarity reversals of seismic reflections, indicating fluids exist at various depths along the plate interface.

  16. Low-frequency flute instabilities of self-pinched ion beams

    SciTech Connect

    Uhm, Han S.; Davidson, Ronald C.

    2005-12-15

    The stability properties of the low-frequency flute instabilities in a self-pinched ion beam propagating through a preformed plasma channel are investigated for long-wavelength and low-frequency perturbations. Consistent with the flute instabilities, the stability analysis is restricted to the surface perturbations on the ion beam. A closed algebraic dispersion relation of the flute instabilities for Bennett [Phys. Rev. 45, 890 (1934)] density profile is obtained, by making use of the energy group model. From the analytical and numerical calculations of the dispersion relation for highly collisional plasma, we find the necessary condition for instability in terms of the fractional current neutralization f. Threshold values of the fractional current neutralization for instability are tabulated for each azimuthal mode number l.

  17. The oblique behavior of low-frequency electromagnetic waves excited by newborn cometary ions

    NASA Technical Reports Server (NTRS)

    Brinca, Armando L.; Tsurutani, Bruce T.

    1989-01-01

    The free energy in oxygen or hydrogen ions freshly created in the solar wind stimulates low-frequency electromagnetic waves whose growth does not always maximize at parallel propagation. Exploration of the wave vector plane discloses the frequent occurrence of islets of oblique growth unconnected to the unstable parallel modes. Contour plots of the growth rate, real frequency, polarization, and magnetic compression characterize the oblique wave behavior for large values of the initial pitch angle of the cometary particles. Although wave-particle (Landau and cyclotron) resonances feed most of the surveyed oblique instabilities, some are seemingly fluidlike. The results, obtained from the numerical solution of the kinetic dispersion and wave equations, imply that newborn ions can easily excite significant oblique hydromagnetic wave activity. Cometary environments provide the adopted plasma model, but the study is helpful in the interpretation of other low-frequency wave observations in space.

  18. Parametric excitation of coupled fast and slow upper hybrid waves by counter-propagating circularly polarized lasers in a magnetized plasma

    NASA Astrophysics Data System (ADS)

    Verma, Kanika; Baliyan, Sweta; Sajal, Vivek; Kumar, Ravindra; Sharma, Navneet K.

    2016-07-01

    The parametric decay of large amplitude non-resonant beating mode of counter-propagating lasers (having a frequency difference ≥ 2 ω p ) into a pair of upper hybrid waves is studied in magnetized plasma. One of the excited upper hybrid waves (known as fast wave) having phase velocity close to c , can be utilized for electron acceleration. The coupled mode equations of fast and slow upper hybrid waves are modelled by solving equation of motion and continuity equation simultaneously (using the density perturbation technique) to derive the dispersion relation for two plasmon decay process. The growth rate of the present excitation process using right circularly polarized beating lasers is higher as compared with the growth rates of the excitation processes using ordinary and extraordinary beating lasers. However, the growth rate is not significant in the case of left circularly polarized beating lasers. The growth rate ˜ 0.15 ω p s - 1 is achieved for right circularly polarized beating lasers having v 1 , 2 / c = 0.1 for scattering angle θ s ˜ 160 ° and applied magnetic field ˜ 90 T. The growth rate of fast upper hybrid wave was reduced with the applied axial magnetic field in the present case. The present work is not only significant for the electron acceleration by fast upper hybrid wave but also for diagnostic purpose.

  19. Low-frequency EEG oscillations associated with information processing in schizophrenia.

    PubMed

    Bates, Alan T; Kiehl, Kent A; Laurens, Kristin R; Liddle, Peter F

    2009-12-01

    Numerous studies have described attenuated event-related potential (ERP) component amplitudes in schizophrenia (e.g., P300, Mismatch Negativity (MMN), Error Negativity/Error-Related Negativity (Ne/ERN)). Functional magnetic resonance imaging (fMRI) studies have typically shown decreased recruitment of diverse brain areas during performance of tasks that elicit the above ERP components. Recent research suggests that phase-resetting of slow-oscillations (e.g., in the delta and theta bands) underlies the potentials observed in ERP averages. Several studies have reported that slow-oscillations are increased in amplitude in people with schizophrenia at rest. Few studies have examined event-related low-frequency oscillations in schizophrenia. We examined event-related evoked and induced delta and theta activity in 17 people with schizophrenia and 17 healthy controls in two go/no-go task variants. We analyzed stimulus-related and response-related oscillations associated with correct-hits, correct-rejects and false-alarms. Our results reveal a pattern of reduced delta and theta activity for task-relevant events in schizophrenia. The findings indicate that while low-frequency oscillations are increased in amplitude at rest, they are not coordinated effectively in schizophrenia during various information processing tasks including target-detection, response-inhibition and error-detection. This slow-oscillation coordination abnormality may help explain the decreased recruitment of brain areas seen in fMRI studies. PMID:19850450

  20. Manipulating neuronal activity with low frequency transcranial ultrasound

    NASA Astrophysics Data System (ADS)

    Moore, Michele Elizabeth

    neurons impose temporal constraints on their response to stimulation. If ultrasound-mediated responses are, in fact, ion channel mediated responses, ultrasound-induced responses should exhibit time-dependence characteristics similar to those of optogenetically-triggered responses. Minimal stimulus duration thresholds and the temporal limits of paired pulse facilitation for ultrasound stimulation were identical to those of optogenetic stimulation. Collectively, these experiments demonstrate an electrophysiological basis for low-frequency transcranial ultrasound stimulation of cerebral cortical neuronal activity.

  1. EDITORIAL: Slow light Slow light

    NASA Astrophysics Data System (ADS)

    Boyd, Robert; Hess, Ortwin; Denz, Cornelia; Paspalakis, Emmanuel

    2010-10-01

    storage, optical memories, quantum information devices, and optical communication systems in which the use of slow light will allow all-optical processing with less wasted heat. To implement these applications, devices such as buffers, memories, interferometers and switches that utilize slow light need to be developed. Future challenges include the need for improved coupling of light into slow light modes, overcoming propagation losses, and mitigating the influence of large dispersion of the group velocity. The collection of papers in this special issue of Journal of Optics features a broad spectrum of articles that highlight actual developments in many of the material types and schemes described above. It represents therefore an excellent up to date snapshot of the current state of the field of slow light research. References [1] Lorentz H A 1880 Uber die Beziehung zwischen der Fortpflanzung des Lichtes und der Körperdichte Wiedemann Ann. 9 641-64 [2] McCall S L and Hahn E L 1967 Self-induced transparency by pulsed coherent light Phys. Rev. Lett. 18 908-11 [3] Vestergaard Hau L, Harris S E, Dutton Z and Behroozi C H 1999 Nature 397 594 [4] Philips D F, Fleischhauer A, Mair A, Walsworth R L and Lukin M D 2001 Storage of light in atomic vapor Phys. Rev. Lett. 86 783-6

  2. A Review of Low Frequency Electromagnetic Wave Phenomena Related to Tropospheric-Ionospheric Coupling Mechanisms

    NASA Technical Reports Server (NTRS)

    Simoes, Fernando; Pfaff, Robert; Berthelier, Jean-Jacques; Klenzing, Jeffrey

    2012-01-01

    Investigation of coupling mechanisms between the troposphere and the ionosphere requires a multidisciplinary approach involving several branches of atmospheric sciences, from meteorology, atmospheric chemistry, and fulminology to aeronomy, plasma physics, and space weather. In this work, we review low frequency electromagnetic wave propagation in the Earth-ionosphere cavity from a troposphere-ionosphere coupling perspective. We discuss electromagnetic wave generation, propagation, and resonance phenomena, considering atmospheric, ionospheric and magnetospheric sources, from lightning and transient luminous events at low altitude to Alfven waves and particle precipitation related to solar and magnetospheric processes. We review in situ ionospheric processes as well as surface and space weather phenomena that drive troposphere-ionosphere dynamics. Effects of aerosols, water vapor distribution, thermodynamic parameters, and cloud charge separation and electrification processes on atmospheric electricity and electromagnetic waves are reviewed. We also briefly revisit ionospheric irregularities such as spread-F and explosive spread-F, sporadic-E, traveling ionospheric disturbances, Trimpi effect, and hiss and plasma turbulence. Regarding the role of the lower boundary of the cavity, we review transient surface phenomena, including seismic activity, earthquakes, volcanic processes and dust electrification. The role of surface and atmospheric gravity waves in ionospheric dynamics is also briefly addressed. We summarize analytical and numerical tools and techniques to model low frequency electromagnetic wave propagation and solving inverse problems and summarize in a final section a few challenging subjects that are important for a better understanding of tropospheric-ionospheric coupling mechanisms.

  3. Low-frequency sound affects active micromechanics in the human inner ear.

    PubMed

    Kugler, Kathrin; Wiegrebe, Lutz; Grothe, Benedikt; Kössl, Manfred; Gürkov, Robert; Krause, Eike; Drexl, Markus

    2014-10-01

    Noise-induced hearing loss is one of the most common auditory pathologies, resulting from overstimulation of the human cochlea, an exquisitely sensitive micromechanical device. At very low frequencies (less than 250 Hz), however, the sensitivity of human hearing, and therefore the perceived loudness is poor. The perceived loudness is mediated by the inner hair cells of the cochlea which are driven very inadequately at low frequencies. To assess the impact of low-frequency (LF) sound, we exploited a by-product of the active amplification of sound outer hair cells (OHCs) perform, so-called spontaneous otoacoustic emissions. These are faint sounds produced by the inner ear that can be used to detect changes of cochlear physiology. We show that a short exposure to perceptually unobtrusive, LF sounds significantly affects OHCs: a 90 s, 80 dB(A) LF sound induced slow, concordant and positively correlated frequency and level oscillations of spontaneous otoacoustic emissions that lasted for about 2 min after LF sound offset. LF sounds, contrary to their unobtrusive perception, strongly stimulate the human cochlea and affect amplification processes in the most sensitive and important frequency range of human hearing. PMID:26064536

  4. Low-frequency sound affects active micromechanics in the human inner ear

    PubMed Central

    Kugler, Kathrin; Wiegrebe, Lutz; Grothe, Benedikt; Kössl, Manfred; Gürkov, Robert; Krause, Eike; Drexl, Markus

    2014-01-01

    Noise-induced hearing loss is one of the most common auditory pathologies, resulting from overstimulation of the human cochlea, an exquisitely sensitive micromechanical device. At very low frequencies (less than 250 Hz), however, the sensitivity of human hearing, and therefore the perceived loudness is poor. The perceived loudness is mediated by the inner hair cells of the cochlea which are driven very inadequately at low frequencies. To assess the impact of low-frequency (LF) sound, we exploited a by-product of the active amplification of sound outer hair cells (OHCs) perform, so-called spontaneous otoacoustic emissions. These are faint sounds produced by the inner ear that can be used to detect changes of cochlear physiology. We show that a short exposure to perceptually unobtrusive, LF sounds significantly affects OHCs: a 90 s, 80 dB(A) LF sound induced slow, concordant and positively correlated frequency and level oscillations of spontaneous otoacoustic emissions that lasted for about 2 min after LF sound offset. LF sounds, contrary to their unobtrusive perception, strongly stimulate the human cochlea and affect amplification processes in the most sensitive and important frequency range of human hearing. PMID:26064536

  5. Minimal basilar membrane motion in low-frequency hearing.

    PubMed

    Warren, Rebecca L; Ramamoorthy, Sripriya; Ciganović, Nikola; Zhang, Yuan; Wilson, Teresa M; Petrie, Tracy; Wang, Ruikang K; Jacques, Steven L; Reichenbach, Tobias; Nuttall, Alfred L; Fridberger, Anders

    2016-07-26

    Low-frequency hearing is critically important for speech and music perception, but no mechanical measurements have previously been available from inner ears with intact low-frequency parts. These regions of the cochlea may function in ways different from the extensively studied high-frequency regions, where the sensory outer hair cells produce force that greatly increases the sound-evoked vibrations of the basilar membrane. We used laser interferometry in vitro and optical coherence tomography in vivo to study the low-frequency part of the guinea pig cochlea, and found that sound stimulation caused motion of a minimal portion of the basilar membrane. Outside the region of peak movement, an exponential decline in motion amplitude occurred across the basilar membrane. The moving region had different dependence on stimulus frequency than the vibrations measured near the mechanosensitive stereocilia. This behavior differs substantially from the behavior found in the extensively studied high-frequency regions of the cochlea. PMID:27407145

  6. Minimal basilar membrane motion in low-frequency hearing

    PubMed Central

    Warren, Rebecca L.; Ramamoorthy, Sripriya; Ciganović, Nikola; Zhang, Yuan; Wilson, Teresa M.; Petrie, Tracy; Wang, Ruikang K.; Jacques, Steven L.; Reichenbach, Tobias; Nuttall, Alfred L.; Fridberger, Anders

    2016-01-01

    Low-frequency hearing is critically important for speech and music perception, but no mechanical measurements have previously been available from inner ears with intact low-frequency parts. These regions of the cochlea may function in ways different from the extensively studied high-frequency regions, where the sensory outer hair cells produce force that greatly increases the sound-evoked vibrations of the basilar membrane. We used laser interferometry in vitro and optical coherence tomography in vivo to study the low-frequency part of the guinea pig cochlea, and found that sound stimulation caused motion of a minimal portion of the basilar membrane. Outside the region of peak movement, an exponential decline in motion amplitude occurred across the basilar membrane. The moving region had different dependence on stimulus frequency than the vibrations measured near the mechanosensitive stereocilia. This behavior differs substantially from the behavior found in the extensively studied high-frequency regions of the cochlea. PMID:27407145

  7. Atomic Oxygen Energy in Low Frequency Hyperthermal Plasma Ashers

    NASA Technical Reports Server (NTRS)

    Banks, Bruce A.; Miller, Sharon K R.; Kneubel, Christian A.

    2014-01-01

    Experimental and analytical analysis of the atomic oxygen erosion of pyrolytic graphite as well as Monte Carlo computational modeling of the erosion of Kapton H (DuPont, Wilmington, DE) polyimide was performed to determine the hyperthermal energy of low frequency (30 to 35 kHz) plasma ashers operating on air. It was concluded that hyperthermal energies in the range of 0.3 to 0.9 eV are produced in the low frequency air plasmas which results in texturing similar to that in low Earth orbit (LEO). Monte Carlo computational modeling also indicated that such low energy directed ions are fully capable of producing the experimentally observed textured surfaces in low frequency plasmas.

  8. Directivity and prediction of low frequency rotor noise

    NASA Technical Reports Server (NTRS)

    Burley, C. L.; Marcolini, M. A.; Jones, H. E.; Splettstoesser, W. R.

    1991-01-01

    Acoustic data obtained over a large horizontal plane under the model rotor and digitally filtered in order to determine the low-frequency content near the blade passage frequency is analyzed. Focus is placed on the directivity of low-frequency noise, and the changes in directivity as a function of the descent glide slope angle and advance ratio are presented and compared with predicted directivity results. The differences between the data and prediction are discussed for two observer positions, one below and on the rotor axis, and the other 60 degrees down from the horizontal. It is demonstrated that for the latter position, blade-vortex interaction noise is strong when it occurs, and the loading at the low frequencies is significantly affected during blade-vortex interactions.

  9. Present and Future Modes of Low Frequency Climate Variability

    SciTech Connect

    Cane, Mark A.

    2014-02-20

    This project addressed area (1) of the FOA, “Interaction of Climate Change and Low Frequency Modes of Natural Climate Variability”. Our overarching objective is to detect, describe and understand the changes in low frequency variability between model simulations of the preindustrial climate and simulations of a doubled CO2 climate. The deliverables are a set of papers providing a dynamical characterization of interannual, decadal, and multidecadal variability in coupled models with attention to the changes in this low frequency variability between pre-industrial concentrations of greenhouse gases and a doubling of atmospheric concentrations of CO2. The principle mode of analysis, singular vector decomposition, is designed to advance our physical, mechanistic understanding. This study will include external natural variability due to solar and volcanic aerosol variations as well as variability internal to the climate system. An important byproduct is a set of analysis tools for estimating global singular vector structures from the archived output of model simulations.

  10. Nonlinear low-frequency electrostatic wave dynamics in a two-dimensional quantum plasma

    NASA Astrophysics Data System (ADS)

    Ghosh, Samiran; Chakrabarti, Nikhil

    2016-08-01

    The problem of two-dimensional arbitrary amplitude low-frequency electrostatic oscillation in a quasi-neutral quantum plasma is solved exactly by elementary means. In such quantum plasmas we have treated electrons quantum mechanically and ions classically. The exact analytical solution of the nonlinear system exhibits the formation of dark and black solitons. Numerical simulation also predicts the possible periodic solution of the nonlinear system. Nonlinear analysis reveals that the system does have a bifurcation at a critical Mach number that depends on the angle of propagation of the wave. The small-amplitude limit leads to the formation of weakly nonlinear Kadomstev-Petviashvili solitons.

  11. Low frequency electric field variations during HF transmissions on a mother-daughter rocket

    NASA Technical Reports Server (NTRS)

    Rosenberg, T. J.; Maynard, M. C.; Holtet, J. A.; Karlsen, N. O.; Egeland, A.; Moe, T. E.; Troim, J.

    1977-01-01

    HF wave propagation experiments were conducted on Mother-Daughter rockets in the polar ionosphere. Swept frequency transmissions from the Mother, nominally covering the range from 0.5 to 5 MHz in both CW and pulse modes, are received by the Daughter. In the most recent rocket of the series, the Mother also contained an AC electric field spectrometer covering the frequency range from 10 Hz to 100 kHz in four decade bands. The low frequency response of the ionosphere with respect to waves emitted from the onboard HF transmitter is examined.

  12. Internal Rot Detection with the Use of Low-Frequency Flaw Detector

    NASA Astrophysics Data System (ADS)

    Proskórnicki, Marek; Ligus, Grzegorz

    2014-12-01

    The issue of rot detection in standing timber or stocked wood is very important in forest management. Rot flaw detection used for that purpose is represented by invasive and non-invasive devices. Non-invasive devices are very accurate, but due to the cost and complicated operation they have not been applied on a large scale in forest management. Taking into account the practical needs of foresters a prototype of low-frequency flaw was developed. The principle of its operation is based on the difference in acoustic wave propagation in sound wood and wood with rot.

  13. Remote sensing planetary waves in the midlatitude mesosphere using low frequency transmitter signals

    NASA Astrophysics Data System (ADS)

    Schmitter, E. D.

    2011-07-01

    Very low and low radio frequency (VLF/LF) propagation responds sensitively to the electron density distribution in the lower ionosphere (upper mesosphere). Whereas propagation paths crossing subpolar and polar regions are frequently affected by forcing from above by particle precipitations, mid- and lowlatitude paths let forcing from below be more prominent. Our observations (2009-2011) show, that the low frequency propagation conditions along the midlatitude path from Sicily to Germany (52° N 8° E) using the NSY 45.9 kHz transmitter (37° N 14° E) prove to be a good proxy of mesosphere planetary wave activity along the propagation path. High absorption events with VLF/LF propagation correlate to the well known winter time D-layer anomaly observed with high frequency (HF) radio waves. VLF/LF propagation calculations are presented which show that the radio signal amplitude variations can be modeled by planetary wave modulated collison frequency and electron density profiles. The other way around wave pressure amplitudes can be inferred from the VLF/LF data.

  14. Listening to the low-frequency gravitational-wave band

    NASA Astrophysics Data System (ADS)

    Hughes, Scott

    2016-03-01

    Ground-based gravitational-wave detectors are beginning to explore the high-frequency band of roughly 10 to 1000 Hz. These three decades in frequency represent one of several astrophysically important wavebands. In this talk, I will focus on the astrophysics of the low-frequency band, from roughly 30 microhertz to 0.1 Hz. This band is expected to be particularly rich with very loud sources. I will survey what we expect to be important sources of low-frequency gravitational waves, and review the scientific payoff that would come from measuring them.

  15. Improvement of the low frequency oscillation model for Hall thrusters

    NASA Astrophysics Data System (ADS)

    Wang, Chunsheng; Wang, Huashan

    2016-08-01

    The low frequency oscillation of the discharge current in Hall thrusters is a major aspect of these devices that requires further study. While the existing model captures the ionization mechanism of the low frequency oscillation, it unfortunately fails to express the dynamic characteristics of the ion acceleration. The analysis in this paper shows this is because of the simplification of the electron equation, which affects both the electric field distribution and the ion acceleration process. Additionally, the electron density equation is revised and a new model that is based on the physical properties of ion movement is proposed.

  16. Excitation of low-frequency waves by auroral electron beams

    NASA Technical Reports Server (NTRS)

    Lin, C. S.; Wong, H. K.; Koga, J.; Burch, J. L.

    1989-01-01

    The electron distribution functions measured by the Dynamics Explorer 1 satellite during an auroral pass in 1981 are used in a linear instability analysis of low-frequency electromagnetic and electrostatic waves near and below the hydrogen gyrofrequency. It is suggested that the low-frequency electric and magnetic noise in the auroral zone might be explained by O and H electromagnetic ion cyclotron waves excited by energetic electron beams. An instability analysis suggests that upward and downward streaming electrons throughout the central plasma sheet region provide the free energy for heating oxygen ion through oxygen electrostatic ion cyclotron waves.

  17. Hemodynamic responses can modulate the brain oscillations in low frequency

    NASA Astrophysics Data System (ADS)

    Lu, Feng-Mei; Wang, Yi-Feng; Yuan, Zhen

    2016-03-01

    Previous studies have showed that the steady-state responses were able to be used as an effective index for modulating the neural oscillations in the high frequency ranges (> 1 Hz). However, the neural oscillations in low frequency ranges (<1 Hz) remain unknown. In this study, a series of fNIRS experimental tests were conducted to validate if the low frequency bands (0.1 Hz - 0.8 Hz) steady-state hemoglobin responses (SSHbRs) could be evoked and modulate the neural oscillation during a serial reaction time (SRT) task.

  18. Low-frequency vibrational modes of riboflavin and related compounds

    NASA Astrophysics Data System (ADS)

    Takahashi, Masae; Ishikawa, Yoichi; Nishizawa, Jun-ichi; Ito, Hiromasa

    2005-01-01

    The low-frequency vibrations of riboflavin and related compounds (alloxazine, lumichrome, lumiflavin as the ring system and D-mannitol as the side-chain system) were observed by far-infrared (terahertz) spectroscopy. Vibrational mode assignments in this spectrally congested range were made using high precision quantum chemical calculations. These resonance frequencies located below 200 cm -1 indicate the existence of motions important for biological reactions. The observed absorption bands in the low-frequency region of riboflavin are assigned to the in-plane and out-of-plane-ring deformations of pyrimidine and isoalloxazine, and to the torsion modes of the ribityl chain.

  19. Improvement of Space Shuttle Main Engine Low Frequency Acceleration Measurements

    NASA Technical Reports Server (NTRS)

    Stec, Robert C.

    1999-01-01

    The noise floor of low frequency acceleration data acquired on the Space Shuttle Main Engines is higher than desirable. Difficulties of acquiring high quality acceleration data on this engine are discussed. The approach presented in this paper for reducing the acceleration noise floor focuses on a search for an accelerometer more capable of measuring low frequency accelerations. An overview is given of the current measurement system used to acquire engine vibratory data. The severity of vibration, temperature, and moisture environments are considered. Vibratory measurements from both laboratory and rocket engine tests are presented.

  20. The global atmospheric response to low-frequency tropical forcing: Zonally averaged basic states

    NASA Technical Reports Server (NTRS)

    Li, Long; Nathan, Terrence R.

    1994-01-01

    The extratropical response to localized, low-frequency tropical forcing is examined using a linearized, non-divergent barotropic model on a sphere. Zonal-mean basic states characterized by solid-body rotation or critical latitudes are considered. An analytical analysis based on WKB and ray tracing methods shows that, in contrast to stationary Rossby waves, westward moving, low-frequency Rossby waves can propagate through the tropical easterlies into the extratropics. It is shown analytically that the difference between the stationary and low-frequency ray paths is proportional to the forcing frequency and inversely proportional to the zonal wavenumber cubed. An expression for the disturbance amplitude is derived that shows the ability of the forced waves to maintain their strength well into middle latitudes depends on their meridional wave scale and northward group velocity, both of which are functions of the slowly varying background flow. A local energetics analysis shows that the combination of energy dispersion from the forcing region and energy extraction from the equatorward flank of the midlatitude jet produces disturbances that have the greatest impact on the extratropical circulation. Under the assumption that the forcing amplitude is independent of frequency, this impact is largest when the tropical forcing period is in the range 10-20 days.

  1. Detection and Analysis of Low-Frequency Sperm Whale Vocalizations with a Towed Array

    NASA Astrophysics Data System (ADS)

    Bohn, Alexander

    Sperm whale vocalizations recorded during a sea test and calibration experiment in the Gulf of Maine on a single towed, horizontal, densely sampled, low-frequency (< 2500 Hz), coherent hydrophone array system are detected and analyzed for signal energy level and other characteristics. The vocalizing individuals are localized in bearing, range, and depth. An algorithm is developed to achieve automatic detection of vocalizations. This analysis is shown to have potential utility despite restriction to only the low-frequency component of the vocalizations by sampling theory. In addition, transmission loss in the New England continental shelf and slope environment is accounted for with an ocean waveguide-acoustic propagation model. Multiple averaged realizations of this model are used to estimate transmission loss as a function of range and depth for transects between the receiver array and vocalizing whales. Comparison of the vocalizations and background noise levels and the estimated transmission loss suggests the sperm whale detection range after coherent array processing exceeds 60 km in low-to-moderate sea states. Low-frequency source levels of vocalizations are estimated using the received levels and the estimated transmission loss, and applications of both this estimate and the receiver-side statistics are discussed.

  2. Constraints on the source parameters of low-frequency earthquakes on the San Andreas Fault

    NASA Astrophysics Data System (ADS)

    Thomas, Amanda M.; Beroza, Gregory C.; Shelly, David R.

    2016-02-01

    Low-frequency earthquakes (LFEs) are small repeating earthquakes that occur in conjunction with deep slow slip. Like typical earthquakes, LFEs are thought to represent shear slip on crustal faults, but when compared to earthquakes of the same magnitude, LFEs are depleted in high-frequency content and have lower corner frequencies, implying longer duration. Here we exploit this difference to estimate the duration of LFEs on the deep San Andreas Fault (SAF). We find that the M ~ 1 LFEs have typical durations of ~0.2 s. Using the annual slip rate of the deep SAF and the average number of LFEs per year, we estimate average LFE slip rates of ~0.24 mm/s. When combined with the LFE magnitude, this number implies a stress drop of ~104 Pa, 2 to 3 orders of magnitude lower than ordinary earthquakes, and a rupture velocity of 0.7 km/s, 20% of the shear wave speed. Typical earthquakes are thought to have rupture velocities of ~80-90% of the shear wave speed. Together, the slow rupture velocity, low stress drops, and slow slip velocity explain why LFEs are depleted in high-frequency content relative to ordinary earthquakes and suggest that LFE sources represent areas capable of relatively higher slip speed in deep fault zones. Additionally, changes in rheology may not be required to explain both LFEs and slow slip; the same process that governs the slip speed during slow earthquakes may also limit the rupture velocity of LFEs.

  3. The evolving interaction of low-frequency earthquakes during transient slip

    PubMed Central

    Frank, William B.; Shapiro, Nikolaï M.; Husker, Allen L.; Kostoglodov, Vladimir; Gusev, Alexander A.; Campillo, Michel

    2016-01-01

    Observed along the roots of seismogenic faults where the locked interface transitions to a stably sliding one, low-frequency earthquakes (LFEs) primarily occur as event bursts during slow slip. Using an event catalog from Guerrero, Mexico, we employ a statistical analysis to consider the sequence of LFEs at a single asperity as a point process, and deduce the level of time clustering from the shape of its autocorrelation function. We show that while the plate interface remains locked, LFEs behave as a simple Poisson process, whereas they become strongly clustered in time during even the smallest slow slip, consistent with interaction between different LFE sources. Our results demonstrate that bursts of LFEs can result from the collective behavior of asperities whose interaction depends on the state of the fault interface. PMID:27152345

  4. The evolving interaction of low-frequency earthquakes during transient slip.

    PubMed

    Frank, William B; Shapiro, Nikolaï M; Husker, Allen L; Kostoglodov, Vladimir; Gusev, Alexander A; Campillo, Michel

    2016-04-01

    Observed along the roots of seismogenic faults where the locked interface transitions to a stably sliding one, low-frequency earthquakes (LFEs) primarily occur as event bursts during slow slip. Using an event catalog from Guerrero, Mexico, we employ a statistical analysis to consider the sequence of LFEs at a single asperity as a point process, and deduce the level of time clustering from the shape of its autocorrelation function. We show that while the plate interface remains locked, LFEs behave as a simple Poisson process, whereas they become strongly clustered in time during even the smallest slow slip, consistent with interaction between different LFE sources. Our results demonstrate that bursts of LFEs can result from the collective behavior of asperities whose interaction depends on the state of the fault interface. PMID:27152345

  5. Low-frequency switching voltage regulators for terrestrial photovoltaic systems

    NASA Technical Reports Server (NTRS)

    Delombard, R.

    1984-01-01

    The photovoltaic technology project and the stand alone applications project are discussed. Two types of low frequency switching type regulators were investigated. The design, operating characteristics and field application of these regulators is described. The regulators are small in size, low in cost, very low in power dissipation, reliable and allow considerable flexibility in system design.

  6. Intrinsic low-frequency variability of the Gulf Stream

    NASA Astrophysics Data System (ADS)

    Quattrocchi, G.; Pierini, S.; Dijkstra, H. A.

    2012-03-01

    In this paper a process study aimed at analyzing the low-frequency variability of intrinsically oceanic origin of the Gulf Stream (GS) and GS extension (GSE) is presented. An eddy-permitting reduced-gravity nonlinear shallow water model is implemented in an idealized North Atlantic Ocean, with schematic boundaries including the essential geometric features of the coastline and a realistic zonal basin width at all latitudes. The forcing is provided by a time-independent climatological surface wind stress obtained from 41 years of monthly ECMWF fields. The model response yields strong intrinsic low-frequency fluctuations on the interannual to decadal time scales. The modelled time-averaged GS/GSE flows are found to exhibit several features that can also be deduced from satellite altimeter data, such as the Florida Current seaward deflection, the GS separation at Cape Hatteras, and the overall structure of the GSE. The intrinsic low-frequency variability yields two preferred states of the GSE differing in latitudinal location that also have their counterpart in the altimeter data. A preliminary analysis of the variability in terms of dynamical systems theory is carried out by using the lateral eddy viscosity as the control parameter. A complex transition sequence from a steady state to irregular low-frequency variability emerges, in which Hopf and global bifurcations can be identified.

  7. Low-frequency scattering from two-dimensional perfect conductors

    NASA Astrophysics Data System (ADS)

    Hansen, Thorkild B.; Yaghjian, Arthur D.

    1992-11-01

    Exact expressions are derived for the leading terms in the low-frequency expansions of the far field scattered by an arbitrarily shaped cylinder with finite cross section, an arbitrarily shaped cylindrical bump on a ground plane, and arbitrarily shaped cylindrical dent in a ground plane. For the cylinder with finite cross section, an expression that is independent of the cylinder shape is derived to describe the leading term in the low-frequency expansion of the TM scattered far field. The explicit expression for the low-frequency TE scattered far field is based on three constants that depend only on the shape of the cylinder. The explicit expressions for the low-frequency diffracted fields of a bump or dent contain one constant that depends only on the shape of the bump or dent. It is noted, that this single constant is the same for both TM and TE polarization and can be derived from the solution to either electrostatic or magnetostatic problem.

  8. Is Reaction Time Variability in ADHD Mainly at Low Frequencies?

    ERIC Educational Resources Information Center

    Karalunas, Sarah L.; Huang-Pollock, Cynthia L.; Nigg, Joel T.

    2013-01-01

    Background: Intraindividual variability in reaction times (RT variability) has garnered increasing interest as an indicator of cognitive and neurobiological dysfunction in children with attention deficit hyperactivity disorder (ADHD). Recent theory and research has emphasized specific low-frequency patterns of RT variability. However, whether…

  9. Very Low Frequency Earthquakes (VLFEs) in Cascadia and Their Interactions with Tremor

    NASA Astrophysics Data System (ADS)

    Ghosh, A.

    2014-12-01

    Very low frequency earthquakes (VLFEs) are discrete seismic events rich in low frequencies (20 - 50 sec) and depleted in high frequencies compared to similar size local events. They are associated with slow earthquakes and so far found in only a handful of subduction zones worldwide. I systematically search and find VLFEs in the Cascadia subduction zone. I use a grid-search moment tensor inversion method to scan for VLFEs in 3-D space and time, locate them and determine their source parameters. They are located downdip of the locked zone, where non-volcanic tremor occurs (Fig. 1). The best estimates of VLFE depths put them near the plate interface. Their focal mechanisms indicate double couple sources and are consistent with shallow dipping thrust movement. Their moment magnitude ranges between 3.3 and 3.5 suggesting that a significant part of seismic moment may be released by such VLFEs during slow earthquakes. Interestingly, most of the VLFEs are located where the slip is the largest in an ETS event. Generally, VLFEs correlates with tremor quite well in space and time. They slowly migrate alongstrike form south to north with tremor. In detail, VLFEs appear to be tracking tremor even during tremor migration of shorter time scales. I am currently expanding the VLFE catalog in space and time to better characterize their spatiotemporal distribution, moment release, and their role in slow earthquakes. VLFEs and their interaction with tremor is providing new insights to the physics of slow earthquakes, underlying processes governing them and fault properties in Cascadia.

  10. Tectonic Tremor and the Collective Behavior of Low-Frequency Earthquakes

    NASA Astrophysics Data System (ADS)

    Frank, W.; Shapiro, N.; Husker, A. L.; Kostoglodov, V.; Campillo, M.; Gusev, A. A.

    2015-12-01

    Tectonic tremor, a long duration, emergent seismic signal observed along the deep roots of plate interfaces, is thought to be the superposition of repetitive shear events called low-frequency earthquakes (LFE) [e.g. Shelly et al., Nature, 2007]. We use a catalog of more than 1.8 million LFEs regrouped into more than 1000 families observed over 2 years in the Guerrero subduction zone in Mexico, considering each family as an individual repetitive source or asperity. We develop a statistical analysis to determine whether the subcatalogs corresponding to different sources represent random Poisson processes or if they exhibit scale-invariant clustering in time, which we interpret as a manifestation of collective behavior. For each individual LFE source, we compare their level of collective behavior during two time periods: during the six-month-long 2006 Mw 7.5 slow-slip event and during a calm period with no observed slow slip. We find that the collective behavior of LFEs depends on distance from the trench and increases when the subduction interface is slowly slipping. Our results suggest that the occurrence of strong episodes of tectonic tremors cannot be simply explained by increased rates of low frequency earthquakes at every individual LFE source but correspond to an enhanced collective behavior of the ensemble of LFE asperities.

  11. Modeling broadband poroelastic propagation using an asymptotic approach

    SciTech Connect

    Vasco, Donald W.

    2009-05-01

    An asymptotic method, valid in the presence of smoothly-varying heterogeneity, is used to derive a semi-analytic solution to the equations for fluid and solid displacements in a poroelastic medium. The solution is defined along trajectories through the porous medium model, in the manner of ray theory. The lowest order expression in the asymptotic expansion provides an eikonal equation for the phase. There are three modes of propagation, two modes of longitudinal displacement and a single mode of transverse displacement. The two longitudinal modes define the Biot fast and slow waves which have very different propagation characteristics. In the limit of low frequency, the Biot slow wave propagates as a diffusive disturbance, in essence a transient pressure pulse. Conversely, at low frequencies the Biot fast wave and the transverse mode are modified elastic waves. At intermediate frequencies the wave characteristics of the longitudinal modes are mixed. A comparison of the asymptotic solution with analytic and numerical solutions shows reasonably good agreement for both homogeneous and heterogeneous Earth models.

  12. Implicit approximate-factorization schemes for the low-frequency transonic equation

    NASA Technical Reports Server (NTRS)

    Ballhaus, W. F.; Steger, J. L.

    1975-01-01

    Two- and three-level implicit finite-difference algorithms for the low-frequency transonic small disturbance-equation are constructed using approximate factorization techniques. The schemes are unconditionally stable for the model linear problem. For nonlinear mixed flows, the schemes maintain stability by the use of conservatively switched difference operators for which stability is maintained only if shock propagation is restricted to be less than one spatial grid point per time step. The shock-capturing properties of the schemes were studied for various shock motions that might be encountered in problems of engineering interest. Computed results for a model airfoil problem that produces a flow field similar to that about a helicopter rotor in forward flight show the development of a shock wave and its subsequent propagation upstream off the front of the airfoil.

  13. Localization of ultra-low frequency waves in multi-ion plasmas of the planetary magnetosphere

    DOE PAGESBeta

    Kim, Eun -Hwa; Johnson, Jay R.; Lee, Dong -Hun

    2015-01-01

    By adopting a 2D time-dependent wave code, we investigate how mode-converted waves at the Ion-Ion Hybrid (IIH) resonance and compressional waves propagate in 2D density structures with a wide range of field-aligned wavenumbers to background magnetic fields. The simulation results show that the mode-converted waves have continuous bands across the field line consistent with previous numerical studies. These waves also have harmonic structures in frequency domain and are localized in the field-aligned heavy ion density well. Lastly, our results thus emphasize the importance of a field-aligned heavy ion density structure for ultra-low frequency wave propagation, and suggest that IIH wavesmore » can be localized in different locations along the field line.« less

  14. Localization of ultra-low frequency waves in multi-ion plasmas of the planetary magnetosphere

    SciTech Connect

    Kim, Eun -Hwa; Johnson, Jay R.; Lee, Dong -Hun

    2015-01-01

    By adopting a 2D time-dependent wave code, we investigate how mode-converted waves at the Ion-Ion Hybrid (IIH) resonance and compressional waves propagate in 2D density structures with a wide range of field-aligned wavenumbers to background magnetic fields. The simulation results show that the mode-converted waves have continuous bands across the field line consistent with previous numerical studies. These waves also have harmonic structures in frequency domain and are localized in the field-aligned heavy ion density well. Lastly, our results thus emphasize the importance of a field-aligned heavy ion density structure for ultra-low frequency wave propagation, and suggest that IIH waves can be localized in different locations along the field line.

  15. Global model of low-frequency chorus (fLHR

    PubMed Central

    Meredith, Nigel P; Horne, Richard B; Li, Wen; Thorne, Richard M; Sicard-Piet, Angélica

    2014-01-01

    Whistler mode chorus is an important magnetospheric emission, playing a dual role in the acceleration and loss of relativistic electrons in the Earth's outer radiation belt. Chorus is typically generated in the equatorial region in the frequency range 0.1–0.8 fce, where fce is the local electron gyrofrequency. However, as the waves propagate to higher latitudes, significant wave power can occur at frequencies below 0.1fce. Since this wave power is largely omitted in current radiation belt models, we construct a global model of low-frequency chorus, fLHRlow-frequency chorus is strongest, with an average intensity of 200 pT2, in the prenoon sector during active conditions at midlatitudes (20°<|λm|<50°) from 4low-frequency chorus wave power will contribute to the acceleration and loss of relativistic electrons and should be taken into account in radiation belt models. Key Points Strong chorus waves can extend below 0.1 times local electron gyrofrequency Low frequency chorus strongest at mid-latitudes in pre-noon sector for L*=4 to 8 Low frequency chorus should be included in radiation belt models PMID:25821274

  16. Low-frequency thermohaline variability in the Subtropical South Atlantic pycnocline during 2002-2013

    NASA Astrophysics Data System (ADS)

    Kolodziejczyk, Nicolas; Reverdin, Gilles; Gaillard, Fabienne; Lazar, Alban

    2014-09-01

    Low-frequency variability of spiciness is observed in the Subtropical South Atlantic over the period 2002-2013 with the Argo gridded product In Situ Analysis System. Within the pycnocline, spiciness anomalies propagate at a mean speed of 0.04 ± 0.02 m s-1, the same speed as the gyre mean circulation, from the Agulhas Retroflection region off South Africa (~35°S-20°E) toward the South American coast (~18°S-35°W). After 2010, propagation is still found, but stationary local spiciness generation is also found over the Subtropical South Atlantic. This spiciness increase is associated with high values of vertical Turner angle below the mixed layer base during late winter. This suggests spice injection resulting from penetrative convective mixing due to air-sea buoyancy loss. These features may have an impact on the low-frequency warm and salty signal produced by the Agulhas leakage in Subtropical South Atlantic and the upper branch of the Atlantic Meridional Overturning Circulation.

  17. Low-frequency thermohaline variability in the Subtropical South Atlantic pycnocline during 2002-2013

    NASA Astrophysics Data System (ADS)

    Kolodziejczyk, Nicolas; Reverdin, Gilles; Gaillard, Fabienne; Lazar, Alban

    2015-04-01

    Low-frequency variability of spiciness is observed in the Subtropical South Atlantic over the period 2002-2013 with the Argo gridded product ISAS. Within the pycnocline, spiciness anomalies propagate at a mean speed of 0.04±0.02 m.s-1, the same speed as the gyre mean circulation, from the Agulhas Retroflection region off South Africa (~35°S-20°E) towards the South American coast (~18°S-35°W). After 2010, propagation is still found, but stationary local spiciness generation is also found over the Subtropical South Atlantic. This spiciness increase is associated with high values of vertical Turner angle below the mixed layer base during late winter. This suggests spice injection resulting from penetrative convective mixing due to air-sea buoyancy loss. These features may have an impact on the low-frequency warm and salty signal produced by the Agulhas leakage in Subtropical South Atlantic and the upper branch of the Atlantic Meridional Overturning Circulation.

  18. Low-frequency spatial wave manipulation via phononic crystals with relaxed cell symmetry

    SciTech Connect

    Celli, Paolo; Gonella, Stefano

    2014-03-14

    Phononic crystals enjoy unique wave manipulation capabilities enabled by their periodic topologies. On one hand, they feature frequency-dependent directivity, which allows directional propagation of selected modes even at low frequencies. However, the stellar nature of the propagation patterns and the inability to induce single-beam focusing represent significant limitations of this functionality. On the other hand, one can realize waveguides by defecting the periodic structure of a crystal operating in bandgap mode along some desired path. Waveguides of this type are only activated in the relatively high and narrow frequency bands corresponding to total bandgaps, which limits their potential technological applications. In this work, we introduce a class of phononic crystals with relaxed cell symmetry and we exploit symmetry relaxation of a population of auxiliary microstructural elements to achieve spatial manipulation of elastic waves at very low frequencies, in the range of existence of the acoustic modes. By this approach, we achieve focusing without modifying the default static properties of the medium and by invoking mechanisms that are well suited to envision adaptive configurations for semi-active wave control.

  19. A low-frequency asymptotic model of seismic reflection from a high-permeability layer

    SciTech Connect

    Silin, Dmitriy; Goloshubin, Gennady

    2009-03-01

    Analysis of compression wave propagation through a high-permeability layer in a homogeneous poroelastic medium predicts a peak of reflection in the low-frequency end of the spectrum. An explicit formula expresses the resonant frequency through the elastic moduli of the solid skeleton, the permeability of the reservoir rock, the fluid viscosity and compressibility, and the reservoir thickness. This result is obtained through a low-frequency asymptotic analysis of the Biot's model of poroelasticity. A new physical interpretation of some coefficients of the classical poroelasticity is a result of the derivation of the main equations from the Hooke's law, momentum and mass balance equations, and the Darcy's law. The velocity of wave propagation, the attenuation factor, and the wave number, are expressed in the form of power series with respect to a small dimensionless parameter. The latter is equal to the product of the kinematic reservoir fluid mobility, an imaginary unit, and the frequency of the signal. Retaining only the leading terms of the series leads to explicit and relatively simple expressions for the reflection and transmission coefficients for a planar wave crossing an interface between two permeable media, as well as wave reflection from a thin highly-permeable layer (a lens). The practical implications of the theory developed here are seismic modeling, inversion, and attribute analysis.

  20. Composite 3D-printed metastructures for low-frequency and broadband vibration absorption.

    PubMed

    Matlack, Kathryn H; Bauhofer, Anton; Krödel, Sebastian; Palermo, Antonio; Daraio, Chiara

    2016-07-26

    Architected materials that control elastic wave propagation are essential in vibration mitigation and sound attenuation. Phononic crystals and acoustic metamaterials use band-gap engineering to forbid certain frequencies from propagating through a material. However, existing solutions are limited in the low-frequency regimes and in their bandwidth of operation because they require impractical sizes and masses. Here, we present a class of materials (labeled elastic metastructures) that supports the formation of wide and low-frequency band gaps, while simultaneously reducing their global mass. To achieve these properties, the metastructures combine local resonances with structural modes of a periodic architected lattice. Whereas the band gaps in these metastructures are induced by Bragg scattering mechanisms, their key feature is that the band-gap size and frequency range can be controlled and broadened through local resonances, which are linked to changes in the lattice geometry. We demonstrate these principles experimentally, using advanced additive manufacturing methods, and inform our designs using finite-element simulations. This design strategy has a broad range of applications, including control of structural vibrations, noise, and shock mitigation. PMID:27410042

  1. Detection and location of shallow very low frequency earthquakes along the Nankai trough and the Ryukyu trench

    NASA Astrophysics Data System (ADS)

    Asano, Y.; Matsuzawa, T.; Obara, K.

    2013-12-01

    We have investigated spatiotemporal distribution of shallow very low frequency earthquakes (VLFEs) along the Nankai trough and the Ryukyu trench. Three component seismograms recorded at broadband stations of the NIED F-net were analyzed by using waveform-correlation and back-projection techniques after processing a band-pass filter (0.02 to 0.05 Hz). Here we used known VLFEs and regular interplate earthquakes near the trench axis as template events. Time series of cross-correlation function (CC) at each station was calculated from continuous waveform data and triggered seismograms of template events with a length of 180 s. Assuming surface wave propagation with a velocity of 3.8 km/s, CCs are back-propagated onto possible origin times and horizontal locations. We obtained VLFE epicenters by performing a grid search in time and space domains with spacing of 1 s and 0.025 degrees, respectively, to maximize the averaged CCs from all stations. At first, we choose grid points with averaged CCs larger than 0.5. If these grid points have similar origin times within 180 s, we assume that these grid points reflect a same event and choose the VLFE candidate having the largest averaged CC. If some grid points are detected in the same time window from different template events, we choose the VLFE candidate with the largest averaged CC from grid points located within 100 km from the template event. VLFEs were finally identified by removing regular earthquakes listed in the JMA catalogue from all candidates. As a result of the analysis for data from October, 2009 to February, 2010, two episodes of VLFE activity were detected. One episode was located east of the M6.8 interplate earthquake which occurred on October 30, 2009 along the Ryukyu trench. The VLFE seismicity was quite active just after the M6.8 earthquake and had been smoothly decreasing with the elapsed time. Such time dependent seismicity may be related to the post-seismic slip following the M6.8 earthquake. Another

  2. Parametric study of a Schamel equation for low-frequency dust acoustic waves in dusty electronegative plasmas

    NASA Astrophysics Data System (ADS)

    Sabetkar, Akbar; Dorranian, Davoud

    2015-08-01

    In this paper, our attention is first concentrated on obliquely propagating properties of low-frequency (ω ≪ ωcd) "fast" and "slow" dust acoustic waves, in the linear regime, in dusty electronegative plasmas with Maxwellian electrons, kappa distributed positive ions, negative ions (following the combination of kappa-Schamel distribution), and negatively charged dust particles. So, an explicit expression for dispersion relation is derived by linearizing a set of dust-fluid equations. The results show that wave frequency ω in long and short-wavelengths limit is conspicuously affected by physical parameters, namely, positive to negative temperature ion ratio (βp), trapping parameter of negative ions (μ), magnitude of the magnetic field B0 (via ωcd), superthermal index ( κn,κp ), and positive ion to dust density ratio (δp). The signature of the penultimate parameter (i.e., κn) on wave frequency reveals that the frequency gap between the modes reduces (escalates) for k kc r ), where kcr is critical wave number. Alternatively, for weakly nonlinear analysis, reductive perturbation theory has been used to construct 1D and 3D Schamel Korteweg-de Vries (S-KdV) equations, whose nonlinearity coefficient prescribes only compressive soliton for all parameter values of interest. The survey manifests that deviation of ions from Maxwellian behavior leads intrinsic properties of solitary waves to be evolved in opposite trend. Additionally, at lower proportion of trapped negative ions, solitary wave amplitude mitigates, whilst the trapping parameter has no effect on both spatial width and the linear wave. The results are discussed in the context of the Earth's mesosphere of dusty electronegative plasma.

  3. Low-frequency noise reduction of lightweight airframe structures

    NASA Technical Reports Server (NTRS)

    Getline, G. L.

    1976-01-01

    The results of an experimental study to determine the noise attenuation characteristics of aircraft type fuselage structural panels were presented. Of particular interest was noise attenuation at low frequencies, below the fundamental resonances of the panels. All panels were flightweight structures for transport type aircraft in the 34,050 to 45,400 kg (75,000 to 100,000 pounds) gross weight range. Test data include the results of vibration and acoustic transmission loss tests on seven types of isotropic and orthotropically stiffened, flat and curved panels. The results show that stiffness controlled acoustically integrated structures can provide very high noise reductions at low frequencies without significantly affecting their high frequency noise reduction capabilities.

  4. Method for sputtering with low frequency alternating current

    DOEpatents

    Timberlake, John R.

    1996-01-01

    Low frequency alternating current sputtering is provided by connecting a low frequency alternating current source to a high voltage transformer having outer taps and a center tap for stepping up the voltage of the alternating current. The center tap of the transformer is connected to a vacuum vessel containing argon or helium gas. Target electrodes, in close proximity to each other, and containing material with which the substrates will be coated, are connected to the outer taps of the transformer. With an applied potential, the gas will ionize and sputtering from the target electrodes onto the substrate will then result. The target electrodes can be copper or boron, and the substrate can be stainless steel, aluminum, or titanium. Copper coatings produced are used in place of nickel and/or copper striking.

  5. Computer simulation of low-frequency electromagnetic data acquisition

    SciTech Connect

    SanFilipo, W.A.; Hohmann, G.W.

    1982-02-01

    Computer simulation of low frequency electromagnetic (LFEM) digital data acquisition in the presence of natural field noise demonstrates several important limitations and considerations. Without the use of a remote reference noise removal scheme it is difficult to obtain an adequate ratio of signal to noise below 0.1 Hz for frequency domain processing and below 0.3 Hz base frequency for time domain processing for a typical source-receiver configuration. A digital high-pass filter substantially facilitates rejection of natural field noise above these frequencies but, at lower frequencies where much longer stacking times are required, it becomes ineffective. Use of a remote reference to subtract natural field noise extends these low-frequency limits a decade, but this technique is limited by the resolution and dynamic range of the instrumentation. Gathering data in short segments so that natural field drift can be offset for each segment allows a higher gain setting to minimize dynamic range problems.

  6. Method for sputtering with low frequency alternating current

    DOEpatents

    Timberlake, J.R.

    1996-04-30

    Low frequency alternating current sputtering is provided by connecting a low frequency alternating current source to a high voltage transformer having outer taps and a center tap for stepping up the voltage of the alternating current. The center tap of the transformer is connected to a vacuum vessel containing argon or helium gas. Target electrodes, in close proximity to each other, and containing material with which the substrates will be coated, are connected to the outer taps of the transformer. With an applied potential, the gas will ionize and sputtering from the target electrodes onto the substrate will then result. The target electrodes can be copper or boron, and the substrate can be stainless steel, aluminum, or titanium. Copper coatings produced are used in place of nickel and/or copper striking. 6 figs.

  7. Diatom response to extremely low-frequency magnetic fields

    SciTech Connect

    Parkinson, W.C.; Sulik, G.L. )

    1992-06-01

    Reports that extremely low-frequency magnetic fields can interfere with normal biological cell function continue to stimulate experimental activity as well as investigations into the possible mechanism of the interaction. The cyclotron resonance' model of Liboff has been tested by Smith et al. using as the biological test system the diatom Amphora coffeiformis. They report enhanced motility of the diatom in response to a low-frequency electromagnetic field tuned to the cyclotron resonance condition for calcium ions. We report here an attempt to reproduce their results. Following their protocol diatoms were seeded onto agar plates containing varying amounts of calcium and exposed to colinear DC and AC magnetic fields tuned to the cyclotron resonant condition for frequencies of 16, 30, and 60 Hz. The fractional motility was compared with that of control plates seeded at the same time from the same culture. We find no evidence of a cyclotron resonance effect.

  8. The very low frequency power spectrum of Centaurus X-3

    NASA Technical Reports Server (NTRS)

    Gruber, D. E.

    1988-01-01

    The long-term variability of Cen X-3 on time scales ranging from days to years has been examined by combining data obtained by the HEAO 1 A-4 instrument with data from Vela 5B. A simple interpretation of the data is made in terms of the standard alpha-disk model of accretion disk structure and dynamics. Assuming that the low-frequency variance represents the inherent variability of the mass transfer from the companion, the decline in power at higher frequencies results from the leveling of radial structure in the accretion disk through viscous mixing. The shape of the observed power spectrum is shown to be in excellent agreement with a calculation based on a simplified form of this model. The observed low-frequency power spectrum of Cen X-3 is consistent with a disk in which viscous mixing occurs about as rapidly as possible and on the largest scale possible.

  9. Theory of low frequency noise transmission through turbines

    NASA Technical Reports Server (NTRS)

    Matta, R. K.; Mani, R.

    1979-01-01

    Improvements of the existing theory of low frequency noise transmission through turbines and development of a working prediction tool are described. The existing actuator-disk model and a new finite-chord model were utilized in an analytical study. The interactive effect of adjacent blade rows, higher order spinning modes, blade-passage shocks, and duct area variations were considered separately. The improved theory was validated using the data acquired in an earlier NASA program. Computer programs incorporating the improved theory were produced for transmission loss prediction purposes. The programs were exercised parametrically and charts constructed to define approximately the low frequency noise transfer through turbines. The loss through the exhaust nozzle and flow(s) was also considered.

  10. An evolutionary sequence of low frequency radio astronomy missions

    NASA Technical Reports Server (NTRS)

    Jones, Dayton L.

    1990-01-01

    Many concepts for space-based low frequency radio astronomy missions are being developed, ranging from simple single-satellite experiments to large arrays on the far side of the moon. Each concept involves a different tradeoff between the range of scientific questions it can answer and the technical complexity of the experiment. Since complexity largely determines the development time, risk, launch vehicle requirements, cost, and probability of approval, it is important to see where the ability to expand the scientific return justifies a major increase in complexity. An evolutionary series of increasingly capable missions, similar to the series of missions for infrared or X-ray astronomy, is advocated. These would range from inexpensive 'piggy-back' experiments on near-future missions to a dedicated low frequency array in earth orbit (or possibly on the lunar nearside) and eventually to an array on the lunar farside.

  11. TorPeDO: A Low Frequency Gravitational Force Sensor

    NASA Astrophysics Data System (ADS)

    McManus, D. J.; Yap, M. J.; Ward, R. L.; Shaddock, D. A.; McClelland, D. E.; Slagmolen, B. J. J.

    2016-05-01

    Second generation gravitational wave detectors are likely to be limited by Newtonian Noise at low frequencies. A dual torsion pendulum sensor aimed at exploring low- frequency gravitational-force noise is being studied at the ANU. This sensor is designed to measure local gravitational forces to high precision and will be limited by Newtonian noise. We report on a controls prototype which has been constructed and suspended, along with initial characterisation and testing of the two torsion pendulums. Large weights at the end of each bar reposition the centres of mass to the same point in space external to both bars. Since both bars have a common suspension point, resonant frequency (≈33.4 mHz), and centre of mass, mechanical disturbances and other noise will affect both bars in the same manner, providing a large mechanical common mode rejection.

  12. A lunar far-side very low frequency array

    NASA Technical Reports Server (NTRS)

    Burns, Jack O. (Editor); Duric, Nebojsa (Editor); Johnson, Stewart (Editor); Taylor, G. Jeffrey (Editor)

    1989-01-01

    Papers were presented to consider very low frequency (VLF) radio astronomical observations from the moon. In part 1, the environment in which a lunar VLF radio array would function is described. Part 2 is a review of previous and proposed low-frequency observatories. The science that could be conducted with a lunar VLF array is described in part 3. The design of a lunar VLF array and site selection criteria are considered, respectively, in parts 4 and 5. Part 6 is a proposal for precursor lunar VLF observations. Finally, part 7 is a summary and statement of conclusions, with suggestions for future science and engineering studies. The workshop concluded with a general consensus on the scientific goals and preliminary design for a lunar VLF array.

  13. Low-frequency computational electromagnetics for antenna analysis

    SciTech Connect

    Miller, E.K. ); Burke, G.J. )

    1991-01-01

    An overview of low-frequency, computational methods for modeling the electromagnetic characteristics of antennas is presented here. The article presents a brief analytical background, and summarizes the essential ingredients of the method of moments, for numerically solving low-frequency antenna problems. Some extensions to the basic models of perfectly conducting objects in free space are also summarized, followed by a consideration of some of the same computational issues that affect model accuracy, efficiency and utility. A variety of representative computations are then presented to illustrate various modeling aspects and capabilities that are currently available. A fairly extensive bibliography is included to suggest further reference material to the reader. 90 refs., 27 figs.

  14. Low-frequency noise assessment metrics -- What do we know?

    SciTech Connect

    Broner, N.

    1994-12-31

    The issue of sound quality in offices and other occupied spaces has been of continuing interest since the 1950s. Existing assessment methods do not adequately account for the low-frequency background sound (< 250 Hz) produced by operating heating, ventilating, and air-conditioning (HVAC) systems, in particular, low-frequency rumble. This paper discusses the results of ASHRAE-sponsored research in which more than 75 HVAC noise samples were collected, normalized, and categorized in terms of sound quality. The results support previous findings that a neutral curve has a slope of approximately {minus}5 decibels (dB) per octave. There is also support for the contention that the balanced noise criterion B (NCB) curves are overly conservative in the region from 63 to 500 Hz and overly permissive below 63 Hz when compared with the room criteria (RC) curves. A modified set of room sound quality (RSQ) curves -- the room sound quality (RSQ) curves -- is proposed.

  15. Low Frequency Waves at and Upstream of Collisionless Shocks

    NASA Astrophysics Data System (ADS)

    Wilson, L. B.

    2016-02-01

    This chapter focuses on the range of low frequency electromagnetic modes observed at and upstream of collisionless shocks in the heliosphere. It discusses a specific class of whistler mode wave observed immediately upstream of collisionless shock ramps, called a whistler precursor. Though these modes have been (and are often) observed upstream of quasi-parallel shocks, the authors limit their discussion to those observed upstream of quasi-perpendicular shocks. The chapter discusses the various ion velocity distributions observed at and upstream of collisionless shocks. It also introduces some terminology and relevant instabilities for ion foreshock waves. The chapter discusses the most common ultra-low frequency (ULF) wave types, their properties, and their free energy sources. It discusses modes that are mostly Alfvénic (i.e., mostly transverse but can be compressive) in nature.

  16. Anomalous low frequency dissipation processes in metal springs

    NASA Astrophysics Data System (ADS)

    DeSalvo, Riccardo; Di Cintio, Arianna; Marchesoni, Fabio; Bhawal, Abhik

    2010-05-01

    The dissipation processes of leaf springs used in seismic isolation chains of Gravitational Wave detectors have been studied. A low frequency phase transition from visco us-like to fractal-like dissipation, controlled by Self Organized Criticality of dislocations, was observed. The new understandings suggest different best practices for the operations of the seismic isolation chains of the second generation of Gravitational Wave observatories and require new techniques and materials for the third generation.

  17. The reduction of low frequency fluctuations in RFP experiments

    SciTech Connect

    Phillips, J.A.; Baker, D.A.; Gribble, R.F.

    1998-09-01

    The low frequency fluctuations seen in RFP experiments are found to be correlated with changes in the toroidal flux measured by diamagnetic loops surrounding the discharge. The correlation of the onset of impurity radiation and x-rays with the crash seen in experiments is caused by plasma bombarding the metal liner associated with this loss of flux. Efforts should be made to design improved stabilizing shells that will reduce the loss of flux and give improved RFP energy confinement times.

  18. Free electron maser experiments in the low-frequency limit

    SciTech Connect

    Drori, R.; Jerby, E.; Shahadi, A.

    1995-12-31

    Table-top free-electron maser (FEM) experiments operating in the low-frequency (< 1 GHz) low-energy ({approximately} 1 keV) limit are reported. These FEM devices employ parallel-stripline non-dispersive waveguides (which support TEM-modes), and planar folded-foil wigglers. Thermionic cathodes and carbon-fiber cold-cathodes are used in these experiments. Results of oscillator and amplifier experiments are presented and compared with theory.

  19. Large-N correlator systems for low frequency radio astronomy

    NASA Astrophysics Data System (ADS)

    Foster, Griffin

    Low frequency radio astronomy has entered a second golden age driven by the development of a new class of large-N interferometric arrays. The low frequency array (LOFAR) and a number of redshifted HI Epoch of Reionization (EoR) arrays are currently undergoing commission and regularly observing. Future arrays of unprecedented sensitivity and resolutions at low frequencies, such as the square kilometer array (SKA) and the hydrogen epoch of reionization array (HERA), are in development. The combination of advancements in specialized field programmable gate array (FPGA) hardware for signal processing, computing and graphics processing unit (GPU) resources, and new imaging and calibration algorithms has opened up the oft underused radio band below 300 MHz. These interferometric arrays require efficient implementation of digital signal processing (DSP) hardware to compute the baseline correlations. FPGA technology provides an optimal platform to develop new correlators. The significant growth in data rates from these systems requires automated software to reduce the correlations in real time before storing the data products to disk. Low frequency, widefield observations introduce a number of unique calibration and imaging challenges. The efficient implementation of FX correlators using FPGA hardware is presented. Two correlators have been developed, one for the 32 element BEST-2 array at Medicina Observatory and the other for the 96 element LOFAR station at Chilbolton Observatory. In addition, calibration and imaging software has been developed for each system which makes use of the radio interferometry measurement equation (RIME) to derive calibrations. A process for generating sky maps from widefield LOFAR station observations is presented. Shapelets, a method of modelling extended structures such as resolved sources and beam patterns has been adapted for radio astronomy use to further improve system calibration. Scaling of computing technology allows for the

  20. Low Frequency Electromagnetic Background Radiation From Electron Acceleration Above Thunderclouds

    NASA Astrophysics Data System (ADS)

    Fullekrug, Martin; Mezentsev, Andrew; Soula, Serge; van der Velde, Oscar; Farges, Thomas

    2013-04-01

    It was recently proposed that the acceleration of electrons during the growth and branching of streamers above thunderclouds initiated by intense lightning discharges could result in detectable low frequency electromagnetic radiation from several tens of kHz up to several hundreds of kHz (Qin et al., GRL, 2012). The intensity of the predicted radiation scales with the streamer density which is particularly large during spectacular sprite occurrences such as jellyfish sprites and/or dancing sprites. Dancing sprites are up to one second long sequences of consecutive sprites or sprite groups which are typically separated by some hundreds of milliseconds and which tend to follow the spatial development of large scale intracloud lightning discharges. A particularly spectacular series of 10 dancing sprite events over a Mediterranean mesoscale convective system was recorded with a low light video camera in south-eastern France during the early morning hours of August 31, 2012. Each dancing sprite event was composed of ~3-4 consecutive sprites or groups of sprites. All of these sprite occurrences were associated with a sudden enhancement ~2 uV/m/Hz-1/2 of the low frequency electromagnetic background radiation as measured with a radio receiver in south-west England. It is estimated that ~1000 streamers at a height of ~40 km are necessary to epxlain the observed electric field strengths. These sudden enhancements are superimposed on a more continuous low frequency electromagnetic background radiation which accompanies each dancing sprite event. It is speculated that this low frequency 'radio glow' results from filamentary streamers near the cloud top as a result of the large scale electrostatic charging of the thundercloud and that it may be used as an indicator for sprite occurrences in future studies.

  1. Length sensing and control for Einstein Telescope Low Frequency

    NASA Astrophysics Data System (ADS)

    Adya, Vaishali; Leavey, Sean; Lück, Harald; Gräf, Christian; Hild, Stefan

    2016-05-01

    In this paper we describe a feasible length sensing and control scheme for the low frequency interferometers of the Einstein Telescope (ET-LF) along with the techniques used to optimise several optical parameters, including the length of the recycling cavities and the modulation frequencies, using two numerical interferometer simulation packages: Optickle and Finesse. The investigations have suggested the use of certain combinations of sidebands to obtain independent information about the different degrees of freedom.

  2. Low-frequency noise in hot-carrier detectors

    NASA Astrophysics Data System (ADS)

    Ivanov, N. I.; Petko, G. V.

    1981-01-01

    The low-frequency noise of hot-carrier detectors has been studied theoretically and experimentallyywith reference to the contributions of each of the following noise sources: thermal noise, shot effect, flicker noise, and noise generated in the input waveguide which is converted to thermoelectromotive force during detection. It is shown that hot-carrier detectors can be used for low-noise detection of electromagnetic radiation in the centimeter and millimeter ranges for various radioelectronic and measurements applications.

  3. A kinetic-MHD model for low frequency phenomena

    SciTech Connect

    Cheng, C.Z.

    1991-07-01

    A hybrid kinetic-MHD model for describing low-frequency phenomena in high beta anisotropic plasmas that consist of two components: a low energy core component and an energetic component with low density. The kinetic-MHD model treats the low energy core component by magnetohydrodynamic (MHD) description, the energetic component by kinetic approach such as the gyrokinetic equation, and the coupling between the dynamics of these two components through plasma pressure in the momentum equation. The kinetic-MHD model optimizes both the physics contents and the theoretical efforts in studying low frequency MHD waves and transport phenomena in general magnetic field geometries, and can be easily modified to include the core plasma kinetic effects if necessary. It is applicable to any magnetized collisionless plasma system where the parallel electric field effects are negligibly small. In the linearized limit two coupled eigenmode equations for describing the coupling between the transverse Alfven type and the compressional Alfven type waves are derived. The eigenmode equations are identical to those derived from the full gyrokinetic equation in the low frequency limit and were previously analyzed both analytically nd numerically to obtain the eigenmode structure of the drift mirror instability which explains successfully the multi-satellite observation of antisymmetric field-aligned structure of the compressional magnetic field of Pc 5 waves in the magnetospheric ring current plasma. Finally, a quadratic form is derived to demonstrate the stability of the low-frequency transverse and compressional Alfven type instabilities in terms of the pressure anisotropy parameter {tau} and the magnetic field curvature-pressure gradient parameter. A procedure for determining the stability of a marginally stable MHD wave due to wave-particle resonances is also presented.

  4. Loads and low frequency dynamics - An ENVIRONET data base

    NASA Technical Reports Server (NTRS)

    Garba, John A.

    1988-01-01

    The loads and low frequency dynamics data base, part of Environet, is described with particular attention given to its development and contents. The objective of the data base is to provide the payload designer with design approaches and design data to meet STS safety requirements. Currently the data base consists of the following sections: abstract, scope, glossary, requirements, interaction with other environments, summary of the loads analysis process, design considerations, guidelines for payload design loads, information data base, and references.

  5. Low-frequency electromagnetic field in a Wigner crystal

    SciTech Connect

    Stupka, Anton

    2013-03-15

    Long-wave low-frequency oscillations are described in a Wigner crystal by generalization of the reverse continuum model for the case of electronic lattice. The internal self-consistent long-wave electromagnetic field is used to describe the collective motions in the system. The eigenvectors and eigenvalues of the obtained system of equations are derived. The velocities of longitudinal and transversal sound waves are found.

  6. Impact of low-frequency sound on historic structures

    NASA Astrophysics Data System (ADS)

    Sutherland, Louis C.; Horonjeff, Richard D.

    2005-09-01

    In common usage, the term soundscape usually refers to portions of the sound spectrum audible to human observers, and perhaps more broadly other members of the animal kingdom. There is, however, a soundscape regime at the low end of the frequency spectrum (e.g., 10-25 Hz), which is inaudible to humans, where nonindigenous sound energy may cause noise-induced vibrations in structures. Such low frequency components may be of sufficient magnitude to pose damage risk potential to historic structures and cultural resources. Examples include Anasazi cliff and cave dwellings, and pueblo structures of vega type roof construction. Both are susceptible to noise induced vibration from low-frequency sound pressures that excite resonant frequencies in these structures. The initial damage mechanism is usually fatigue cracking. Many mechanisms are subtle, temporally multiphased, and not initially evident to the naked eye. This paper reviews the types of sources posing the greatest potential threat, their low-frequency spectral characteristics, typical structural responses, and the damage risk mechanisms involved. Measured sound and vibration levels, case history studies, and conditions favorable to damage risk are presented. The paper concludes with recommendations for increasing the damage risk knowledge base to better protect these resources.

  7. Low-frequency sea waves generated by atmospheric convection cells

    NASA Astrophysics Data System (ADS)

    de Jong, M. P. C.; Battjes, J. A.

    2004-01-01

    The atmospheric origin of low-frequency sea waves that cause seiches in the Port of Rotterdam is investigated using hydrological and meteorological observations. These observations, combined with weather charts, show that all significant seiche events coincide with the passage of a low-pressure area and a cold front. Following these front passages, increased wind speed fluctuations occur with periods on the order of 1 hour. The records show that enhanced low-frequency wave energy at sea and the seiche events in the harbor occur more or less simultaneously with these strong wind speed fluctuations. These oscillatory wind speed changes are due to convection cells that arise in an unstable lower atmosphere in the area behind a cold front, where cold air moves over the relatively warm sea surface. It is shown that the moving system of a cold front and trailing convection cells generates forced low-frequency waves at sea that can cause seiche events inside the harbor. The occurrence of such events may be predictable operationally on the basis of a criterion for the difference in temperature between the air in the upper atmosphere and the water at the sea surface.

  8. Relativistic runaway breakdown in low-frequency radio

    NASA Astrophysics Data System (ADS)

    Füllekrug, Martin; Roussel-Dupré, Robert; Symbalisty, Eugene M. D.; Chanrion, Olivier; Odzimek, Anna; van der Velde, Oscar; Neubert, Torsten

    2010-01-01

    The electromagnetic radiation emitted by an electron avalanche beam resulting from relativistic runaway breakdown within the Earth's atmosphere is investigated. It is found from theoretical modeling with a computer simulation that the electron beam emits electromagnetic radiation which is characterized by consecutive broadband pulses in the low-frequency radio range from ˜10 to 300 kHz at a distance of ˜800 km. Experimental evidence for the existence of consecutive broadband pulses is provided by low-frequency radio observations of sprite-producing lightning discharges at a distance of ˜550 km. The measured broadband pulses occur ˜4-9 ms after the sprite-producing lightning discharge, they exhibit electromagnetic radiation which mainly spans the frequency range from ˜50 to 350 kHz, and they exhibit complex waveforms without the typical ionospheric reflection of the first hop sky wave. Two consecutive pulses occur ˜4.5 ms and ˜3 ms after the causative lightning discharge and coincide with the sprite luminosity. It is concluded that relativistic runaway breakdown within the Earth's atmosphere can emit broadband electromagnetic pulses and possibly generates sprites. The source location of the broadband pulses can be determined with an interferometric network of wideband low-frequency radio receivers to lend further experimental support to the relativistic runaway breakdown theory.

  9. Solar observations with a low frequency radio telescope

    NASA Astrophysics Data System (ADS)

    Myserlis, I.; Seiradakis, J.; Dogramatzidis, M.

    2012-01-01

    We have set up a low frequency radio monitoring station for solar bursts at the Observatory of the Aristotle University in Thessaloniki. The station consists of a dual dipole phased array, a radio receiver and a dedicated computer with the necessary software installed. The constructed radio receiver is based on NASA's Radio Jove project. It operates continuously, since July 2010, at 20.1 MHz (close to the long-wavelength ionospheric cut-off of the radio window) with a narrow bandwidth (~5 kHz). The system is properly calibrated, so that the recorded data are expressed in antenna temperature. Despite the high interference level of an urban region like Thessaloniki (strong broadcasting shortwave radio stations, periodic experimental signals, CBs, etc), we have detected several low frequency solar radio bursts and correlated them with solar flares, X-ray events and other low frequency solar observations. The received signal is monitored in ordinary ASCII format and as audio signal, in order to investigate and exclude man-made radio interference. In order to exclude narrow band interference and calculate the spectral indices of the observed events, a second monitoring station, working at 36 MHz, is under construction at the village of Nikiforos near the town of Drama, about 130 km away of Thessaloniki. Finally, we plan to construct a third monitoring station at 58 MHz, in Thessaloniki. This frequency was revealed to be relatively free of interference, after a thorough investigation of the region.

  10. High Efficiency Mode Converter for Low-Frequency Gyrotron

    NASA Astrophysics Data System (ADS)

    Minami, Ryutaro; Kariya, Tsuyoshi; Imai, Tsuyoshi; Mitsunaka, Yoshika; Sakamoto, Keishi

    2011-03-01

    A high efficiency quasi-optical (QO) mode converter for high-power, low-frequency gyrotron have been designed and tested. For low-frequency gyrotrons, the scales of the mode converter are comparatively small on the wavelength scale, thus causing significant diffraction losses. Over-1 MW power gyrotron with TE8,3 cavity at 28 GHz have been developed, which has a high efficiency mode converter designed by the use of numerical methods for launcher optimization. This calculation is sufficiently optimized to maximize the fractional Gaussian content of the far field. The total transmission efficiency from the mode converter to output window is 94.7%. For the experimental result of first tube, the output power of more than 1 MW has been obtained with about 40% efficiency and output burn pattern agrees fairly with the calculated profiles, which imply the design appropriateness. Besides, the frequency dependence for diffraction loss is discussed, and these results give the guiding design principle of the mode converter for high-power, low-frequency and long-pulse gyrotrons.

  11. Measurement of Flux Density of Cas A at Low Frequencies

    NASA Astrophysics Data System (ADS)

    Patil, Ajinkya; Fisher, R.

    2012-01-01

    Cas A is used as a flux calibrator throughout the radio spectrum. Therefore it is important to know the spectral and secular variations in its flux density. Earlier observations by Scott et. al. (1969) and Baars et. al. (1972) suggested a secular decrease in flux density of Cas A at a rate of about 1% per year at all frequencies. However later observations by Erickson & Perley (1975) and Read (1977) indicated anomalously high flux from Cas A at 38 MHz. Also, these observations suggested that the original idea of faster decay of the flux density rate at low frequencies may be in error or that something more complex than simple decay is affecting the flux density at low frequencies. The source changes at 38 MHz still remains a mystery. We intend to present the results of follow up observations made from 1995 to 1998 with a three element interferometer in Green Bank operating in frequency range 30 to 120 MHz. We will discuss the problems at such low frequencies due to large beamwidth and unstable ionosphere. We will also discuss the strategies we have used so far to to find the flux density of Cas A by calculating the ratio of flux density of Cas A to that of Cyg A, assuming flux density of Cyg A to be constant. Above mentioned work was performed in summer student program sponsored by National Radio Astronomy Observatory.

  12. Frequency-specific alterations in the fractional amplitude of low-frequency fluctuations in amyotrophic lateral sclerosis.

    PubMed

    Ma, Xujing; Zhang, Jiuquan; Zhang, Youxue; Chen, Heng; Li, Rong; Long, Zhiliang; Zheng, Junjie; Wang, Jian; Chen, Huafu

    2016-08-01

    This study used resting-state functional magnetic resonance imaging and fractional amplitude of low-frequency fluctuations (fALFF) method to investigate low-frequency spontaneous neural activity at the bands of slow-5 (0.01-0.027 Hz) and slow-4 (0.027-0.073 Hz) in 20 patients with amyotrophic lateral sclerosis (ALS) and 20 healthy controls. We determined that, at slow-5 band, patients with ALS showed increased fALFF in the right middle frontal gyrus and decreased fALFF in the left middle occipital gyrus. However, compared with healthy controls, patients with ALS exhibited higher fALFF in the right caudate nucleus, left superior frontal gyrus, and right anterior cingulate cortex and lower fALFF in the right inferior occipital gyrus and bilateral middle occipital gyrus at slow-4 band. Furthermore, the fALFF value in the left superior frontal gyrus at slow-4 band was negatively correlated with functional rating scale-revised score. Our results demonstrated that the fALFF changes in ALS were widespread and frequency dependent. These findings may provide a novel way to look into the pathophysiology mechanisms underlying ALS. PMID:27139743

  13. Low frequency signals analysis from broadband seismometers records

    NASA Astrophysics Data System (ADS)

    Hsu, Po-Chin

    2016-04-01

    Broadband seismometers record signals over a wide frequency band, in which the high-frequency background noise is usually associated with human activities, such as cars, trains and factory-related activities. Meanwhile, the low-frequency signals are generally linked to the microseisms, atmospheric phenomena and oceanic wave movement. In this study, we selected the broadband seismometer data recorded during the pass of the typhoons with different moving paths, such as Doksuri in 2012, Trami and Kong-Rey in 2013, Hagibis and Matmo in 2014. By comparing the broadband seismic data, the meteorological information, and the marine conditions, we attempt to understand the effect of the meteorological conditions on the low-frequency noise. The result shows that the broadband station located along the southwestern coast of Taiwan usually have relatively higher background noise value, while the inland stations were characterized by lower noise energy. This rapid decay of the noise energy with distance from the coastline suggest that the low frequency noise could be correlated with the oceanic waves. In addition, the noise energy level increases when the distance from the typhoon and the station decreases. The enhanced frequency range is between 0.1~0.3 Hz, which is consistent with the effect caused by the interference of oceanic waves as suggested by the previous studies. This observation indicates that when the pass of typhoon may reinforce the interaction of oceanic waves and caused some influence on the seismic records. The positive correlation between the significant wave height and the noise energy could also give evidence to this observation. However, we found that the noise energy is not necessarily the strongest when the distance from typhoon and the station is the shortest. This phenomenon seems to be related to the typhoon path. When the typhoon track is perpendicular to the coastline, the change of noise energy is generally more significantly; whereas less energy

  14. Low-frequency stimulation of the external globus palladium produces anti-epileptogenic and anti-ictogenic actions in rats

    PubMed Central

    Cheng, Hui; Kuang, Yi-fang; Liu, Yang; Wang, Yi; Xu, Zheng-hao; Gao, Feng; Zhang, Shi-hong; Ding, Mei-ping; Chen, Zhong

    2015-01-01

    Aim: To investigate the anti-epileptic effects of deep brain stimulation targeting the external globus palladium (GPe) in rats. Methods: For inducing amygdala kindling and deep brain stimulation, bipolar stainless-steel electrodes were implanted in SD rats into right basolateral amygdala and right GPe, respectively. The effects of deep brain stimulation were evaluated in the amygdala kindling model, maximal electroshock model (MES) and pentylenetetrazole (PTZ) model. Moreover, the background EEGs in the amygdala and GPe were recorded. Results: Low-frequency stimulation (0.1 ms, 1 Hz, 15 min) at the GPe slowed the progression of seizure stages and shortened the after-discharge duration (ADD) during kindling acquisition. Furthermore, low-frequency stimulation significantly decreased the incidence of generalized seizures, suppressed the average stage, and shortened the cumulative ADD and generalized seizure duration in fully kindled rats. In addition, low-frequency stimulation significantly suppressed the average stage of MES-induced seizures and increased the latency to generalized seizures in the PTZ model. High-frequency stimulation (0.1 ms, 130 Hz, 5 min) at the GPe had no anti-epileptic effect and even aggravated epileptogenesis induced by amygdala kindling. EEG analysis showed that low-frequency stimulation at the GPe reversed the increase in delta power, whereas high-frequency stimulation at the GPe had no such effect. Conclusion: Low-frequency stimulation, but not high-frequency stimulation, at the GPe exerts therapeutic effect on temporal lobe epilepsy and tonic-colonic generalized seizures, which may be due to interference with delta rhythms. The results suggest that modulation of GPe activity using low-frequency stimulation or drugs may be a promising epilepsy treatment. PMID:26095038

  15. A mechanism for weak double layers and coherent low-frequency electrostatic wave activity in the solar wind

    NASA Astrophysics Data System (ADS)

    Singh Lakhina, Gurbax; Singh, Satyavir

    2016-07-01

    A mechanism for the weak double layers and coherent low-frequency electrostatic wave activity observed by Wind spacecraft in the solar wind at 1 AU is proposed in terms of ion-acoustic solitons and double layers. The solar wind plasma is modelled by a three component plasma consisting of fluid hot protons, hot alpha particles streaming with respect to protons, and suprathermal electrons having κ- distribution. This system supports two types of, slow and fast, ion-acoustic solitary waves. The fast ion-acoustic mode is similar to the ion-acoustic mode of proton-electron plasma, and can support only positive potential solitons. The slow ion-acoustic mode is a new mode that occurs due to the presence of alpha particles. This mode can support both positive and negative solitons and double layers. An increase of the κ- index leads to an increase in the critical Mach number, maximum Mach number and the maximum amplitude of both slow and fast ion-acoustic solitons. The slow ion-acoustic double layer can explain the amplitudes and widths, but not shapes, of the weak double layers (WDLs) observed in the solar wind at 1 AU by Wind spacecraft. The Fourier transform of the slow ion-acoustic solitons/double layers would produce broadband low-frequency electrostatic waves having main peaks between 0.35 kHz to 1.6 kHz, with electric field in the range of E = (0.01 - 0.7 ) mV/m, in excellent agreement with the observed low-frequency electrostatic wave activity in the solar wind at 1 AU.

  16. The mechanism of growth of the low-frequency East Asia-Pacific teleconnection and the triggering role of tropical intraseasonal oscillation

    NASA Astrophysics Data System (ADS)

    Wang, Jiabao; Wen, Zhiping; Wu, Renguang; Guo, Yuanyuan; Chen, Zesheng

    2015-08-01

    The East Asia-Pacific (EAP) pattern is a well-known meridional teleconnection over East Asia during boreal summer. In this study, the mechanism for growth of the EAP on intraseasonal timescale is investigated through a vorticity budget. It is found that the beta-effect and high-frequency transient eddies have primary contributions to the growth of the low-frequency EAP. The former leads to a westward shift of disturbances associated with the low-frequency EAP and the latter favors an amplification of disturbances, respectively. The interaction between low-frequency disturbances and zonal flow has a damping effect by dragging disturbances eastward. The impact of boreal summer intraseasonal oscillation (BSISO) on the triggering of the low-frequency EAP is also examined in this study based on observational analysis and a linear model experiment. It is shown that an elongated anomalous convection band located in the vicinity of Philippines associated with the dominant mode of BSISO has a significant impact on the initiation of low-frequency EAP via Rossby wave propagation, whereas anomalous convection located over the North Indian Ocean has a limited impact. Based on the results of present study, the low-frequency EAP could be a self-sustained mode, and the BSISO plays a substantial role in triggering the low-frequency EAP.

  17. The mechanism of growth of the low-frequency East Asia-Pacific teleconnection and the triggering role of tropical intraseasonal oscillation

    NASA Astrophysics Data System (ADS)

    Wang, Jiabao; Wen, Zhiping; Wu, Renguang; Guo, Yuanyuan; Chen, Zesheng

    2016-06-01

    The East Asia-Pacific (EAP) pattern is a well-known meridional teleconnection over East Asia during boreal summer. In this study, the mechanism for growth of the EAP on intraseasonal timescale is investigated through a vorticity budget. It is found that the beta-effect and high-frequency transient eddies have primary contributions to the growth of the low-frequency EAP. The former leads to a westward shift of disturbances associated with the low-frequency EAP and the latter favors an amplification of disturbances, respectively. The interaction between low-frequency disturbances and zonal flow has a damping effect by dragging disturbances eastward. The impact of boreal summer intraseasonal oscillation (BSISO) on the triggering of the low-frequency EAP is also examined in this study based on observational analysis and a linear model experiment. It is shown that an elongated anomalous convection band located in the vicinity of Philippines associated with the dominant mode of BSISO has a significant impact on the initiation of low-frequency EAP via Rossby wave propagation, whereas anomalous convection located over the North Indian Ocean has a limited impact. Based on the results of present study, the low-frequency EAP could be a self-sustained mode, and the BSISO plays a substantial role in triggering the low-frequency EAP.

  18. Wide-band, low-frequency pulse profiles of 100 radio pulsars with LOFAR

    NASA Astrophysics Data System (ADS)

    Pilia, M.; Hessels, J. W. T.; Stappers, B. W.; Kondratiev, V. I.; Kramer, M.; van Leeuwen, J.; Weltevrede, P.; Lyne, A. G.; Zagkouris, K.; Hassall, T. E.; Bilous, A. V.; Breton, R. P.; Falcke, H.; Grießmeier, J.-M.; Keane, E.; Karastergiou, A.; Kuniyoshi, M.; Noutsos, A.; Osłowski, S.; Serylak, M.; Sobey, C.; ter Veen, S.; Alexov, A.; Anderson, J.; Asgekar, A.; Avruch, I. M.; Bell, M. E.; Bentum, M. J.; Bernardi, G.; Bîrzan, L.; Bonafede, A.; Breitling, F.; Broderick, J. W.; Brüggen, M.; Ciardi, B.; Corbel, S.; de Geus, E.; de Jong, A.; Deller, A.; Duscha, S.; Eislöffel, J.; Fallows, R. A.; Fender, R.; Ferrari, C.; Frieswijk, W.; Garrett, M. A.; Gunst, A. W.; Hamaker, J. P.; Heald, G.; Horneffer, A.; Jonker, P.; Juette, E.; Kuper, G.; Maat, P.; Mann, G.; Markoff, S.; McFadden, R.; McKay-Bukowski, D.; Miller-Jones, J. C. A.; Nelles, A.; Paas, H.; Pandey-Pommier, M.; Pietka, M.; Pizzo, R.; Polatidis, A. G.; Reich, W.; Röttgering, H.; Rowlinson, A.; Schwarz, D.; Smirnov, O.; Steinmetz, M.; Stewart, A.; Swinbank, J. D.; Tagger, M.; Tang, Y.; Tasse, C.; Thoudam, S.; Toribio, M. C.; van der Horst, A. J.; Vermeulen, R.; Vocks, C.; van Weeren, R. J.; Wijers, R. A. M. J.; Wijnands, R.; Wijnholds, S. J.; Wucknitz, O.; Zarka, P.

    2016-02-01

    Context. LOFAR offers the unique capability of observing pulsars across the 10-240 MHz frequency range with a fractional bandwidth of roughly 50%. This spectral range is well suited for studying the frequency evolution of pulse profile morphology caused by both intrinsic and extrinsic effects such as changing emission altitude in the pulsar magnetosphere or scatter broadening by the interstellar medium, respectively. Aims: The magnitude of most of these effects increases rapidly towards low frequencies. LOFAR can thus address a number of open questions about the nature of radio pulsar emission and its propagation through the interstellar medium. Methods: We present the average pulse profiles of 100 pulsars observed in the two LOFAR frequency bands: high band (120-167 MHz, 100 profiles) and low band (15-62 MHz, 26 profiles). We compare them with Westerbork Synthesis Radio Telescope (WSRT) and Lovell Telescope observations at higher frequencies (350 and 1400 MHz) to study the profile evolution. The profiles were aligned in absolute phase by folding with a new set of timing solutions from the Lovell Telescope, which we present along with precise dispersion measures obtained with LOFAR. Results: We find that the profile evolution with decreasing radio frequency does not follow a specific trend; depending on the geometry of the pulsar, new components can enter into or be hidden from view. Nonetheless, in general our observations confirm the widening of pulsar profiles at low frequencies, as expected from radius-to-frequency mapping or birefringence theories. We offer this catalogue of low-frequency pulsar profiles in a user friendly way via the EPN Database of Pulsar Profiles, http://www.epta.eu.org/epndb/

  19. Chronic effects of low-frequency low-intensity electrical stimulation of stretched human muscle

    NASA Astrophysics Data System (ADS)

    Shenkman, Boris S.; Lyubaeva, Ekaterina V.; Popov, Daniil V.; Netreba, Aleksey I.; Bravy, Yan R.; Tarakin, Pavel P.; Lemesheva, Yulia S.; Vinogradova, Olga L.

    2007-02-01

    Effects of low-frequency electrical stimulation, which is currently considered to be a possible countermeasure for long-duration spaceflights, with and without stretch were evaluated. Twelve young male volunteers were randomly distributed into two groups. In one group anterior thigh muscles—knee extensors of both legs were stimulated with frequency of 15 Hz for 4.5 wks, six times a week; each session was 6-h long. In the other group, electrical stimulation with the same parameters was applied to stretched knee extensors. Following stimulation the subjects exhibited an increase in fatigue resistance, and in the succinate dehydrogenase activity and a 10% gain in the percentage of muscle fibers with slow myosin heavy chain isoforms. In a stimulated group the peak voluntary strength went down significantly, the CSA of fast muscle fibers in m. quadriceps femoris became slightly less in size (10%). Electrical stimulation of the stretched muscles induced an insignificant decline in their strength and an increase of cross-sectional area of muscle fibers of both types. Thus chronic low-frequency electrical stimulation may be proposed as a candidate countermeasure against muscle strength and mass loss if it is combined with stretch.

  20. Low-frequency source parameters of twelve large earthquakes

    NASA Astrophysics Data System (ADS)

    Harabaglia, Paolo

    1993-06-01

    A global survey of the low-frequency (1-21 mHz) source characteristics of large events are studied. We are particularly interested in events unusually enriched in low-frequency and in events with a short-term precursor. We model the source time function of 12 large earthquakes using teleseismic data at low frequency. For each event we retrieve the source amplitude spectrum in the frequency range between 1 and 21 mHz with the Silver and Jordan method and the phase-shift spectrum in the frequency range between 1 and 11 mHz with the Riedesel and Jordan method. We then model the source time function by fitting the two spectra. Two of these events, the 1980 Irpinia, Italy, and the 1983 Akita-Oki, Japan, are shallow-depth complex events that took place on multiple faults. In both cases the source time function has a length of about 100 seconds. By comparison Westaway and Jackson find 45 seconds for the Irpinia event and Houston and Kanamori about 50 seconds for the Akita-Oki earthquake. The three deep events and four of the seven intermediate-depth events are fast rupturing earthquakes. A single pulse is sufficient to model the source spectra in the frequency range of our interest. Two other intermediate-depth events have slower rupturing processes, characterized by a continuous energy release lasting for about 40 seconds. The last event is the intermediate-depth 1983 Peru-Ecuador earthquake. It was first recognized as a precursive event by Jordan. We model it with a smooth rupturing process starting about 2 minutes before the high frequency origin time superimposed to an impulsive source.

  1. Low-Frequency Electromagnetic Exploration for Groundwater on Mars

    NASA Technical Reports Server (NTRS)

    Grimm, Robert E.

    2002-01-01

    Water with even a small amount of dissolved solids has an electrical conductivity orders of magnitude higher than dry rock and is therefore a near-ideal exploration target on Mars for low frequency, diffusive electromagnetic methods. Models of the temperature- and frequency-dependent electrical properties of rock-ice-water mixtures are used to predict the electromagnetic response of the Martian subsurface. Detection of ice is difficult unless it is massively segregated. In contrast, liquid water profoundly affects soundings, and even a small amount of adsorbed water in the cryosphere can be detected. Subcryospheric water is readily distinguishable at frequencies as low as 100 Hz for fresh water to 10 mHz for brines. These responses can be measured using either natural or artificial sources. Ultra low frequency signals from solar wind and diurnal-heating perturbations of the ionosphere are likely, and disturbances of regional crustal magnetic fields may also be observable. Spherics, or extremely to very low frequency signals from lightning discharge, would provide optimal soundings; however, lightning may be the least likely of the possible natural sources. Among the active techniques, only the time-domain electromagnetic (TDEM) method can accommodate a closely spaced transmitter and receiver and sound to depths of hundreds of meters or more. A ground- or aircraft-based TDEM system of several kilograms can detect water to a depth of several hundred meters, and a system of tens of kilograms featuring a large, fixed, rover- or ballistically deployed loop can detect water to several kilometers depth.

  2. Low Frequency Radio Astronomical Antennas for the Lunar Environment

    NASA Astrophysics Data System (ADS)

    Burns, Jack O.; Lazio, J.; ROLSS DALI Teams

    2009-01-01

    Low radio frequencies ( MHz) represent the last of the relatively unexplored wavebands in the electromagnetic spectrum for astrophysics. Such observations are very challenging from the surface of the Earth because of an abundance of human-made radio interference (e.g., FM bands, TV channels) and because of ionospheric refraction. The lunar farside presents a unique opportunity to fully open this cosmic window because of the demonstrated radio-quiet environment. The ultimate science goal of a lunar farside low frequency telescope is to explore a new frontier in cosmology, the so-called Dark Ages. This era occurs between Recombination (at z 1100) when the universe first becomes transparent (producing what we observe today as the CMB) and Reionization when the first stars and galaxies form (at z 10-20). During the Dark Ages, the universe was unlit by any star and the only detectable signal is likely to arise from neutral hydrogen absorption against the CMB (from the collapse of the first structures). Observing this absorption signal would be a powerful probe of fundamental cosmology. During the Dark Ages (z 20 - 150), when the 21-cm (1.4 GHz) neutral hydrogen line is redshifted into the low frequency radio band (10-30 MHz, 10-30 m), the absorption signal has the potential to be the richest of all cosmological data sets. In this poster, we will discuss the opportunities and options for low frequency radio antennas in both lunar orbit and on the lunar surface. We are investigating a novel concept to deploy a large number of low-mass antennas deposited on sheets of polyimide film. We will also describe results of laboratory vacuum testing at U. Colorado on polyimide film cycled between -150 C and 100 C, and exposed to far-ultraviolet light, with conditions like those on the lunar surface.

  3. Effect of low frequency transcutaneous magnetic stimulation on sensory and motor transmission.

    PubMed

    Leung, Albert; Shukla, Shivshil; Lee, Jacquelyn; Metzger-Smith, Valerie; He, Yifan; Chen, Jeffrey; Golshan, Shahrokh

    2015-09-01

    Peripheral nerve injury diminishes fast conducting large myelinated afferent fibers transmission but enhances smaller pain transmitting fibers firing. This aberrant afferent neuronal behavior contributes to development of chronic post-traumatic peripheral neuropathic pain (PTP-NP). Non-invasive dynamic magnetic flux stimulation has been implicated in treating PTP-NP, a condition currently not adequately addressed by other therapies including transcutaneous electrical nerve stimulation (TENS). The current study assessed the effect of low frequency transcutaneous magnetic stimulation (LFTMS) on peripheral sensory thresholds, nerve conduction properties, and TENS induced fast afferent slowing effect as measured by motor and sensory conduction studies in the ulnar nerve. Results indicated sham LFTMS with TENS (Sham + TENS) significantly (P = 0.02 and 0.007, respectively) reduces sensory conduction velocity (CV) and increases sensory onset latency (OL), and motor peak latency (PL) whereas, real LFTMS with TENS (Real + TENS) reverses effects of TENS on sensory CV and OL, and significantly (P = 0.036) increases the sensory PL. LFTMS alone significantly (P < 0.05) elevates sensory PL and onset-to-peak latency. LFTMS appears to reverse TENS slowing effect on fast conducting fibers and casts a selective peripheral modulatory effect on slow conducting pain afferent fibers. PMID:25989482

  4. Compton interaction of free electrons with intense low frequency radiation

    NASA Technical Reports Server (NTRS)

    Illarionov, A. F.; Kompaneyets, D. A.

    1978-01-01

    Electron behavior in an intense low frequency radiation field, with induced Compton scattering as the primary mechanism of interaction, is investigated. Evolution of the electron energy spectrum is studied, and the equilibrium spectrum of relativistic electrons in a radiation field with high brightness temperature is found. The induced radiation pressure and heating rate of an electron gas are calculated. The direction of the induced pressure depends on the radiation spectrum. The form of spectrum, under the induced force can accelerate electrons to superrelativistic energies is found.

  5. Low frequency sound radiation from finite stiffened plates

    NASA Astrophysics Data System (ADS)

    Keltie, Richard F.

    1993-07-01

    The purpose of the research effort reported herein was to assess the feasibility of developing efficient low frequency acoustic radiators using flexural vibration of submerged stiffened plates. Candidate radiator geometries were identified at NUWC using an infinite plate model. A finite plate implementation of these models was then examined by the author using an analysis capability previously developed. The purpose of this examination was to study the extent to which infinite plate results could be achieved by a finite radiator, and to obtain an estimate of the effects of plate size and number of attached ribs on the radiation characteristics.

  6. Access to Strain and Other Low Frequency Geophysical Observations

    NASA Astrophysics Data System (ADS)

    Prescott, W.; Hodgkinson, K.; Neuhauser, D.; Silverman, S.; Stites, N. P.; Zuzlewski, S.

    2001-12-01

    With National Earthquake Hazards Program (NEHRP) funding, the U.S. Geological Survey (USGS ) has supported various fault monitoring efforts. In addition to monitoring at seismic frequency bands, the USGS also has supported strain, creep, water level and other instrumental systems operating at "low frequency". Principal investigators include both USGS and academic scientists. The typical observation interval for these data sets is 10 minutes (0.00167 hz). Raw data from most of these instruments are now available at the Northern California Earthquake Data Center (NCEDC). NCEDC is a joint effort of the University of California, Berkeley and the USGS. Raw low frequency data at NCEDC are stored in the SEED format, a standard format adopted for seismic data. And the raw low frequency data can be accessed with seismic querying tools. In a companion effort, the USGS is improving access to processed versions of these data. The raw low frequency data are processed to remove obvious instrumental offsets and spurious outliers. In the case of dilatometer data, the effects of air pressure variations are also removed from the time series. The resulting processed time series are available as plots (most recent week, most recent month, most recent year, and complete time series) and as downloadable files in either a tabular file or an eXtensible Markup Language (XML) file. XML is a standard format containing markup "tags" to identify the fields. The XML file contains all of the information known about the sensors as well as the observations. For example, in addition to the date, time and value, the XML file may contain investigator name and contact information, latitude, longitude, elevation, instrument manufacturer, serial number, digitizer manufacturer, serial number, orientation, and scale factors. The tabular file is a simple space-delimited file containing just the date, time and observed value fields. The XML file is large, but compresses well. For 10 minutes samples, XML

  7. Low-frequency fluid waves in fractures and pipes

    SciTech Connect

    Korneev, Valeri

    2010-09-01

    Low-frequency analytical solutions have been obtained for phase velocities of symmetrical fluid waves within both an infinite fracture and a pipe filled with a viscous fluid. Three different fluid wave regimes can exist in such objects, depending on the various combinations of parameters, such as fluid density, fluid viscosity, walls shear modulus, channel thickness, and frequency. Equations for velocities of all these regimes have explicit forms and are verified by comparisons with the exact solutions. The dominant role of fractures in rock permeability at field scales and the strong amplitude and frequency effects of Stoneley guided waves suggest the importance of including these wave effects into poroelastic theories.

  8. High-. beta. theory of low-frequency magnetic pulsations

    SciTech Connect

    Migliuolo, S.

    1983-03-01

    The theory of low-frequency (compared to ion cyclotron) arbitrary-..beta.. modes is developed for the following system: a two-component (hot and cold) inhomogeneous plasma, and a straight inhomogeneous magnetic field. This system is taken to model the magnetosphere, near the geomagnetic equator. The stability properties of three modes are presented in detail: the drift-compressional mode (driven by pressure gradients) the firehose mode (driven by T/sub parallel/>T/sub perpendicular/), and the drift mirror mode (driven by T/sub perpendicular/>T/sub parallel/). Comparisons to earlier models and to one observed event are also presented.

  9. Locally resonant periodic structures with low-frequency band gaps

    NASA Astrophysics Data System (ADS)

    Cheng, Zhibao; Shi, Zhifei; Mo, Y. L.; Xiang, Hongjun

    2013-07-01

    Presented in this paper are study results of dispersion relationships of periodic structures composited of concrete and rubber, from which the frequency band gap can be found. Two models with fixed or free boundary conditions are proposed to approximate the bound frequencies of the first band gap. Studies are conducted to investigate the low-frequency and directional frequency band gaps for their application to engineering. The study finds that civil engineering structures can be designed to block harmful waves, such as earthquake disturbance.

  10. Fetal exposure to low frequency electric and magnetic fields

    NASA Astrophysics Data System (ADS)

    Cech, R.; Leitgeb, N.; Pediaditis, M.

    2007-02-01

    To investigate the interaction of low frequency electric and magnetic fields with pregnant women and in particular with the fetus, an anatomical voxel model of an 89 kg woman at week 30 of pregnancy was developed. Intracorporal electric current density distributions due to exposure to homogeneous 50 Hz electric and magnetic fields were calculated and results were compared with basic restrictions recommended by ICNIRP guidelines. It could be shown that the basic restriction is met within the central nervous system (CNS) of the mother at exposure to reference level of either electric or magnetic fields. However, within the fetus the basic restriction is considerably exceeded. Revision of reference levels might be necessary.

  11. Nonlinear behavior of electrodynamic loudspeaker suspension at low frequencies

    NASA Astrophysics Data System (ADS)

    Feng, ZiXin; Shen, Yong; Heng, Wei; Liu, YunFeng

    2013-07-01

    The suspension of electrodynamic loudspeakers includes a surround of the cone and a spider, and it is characterized by the mechanic stiffness in the lumped-parameter model. By solving the nonlinear differential equation of motion which considers the nonlinearity of suspension at low frequencies numerically and measuring different kinds of surrounds and spiders, the nonlinear behavior of suspension is theoretically and experimentally studied. Since the nonlinear stiffness of spiders and surrounds can be measured and fitted respectively before assembled into loudspeakers, which spider works best with which surround is studied. The performance of loudspeakers such as harmonic distortion based on the nonlinear parameters can be predicted.

  12. Imaging of the interaction of low frequency electric fields with biological tissues by optical coherence tomography

    NASA Astrophysics Data System (ADS)

    Peña, Adrian F.; Devine, Jack; Doronin, Alexander; Meglinski, Igor

    2014-03-01

    We report the use of conventional Optical Coherence Tomography (OCT) for visualization of propagation of low frequency electric field in soft biological tissues ex vivo. To increase the overall quality of the experimental images an adaptive Wiener filtering technique has been employed. Fourier domain correlation has been subsequently applied to enhance spatial resolution of images of biological tissues influenced by low frequency electric field. Image processing has been performed on Graphics Processing Units (GPUs) utilizing Compute Unified Device Architecture (CUDA) framework in the frequencydomain. The results show that variation in voltage and frequency of the applied electric field relates exponentially to the magnitude of its influence on biological tissue. The magnitude of influence is about twice more for fresh tissue samples in comparison to non-fresh ones. The obtained results suggest that OCT can be used for observation and quantitative evaluation of the electro-kinetic changes in biological tissues under different physiological conditions, functional electrical stimulation, and potentially can be used non-invasively for food quality control.

  13. A mathematical model of extremely low frequency ocean induced electromagnetic noise

    NASA Astrophysics Data System (ADS)

    Dautta, Manik; Faruque, Rumana Binte; Islam, Rakibul

    2016-07-01

    Magnetic Anomaly Detection (MAD) system uses the principle that ferromagnetic objects disturb the magnetic lines of force of the earth. These lines of force are able to pass through both water and air in similar manners. A MAD system, usually mounted on an aerial vehicle, is thus often employed to confirm the detection and accomplish localization of large ferromagnetic objects submerged in a sea-water environment. However, the total magnetic signal encountered by a MAD system includes contributions from a myriad of low to Extremely Low Frequency (ELF) sources. The goal of the MAD system is to detect small anomaly signals in the midst of these low-frequency interfering signals. Both the Range of Detection (Rd) and the Probability of Detection (Pd) are limited by the ratio of anomaly signal strength to the interfering magnetic noise. In this paper, we report a generic mathematical model to estimate the signal-to-noise ratio or SNR. Since time-variant electro-magnetic signals are affected by conduction losses due to sea-water conductivity and the presence of air-water interface, we employ the general formulation of dipole induced electromagnetic field propagation in stratified media [1]. As a first step we employ a volumetric distribution of isolated elementary magnetic dipoles, each having its own dipole strength and orientation, to estimate the magnetic noise observed by a MAD system. Numerical results are presented for a few realizations out of an ensemble of possible realizations of elementary dipole source distributions.

  14. Near-field Observations of Very-low-frequency Earthquakes on the San Andreas Fault

    NASA Astrophysics Data System (ADS)

    Peña-Castro, A. F.; Harrington, R. M.; Cochran, E. S.

    2015-12-01

    Fault rupture at varying time scales has been detected in multiple subduction zones, e.g., in slow-slip events (SSEs), very-low-frequency earthquakes (VLFEs), and low-frequency earthquakes (LFEs) or tectonic tremor. However, only LFEs or tremor have been identified and studied in detail along strike-slip faults, like the San Andreas Fault (SAF). Here, we present evidence for VLFEs on the SAF near Parkfield, California. Using data from permanent broadband stations and a temporary deployment of 13 broadband stations installed in 2010-2011 near Cholame, California, we detect 5 VLFEs, with 1 VLFE occurring unambiguosly when there is visible tremor activity. We check that the signals we detect are local by confirming that they appear only on stations within a 70 km radius, and removing time periods when teleseismic events occur, as identified in the global Centroid Moment Tensor (CMT) and the Northern California Seismic Network (NCSN) catalogs. VLFEs have to-date been observed to only occur simultaneously in time and space with tremor activity, but our detections suggests that VLFEs can occur independent of tremor along strike-slip faults. This may indicate that the slipping patches that produce slow earthquakes in transform faults have different mechanical properties than the patches in subducting plates, althought it does not rule out that VLFEs are only observed with tremor in subduction zones simply due to detection methods. An approximate estimation of the apparent velocity, based on a grid-search location using variance reduction, suggests that the observed phase velocity of the VLFEs is ~ 3km/s, corresponding to surface waves. We perform a focal mechanism inversion with a grid search to find a more precise location, depth and orientation of the VLFEs. These results provide new insight into the behavior of the SAF and more generally contribute to an improved understanding of transform fault systems.

  15. A new approach to imaging with low-frequency electromagnetic fields

    SciTech Connect

    Ki Ha Lee; Ganquan Xie )

    1993-06-01

    The authors present a new method for interpreting electromagnetic (EM) data using ray tomography. Direct application of ray tomography to low-frequency EM data is difficult because of the diffusive nature of the field. Diffusive EM fields can, however, be mathematically transformed to wavefields defined in a time-like variable. The transform uniquely relates a field satisfying a diffusion equation in time, or in frequency, to an integral of the corresponding wavefield. If the corresponding wavefields can be computed from low-frequency EM data, one should be able to interpret these data using techniques developed for the wavefields. To test the idea, numerically calculated transient magnetic fields were first transformed to wavefields. The typical window of the time-domain data required for the transform is 1.5 decades. Traveltimes from a source to the receivers were estimated from the reconstructed wavefields. Time-domain data with a Gaussian noise of 3 percent gave a traveltime resolution of better than one percent. For the tomographic inversion, the cross-section between the transmitter and receiver boreholes is divided into a number of rectangular elements, and a continuous slowness is assigned to each of these elements. A functional is formulated by invoking Fermat's principle for the traveltime data. Imposing a stationary condition on the functional gives an iterative procedure for the slowness model. Rays are allowed to bend smoothly within each cell. Incorporating smoothly bending rays is extremely important when the velocity contrast is large. A model with a conductivity contrast of ten (10) has been successfully imaged in 120 iterations with 5 CPU hours on a SUN SPARCstation 2.

  16. The Low Frequency Aeroacoustics of Buried Nozzle Systems

    NASA Astrophysics Data System (ADS)

    Taylor, M. V.; Crighton, D. G.; Cargill, A. M.

    1993-05-01

    A simplified model of a "buried nozzle" aeroengine system is considered. The primary flow issues into a co-annular flow within a mixing chamber, and then the co-annular flow issues into the ambient medium from a secondary nozzle. Within the mixing chamber only fine scale mixing takes place, and shear layers within the mixing chamber and downstream of the secondary nozzle are assumed to sustain large scale instability waves. Excitation of this system is provided by low frequency plane waves, incident from upstream on the primary nozzle (and emanating from combustion processes in the hot core of an aeroengine). The response of this system, in the acoustic far field and in the mixing chamber, is obtained analytically from the asymptotic solution, at low frequency, of model sub-problems the solutions of which determine the wave reflection and transmission processes at the primary and secondary nozzles. In these sub-problems the shear layers are represented by vortex sheets and the nozzle walls by semi-infinite circular ducts, with Kutta conditions imposed on the unsteady flow at the primary and secondary nozzle lips. Analytical descriptions are given of the various wave modes (quasi-plane acoustic waves, and instability waves localized on the primary and secondary shear layers), of the acoustic field strength and directivity (essentially monopole, dipole and quadrupole fields), and of the conditions under which near-resonant response may occur, with large amplitudes of the perturbations in the mixing chamber and in the acoustic field.

  17. Dielectric behavior of some ferrofluids in low-frequency fields.

    PubMed

    Malaescu, I; Marin, C N

    2002-07-01

    The dielectric behavior of a ferrofluid with magnetite particles dispersed in kerosene was analyzed taking into account the Schwarz model, concerning the low-frequency dielectric behavior in systems consisting of colloidal particles suspended in electrolytes. For this reason, the complex dielectric permittivity and dielectric loss factor, in the frequency range of 10 Hz-500 kHz, at different temperatures between 20 degrees C and 100 degrees C were measured. Based on these experimental results, the experimental dependencies on both temperature of the relaxation time and activation energy of the relaxation process were analyzed. The obtained results show that the Schwarz model can be applied, in order to explain the low-frequency dielectric behavior of a ferrofluid with magnetite particles in kerosene, if the change of counterion concentration at the surface of colloidal particles is taken into account. Consequently, it is shown that the dielectric spectroscopy can be used in order to analyze the presence of particle agglomerations within ferrofluids. PMID:16290703

  18. Low-Frequency Electromagnetic Thermal Fluctuations in the Solar Wind

    NASA Astrophysics Data System (ADS)

    Gaelzer, R.; Yoon, P. H.; Ziebell, L. F.; Pavan, J.

    2012-12-01

    It is well known that the solar wind proton temperature anisotropy is constrained in the temperature ratio vs. beta parameter space by the mirror/proton-cyclotron and parallel/oblique firehose instability threshold conditions (Hellinger et al., 2006). However, the actual solar wind is found in the parameter regime stable to these instabilities (Bale et al., 2009). Since no waves can be generated in the purely collisionless and stable plasma, the source of the low-frequency electromagnetic fluctuations in the solar wind must be owing to spontaneous thermal effects. The problem of the spontaneously emitted electromagnetic waves from magnetized plasmas is generally poorly understood (Araneda et al., 2011). In the present paper, we formulate the theory of spontaneous thermal emission of electromagnetic radiation in the vicinity of the low-frequency modes of Alfvén, ion-cyclotron, and whistler modes. We carry out a statistical analysis by varying the temperature anisotropy and parallel beta and compare the theoretical fluctuation intensity against the observation such as that reported by Bale et al. (2009). Hellinger et al., GRL, 33, L09101 (2006). Bale et al., PRL, 103, 211101 (2009). Araneda et al., Space Sci. Rev., DOI:10.1007/s11214-011-9773-0 (2011).

  19. Characterizing low frequency plasma waves at Mars with MAVEN

    NASA Astrophysics Data System (ADS)

    Ruhunusiri, Suranga; Halekas, Jasper; Connerney, Jack; Espley, Jared; Larson, Davin; Mitchell, David L.

    2015-04-01

    We use the measurements from the Solar Wind Ion Analyzer (SWIA) and the magnetometer (MAG) instruments aboard the MAVEN spacecraft to characterize plasma waves in the Martian magnetosphere. SWIA is a toroidal energy analyzer that measures 3-d ion velocity distributions, and we use it for measuring ion moment fluctuations. MAG instrument, on the other hand, is a fluxgate magnetometer, and we use it for measuring magnetic field fluctuations. Mars is unique in the solar system because of two characteristics: it only has an induced magnetosphere with strong crustal fields at low altitudes, and it has an extended atmosphere due to its lower gravity. Due to these two characteristics, Mars presents a unique environment to study the interaction of a planetary magnetosphere and an exosphere with the solar wind. One consequence of this interaction is the excitation of low frequency plasma waves which have highest power near and below the proton gyrofrequency. Studying these waves is of interest because they can play a vital role in the mass and energy transport in the Martian magnetosphere. In this investigation, we use both ion moment fluctuations (density and velocity) and the magnetic field fluctuations to characterize these low frequency plasma waves.

  20. Low-frequency terrestrial tensor gravitational-wave detector

    NASA Astrophysics Data System (ADS)

    Paik, Ho Jung; Griggs, Cornelius E.; Vol Moody, M.; Venkateswara, Krishna; Lee, Hyung Mok; Nielsen, Alex B.; Majorana, Ettore; Harms, Jan

    2016-04-01

    Terrestrial gravitational-wave (GW) detectors are mostly based on Michelson-type laser interferometers with arm lengths of a few km and signal bandwidths of tens of Hz to a few kHz. Many conceivable sources would emit GWs below 10 Hz. A low-frequency tensor GW detector can be constructed by combining six magnetically levitated superconducting test masses. Seismic noise and Newtonian gravity noise are serious obstacles in constructing terrestrial GW detectors at such low frequencies. By using the transverse nature of GWs, a full tensor detector, which can in principle distinguish GWs from near-field Newtonian gravity, can be constructed. Such a tensor detector is sensitive to GWs coming from any direction with any polarization; thus a single antenna is capable of resolving the source direction and polarization. We present a design concept of a tensor GW detector that could reach a strain sensitivity of 10-19-10-20 Hz-1/2 at 0.2-10 Hz, compute its intrinsic detector noise, and discuss procedures of mitigating the seismic and Newtonian noise.

  1. Online detection of low-frequency functional connectivity

    NASA Astrophysics Data System (ADS)

    Peltier, Scott J.; LaConte, Stephen M.; Hu, Xiaoping

    2004-04-01

    Synchronized oscillations in resting state timecourses have been detected in recent fMRI studies. These oscillations are low frequency in nature (<0.08 Hz), and seem to be a property of symmetric cortices. These fluctuations are important as a pontential signal of interest, which could indicate connectivity between functionally related areas of the brain. It has also been shown that the synchronized oscillations decrease in some spontaneous pathological states (such as cocaine injection). Thus, detection of these functional connectivity patterns may help to serve as a guage of normal brain activity. Currently, functional connectivity detection is applied only in offline post-processing analysis. Online detection methods have been applied to detect task activation in functional MRI. This allows real-time analysis of fMRI results, and could be important in detecting short-term changes in functional states. In this work, we develop an outline algorithm to detect low frequency resting state functional connectivity in real time. This will extend connectivity analysis to allow online detection of changes in "resting state" brain networks.

  2. Low-frequency 1/f noise in graphene devices.

    PubMed

    Balandin, Alexander A

    2013-08-01

    Low-frequency noise with a spectral density that depends inversely on frequency has been observed in a wide variety of systems including current fluctuations in resistors, intensity fluctuations in music and signals in human cognition. In electronics, the phenomenon, which is known as 1/f noise, flicker noise or excess noise, hampers the operation of numerous devices and circuits, and can be a significant impediment to the development of practical applications from new materials. Graphene offers unique opportunities for studying 1/f noise because of its two-dimensional structure and widely tunable two-dimensional carrier concentration. The creation of practical graphene-based devices will also depend on our ability to understand and control the low-frequency noise in this material system. Here, the characteristic features of 1/f noise in graphene and few-layer graphene are reviewed, and the implications of such noise for the development of graphene-based electronics including high-frequency devices and sensors are examined. PMID:23912107

  3. Sintering of ceramics using low frequency rf power

    SciTech Connect

    Caughman, J.B.O.; Hoffman, D.J.; Baity, F.W.; Akerman, M.A.; Forrester, S.C.; Kass, M.D.

    1995-07-01

    Sintering with low frequency rf power ({approximately}50 MHz) is a new technique with unique capabilities that has been used to sinter a variety of ceramic materials, including zirconia-toughened alumina, alumina, silicon carbide, and boron carbide. Processing with low frequencies offers many advantages compared to processing with conventional microwave frequencies (915 MHz and 2.45 GHz). Because of the longer wavelength, the rf electric field penetrates materials more than microwaves. This effect allows the processing of a wider variety of materials and allows for an increase in the physical size of the material being processed. In addition, the material is heated in a single mode cavity with a uniform electric field, which reduces the occurrence of hot-spot generation and thermal runaway effects. This technique has been used to sinter large crack-free alumina samples (3 inch square) to > 97% density. The sintering and/or annealing of a number of carbide materials has been demonstrated as well, including silicon carbide, boron carbide, tungsten carbide, and titanium carbide.

  4. Sintering of ceramics using low frequency RF powder

    SciTech Connect

    Caughman, J.B.O.; Hoffman, D.J.; Baity, F.W.; Akerman, M.A.

    1995-12-31

    Sintering with low frequency rf power ({approximately}50 MHz) is a new technique with unique capabilities that has been used to sinter a variety of ceramic materials, including zirconia-toughened alumina, alumina, silicon carbide, and boron carbide. Processing with low frequencies offers many advantages compared to processing with conventional microwave frequencies (915 MHz and 2.45 GHz). Because of the longer wavelength, the rf electric field penetrates materials more than microwaves. This effect allows the processing of a wider variety of materials and allows for an increase in the physical size of the material being processed. In addition, the material is heated in a single mode cavity with a uniform electric field, which reduces the occurrence of hot-spot generation and thermal runaway effects. This technique has been used to sinter large crack-free alumina samples (3 inches square) to >97% density. The sintering and/or annealing of a number of carbide materials has been demonstrated as well, including silicon carbide, boron carbide, tungsten carbide, and titanium carbide.

  5. Powerful Low-Frequency Vibrators for Active Seismology

    SciTech Connect

    Alekseev, A.S.; Chichinin, I.S.; Korneev, V.A.

    2003-12-01

    In the past two decades, active seismology studies in Russia have made use of powerful (40- and 100-ton) low-frequency vibrators. These sources create a force amplitude of up to 100 tons and function in the 1.5 3, 3 6, and 5 10 Hz frequency bands. The mobile versions of the vibrator have a force amplitude of 40 tons and a 6 12 Hz frequency band. Recording distances for the 100-ton vibrator are as large as 350 km, enabling the refracted waves to penetrate down to 50 km depths. Vibrator operation sessions are highly repeatable, having distinct summer or winter spectral patterns. A long profile of seismic records allows estimation of fault zone depths using changes in recorded spectra. Other applications include deep seismic profiling, seismic hazard mapping, structural testing, stress-induced anisotropy studies, seismic station calibration, and large-structure integrity testing. The theoretical description of the low-frequency vibrator is given in the appendices, which contain numerical examples.

  6. A Digital Backend for the Low Frequency All Sky Monitor

    NASA Astrophysics Data System (ADS)

    Dartez, L. P.

    2014-04-01

    The Low Frequency All Sky Monitor (LoFASM) is a distributed array of dipole antennas that are sensitive to radio frequencies from 10 to 88 MHz. The primary science goals of LoFASM are the detection and study of low-frequency radio transients, a high priority science goal as deemed by the National Research Council's decadal survey. LoFASM consists of antennas and front-end electronics that were originally developed for the Long Wavelength Array (LWA) by the U.S. Naval Research Lab, the University of New Mexico, Virginia Tech, and the Jet Propulsion Laboratory. LoFASM, funded by the U.S. Department of Defense, will initially consist of four stations, each consisting of 12 dual-polarization dipole antennas. In a single station, RF signals from each of the individual LoFASM dipoles are combined in phase in order to synthesize LoFASM's beam. The LoFASM RF signals are phased up so that the resulting beam is sensitive to radio emission that originates from the zenith and RF signals approaching from the horizon are attenuated. Digitally, this is achieved using a full Stokes 100MHz correlating spectrometer constructed using field programmable gate array (FPGA) technology. In this thesis I will describe the design and usage of the LoFASM Correlator.

  7. Low frequency mechanical actuation accelerates reperfusion in-vitro

    PubMed Central

    2013-01-01

    Background Rapid restoration of vessel patency after acute myocardial infarction is key to reducing myocardial muscle death and increases survival rates. Standard therapies include thrombolysis and direct PTCA. Alternative or adjunctive emergency therapies that could be initiated by minimally trained personnel in the field are of potential clinical benefit. This paper evaluates a method of accelerating reperfusion through application of low frequency mechanical stimulus to the blood carrying vessels. Materials and method We consider a stenosed, heparinized flow system with aortic-like pressure variations subject to direct vessel vibration at the occlusion site or vessel deformation proximal and distal to the occlusion site, versus a reference system lacking any form of mechanical stimulus on the vessels. Results The experimental results show limited effectiveness of the direct mechanical vibration method and a drastic increase in the patency rate when vessel deformation is induced. For vessel deformation at occlusion site 95% of clots perfused within 11 minutes of application of mechanical stimulus, for vessel deformation 60 centimeters from the occlusion site 95% percent of clots perfused within 16 minutes of stimulus application, while only 2.3% of clots perfused within 20 minutes in the reference system. Conclusion The presented in-vitro results suggest that low frequency mechanical actuation applied during the pre-hospitalization phase in patients with acute myocardial infarction have potential of being a simple and efficient adjunct therapy. PMID:24257116

  8. Low-Frequency, Low-G MEMS Piezoelectric Energy Harvester

    NASA Astrophysics Data System (ADS)

    Xu, R.; Kim, S. G.

    2015-12-01

    This paper reports the design, modeling and fabrication of a novel MEMS device for low-frequency, low-g vibration energy harvesting. The new design is based on bi-stable buckled beam structure. To implement the design at MEMS scale, we further proposed to employ residual stress in micro-fabricated thin films. With an electromechanical lumped model, the multi-layer beam could be designed to achieve bi-stability with desired frequency range and excitation amplitude. A macro-scale prototype has been built and tested to verifies the prediction of the performance enhancement of the bi-stable beam at low frequencies. A MEMS scale prototype has been fabricated and tested to verify the frequency range at low excitation amplitude. The MEMS device shows wide operating frequency range from 50Hz to 150Hz at 0.2g without external proof mass. The same device with external proof mass has lower frequency range (< 10Hz) with boosted deflection amplitude.

  9. A new era for low frequency Galactic center transient monitoring

    NASA Astrophysics Data System (ADS)

    Kassim, N. E.; Hyman, S. D.; Intema, H.; Lazio, T. J. W.

    2014-05-01

    An upgrade of the low frequency observing system of the VLA developed by NRL and NRAO, called low band (LB), will open a new era of Galactic center (GC) transient monitoring. Our previous searches using the VLA and GMRT have revealed a modest number of radio-selected transients, but have been severely sensitivity and observing time limited. The new LB system, currently accessing the 236--492 MHz frequency range, promises ≥5 × improved sensitivity over the legacy VLA system. The new system is emerging from commissioning in time to catch any enhanced sub-GHz emission from the G2 cloud event, and we review existing limits based on recent observations. We also describe a proposed 24/7 commensal system, called the LOw Band Observatory (LOBO). LOBO offers over 100 VLA GC monitoring hours per year, possibly revealing new transients and helping validate ASTRO2010's anticipation of a new era of transient radio astronomy. A funded LOBO pathfinder called the VLA Low Frequency Ionosphere and Transient Experiment (VLITE) is under development. Finally, we consider the impact of LB and LOBO on our GC monitoring program.

  10. Gravitational dynamos and the low-frequency geomagnetic secular variation.

    PubMed

    Olson, P

    2007-12-18

    Self-sustaining numerical dynamos are used to infer the sources of low-frequency secular variation of the geomagnetic field. Gravitational dynamo models powered by compositional convection in an electrically conducting, rotating fluid shell exhibit several regimes of magnetic field behavior with an increasing Rayleigh number of the convection, including nearly steady dipoles, chaotic nonreversing dipoles, and chaotic reversing dipoles. The time average dipole strength and dipolarity of the magnetic field decrease, whereas the dipole variability, average dipole tilt angle, and frequency of polarity reversals increase with Rayleigh number. Chaotic gravitational dynamos have large-amplitude dipole secular variation with maximum power at frequencies corresponding to a few cycles per million years on Earth. Their external magnetic field structure, dipole statistics, low-frequency power spectra, and polarity reversal frequency are comparable to the geomagnetic field. The magnetic variability is driven by the Lorentz force and is characterized by an inverse correlation between dynamo magnetic and kinetic energy fluctuations. A constant energy dissipation theory accounts for this inverse energy correlation, which is shown to produce conditions favorable for dipole drift, polarity reversals, and excursions. PMID:18048345

  11. Micromachined low frequency rocking accelerometer with capacitive pickoff

    DOEpatents

    Lee, Abraham P.; Simon, Jonathon N.; McConaghy, Charles F.

    2001-01-01

    A micro electro mechanical sensor that uses capacitive readout electronics. The sensor involves a micromachined low frequency rocking accelerometer with capacitive pickoff fabricated by deep reactive ion etching. The accelerometer includes a central silicon proof mass, is suspended by a thin polysilicon tether, and has a moving electrode (capacitor plate or interdigitated fingers) located at each end the proof mass. During movement (acceleration), the tethered mass moves relative to the surrounding packaging, for example, and this defection is measured capacitively by a plate capacitor or interdigitated finger capacitor, having the cooperating fixed electrode (capacitor plate or interdigitated fingers) positioned on the packaging, for example. The micromachined rocking accelerometer has a low frequency (<500 Hz), high sensitivity (.mu.G), with minimal power usage. The capacitors are connected to a power supply (battery) and to sensor interface electronics, which may include an analog to digital (A/D) converter, logic, RF communication link, antenna, etc. The sensor (accelerometer) may be, for example, packaged along with the interface electronics and a communication system in a 2".times.2".times.2" cube. The proof mass may be asymmetric or symmetric. Additional actuating capacitive plates may be used for feedback control which gives a greater dynamic range.

  12. Technologies for Low Frequency Radio Observations of the Cosmic Dawn

    NASA Technical Reports Server (NTRS)

    Jones, Dayton L.

    2014-01-01

    The Jet Propulsion Laboratory (JPL) is developing concepts and technologies for low frequency radio astronomy space missions aimed at observing highly redshifted neutral Hydrogen from the Dark Ages. This is the period of cosmic history between the recombination epoch when the microwave background radiation was produced and the re-ionization of the intergalactic medium by the first generation of stars (Cosmic Dawn). This period, at redshifts greater than about 20, is a critical epoch for the formation and evolution of large-scale structure in the universe. The 21-cm spectral line of Hydrogen provides the most promising method for directly studying the Dark Ages, but the corresponding frequencies at such large redshifts are only tens of MHz and thus require space-based observations to avoid terrestrial RFI and ionospheric absorption and refraction. This paper reports on the status of several low frequency technology development activities at JPL, including deployable bi-conical dipoles for a planned lunar-orbiting mission, and both rover-deployed and inflation-deployed long dipole antennas for use on the lunar surface.

  13. Low-frequency electromagnetic technique for nondestructive evaluation

    NASA Astrophysics Data System (ADS)

    Dalichaouch, Yacine; Singsaas, Alan L.; Putris, Firas; Perry, Alexander R.; Czipott, Peter V.

    2000-05-01

    We have developed a low frequency electromagnetic technique using sensitive room temperature magnetoresistive (MR) sensors for a variety of nondestructive evaluation (NDE) applications. These applications include the NDE of medical implants and aircraft structures, the detection of cracks and corrosion in metals, the detection of ferromagnetic foreign objects in the eye and the brain, and the noninvasive determination of iron content in the liver. Our technique consists of applying a low frequency ac magnetic field to the sample and detecting the sample response. The low excitation frequency enables us to probe deep into metal structures; the sensitivity of the MR sensor allows us to detect weak responses from the sample without applying too large an excitation field, particularly in the case of human tissue. The MR sensors are small and relatively inexpensive compared to other sensitive magnetic field sensors such as fluxgates and superconducting quantum interference devices or SQUIDs; hence the resulting NDE instrument will be compact and cost-efficient, enabling its commercialization for practical applications. In this paper, we focus primarily on NDE of orthopedic implants.

  14. Mechanically-tunable composite filter at low frequencies

    NASA Astrophysics Data System (ADS)

    Wheeland, Sara; Amirkhizi, Alireza V.; Nemat-Nasser, Sia

    2010-04-01

    Previous studies into the possibility of a plasmonic medium of a coiled conductor array in air have shown promise. This work serves to evaluate the possibility of creating a mechanically-tunable composite filter at low frequencies. Copper springs were created with varying starting pitches using a coil winder. These springs were then embedded into a flexible host polymer. The mechanical and electromagnetic properties of each spring design were predicted and tested. Two horn antennas were used to characterize the overall electromagnetic (EM) properties of the composite. The pitch of each spring was increased mechanically through application of force to the entire polymermetal composite at equal intervals, with an EM test completed at each step. Using an Agilent 8510C Vector Network Analyzer (VNA), the frequency spectrum within the microwave range was scanned. Relative amplitude and phase measurements were taken at equal frequency and pitch steps. With no polymer surrounding the springs, plasmon turn-on frequencies were observed to span the microwave bands as the pitch of the springs were increased. Similar results are expected with the springs embedded in a polymeric matrix. These results suggest a method of creating a mechanically-tunable composite filter for use at low frequencies.

  15. Low Frequency noise of nanowire bioFETs

    NASA Astrophysics Data System (ADS)

    Rajan, Nitin; Chen, Jin; Routenberg, David; Reed, Mark

    2010-03-01

    In this study we characterize the low frequency noise of top-down fabricated silicon nanowire FETs with exposed channels used as biological sensors. Understanding their low frequency noise behavior is important because signal-to-noise ratio limits the sensitivity of these devices when attempting to detect low analyte concentrations. Using noise spectroscopy we quantitatively demonstrate that a wet orientation dependent etch (ODE) using tetramethylammonium hydroxide yields a lower surface state density and thus better noise performance that common plasma-based etch processes. To thoroughly characterize and accurately model the noise of fabricated silicon nanowires using the wet ODE, we carry out 1/f noise measurements from subthreshold to strong inversion as well as noise measurements at different temperatures. We observe an increase in the noise amplitude at lower temperatures, the increase being more pronounced in the subthreshold region. We also observe a change in the noise profile, indicating a change in the dominant mechanism giving rise to 1/f noise, as the temperature is lowered.

  16. Low-frequency 1/f noise in graphene devices

    NASA Astrophysics Data System (ADS)

    Balandin, Alexander A.

    2013-08-01

    Low-frequency noise with a spectral density that depends inversely on frequency has been observed in a wide variety of systems including current fluctuations in resistors, intensity fluctuations in music and signals in human cognition. In electronics, the phenomenon, which is known as 1/f noise, flicker noise or excess noise, hampers the operation of numerous devices and circuits, and can be a significant impediment to the development of practical applications from new materials. Graphene offers unique opportunities for studying 1/f noise because of its two-dimensional structure and widely tunable two-dimensional carrier concentration. The creation of practical graphene-based devices will also depend on our ability to understand and control the low-frequency noise in this material system. Here, the characteristic features of 1/f noise in graphene and few-layer graphene are reviewed, and the implications of such noise for the development of graphene-based electronics including high-frequency devices and sensors are examined.

  17. Low Frequency Vibration Energy Harvesting using Diamagnetically Stabilized Magnet Levitation

    NASA Astrophysics Data System (ADS)

    Palagummi, Sri Vikram

    Over the last decade, vibration-based energy harvesting has provided a technology push on the feasibility of self-powered portable small electronic devices and wireless sensor nodes. Vibration energy harvesters in general transduce energy by damping out the environmentally induced relative emotion through either a cantilever beam or an equivalent suspension mechanism with one of the transduction mechanisms, like, piezoelectric, electrostatic, electromagnetic or magnetostrictive. Two major challenges face the present harvesters in literature, one, they suffer from the unavoidable mechanical damping due to internal friction present in the systems, second, they cannot operate efficiently in the low frequency range (< 10 Hz), when most of the ambient vibrational energy is in this low frequency broadband range. Passive and friction free diamagnetically stabilized magnet levitation mechanisms which can work efficiently as a vibration energy harvester in the low frequency range are discussed in this work. First, a mono-stable vertical diamagnetic levitation (VDL) based vibration energy harvester (VEH) is discussed. The harvester consists of a lifting magnet (LM), a floating magnet (FM) and two diamagnetic plates (DPs). The LM balances out the weight of the FM and stability is brought about by the repulsive effect of the DPs, made of pyrolytic graphite. Two thick cylindrical coils, placed in grooves which are engraved in the DPs, are used to convert the mechanical energy into electrical energy. Experimental frequency response of the system is validated by the theoretical analysis which showed that the VEH works in a low frequency range but sufficient levitation gap was not achieved and the frequency response characteristic of the system was effectively linear. To overcome these challenges, the influence of the geometry of the FM, the LM, and the DP were parametrically studied to assess their effects on the levitation gap, size of the system and the natural frequency. For

  18. Low-frequency electromagnetic waves driven by gyrotropic gyrating ion beams

    NASA Technical Reports Server (NTRS)

    Sharma, O. P.; Patel, V. L.

    1986-01-01

    The origin of left- and right-hand-polarized low-frequency waves in space plasmas is analyzed. It has been shown that a gyrotropic gyrating ion beam, a ring in velocity space, can excite electromagnetic modes in the plasma near the beam gyrofrequency. It excites left-hand-polarized shear Alfven waves and their harmonics via the coupling of Alfven modes with the beam modes. It can also excite right-hand-polarized fast-mode magnetosonic waves and their harmonics as well. The excitation is possible for beam ions heavier than the plasma ions. The growth rate varies as one-third power of the beam density and decreases with the angle of wave propagation with respect to the ambient magnetic field. The nonlocality has a stabilizing effect on the instability. The predicted values of the wave frequencies compare reasonably well with those observed in satellite data.

  19. Excitation of low frequency waves by streaming ions via anomalous cyclotron resonance

    NASA Technical Reports Server (NTRS)

    Wu, C. S.; Dillenburg, D.; Gaffey, J. D., Jr.; Ziebell, L. F.; Goedert, J.; Freund, H. P.

    1978-01-01

    The effect of a small population of streaming ions on low-frequency waves with frequencies below the ion cyclotron frequency is analyzed for three modes of interest: Alfven waves, magnetosonic waves, and ion-cyclotron waves. The instability mechanism is the anomalous cyclotron resonance of the waves with the streaming ions. Conditions for excitation of the three types of waves are derived and expressions for the growth rates are obtained. Excitation of Alfven waves is possible even if the ratio of the densities of the streaming ions to the thermal ions is very small. For magnetosonic waves, excitation can easily occur if waves are propagating parallel or nearly parallel to the ambient magnetic field. As for ion-cyclotron waves, it is found that for the ion-whistler branch the excitation is suppressed over a broader range of wave frequencies than for the fast magnetosonic branch.

  20. Generation of infrasonic waves by low-frequency dust acoustic perturbations in the Earth's lower ionosphere

    SciTech Connect

    Kopnin, S. I.; Popel, S. I.

    2008-06-15

    It is shown that, during Perseid, Geminid, Orionid, and Leonid meteor showers, the excitation of low-frequency dust acoustic perturbations by modulational instability in the Earth's ionosphere can lead to the generation of infrasonic waves. The processes accompanying the propagation of these waves are considered, and the possibility of observing the waves from the Earth's surface is discussed, as well as the possible onset of acoustic gravitational vortex structures in the region of dust acoustic perturbations. The generation of such structures during Perseid, Geminid, Orionid, and Leonid meteor showers can show up as an increase in the intensity of green nightglow by an amount on the order of 10% and can be attributed to the formation of nonlinear (vortex) structures at altitudes of 110-120 km.

  1. Excitations of low-frequency hydromagnetic waves by freshly created ions in the solar wind

    NASA Technical Reports Server (NTRS)

    Price, C. P.; Gaffey, J. D.; Dong, J. Q.

    1988-01-01

    Low-frequency hydromagnetic waves excited by newborn ions in the solar wind plasma are studied. The freshly created ions appear in the solar wind frame with a ring beam distribution. Both Alfven and fast magnetosonic waves are made unstable by the presence of the newborn ions. The dependence of the growth rate of both waves on the newborn ion density, the angle between the interplanetary magnetic field (IMF) and solar wind flow, and the angle of wave propagation relative to the IMF is investigated. Analytic approximations for the growth rates are presented, and numerical solutions of the dispersion equation are shown. The approximations are quite close to the numerically determined growth rates. It is found that the waves grow preferentially in the direction parallel to the IMF, and that the growth rates increase with both newborn ion density and the angle between the IMF and the solar wind flow.

  2. High- and low-frequency phonon modes in dipolar quantum gases trapped in deep lattices

    NASA Astrophysics Data System (ADS)

    Maluckov, Aleksandra; Gligorić, Goran; Hadžievski, Ljupčo; Malomed, Boris A.; Pfau, Tilman

    2013-02-01

    We study normal modes propagating on top of the stable uniform background in arrays of dipolar Bose-Einstein condensate (BEC) droplets trapped in a deep optical lattice. Both the on-site mean-field dynamics of the droplets and their displacement due to the repulsive dipole-dipole interactions (DDIs) are taken into account. Dispersion relations for two modes, viz., high- and low- frequency counterparts of optical and acoustic phonon modes in condensed matter, are derived analytically and verified by direct simulations, for both cases of the repulsive and attractive contact interactions. The (counterpart of the) optical-phonon branch does not exist without the DDIs. These results are relevant in the connection to emerging experimental techniques enabling real-time imaging of the condensate dynamics and direct experimental measurement of phonon dispersion relations in BECs.

  3. Low Frequency-SAFT Inspection Methodology for Coarse-Grained Steel Rail Components (Manganese Steel Frogs)

    SciTech Connect

    Diaz, Aaron A.; Andersen, Eric S.; Samuel, Todd J.

    2004-11-01

    In the rail industry, sections of high strength Manganese steel are employed at critical locations in railroad networks. Ultrasonic inspections of Manganese steel microstructures are difficult to inspect with conventional means, as the propagation medium is highly attenuative, coarse-grained, anisotropic and nonhomogeneous in nature. Current in-service inspection methods are ineffective while pre-service X-ray methods (used for full-volumetric examinations of components prior to shipment) are time-consuming, costly, require special facilities and highly trained personnel for safe operations, and preclude manufacturers from inspecting statistically meaningful numbers of frogs for effective quality assurance. In-service examinations consist of visual inspections only and by the time a defect or flaw is visually detected, the structural integrity of the component may already be compromised, and immediate repair or replacement is required. A novel ultrasonic inspection technique utilizing low frequency ultrasound (100 to 500 kHz) combined with a synthetic aperture focusing technique (SAFT) for effective reduction of signal clutter and noise, and extraction of important features in the data, has proven to be effective for these coarse grained steel components. Results from proof-of-principal tests in the laboratory demonstrate an effective means to detect and localize reflectors introduced as a function of size and depth from the top of the frog rail. Using non-optimal, commercially available transducers coupled with the low-frequency/SAFT approach, preliminary evaluations were conducted to study the effects of the material microstructure on ultrasonic propagation, sensitivity and resolution in thick section frog components with machined side-drilled holes. Results from this study will be presented and discussed.

  4. Low frequency radio synthesis imaging of the galactic center region

    NASA Astrophysics Data System (ADS)

    Nord, Michael Evans

    2005-11-01

    The Very Large Array radio interferometer has been equipped with new receivers to allow observations at 330 and 74 MHz, frequencies much lower than were previously possible with this instrument. Though the VLA dishes are not optimal for working at these frequencies, the system is successful and regular observations are now taken at these frequencies. However, new data analysis techniques are required to work at these frequencies. The technique of self- calibration, used to remove small atmospheric effects at higher frequencies, has been adapted to compensate for ionospheric turbulence in much the same way that adaptive optics is used in the optical regime. Faceted imaging techniques are required to compensate for the noncoplanar image distortion that affects the system due to the wide fields of view at these frequencies (~2.3° at 330 MHz and ~11° at 74 MHz). Furthermore, radio frequency interference is a much larger problem at these frequencies than in higher frequencies and novel approaches to its mitigation are required. These new techniques and new system are allowing for imaging of the radio sky at sensitivities and resolutions orders of magnitude higher than were possible with the low frequency systems of decades past. In this work I discuss the advancements in low frequency data techniques required to make high resolution, high sensitivity, large field of view measurements with the new Very Large Array low frequency system and then detail the results of turning this new system and techniques on the center of our Milky Way Galaxy. At 330 MHz I image the Galactic center region with roughly 10 inches resolution and 1.6 mJy beam -1 sensitivity. New Galactic center nonthermal filaments, new pulsar candidates, and the lowest frequency detection to date of the radio source associated with our Galaxy's central massive black hole result. At 74 MHz I image a region of the sky roughly 40° x 6° with, ~10 feet resolution. I use the high opacity of H II regions at 74

  5. Low-Frequency Electromagnetic Sounding for Planetary Volatiles (Invited)

    NASA Astrophysics Data System (ADS)

    Grimm, R. E.

    2013-12-01

    EM sounding is divided by loss tangent << 1 (surface-penetrating radars) and >> 1 (inductive methods). The former have high resolution and responses dominated by dielectric permittivity. They have been useful for sounding the polar caps of Mars and are very promising to image the shells of icy satellites as well as the uppermost crusts of silicate bodies. The latter have poorer resolution but greater penetration depth, responses dominated by electrical conductivity, and are the subject of this talk. Low-frequency inductive methods are further divided by comparing the source-receiver separation to the skin depth. Large separations are parametric in frequency so that the variation of EM response with frequency is translated to change in conductivity with depth. Parametric soundings can exploit natural sources from the solar wind, magnetosphere, ionosphere, or atmosphere. Small source-sensor separations are geometric with transmitter-receiver positions: both conductivity and permittivity can be recovered as a function of frequency (a dielectric spectrum), but at greater resource requirements. Subsurface liquid water is an optimal low-frequency EM target because even small quantities of dissolved ions make it a powerful electrical conductor compared to dry, resistive, silicate crusts. Water at kms or even tens of kms can be detected using the magnetotelluric, geomagnetic-depth sounding, or wave-tilt methods: these are all natural-source soundings using different combinations of field components and receiver geometries. If natural sources are weak or absent, a transmitter can be used to obtain high SNR; the time-domain EM (TDEM) method has been used extensively for terrestrial groundwater exploration. Using a ballistically deployed 200-m diameter transmitter loop, TDEM can detect groundwater at depths of several km. If landed in a region of strong local crustal magnetism, the characteristic Larmor frequency of liquid water can be detected with a TDEM-like setup using

  6. The Challenge of Low-Frequency ENSO Variability (Invited)

    NASA Astrophysics Data System (ADS)

    Cole, J. E.; Ault, T. R.; Thompson, D. M.

    2010-12-01

    The tropical Pacific is connected to patterns of drought and flooding throughout the world. Global climate models give ambiguous projections about changes in this region, yet anticipating these changes will be crucial to adaptation strategies during the coming decades. Here, we use proxy, observational, and climate model data to address two fundamental questions that are not resolved in our view. First, what is the relationship between frequency and variance in the tropical Pacific? And, second, do observations, climate models, and proxy records provide a consistent view of tropical Pacific SST variability across interannual, decadal, and centennial timescales? To explore these questions, we analyze the power spectra of proxy records from the tropical Pacific as well as observational and climate model data from the same region. Observations suggest that variance is concentrated within the canonical 2-7 year window, but results are ambiguous on longer timescales due to the shortness of the records and other limitations of instrumental data. High-resolution coral δ18O records suggest that the window of ENSO variance extends through multidecadal periods, with exceptionally strong variability concentrated in the decadal band during the late 19th century. Lower-resolution proxy records imply a continuum of variance that increases at lower frequencies through the multicentury time scale. These paleoclimatic analyses suggest that the 20th century does not capture the full range of ENSO variability, even in the last millennium. Climate models, in contrast, support ENSO variability at timescales of 2-5 years, with many models exhibiting overly regular oscillations between El Niño and La Niña conditions. Proxies and observations argue for more energetic low-frequency variability than is seen in the variance spectra of simulated tropical Pacific SST, suggesting that GCMs may be missing key low-frequency behaviors in their internal variability. GCMs that do not capture the

  7. Physiological and content considerations for a second low frequency channel for bass management, subwoofers, and low frequency enhancement (LFE)

    NASA Astrophysics Data System (ADS)

    Miller, Robert E. (Robin)

    2005-04-01

    Perception of very low frequencies (VLF) below 125 Hz reproduced by large woofers and subwoofers (SW), encompassing 3 octaves of the 10 regarded as audible, has physiological and content aspects. Large room acoustics and vibrato add VLF fluctuations, modulating audible carrier frequencies to >1 Hz. By convention, sounds below 90 Hz produce no interaural cues useful for spatial perception or localization, therefore bass management redirects the VLF range from main channels to a single (monaural) subwoofer channel, even if to more than one subwoofer. Yet subjects claim they hear a difference between a single subwoofer channel and two (stereo bass). If recordings contain spatial VLF content, is it possible physiologically to perceive interaural time/phase difference (ITD/IPD) between 16 and 125 Hz? To what extent does this perception have a lifelike quality; to what extent is it localization? If a first approximation of localization, would binaural SWs allow a higher crossover frequency (smaller satellite speakers)? Reported research supports the Jeffress model of ITD determination in brain structures, and extending the accepted lower frequency limit of IPD. Meanwhile, uncorrelated very low frequencies exist in all tested multi-channel music and movie content. The audibility, recording, and reproduction of uncorrelated VLF are explored in theory and experiments.

  8. Space applications of superconductivity - Low frequency superconducting sensors

    NASA Technical Reports Server (NTRS)

    Zimmerman, J. E.

    1980-01-01

    Although this paper deals with several low-frequency instruments and devices, most of the discussion relates to SQUID (Superconducting QUantum Interference Device) magnetometers and gradiometers, since these are perceived as the instruments with the greatest potential for space applications. The discussion covers SQUID for magnetic field measurements; present state of the art of SQUID technology; ultimate potential performance; applications to magnetic measurements in space; SQUID galvanometers, voltage and current sensors, and wide-band amplifiers; magnetic shielding, and superconducting dc transformer. SQUIDS are superior to all other magnetic sensors in sensitivity, frequency response, range, and linearity. It is suggested that SQUID instruments, both magnetometers and gradiometers, would be valuable in studies of the dynamics of interplanetary and planetary fields. SQUID gradiometers are useful for detection and mapping of magnetic anomalies at short to moderate ranges.

  9. Do GCM's Predict the Climate.... Or the Low Frequency Weather?

    NASA Astrophysics Data System (ADS)

    Lovejoy, S.; Varon, D.; Schertzer, D. J.

    2011-12-01

    Over twenty-five years ago, a three-regime scaling model was proposed describing the statistical variability of the atmosphere over time scales ranging from weather scales out to ≈ 100 kyrs. Using modern in situ data reanalyses, monthly surface series (at 5ox5o), 8 "multiproxy" (yearly) series of the Northern hemisphere from 1500- 1980, and GRIP and Vostok paleotemperatures at 5.2 and ≈ 100 year resolutions (over the past 91-420 kyrs), we refine the model and show how it can be understood with the help of new developments in nonlinear dynamics, especially multifractals and cascades. In a scaling range, mean fluctuations in state variables such as temperature ΔT ≈ ΔtH the where Δt is the duration. At small (weather) scales the fluctuation exponents are generally H>0; they grow with scale. At longer scales Δt >τw (≈ 10 days) they change sign, the fluctuations decrease with scale; this is the low variability, "low frequency weather" regime the spectrum is a relatively flat "plateau", it's variability is that of the usual idea of "long term weather statistics". Finally for longer times, Δt>τc ≈ 10 - 100 years, again H>0, the variability again increases with scale. This is the true climate regime. These scaling regimes allow us to objectively define the weather as fluctuations over periods <τw, "climate states", as fluctuations at scale τc and "climate change" as the fluctuations at longer periods >τc). We show that the intermediate regime is the result of the weather regime undergoing a "dimensional transition": at temporal scales longer than the typical lifetime of planetary structures (τw), the spatial degrees of freedom are rapidly quenched, only the temporal degrees of freedom are important. This low frequency weather regime has statistical properties well reproduced not only by weather cascade models, but also by control runs (i.e. without climate forcing) of GCM's (including IPSL and ECHAM GCM's). In order for GCM's to go beyond simply

  10. Low frequency drift instabilities in a dusty plasma

    SciTech Connect

    Rosenberg, M.; Krall, N.A.

    1996-02-01

    Low frequency drift instabilities are investigated in a dusty magnetized plasma with negatively charged grains in which locally there is an electron density gradient which is opposite in sign to a dust density gradient. Frequencies less than the ion gyrofrequency but much larger than the dust gyrofrequency are considered. Two different equilibria are considered that are characterized by {rho}{sub {ital d}}{lt_or_gt}{ital L}{sub {ital nd}}, where {rho}{sub {ital d}} is the dust gyroradius and {ital L}{sub {ital nd}} is the dust density scale length. Instabilities analogous to the universal instability and to the lower-hybrid-drift instability (with the lower-hybrid frequency in this case associated with the dust) are investigated. Possible applications to dusty space plasmas such as the spoke regions of Saturn{close_quote}s B-ring are discussed. {copyright} {ital 1996 American Institute of Physics.}

  11. Low frequency mechanical modes of viruses with atomic detail

    NASA Astrophysics Data System (ADS)

    Dykeman, Eric; Sankey, Otto

    2008-03-01

    The low frequency mechanical modes of viruses can provide important insights into the large global motions that a virus may exhibit. Recently it has been proposed that these large global motions may be excited using impulsive stimulated Raman scattering producing permanent damage to the virus. In order to understand the coupling of external probes to the capsid, vibrational modes with atomic detail are essential. The standard approach to find the atomic modes of a molecule with N atoms requires the formation and diagonlization of a 3Nx3N matrix. As viruses have 10^5 or more atoms, the standard approach is difficult. Using ideas from electronic structure theory, we have developed a method to construct the mechanical modes of large molecules such as viruses with atomic detail. Application to viruses such as the cowpea chlorotic mottle virus, satellite tobacco necrosis virus, and M13 bacteriophage show a fairly complicated picture of the mechanical modes.

  12. Characterisation of wind farm infrasound and low-frequency noise

    NASA Astrophysics Data System (ADS)

    Zajamšek, Branko; Hansen, Kristy L.; Doolan, Con J.; Hansen, Colin H.

    2016-05-01

    This paper seeks to characterise infrasound and low-frequency noise (ILFN) from a wind farm, which contains distinct tonal components with distinguishable blade-pass frequency and higher harmonics. Acoustic measurements were conducted at dwellings in the vicinity of the wind farm and meteorological measurements were taken at the wind farm location and dwellings. Wind farm ILFN was measured frequently under stable and very stable atmospheric conditions and was also found to be dependent on the time of year. For noise character assessment, wind farm ILFN was compared with several hearing thresholds and also with the spectra obtained when the wind farm was not operating. Wind farm ILFN was found to exceed the audibility threshold at distances up to 4 km from the wind farm and to undergo large variations in magnitude with time.

  13. Analysis of Low Frequency Oscillations in Magnetron Injection Guns

    NASA Astrophysics Data System (ADS)

    Pu, Youlei; Luo, Yong; Yan, Ran; Liu, Guo; Jiang, Wei

    2012-02-01

    In our gyro-TWT experiments, low-frequency oscillations (LFOs) had been observed. LFOs is a physical phenomenon usually caused by the electrons trapped between the magnetron injection guns (MIGs) and the interaction region. In this paper, the formation procedure and physical mechanism of LFOs are reported. Available methods including optimizing the magnetic field distribution in the beam compression region and loading bevel cuts on the second anode are involved to capture the trapped electrons, suppress the LFOs and improve the helical electron beam quality. Simulations and experimental results are in good agreement with each other and also reveal the reasonableness of this means. Finally, the influence of current capture ratio on LFOs and the beam quality are studied. With the current capture ratio increasing, the amplitude of LFOs decreases, the pitch factor maintains a constant about 1.2 and we also demonstrate a low transverse velocity spread about 3%.

  14. Characterization of microstructure with low frequency electromagnetic techniques

    SciTech Connect

    Cherry, Matthew R.; Sathish, Shamachary; Pilchak, Adam L.; Blodgett, Mark P.; Cherry, Aaron J.

    2014-02-18

    A new computational method for characterizing the relationship between surface crystallography and electrical conductivity in anisotropic materials with low frequency electromagnetic techniques is presented. The method is discussed from the standpoint of characterizing the orientation of a single grain, as well as characterizing statistical information about grain ensembles in the microstructure. Large-area electron backscatter diffraction (EBSD) data was obtained and used in conjunction with a synthetic aperture approach to simulate the eddy current response of beta annealed Ti-6Al-4V. Experimental eddy current results are compared to the computed eddy current approximations based on electron backscatter diffraction (EBSD) data, demonstrating good agreement. The detectability of notches in the presence of noise from microstructure is analyzed with the described simulation method and advantages and limitations of this method are discussed relative to other NDE techniques for such analysis.

  15. Low-frequency Electrical Response to Microbial Induced Sulfide Precipitation

    SciTech Connect

    Ntarlagiannis, Dimitrios; Williams, Kenneth H.; Slater, Lee D.; Hubbard, Susan S.

    2005-11-19

    We investigated the sensitivity of low-frequency electrical measurements to microbeinduced metal sulfide precipitation. Three identical sand-packed monitoring columns were used; a geochemical column, an electrical column and a control column. In the first experiment, continuous upward flow of nutrients and metals in solution was established in each column. Cells of Desulfovibrio vulgaris (D. vulgaris) were injected into the center of the geochemical and electrical columns. Geochemical sampling and post-experiment destructive analysis showed that microbial induced sulfate reduction led to metal precipitation on bacteria cells, forming motile biominerals. Precipitation initially occurred in the injection zone, followed by chemotactic migration of D. vulgaris and ultimate accumulation around the nutrient source at the column base.

  16. Low-frequency microwave radiometer for N-ROSS

    NASA Technical Reports Server (NTRS)

    Hollinger, J. P.; Lo, R. C.

    1985-01-01

    The all weather, global determination of sea surface temperature (SST) has been identified as a requirement needed to support naval operations. The target SST accuracy is + or - 1.0 K with a surface resolution of 10 km. Investigations of the phenomenology and technology of remote passive microwave sensing of the ocean environment over the past decade have demonstrated that this objective is presently attainable. Preliminary specification and trade off studies were conducted to define the frequency, polarization, scan geometry, antenna size, and other esstential parameters of the low frequency microwave radiometer (LFMR). It will be a dual polarized, dual frequency system at 5.2 and 10.4 GHz using a 4.9 meter deployable mesh surface antenna. It is to be flown on the Navy-Remote Ocean Sensing System (N-ROSS) satellite scheduled to be launched in late 1988.

  17. Method for imaging with low frequency electromagnetic fields

    DOEpatents

    Lee, Ki H.; Xie, Gan Q.

    1994-01-01

    A method for imaging with low frequency electromagnetic fields, and for interpreting the electromagnetic data using ray tomography, in order to determine the earth conductivity with high accuracy and resolution. The imaging method includes the steps of placing one or more transmitters, at various positions in a plurality of transmitter holes, and placing a plurality of receivers in a plurality of receiver holes. The transmitters generate electromagnetic signals which diffuse through a medium, such as earth, toward the receivers. The measured diffusion field data H is then transformed into wavefield data U. The traveltimes corresponding to the wavefield data U, are then obtained, by charting the wavefield data U, using a different regularization parameter .alpha. for each transform. The desired property of the medium, such as conductivity, is then derived from the velocity, which in turn is constructed from the wavefield data U using ray tomography.

  18. Method for imaging with low frequency electromagnetic fields

    DOEpatents

    Lee, K.H.; Xie, G.Q.

    1994-12-13

    A method is described for imaging with low frequency electromagnetic fields, and for interpreting the electromagnetic data using ray tomography, in order to determine the earth conductivity with high accuracy and resolution. The imaging method includes the steps of placing one or more transmitters, at various positions in a plurality of transmitter holes, and placing a plurality of receivers in a plurality of receiver holes. The transmitters generate electromagnetic signals which diffuse through a medium, such as earth, toward the receivers. The measured diffusion field data H is then transformed into wavefield data U. The travel times corresponding to the wavefield data U, are then obtained, by charting the wavefield data U, using a different regularization parameter [alpha] for each transform. The desired property of the medium, such as conductivity, is then derived from the velocity, which in turn is constructed from the wavefield data U using ray tomography. 13 figures.

  19. Stochastic regimes in very-low-frequency fluidic oscillator

    NASA Astrophysics Data System (ADS)

    Tesař, Václav

    2016-03-01

    Paper discusses interesting unexpected stochastic regimes discovered in a fluidic oscillator designed for operation at very low oscillation frequencies - without the inconvenience of the long feedback loops needed in standard low-frequency oscillator designs. The new oscillator contains a pair of bistable turn-down active valves operating in anti-parallel — essentially analogous to Abraham & Bloch electric "multibrateur" invented in 1919. Three different self-excited oscillation regimes were found. In the order of increasing supplied flow rate, these regimes are characterised by: (A) generation of stochastic-duration multi-pulse packs, (B) generation of individual pulses with a degree of periodicity, and (C) regime with randomly appearing flow pulses separated by intervals of the order of seconds.

  20. Quantification of Low Frequency Magnetic Fields Generated by Household Appliances

    NASA Astrophysics Data System (ADS)

    Tanaka, Kenji; Mizuno, Yukio; Naito, Katsuhiko

    Resultant low frequency magnetic field generated was measured as a function of distance from them over broadband range (40-800Hz) and harmonic range (100-800Hz) on eleven kinds and more than two hundred recent household appliances in total. The relationship between magnetic filed measured and the power consumption of household appliances was also examined. As a result, it is verified that magnetic field from appliances is drastically reduced as the distance goes. And, any clear dependence of what on the power consumption of appliances is not recognized. Furthermore, from harmonic component analysis by using FFT for current flowing in appliances, it is assumed that relatively large amount of harmonic components with odd numbers is contained in the magnetic field.

  1. A very low frequency radio astronomy observatory on the Moon

    NASA Technical Reports Server (NTRS)

    Douglas, James N.; Smith, Harlan J.

    1988-01-01

    Because of terrestrial ionospheric absorption, very little is known of the radio sky beyond 10 m wavelength. An extremely simple, low cost very low frequency radio telescope is proposed, consisting of a large array of short wires laid on the lunar surface, each wire equipped with an amplifier and a digitizer, and connected to a common computer. The telescope could do simultaneous multifrequency observations of much of the visible sky with high resolution in the 10 to 100 m wavelength range, and with lower resolution in the 100 to 1000 m range. It would explore structure and spectra of galactic and extragalactic point sources, objects, and clouds, and would produce detailed quasi-three-dimensional mapping of interstellar matter within several thousand parsecs of the Sun.

  2. Multimode guidance project low frequency ECM simulator: Hardware description

    NASA Astrophysics Data System (ADS)

    Kaye, H. M.

    1982-10-01

    The Multimode Guidance(MMG) Project, part of the Army/Navy Area Defense SAM Technology Prototyping Program, was established to conduct a feasibility demonstration of multimode guidance concepts. Prototype guidance units for advanced, long range missiles are being built and tested under MMG Project sponsorship. The Johns Hopkins University Applied Physics Laboratory has been designated as Government Agent for countermeasures for this project. In support of this effort, a family of computer-controlled ECM simulators is being developed for validation of contractor's multimode guidance prototype designs. The design of the Low Frequency ECM Simulator is documented in two volumes. This report, Volume A, describes the hardware design of the simulator; Volume B describes the software design. This computer-controlled simulator can simulate up to six surveillance frequency jammers in B through F bands and will be used to evaluate the performance of home-on-jamming guidance modes in multiple jammer environments.

  3. Low Frequency Thermal Conductivity in Micro Phononic Crystals

    NASA Astrophysics Data System (ADS)

    Anjos, Virgilio; Arantes, Alison

    2015-03-01

    We study theoretically the cumulative thermal conductivity of a micro phononic crystal at low temperature regime. The phononic crystal considered presents carbon microtubes inclusions arranged periodically in a two-dimensional square lattice embebed in soft elastic matrix. Moderate and high impedance mismatch are considered concerning the material composition. The low frequency phonon spectra (up to tens of GHz) are obtained solving the generalized wave equation for inhomogeneous media within the Plane Wave Expansion method. We consider low temperatures in order to increase the participation of GHz thermal phonons. We observed suppression in the cumulative thermal conductivity at the band gap region and thus a reduction of thermal conductivity of the phononic crystal when compared with the bulk matrix. The authors would like to thank the Brazilian agencies, National Council of Technological and Scientific Development (CNPq), Foundation for Research Support of Minas Gerais (FAPEMIG) and CAPES for their support.

  4. Dielectric dispersion of Y-type hexaferrites at low frequencies

    NASA Astrophysics Data System (ADS)

    Abo El Ata, A. M.; Attia, S. M.

    2003-02-01

    A series of polycrystalline Y-type hexaferrites with composition Ba 2Ni 2- xZn xFe 12O 22 (where 0.0⩽ x⩽2.0) were prepared by the standard ceramic method to study the effect of the frequency, temperature and composition on their AC electrical conductivity σ' AC, and dielectric properties. It was found that, the AC conductivity shows dispersion at high frequencies. This dispersion was attributed to the interfacial polarization arising from the inhomogeneous structure of the material. At low frequencies the dielectric constant, ɛ', is abnormally high and decreases rapidly with increasing frequency. Dielectric relaxation peaks were observed on the tan δ( F) curves. The results of the dielectric constant and dielectric loss were explained on the basis of the assumption that the mechanism of dielectric polarization is similar to that of the conduction process.

  5. Modeling and investigative studies of Jovian low frequency emissions

    NASA Technical Reports Server (NTRS)

    Menietti, J. D.; Green, James L.; Six, N. Frank; Gulkis, S.

    1986-01-01

    Jovian decametric (DAM) and hectometric (HOM) emissions were first observed over the entire spectrum by the Voyager 1 and 2 flybys of the planet. They display unusual arc-like structures on frequency-versus-time spectrograms. Software for the modeling of the Jovian plasma and magnetic field environment was performed. In addition, an extensive library of programs was developed for the retrieval of Voyager Planetary Radio Astronomy (PRA) data in both the high and low frequency bands from new noise-free, recalibrated data tapes. This software allows the option of retrieving data sorted with respect to particular sub-Io longitudes. This has proven to be invaluable in the analyses of the data. Graphics routines were also developed to display the data on color spectrograms.

  6. The effect of islands on low frequency equatorial motions

    NASA Technical Reports Server (NTRS)

    Cane, M. A.; Du Penhoat, Y.

    1982-01-01

    A complete analytic solution is presented for the influence of equatorial islands on steady low-frequency waves. If the island is small (the meridional extent is much less than the equatorial radius of deformation, R), the waves pass it almost undisturbed, with the mass flux incident on the upstream side flowing around it nearly equally to the north and to the south and continuing on downstream in the lee of the island. For large islands (comparable in extent with R or larger), the principal response is organized as it would be if the island barrier were meridionally infinite. An incident Kelvin wave is largely reflected as long Rossby waves; symmetric long Rossby waves are reflected as equatorial Kelvin waves, while antisymmetric ones stop at the island barrier. In all cases, a boundary current composed of short Rossby waves forms at the eastern side of the island and accomplishes the required meridional redistribution of the zonal mass flux.

  7. Infrasonic and low-frequency insert earphone hearing threshold.

    PubMed

    Kuehler, Robert; Fedtke, Thomas; Hensel, Johannes

    2015-04-01

    Low-frequency and infrasonic pure-tone monaural hearing threshold data down to 2.5 Hz are presented. These measurements were made by means of a newly developed insert-earphone source. The source is able to generate pure-tone sound pressure levels up to 130 dB between 2 and 250 Hz with very low harmonic distortions. Behavioral hearing thresholds were determined in the frequency range from 2.5 to 125 Hz for 18 otologically normal test persons. The median hearing thresholds are comparable to values given in the literature. They are intended for stimulus calibration in subsequent brain imaging investigations. PMID:25920888

  8. Membrane-constrained acoustic metamaterials for low frequency sound insulation

    NASA Astrophysics Data System (ADS)

    Wang, Xiaole; Zhao, Hui; Luo, Xudong; Huang, Zhenyu

    2016-01-01

    We present a constrained membrane-type acoustic metamaterial (CMAM) that employs constraint sticks to add out-of-plane dimensions in the design space of MAM. A CMAM sample, which adopts constraint sticks to suppress vibrations at the membrane center, was fabricated to achieve a sound transmission loss (STL) peak of 26 dB at 140 Hz, with the static areal density of 6.0 kg/m2. The working mechanism of the CMAM as an acoustic metamaterial is elucidated by calculating the averaged normal displacement, the equivalent areal density, and the effective dynamic mass of a unit cell through finite element simulations. Furthermore, the vibration modes of the CMAM indicate that the eigenmodes related to STL dips are shifted into high frequencies, thus broadening its effective bandwidth significantly. Three samples possessing the same geometry and material but different constraint areas were fabricated to illustrate the tunability of STL peaks at low frequencies.

  9. Interaction of extremely low-frequency electromagnetic fields with humans

    SciTech Connect

    Tenforde, T.S.

    1990-04-01

    Public concern has grown in recent years concerning the possible health effects of extremely low-frequency (ELF) electromagnetic fields to which we are exposed in all aspects of everyday life. By definition ELF refers to the range of electromagnetic field frequencies below 300 Hz, which includes the power transmission and distribution frequencies used throughout the world. In materials with the electrical and magnetic properties of living tissues, these fields have a long wavelength (5000 m) and skin depth (150 m). As a consequence, in their interactions with humans and other living organisms ELF fields behave as though they are composed of independent electric and magnetic fields components. This paper discusses ELF fields and their interactions with humans and other living organisms as well as their biological effects.

  10. The Noisiness of Low Frequency Bands of Noise

    NASA Technical Reports Server (NTRS)

    Lawton, B. W.

    1975-01-01

    The relative noisiness of low frequency 1/3-octave bands of noise was examined. The frequency range investigated was bounded by the bands centered at 25 and 200 Hz, with intensities ranging from 50 to 95 db (SPL). Thirty-two subjects used a method of adjustment technique, producing comparison band intensities as noisy as 100 and 200 Hz standard bands at 60 and 72 db. The work resulted in contours of equal noisiness for 1/3-octave bands, ranging in intensity from approximately 58 to 86 db (SPL). These contours were compared with the standard equal noisiness contours; in the region of overlap, between 50 and 200 Hz, the agreement was good.

  11. Low Frequency Radiophysics of the Sun and Heliosphere

    NASA Astrophysics Data System (ADS)

    Bastian, T. S.

    2006-08-01

    The solar corona and heliosphere offer rich environments for exploration via low frequency radio techniques. Recent progress in observing and understanding both direct and indirect radio phenomena associated with flares and coronal mass ejections is discussed. In the case of flares, radio diagnostics of coronal energy release and particle acceleration are discussed. In the case of coronal mass ejections (CMEs), direct and indirect signatures of their initiation and acceleration are discussed. Magnetic field measurements of CMEs using direct measurements of their incoherent synchrotron radiation, or using observations of Faraday rotation via trans-illumination of the CME using background sidereal sources or spacecraft beacons, is also discussed. Finally, recent work on theory and observations of type II radio bursts, signatures of interplanetary shocks driven by CMEs , is briefly reviewed.

  12. Man-induced low-frequency seismic events in Italy

    NASA Astrophysics Data System (ADS)

    Latorre, Diana; Amato, Alessandro; Cattaneo, Marco; Carannante, Simona; Michelini, Alberto

    2014-12-01

    Unconventional seismic events in Italy are detected by scanning three years of continuous waveforms recorded by the Italian National Seismic Network. Cross correlation of signal templates with continuous seismic records has evidenced unusual events with similar low-frequency characteristics in several Italian regions. Spectral analysis and spatiotemporal distribution of these events, some of which are previously interpreted as tectonic long-period transients, suggest that they are not natural, but produced by huge cement factories. Since there are at least 57 full-cycle cement plants operating in Italy, each affecting areas of about 1250 to 2800 km2, we argue that significant portions of the Italian territory (23% to 51%) can be affected by this man-made noise. Seismic noise analyses, such as those used for microzonation or crustal structure investigations, as well as data mining techniques used to retrieve anomalous transient signals, should thus take into account this peculiar and pervasive source of seismic waves.

  13. Low-frequency oscillations of the level of enclosed sea

    NASA Astrophysics Data System (ADS)

    Korotaev, G. K.

    2015-07-01

    This work studies the variable surface level of a small enclosed basin that is related to the problem of interpreting satellite altimetric data, which assist in observations only of a deviation of a marine basin from unknown average condition needed to be calculated for the reconstruction of the sea-level topography. The reconstruction of unknown average condition becomes especially uncertain for the enclosed basins with significant level oscillations due to a variable water balance and requires the attraction of a priori physical concepts. This work reveals the general principles of response of the sea level to the low-frequency changes of the water exchange through the boundary of the basin with a rather arbitrary morphology.

  14. Low-frequency ac electro-flow-focusing microfluidic emulsification

    NASA Astrophysics Data System (ADS)

    He, Peng; Kim, Haejune; Luo, Dawei; Marquez, Manuel; Cheng, Zhengdong

    2010-04-01

    Applications of electric field, using either dc or high-frequency ac field, have shown many advantages in emulsification. We further develop this technique by a detailed study on low-frequency ac electro-flow-focusing (EFF) microfluidic emulsification. Counter-intuitively, the droplet size variation is not monotonic with the electric field, in contrary to the dc-EFF emulsification. This phenomenon originates from a relaxation oscillation of flow rate through the Taylor cone. Particularly, a continuous droplet size decrease was obtained at the voltage ramp-up stage. This emulsification process was modeled in analog to the accumulation and release of charges in an RC electric circuit with an adjustable resistor.

  15. Low-Frequency Waves in the Outer Heliosphere

    NASA Astrophysics Data System (ADS)

    Zank, G. P.; Hunana, P.; Goldstein, M. L.

    2013-12-01

    The outer heliosphere beyond some 10 AU is dominated by mass by interstellar neutral Hydrogen (H). Neutral H is coupled to the background solar wind plasma by charge exchange processes, which leads to the creation of a suprathermal pickup ion (PUI) population. The initially unstable ring beam PUI distribution is isotropized to form a filled shell distribution. The PUI contribution to the thermal solar wind pressure/temperature is sizable and in fact can dominate that of the colder thermal solar wind protons. Furthermore, the PUI distribution does not equilibrate or thermalize with the background solar wind plasma, and should therefore be regarded as a distinct distribution. Based on an elaboration of a three-fluid model, we investigate the effect of PUIs on the low-frequency wave properties of the outer heliosphere. To maintain tractability initially, we neglect the electron mass and obtain a 10th-order dispersion relation (compared to the 6th-order two-fluid dispersion relation). Our analysis reveals the existence of several wave modes that do not have a standard solar wind two-fluid plasma counterpart. For the zero angle case, of the 10 solutions, it is possible to obtain 6 analytic solutions for a general value of the ratio of electron number density to the thermal proton number density, four of which are dispersive Alfven waves (left and right polarized, two forward and two backward). Two other solutions are finite frequency modes, and the remaining four modes are magnetoacoustic modes (2 forward, 2 backward). One solution is a (magneto)acoustic wave with the usual 2-fluid sound speed, and the second is a (magneto)acoustic mode with a sound speed associated with the PUI distribution. Both (magneto)acoustic modes couple to the 'other fluids.' We present an analysis of the full dispersion relation for wave modes in the outer heliosphere, clarifying the role of PUIs in determining the properties of low-frequency fluctuations.

  16. Remote tracking of a magnetic receiver using low frequency beacons

    NASA Astrophysics Data System (ADS)

    Sheinker, Arie; Ginzburg, Boris; Salomonski, Nizan; Frumkis, Lev; Kaplan, Ben-Zion

    2014-10-01

    Low frequency magnetic fields feature high penetration ability, which allows communication, localization, and tracking in environments where radio or acoustic waves are blocked or distorted by multipath interferences. In the present work, we propose a method for tracking a magnetic receiver using beacons of low frequency magnetic field, where the receiver includes a tri-axial search-coil magnetometer. Measuring the beacons’ magnetic fields and calculating the total-field signals enables localization without restrictions on magnetometer orientation, allowing on-the-move tracking. The total-field signals are used by a global search method, e.g., simulated annealing (SA) algorithm, to localize the receiver. The magnetic field produced by each beacon has a dipole structure and is governed by the beacon’s position and magnetic moment. We have investigated two different methods for estimating beacons’ magnetic moments prior to localization. The first method requires directional measurements, whereas for the second method the total-field signal is used. Effectiveness of these methods has been proved in numerous field tests. In the present work, we introduce a method for tracking a moving receiver by successive localizations. Using previous localization as a starting point of the search method for the next localization can reduce execution time and chances for divergence. The proposed method has been tested using numerous computer simulations. Successful system operation has been verified in field conditions. The good tracking capability together with simple implementation makes the proposed method attractive for real-time, low power field applications, such as mobile robots navigation.

  17. Concealed weapons detection using low-frequency magnetic imaging

    NASA Astrophysics Data System (ADS)

    Zollars, Byron G.; Sallee, Bradley; Durrett, Michael G.; Cruce, Clay; Hallidy, William

    1997-02-01

    Military personnel, law-enforcement officers, and civilians face ever-increasing dangers from persons carrying concealed handguns and other weapons. In direct correspondence with this danger is a need for more sophisticated means of detecting concealed weapons. We have developed a novel concealed-weapons detector based on the principle of low- frequency magnetic imaging. The detector is configured as a portal, and constructs an image of electrically conductive objects transported through it with a potential spatial resolution of approximately 1 inch. Measurements on a breadboard version of the weapons detector have, to date, yielded a resolution of 2 inches. In operation, magnetic dipole radiation, emitted by transmitting antennas in the perimeter of the portal, is scattered from conductive objects and is picked up by receive antennas, also positioned around the portal. With sufficient measurements, each with a different geometry, a solution to the inverse scattering problem can be found. The result is an image of conductive objects in the detector. The detector is sensitive to all metals, semiconductors, and conductive composites. The measured conductivity image formed by the detector is combined with the video signal from a visible CCD camera to form a composite image of persons transiting the detector portal and the conductive objects they are carrying. Accompanying image recognition software could be used to determine the threat level of objects based upon shape, conductivity, and placement on the person of the carrier, and provide cueing, logging, or alarm functions to the operator if suspect weapons are identified. The low- power, low-frequency emissions from the detector are at levels considered safe to humans and medical implants..

  18. Numerical and experimental characterizations of low frequency MEMS AE sensors

    NASA Astrophysics Data System (ADS)

    Saboonchi, Hossain; Ozevin, Didem

    2013-04-01

    In this paper, new MEMS Acoustic Emission (AE) sensors are introduced. The transduction principle of the sensors is capacitance due to gap change. The sensors are numerically modeled using COMSOL Multiphysics software in order to estimate the resonant frequencies and capacitance values, and manufactured using MetalMUMPS process. The process includes thick metal layer (20 μm) made of nickel for freely vibration layer and polysilicon layer as the stationary layer. The metal layer provides a relatively heavy mass so that the spring constant can be designed high for low frequency sensor designs in order to increase the collapse voltage level (proportional to the stiffness), which increases the sensor sensitivity. An insulator layer is deposited between stationary layer and freely vibration layer, which significantly reduces the potential of stiction as a failure mode. As conventional AE sensors made of piezoelectric materials cannot be designed for low frequencies (<300 kHz) with miniature size, the MEMS sensor frequencies are tuned to 50 kHz and 200 kHz. The each sensor contained several parallel-connected cells with an overall size of approximately 250μm × 500 μm. The electromechanical characterizations are performed using high precision impedance analyzer and compared with the numerical results, which indicate a good fit. The initial mechanical characterization tests in atmospheric pressure are conducted using pencil lead break simulations. The proper sensor design reduces the squeeze film damping so that it does not require any vacuum packaging. The MEMS sensor responses are compared with similar frequency piezoelectric AE sensors.

  19. Biophysical characterization of low-frequency ultrasound interaction with dental pulp stem cells

    PubMed Central

    2013-01-01

    Background Low-intensity ultrasound is considered an effective non-invasive therapy to stimulate hard tissue repair, in particular to accelerate delayed non-union bone fracture healing. More recently, ultrasound has been proposed as a therapeutic tool to repair and regenerate dental tissues. Our recent work suggested that low-frequency kilohertz-range ultrasound is able to interact with dental pulp cells which could have potential to stimulate dentine reparative processes and hence promote the viability and longevity of teeth. Methods In this study, the biophysical characteristics of low-frequency ultrasound transmission through teeth towards the dental pulp were explored. We conducted cell culture studies using an odontoblast-like/dental pulp cell line, MDPC-23. Half of the samples underwent ultrasound exposure while the other half underwent ‘sham treatment’ where the transducer was submerged into the medium but no ultrasound was generated. Ultrasound was applied directly to the cell cultures using a therapeutic ultrasound device at a frequency of 45 kHz with intensity settings of 10, 25 and 75 mW/cm2 for 5 min. Following ultrasound treatment, the odontoblast-like cells were detached from the culture using a 0.25% Trypsin/EDTA solution, and viable cell numbers were counted. Two-dimensional tooth models based on μ-CT 2D images of the teeth were analyzed using COMSOL as the finite element analysis platform. This was used to confirm experimental results and to demonstrate the potential theory that with the correct combination of frequency and intensity, a tooth can be repaired using small doses of ultrasound. Frequencies in the 30 kHz–1 MHz range were analyzed. For each frequency, pressure/intensity plots provided information on how the intensity changes at each point throughout the propagation path. Spatial peak temporal average (SPTA) intensity was calculated and related to existing optimal spatial average temporal average (SATA) intensity deemed effective

  20. X-ray Thomson scattering in warm dense matter at low frequencies

    SciTech Connect

    Murillo, Michael S.

    2010-03-15

    The low-frequency portion of the x-ray Thomson scattering spectrum is determined by electrons that follow the slow ion motion. This ion motion is characterized by the ion-ion dynamic structure factor, which contains a wealth of information about the ions, including structure and collective modes. The frequency-integrated (diffraction) contribution is considered first. An effective dressed-particle description of warm dense matter is derived from the quantum Ornstein-Zernike equations, and this is used to identify a Yukawa model for warm dense matter. The efficacy of this approach is validated by comparing a predicted structure with data from the extreme case of a liquid metal; good agreement is found. A Thomas-Fermi model is then introduced to allow the separation of bound and free states at finite temperatures, and issues with the definition of the ionization state in warm dense matter are discussed. For applications, analytic structure factors are given on either side of the Kirkwood line. Finally, several models are constructed for describing the slow dynamics of warm dense matter. Two classes of models are introduced that both satisfy the basic sum rules. One class of models is the 'plasmon-pole'-like class, which yields the dispersion of ion-acoustic waves. Damping is then included via generalized hydrodynamics models that incorporate viscous contributions.

  1. X-ray Thomson scattering in warm dense matter at low frequencies.

    PubMed

    Murillo, Michael S

    2010-03-01

    The low-frequency portion of the x-ray Thomson scattering spectrum is determined by electrons that follow the slow ion motion. This ion motion is characterized by the ion-ion dynamic structure factor, which contains a wealth of information about the ions, including structure and collective modes. The frequency-integrated (diffraction) contribution is considered first. An effective dressed-particle description of warm dense matter is derived from the quantum Ornstein-Zernike equations, and this is used to identify a Yukawa model for warm dense matter. The efficacy of this approach is validated by comparing a predicted structure with data from the extreme case of a liquid metal; good agreement is found. A Thomas-Fermi model is then introduced to allow the separation of bound and free states at finite temperatures, and issues with the definition of the ionization state in warm dense matter are discussed. For applications, analytic structure factors are given on either side of the Kirkwood line. Finally, several models are constructed for describing the slow dynamics of warm dense matter. Two classes of models are introduced that both satisfy the basic sum rules. One class of models is the "plasmon-pole"-like class, which yields the dispersion of ion-acoustic waves. Damping is then included via generalized hydrodynamics models that incorporate viscous contributions. PMID:20365878

  2. X-ray Thomson scattering in warm dense matter at low frequencies

    NASA Astrophysics Data System (ADS)

    Murillo, Michael

    2010-11-01

    The low-frequency portion of the x-ray Thomson scattering spectrum is determined by electrons that follow the slow ion motion. This ion motion is characterized by the ion-ion dynamic structure factor, which contains a wealth of information about the ions, including structure and collective modes. The frequency-integrated (diffraction) contribution is considered first. An effective dressed-particle description of warm dense matter is derived from the quantum Ornstein-Zernike equations, and this is used to identify a Yukawa model for warm dense matter. The efficacy of this approach is validated by comparing a predicted structure factor with data for the extreme case of a liquid metal. A Thomas-Fermi model is then introduced to allow the separation of bound and free states at finite temperatures, and issues with the definition of the ionization state in warm dense matter are discussed. For applications, analytic structure factors are given on either side of the Kirkwood line. Finally, several models are constructed for describing the slow dynamics of warm dense matter. Two classes of models are introduced that both satisfy the basic sum rules. One class of models is the ``plasmon-pole''-like class, which yields the dispersion of ion-acoustic waves. Damping is then included via generalized hydrodynamics models that incorporate viscous contributions. This suggests a method by which viscous transport properties can be measured.

  3. X-ray Thomson scattering in warm dense matter at low frequencies

    NASA Astrophysics Data System (ADS)

    Murillo, Michael S.

    2010-03-01

    The low-frequency portion of the x-ray Thomson scattering spectrum is determined by electrons that follow the slow ion motion. This ion motion is characterized by the ion-ion dynamic structure factor, which contains a wealth of information about the ions, including structure and collective modes. The frequency-integrated (diffraction) contribution is considered first. An effective dressed-particle description of warm dense matter is derived from the quantum Ornstein-Zernike equations, and this is used to identify a Yukawa model for warm dense matter. The efficacy of this approach is validated by comparing a predicted structure with data from the extreme case of a liquid metal; good agreement is found. A Thomas-Fermi model is then introduced to allow the separation of bound and free states at finite temperatures, and issues with the definition of the ionization state in warm dense matter are discussed. For applications, analytic structure factors are given on either side of the Kirkwood line. Finally, several models are constructed for describing the slow dynamics of warm dense matter. Two classes of models are introduced that both satisfy the basic sum rules. One class of models is the “plasmon-pole”-like class, which yields the dispersion of ion-acoustic waves. Damping is then included via generalized hydrodynamics models that incorporate viscous contributions.

  4. Tidal modulation and triggering of low-frequency earthquakes in northern Cascadia

    NASA Astrophysics Data System (ADS)

    Royer, A. A.; Thomas, A. M.; Bostock, M. G.

    2015-01-01

    We analyze the influence of Earth and ocean tides on the triggering of low-frequency earthquakes (LFEs) in northern Cascadia using three LFE catalogs for southern Vancouver Island and Washington state from episodic tremor and slip events between 2003 and 2013. Sensitivities of LFE families to tidally induced fault normal stress, updip shear stress (UDSS), and corresponding time derivatives are computed and their geographic variability is mapped. We find localized areas showing higher sensitivity to UDSS than their surroundings, suggesting that tidal sensitivity depends on laterally heterogeneous physical properties such as variable pore fluid pressures and frictional properties along the plate interface. We observe that sensitivity of LFEs to UDSS rises dramatically from near zero on the first day of strong activity to a maximum ˜4 days later. In addition, the peak LFE rate transitions from a correlation with peak tidal shear stress rate to a correlation with peak tidal shear stress through large slow slip events. We identify 64 Rapid-Tremor-Reversals (RTRs) that start a few days after the main slip front. The RTRs have an average stress drop of ˜0.8 kPa and a majority (72%) occurs during periods of large positive UDSS. The combined observations imply that RTRs play an important role in slow slip processes and that modulation of creep rate due to tidal stress and tidal triggering of secondary events are jointly responsible for the observed tidal sensitivity.

  5. Scaled energy of deep low-frequency tremor in southwest Japan

    NASA Astrophysics Data System (ADS)

    Horino, K.; Hiramatsu, Y.; Mizukami, T.; Obara, K.; Matsuzawa, T.

    2014-12-01

    In southwestern Japan, deep low-frequency tremor occurs along the subduction zone of Philippine Sea plate. Sources of the tremor show a belt-like distribution of about 30-40 km in depth [Obara, 2002], where the transition from unstable to stable slip occurs at the plate interface. The ratio of radiated energy and seismic moment is called the scaled energy, and it has been often used for comparing the dynamic characteristics of small and large earthquakes. The scaled energy of slow earthquake is in the order of 10-11-10-9 [e.g. Ide et al., 2008; Maeda and Obara, 2009]. This value is about 5 orders smaller than regular earthquakes, indicating that the source process of slow earthquakes is different from that of regular earthquakes. We investigate here the scaled energy of tremor in southwestern Japan to understand the source process of the tremor. We analyze tremor recorded by NIED Hi-net, with Hybrid catalog [Maeda and Obara, 2009] and Hybrid Clustering catalog [Obara et al., 2010], from January 2001 to June 2011 in southwest Japan. We follow the procedure of Maeda and Obara [2009] to estimate the radiated energy of tremor. The values of the scaled energy of tremor are in the range of 10-8-10-6 and are smaller than those of regular earthquakes by order of 2. Multiplying the scaled energy by the rigidity provides that the apparent stress drop of tremor is lower than that of regular earthquakes. This result indicates that tremor occurs in the condition of lower-stress and strain than regular earthquake. Furthermore, the obtained value is 2 orders larger than the scaled energy of very low-frequency earthquake [Maeda and Obara, 2009]. These comparisons imply that each slip phenomenon of slow earthquake family has different dynamic characteristics. We recognize heterogeneous distribution of the scaled energy along the strike and the dip directions of the plate. Shallower events show larger value of the scaled energy. This indicates that the source process of the tremor

  6. Low-Frequency Electromagnetic Backscatter from Buried Tunnels

    SciTech Connect

    Casey, K; Pao, H

    2006-06-21

    This progress report is submitted under a contract between the Special Project Office of DARPA and Lawrence Livermore National Laboratory. The Project Manager at DARPA is Dr. Michael Zatman. Our purpose under this contract is to investigate interactions between electromagnetic waves and a class of buried targets located in multilayered media with rough interfaces. In this report, we investigate three preliminary problems. In each case our specific goal is to understand various aspects of the electromagnetic wave interaction mechanisms with targets in layered media. The first problem, discussed in Section 2, is that of low-frequency electromagnetic backscattering from a tunnel that is cut into a lossy dielectric half-space. In this problem, the interface between the upper (free space) region and the lower (ground) region is smooth. The tunnel is assumed to be a cylindrical free-space region of infinite extent in its axial direction and with a diameter that is small in comparison to the free-space wavelength. Because its diameter is small, the tunnel can be modeled as a buried ''wire'' described by an equivalent impedance per unit length. In Section 3 we extend the analysis to include a statistically rough interface between the air and ground regions. The interface is modeled as a random-phase screen. Such a screen reduces the coherent power in a plane wave that is transmitted through it, scattering some of the total power into an incoherent field. Our analysis of this second problem quantifies the reduction in the coherent power backscattered from the buried tunnel that is caused by the roughness of the air-ground interface. The problem of low-frequency electromagnetic backscattering from two buried tunnels, parallel to each other but at different locations in the ground, is considered in Section 4. In this analysis, we wish to determine the conditions under which the presence of more than one tunnel can be detected via backscattering. Section 5 concludes the report

  7. Recreational scuba divers' aversion to low-frequency underwater sound.

    PubMed

    Fothergill, D M; Sims, J R; Curley, M D

    2001-01-01

    Increasing use of active low-frequency sonar by submarines and ships raises the risk of accidental exposure of recreational divers to low-frequency underwater sound (LFS). This study aimed to characterize the subjective responses of recreational scuba divers to LFS to ascertain the extent to which LFS may impact their enjoyment, comfort, or time spent underwater. Seventeen male and nine female recreational scuba divers participated. Diving was conducted in an acoustically transparent tank located within a larger anechoic pool. Subjects wore scuba gear and were positioned I m below the surface in a prone position. The sound transducer was located 4 m directly below the diver's head. Sound exposures consisted of three signal types (pure tone, 30 Hz hyperbolic sweep up, and 30 Hz hyperbolic sweep down) each presented at six center frequencies from 100 to 500 Hz and six sound pressure levels(SPL) ranging from 130 to 157 dB re 1 microPa. The duration of each sound exposure was 7 s. Subjects responded via an underwater console to rate aversion to LFS on a category-ratio scale, and to indicate the presence or absence of vibration of any body part. Aversion to LFS and the percent incidence of vibration increased as the SPL increased. The percent incidence of vibration decreased linearly with increasing frequency. At the highest SPL the probability that an aversion rating would exceed Very Severe (7 on the category-ratio scale) was predicted to be 19%. There was no significant difference in aversion among signal types. The 100 Hz frequency was the most aversive frequency (P < 0.05). A plot of aversion vs. frequency showed a U-shaped function with minimum aversion at 250 Hz. In conclusion, diver aversion to LFS is dependent upon SPL and center frequency. The highest aversion rating was given for 100 Hz, this frequency corresponded with the greatest probability of detecting vibration. Factors other than vibration seem to account for aversion to the highest frequencies. Our

  8. Long-lasting hyperpolarization underlies seizure reduction by low frequency deep brain electrical stimulation

    PubMed Central

    Toprani, Sheela; Durand, Dominique M

    2013-01-01

    Mesial temporal lobe epilepsy (MTLE) is a common medically refractory neurological disease. Deep brain electrical stimulation (DBS) of grey matter has been used for MTLE with limited success. However, stimulation of a white matter tract connecting the hippocampi, the ventral hippocampal commissure (VHC), with low frequencies that simulate interictal discharges has shown promising results, with seizure reduction greater than 98% in bilateral hippocampi during stimulation and greater than 50% seizure reduction in bilateral hippocampi after treatment. A major hurdle to the implementation and optimization of this treatment is that the mechanisms of seizure reduction by low frequency electrical stimulation (LFS) are not known. The goal of this study is to understand how commissural fibre tract stimulation reduces bilateral hippocampal epileptic activity in an in vitro slice preparation containing bilateral hippocampi connected by the VHC. It is our hypothesis that electrical stimuli induce hyperpolarization lasting hundreds of milliseconds following each pulse which reduces spontaneous epileptic activity during each inter-stimulus interval (ISI). Stimulus-induced long-lasting-hyperpolarization (LLH) can be mediated by GABAB inhibitory post-synaptic potentials (IPSPs) or slow after-hyperpolarization (sAHP). To test the role of LLH in effective bilateral seizure reduction by fibre tract stimulation, we measured stimulus-induced hyperpolarization during LFS of the VHC using electrophysiology techniques. Antagonism of the GABAB IPSP and/or sAHP diminished stimulus-induced hyperpolarization concurrently with LFS efficacy (greater than 50% reduction). Blocking both the GABAB IPSP and sAHP simultaneously eliminated the effect of electrical stimulation on seizure reduction entirely. These data show that LFS of the VHC is an effective protocol for bilateral hippocampal seizure reduction and that its efficacy relies on the induction of long-lasting hyperpolarization mediated

  9. Ocean seismo-acoustics. Low-frequency underwater acoustics

    SciTech Connect

    Akal, T.; berkson, J.M.

    1986-01-01

    This book presents information on seismo-acoustic propagation in seawater and sea beds that includes theoretical developments, modelling and experiments, and fluctuations. Boundary scatteiring, seismo-acoustic waves and seismo-acoustic noise are discussed. Technology and new approaches in seismo-acoustic measurements are presented.

  10. Ultra low frequency waves at the Earth's bow shock

    NASA Technical Reports Server (NTRS)

    Russell, C. T.; Farris, M. H.

    1995-01-01

    The Earth's bow shock is a bountiful generator of waves. Some of these waves have group velocities that exceed the solar wind velocity directed into the shock and can propagate upstream against the flow. Upstream whistlers observed close to one Hertz in the spacecraft frame have been seen many Earth radii upstream. A second whistler mode wave, called the precursor, propagates upstream along the shock normal but is phase standing in the solar wind flow. The damping of both whistler mode waves is consistent with Landau damping. At low Mach numbers the precursor is connected to the non-coplanarity component in the shock ramp. At higher Mach numbers the upstream waves cannot propagate upstream and ion reflection becomes more important in providing free energy for wave particle interactions. The non-coplanarity component is still present but it now initiates a downstream wave train. Generally the waves just downstream from the bow shock are left hand circularly polarized ion cyclotron waves propagating along the magnetic field at the Alfven velocity. When the upstream Mach number is high and the helium content of the plasma is high, mirror mode waves are observed.

  11. Amplitude of Low-Frequency Fluctuations in Multiple-Frequency Bands in Acute Mild Traumatic Brain Injury

    PubMed Central

    Zhan, Jie; Gao, Lei; Zhou, Fuqing; Bai, Lijun; Kuang, Hongmei; He, Laichang; Zeng, Xianjun; Gong, Honghan

    2016-01-01

    Functional disconnectivity during the resting state has been observed in mild traumatic brain injury (mTBI) patients during the acute stage. However, it remains largely unknown whether the abnormalities are related to specific frequency bands of the low-frequency oscillations (LFO). Here, we used the amplitude of low-frequency fluctuations (ALFF) to examine the amplitudes of LFO in different frequency bands (slow-5: 0.01–0.027 Hz; slow-4: 0.027–0.073 Hz; and typical: 0.01–0.08 Hz) in patients with acute mTBI. A total of 24 acute mTBI patients and 24 age-, sex-, and education-matched healthy controls participated in this study. In the typical band, acute mTBI patients showed lower standardized ALFF in the right middle frontal gyrus and higher standardized ALFF in the right lingual/fusiform gyrus and left middle occipital gyrus. Further analyses showed that the difference between groups was concentrated in a narrower (slow-4) frequency band. In the slow-5 band, mTBI patients only exhibited higher standardized ALFF in the occipital areas. No significant correlation between the mini-mental state examination score and the standardized ALFF value was found in any brain region in the three frequency bands. Finally, no significant interaction between frequency bands and groups was found in any brain region. We concluded that the abnormality of spontaneous brain activity in acute mTBI patients existed in the frontal lobe as well as in distributed brain regions associated with integrative, sensory, and emotional roles, and the abnormal spontaneous neuronal activity in different brain regions could be better detected by the slow-4 band. These findings might contribute to a better understanding of local neural psychopathology of acute mTBI. Future studies should take the frequency bands into account when measuring intrinsic brain activity of mTBI patients. PMID:26869907

  12. The application of low frequency longitudinal guided wave mode for the inspection of multi-hole steel floral pipes

    NASA Astrophysics Data System (ADS)

    Liu, Z. H.; Xie, X. D.; Wu, B.; Li, Y. H.; He, C. F.

    2012-03-01

    Shed-pipe grouting technology, an effective advanced supporting method, is often used in the excavation of soft strata. Steel floral pipes are one of the key load-carrying components of shed-pipe grouting supporting structures. Guided waves are a very attractive methodology to inspect multi-hole steel floral pipes as they offer long range inspection capability, mode and frequency tuning, and cost effectiveness. In this contribution, preliminary experiments are described for the inspection of steel floral pipes using a low frequency longitudinal guided wave mode, L(0,2). The relation between the number of grouting holes and the peak-to-peak amplitude of the first end-reflected signal was obtained. The effect of the grouting holes in steel floral pipes on the propagation velocity of the L(0,2) mode at 30 kHz was analyzed. Experimental results indicate that the typical grouting holes in steel floral pipe have no significant effect on the propagation of this mode. As a result, low frequency longitudinal guided wave modes have potential for the non-destructive long range inspection of multi-hole steel floral pipes. Furthermore, the propagation velocity of the investigated L(0,2) mode at 30 kHz decreases linearly with the increase of the number of grouting holes in a steel floral pipe. It is also noticeable that the effect of the grouting holes cumulates along with the increase in the number of grouting holes and subsequent increase in reflection times of longitudinal guided waves in the steel floral pipe. The application potential of the low frequency longitudinal guided wave technique for the inspection of embedded steel floral pipes is discussed.

  13. Progress on the Low Frequency All Sky Monitor

    NASA Astrophysics Data System (ADS)

    Murray, James; Jenet, Fredrick; Craig, Joseph; Creighton, Teviet David; Percy Dartez, Louis; Ford, Anthony J.; Hernandez, Andrés; Hicks, Brian; Hinojosa, Jesus; Jaramillo, Ricardo; Kassim, Namir E.; Lazio, Joseph; Lunsford, Grady; Miller, Rossina B.; Ray, Paul S.; Rivera, Jesus; Taylor, Gregory B.; Teitelbaum, Lawrence; CenterAdvanced Radio Astronomy, University of Texas at Brownsville, University of New Mexico, Naval Research Laboratory, Jet Propulsion Laborator

    2015-01-01

    The Low Frequency All Sky Monitor (LoFASM) is a system of geographically separated radio arrays dedicated to the study of radio transients. LoFASM consists of four stations, each comprised of 12 cross-dipole antennas designed to operate between 10-88MHz. The antennas and front end electronics for LoFASM were designed by the Naval Research Laboratory for the Long Wavelength Array (LWA) project (cf. Hicks et al. PASP 124, 1090 (2012)). All four stations are currently operational and in the commissioning stage . Over the last 3 years, undergraduate and graduate students from the University of Texas at Brownsville's Center for Advanced Radio Astronomy have been establishing these stations around the continental US, consisting of sites located in Port Mansfield, Texas, the LWA North Arm site of the LWA1 Radio Observatory in New Mexico, adjacent to the North Arm of the Very Large Array, the Green Bank Radio Observatory, West Virginia, and the Goldstone Deep Space Communications Complex, California. In combination with the establishment of these sites was the development of the analog hardware, which consists of custom RF splitter/combiners and a custom amplifier and filter chain designed at Center for Advanced Radio Astronomy (CARA). This poster will expound on progress in site installation and the development of the analog signal chain, specifically the redesigned analog receiving system.

  14. Planck early results. V. The Low Frequency Instrument data processing

    NASA Astrophysics Data System (ADS)

    Zacchei, A.; Maino, D.; Baccigalupi, C.; Bersanelli, M.; Bonaldi, A.; Bonavera, L.; Burigana, C.; Butler, R. C.; Cuttaia, F.; de Zotti, G.; Dick, J.; Frailis, M.; Galeotta, S.; González-Nuevo, J.; Górski, K. M.; Gregorio, A.; Keihänen, E.; Keskitalo, R.; Knoche, J.; Kurki-Suonio, H.; Lawrence, C. R.; Leach, S.; Leahy, J. P.; López-Caniego, M.; Mandolesi, N.; Maris, M.; Matthai, F.; Meinhold, P. R.; Mennella, A.; Morgante, G.; Morisset, N.; Natoli, P.; Pasian, F.; Perrotta, F.; Polenta, G.; Poutanen, T.; Reinecke, M.; Ricciardi, S.; Rohlfs, R.; Sandri, M.; Suur-Uski, A.-S.; Tauber, J. A.; Tavagnacco, D.; Terenzi, L.; Tomasi, M.; Valiviita, J.; Villa, F.; Zonca, A.; Banday, A. J.; Barreiro, R. B.; Bartlett, J. G.; Bartolo, N.; Bedini, L.; Bennett, K.; Binko, P.; Borrill, J.; Bouchet, F. R.; Bremer, M.; Cabella, P.; Cappellini, B.; Chen, X.; Colombo, L.; Cruz, M.; Curto, A.; Danese, L.; Davies, R. D.; Davis, R. J.; de Gasperis, G.; de Rosa, A.; de Troia, G.; Dickinson, C.; Diego, J. M.; Donzelli, S.; Dörl, U.; Efstathiou, G.; Enßlin, T. A.; Eriksen, H. K.; Falvella, M. C.; Finelli, F.; Franceschi, E.; Gaier, T. C.; Gasparo, F.; Génova-Santos, R. T.; Giardino, G.; Gómez, F.; Gruppuso, A.; Hansen, F. K.; Hell, R.; Herranz, D.; Hovest, W.; Huynh, M.; Jewell, J.; Juvela, M.; Kisner, T. S.; Knox, L.; Lähteenmäki, A.; Lamarre, J.-M.; Leonardi, R.; León-Tavares, J.; Lilje, P. B.; Lubin, P. M.; Maggio, G.; Marinucci, D.; Martínez-González, E.; Massardi, M.; Matarrese, S.; Meharga, M. T.; Melchiorri, A.; Migliaccio, M.; Mitra, S.; Moss, A.; Nørgaard-Nielsen, H. U.; Pagano, L.; Paladini, R.; Paoletti, D.; Partridge, B.; Pearson, D.; Pettorino, V.; Pietrobon, D.; Prézeau, G.; Procopio, P.; Puget, J.-L.; Quercellini, C.; Rachen, J. P.; Rebolo, R.; Robbers, G.; Rocha, G.; Rubiño-Martín, J. A.; Salerno, E.; Savelainen, M.; Scott, D.; Seiffert, M. D.; Silk, J. I.; Smoot, G. F.; Sternberg, J.; Stivoli, F.; Stompor, R.; Tofani, G.; Toffolatti, L.; Tuovinen, J.; Türler, M.; Umana, G.; Vielva, P.; Vittorio, N.; Vuerli, C.; Wade, L. A.; Watson, R.; White, S. D. M.; Wilkinson, A.

    2011-12-01

    We describe the processing of data from the Low Frequency Instrument (LFI) used in production of the Planck Early Release Compact Source Catalogue (ERCSC). In particular, we discuss the steps involved in reducing the data from telemetry packets to cleaned, calibrated, time-ordered data (TOD) and frequency maps. Data are continuously calibrated using the modulation of the temperature of the cosmic microwave background radiation induced by the motion of the spacecraft. Noise properties are estimated from TOD from which the sky signal has been removed using a generalized least square map-making algorithm. Measured 1/f noise knee-frequencies range from ~100 mHz at 30 GHz to a few tens of mHz at 70GHz. A destriping code (Madam) is employed to combine radiometric data and pointing information into sky maps, minimizing the variance of correlated noise. Noise covariance matrices required to compute statistical uncertainties on LFI and Planck products are also produced. Main beams are estimated down to the ≈-10dB level using Jupiter transits, which are also used for geometrical calibration of the focal plane. Corresponding author: A. Zacchei, e-mail: zacchei@oats.inaf.it

  15. Unusual Low-frequency Magnetic Perturbations in TFTR

    SciTech Connect

    H. Takahashi; E.D. Fredrickson; M.S. Chance

    2001-02-12

    Low-frequency magnetic perturbations (less than or equal to 30 kHz) observed in the Tokamak Fusion Test Reactor (TFTR) tokamak do not always conform to expectations from Magneto-Hydro-Dynamic (MHD) modes. The discrepancy between observations and expectations arises from the existence of three classes of magnetic perturbations in TFTR: (1) 'Edge Originated Magnetic Perturbations' (EOMP's), (2) 'Kink-like Modes' (KLM's), and (3) Tearing Modes (TM's). The EOMP class has unusual magnetic phenomenon including up/down asymmetry in poloidal intensity variation that MHD modes alone cannot generate. The contributions of MHD modes in plasma edge regions are too small to explain the magnitude of observed EOMP perturbations. At least two-thirds, possibly nearly all, of magnetic perturbations in a typical EOMP originate from sources other than MHD modes. An EOMP has a unity toroidal harmonic number and a poloidal harmonic number close to a discharge's edge q-value. It produces little temperature fluctuations, except possibly in edge regions. The KLM class produces temperature fluctuations, mostly confined within the q=1 surface with an ideal-mode-like structure, but generates little external magnetic perturbations. The TM class conforms generally to expectations from MHD modes. We propose that current flowing in the Scrape-off-layer (SOL) plasma is a possible origin of EOMP's.

  16. Low-frequency dielectric dispersion of bacterial cell suspensions.

    PubMed

    Asami, Koji

    2014-07-01

    Dielectric spectra of Escherichia coli cells suspended in 0.1-10 mM NaCl were measured over a frequency range of 10 Hz to 10 MHz. Low-frequency dielectric dispersion, so-called the α-dispersion, was found below 10 kHz in addition to the β-dispersion, due to interfacial polarization, appearing above 100 kHz. When the cells were killed by heating at 60°C for 30 min, the β-dispersion disappeared completely, whereas the α-dispersion was little influenced. This suggests that the plasma (or inner) membranes of the dead cells are no longer the permeability barrier to small ions, and that the α-dispersion is not related to the membrane potential due to selective membrane permeability of ions. The intensity of the α-dispersion depended on both of the pH and ionic strength of the external medium, supporting the model that the α-dispersion results from the deformation of the ion clouds formed outside and inside the cell wall containing charged residues. PMID:24835050

  17. Low-frequency combustion oscillations in a model afterburner

    SciTech Connect

    Macquisten, M.A.; Dowling, A.P. )

    1993-08-01

    Low-frequency combustion oscillations, involving the interaction between longitudinal acoustic waves and unsteady combustion, are investigated for a model afterburner. An experimental rig, in which a confined flame is stabilized in the wake of a conical gutter, is run with inlet conditions representative of an engine afterburner. Results are presented for inlet Mach numbers in the range of 0.15--0.27, with inlet temperatures up to 630 K. Comparison is made between theory and experiment. Although the theory was developed from low Mach number data, it is found to apply equally well at these faster flow rates. The theory is able to predict the frequency of the instability and the mode shape, accurately reproducing the changes due to variations in the inlet Mach number and temperature. The effect of altering the downstream boundary condition by replacing the open end by a choked nozzle is also investigated. Such a change is found to be highly destabilizing, both experimentally and theoretically. Again, predictions from the theory are in good agreement with the observations.

  18. Extremely Low Frequency Signal Acquisition, Recording and Analysis

    NASA Astrophysics Data System (ADS)

    Zantis, Franz P.; Hribik, Ján; Ďuračková, Daniela

    2016-05-01

    Our environment is permeated by electrical and magnetic alternating waves in the frequency range above the AC voltage of 50 Hz and also in the radio frequency range. Much attention from the public is given to these waves. Through numerous studies and publications about this type of oscillations and waves it is largely known from which sources they occur and which impact they have. However, very little information could be found about electrical and magnetic alternating waves in the frequency range below 50 Hz. The aim of this research is to demonstrate that these signals exist and also to show how the signals look like and where and when they occur. This article gives an overview of the occurrence of these ELF (Extremely Low Frequencies) signals, their specific properties in view of the time domain and in view of the frequency domain and of the possible sources of these waves. Precise knowledge of the structures of the ELF signals allows conclusions about their potential to cause electromagnetic interference in electronic systems. Also other effects in our environment, eg on flora and fauna could be explained.

  19. Low-frequency meandering piezoelectric vibration energy harvester.

    PubMed

    Berdy, David F; Srisungsitthisunti, Pornsak; Jung, Byunghoo; Xu, Xianfan; Rhoads, Jeffrey F; Peroulis, Dimitrios

    2012-05-01

    The design, fabrication, and characterization of a novel low-frequency meandering piezoelectric vibration energy harvester is presented. The energy harvester is designed for sensor node applications where the node targets a width-to-length aspect ratio close to 1:1 while simultaneously achieving a low resonant frequency. The measured power output and normalized power density are 118 μW and 5.02 μW/mm(3)/g(2), respectively, when excited by an acceleration magnitude of 0.2 g at 49.7 Hz. The energy harvester consists of a laser-machined meandering PZT bimorph. Two methods, strain-matched electrode (SME) and strain-matched polarization (SMP), are utilized to mitigate the voltage cancellation caused by having both positive and negative strains in the piezoelectric layer during operation at the meander's first resonant frequency. We have performed finite element analysis and experimentally demonstrated a prototype harvester with a footprint of 27 x 23 mm and a height of 6.5 mm including the tip mass. The device achieves a low resonant frequency while maintaining a form factor suitable for sensor node applications. The meandering design enables energy harvesters to harvest energy from vibration sources with frequencies less than 100 Hz within a compact footprint. PMID:22622969

  20. A Statistical Approach to Autocorrelation Detection of Low Frequency Earthquakes

    NASA Astrophysics Data System (ADS)

    Aguiar, A. C.; Beroza, G. C.

    2012-12-01

    We have analyzed tremor data during the April, 2006 tremor episode in the Nankai Trough in SW Japan using the auto-correlation approach of Brown et al. (2008), which detects low frequency earthquakes (LFEs) based on pair-wise matching. We have found that the statistical behavior of the autocorrelations of each station is different and for this reason we have based our LFE detection method on the autocorrelation of each station individually. Analyzing one station at a time assures that the detection threshold will only depend on the station being analyzed. Once detections are found on each station individually, using a low detection threshold based on a Gaussian distribution of the correlation coefficients, the results are compared within stations and declared a detection if they are found in a statistically significant number of the stations, following multinomial statistics. We have compared our detections using the single station method to the detections found by Shelly et al. (2007) for the 2006 April 16 events and find a significant number of similar detections as well as many new detections that were not found using templates from known LFEs. We are working towards developing a sound statistical basis for event detection. This approach should improve our ability to detect LFEs within weak tremor signals where they are not already identified, and should be applicable to earthquake swarms and sequences in general.

  1. Extremely Low Frequency Magnetic Field Modulates the Level of Neurotransmitters

    PubMed Central

    Chung, Yoon Hee; Lee, Young Joo; Lee, Ho Sung; Chung, Su Jin; Lim, Cheol Hee; Oh, Keon Woong; Sohn, Uy Dong

    2015-01-01

    This study was aimed to observe that extremely low frequency magnetic field (ELF-MF) may be relevant to changes of major neurotransmitters in rat brain. After the exposure to ELF-MF (60 Hz, 2.0 mT) for 2 or 5 days, we measured the levels of biogenic amines and their metabolites, amino acid neurotransmitters and nitric oxide (NO) in the cortex, striatum, thalamus, cerebellum and hippocampus. The exposure of ELF-MF for 2 or 5 days produced significant differences in norepinephrine and vanillyl mandelic acid in the striatum, thalamus, cerebellum and hippocampus. Significant increases in the levels of serotonin and 5-hydroxyindoleacetic acid were also observed in the striatum, thalamus or hippocampus. ELF-MF significantly increased the concentration of dopamine in the thalamus. ELF-MF tended to increase the levels of amino acid neurotransmitters such as glutamine, glycine and γ -aminobutyric acid in the striatum and thalamus, whereas it decreased the levels in the cortex, cerebellum and hippocampus. ELF-MF significantly increased NO concentration in the striatum, thalamus and hippocampus. The present study has demonstrated that exposure to ELF-MFs may evoke the changes in the levels of biogenic amines, amino acid and NO in the brain although the extent and property vary with the brain areas. However, the mechanisms remain further to be characterized. PMID:25605992

  2. WFC3 Low-Frequency Flat Field Corrections

    NASA Astrophysics Data System (ADS)

    Mack, Jennifer

    2010-07-01

    Multiple dithered observations of the globular cluster Omega Centauri (NGC 5139) have been used to measure inflight corrections to the WFC3 UVIS and IR ground flat fields for a subset of key filters. To obtain an adequate characterization of the flat field over the detector field of view (FOV), 9 pointings were obtained for each filter using a 3x3 box dither pattern with steps of approximately 25% of the FOV. By measuring relative changes in the brightness of a star over different portions of the detector, low-frequency spatial variations in the detector response (L-flats) have been used to correct the flat fields obtained during ground testing. The broad wavelength range covered by these observations allow an interpolation of the L-flat correction for the remaining wide, medium and narrow-band filters, assuming a simple linear dependence with pivot wavelength. Initial results indicate that the required L-flat corrections are ±1.5% (standard deviation) in the IR and ±1.0% in the UVIS, and that the photometric response for a given star after applying the L-flat correction is now stable to better than 1% for any position in the field of view. Followup observations of the same field at multiple orientations will be used to verify the accuracy of the L-flat solutions and to quantify any temporal changes in the detector response while in orbit.

  3. Progress on the Low Frequency All Sky Monitor

    NASA Astrophysics Data System (ADS)

    Ford, Anthony; Jenet, F.; Craig, J.; Creighton, T. D.; Dartez, L. P.; Hicks, B.; Hinojosa, J.; Jaramillo, R.; Kassim, N. E.; Lunsford, G.; Miller, R. B.; Murray, J.; Ray, P. S.; Rivera, J.; Taylor, G. B.

    2013-01-01

    The Low Frequency All Sky Monitor is a system of geographically separated radio arrays dedicated to the study of radio transients. LoFASM consists of four stations, each comprised of 12 cross-dipole antennas designed to operate between 5-88MHz. The antennas and front end electronics for LoFASM were designed by the Naval Research Laboratory for the Long Wavelength Array project. Over the last year, undergraduate students from the University of Texas at Brownsville’s Center for Advanced Radio Astronomy have been establishing these stations around the continental US, consisting of sites located in Port Mansfield, Texas, the LWA North Arm site of the LWA1 Radio Observatory in New Mexico, adjacent to the North Arm of the Very Large Array, the Green Bank Radio Observatory, West Virginia, and NASA’s Goldstone tracking complex in California. In combination with the establishment of these sites was the development of the analog hardware, which consists of commercial off-the-shelf RF splitter/combiners and a custom amplifier and filter chain designed by colleagues at the University of New Mexico. This poster will expound on progress in site installation and development of the analog signal chain.

  4. Low-frequency nuclear quadrupole resonance with a dc SQUID

    SciTech Connect

    Chang, J.W.

    1991-07-01

    Conventional pure nuclear quadrupole resonance (NQR) is a technique well suited for the study of very large quadrupolar interactions. Numerous nuclear magnetic resonance (NMR) techniques have been developed for the study of smaller quadrupolar interactions. However, there are many nuclei which have quadrupolar interactions of intermediate strength. Quadrupolar interactions in this region have traditionally been difficult or unfeasible to detect. This work describes the development and application of a SQUID NQR technique which is capable of measuring intermediate strength quadrupolar interactions, in the range of a few hundred kilohertz to several megahertz. In this technique, a dc SQUID (Superconducting QUantum Interference Device) is used to monitor the longitudinal sample magnetization, as opposed to the transverse magnetization, as a rf field is swept in frequency. This allows the detection of low-frequency nuclear quadrupole resonances over a very wide frequency range with high sensitivity. The theory of this NQR technique is discussed and a description of the dc SQUID system is given. In the following chapters, the spectrometer is discussed along with its application to the study of samples containing half-odd-integer spin quadrupolar nuclei, in particular boron-11 and aluminum-27. The feasibility of applying this NQR technique in the study of samples containing integer spin nuclei is discussed in the last chapter. 140 refs., 46 figs., 6 tabs.

  5. A Study of Low Frequency Earthquakes in Washington State

    NASA Astrophysics Data System (ADS)

    Royer, A. A.; Bostock, M. G.

    2012-12-01

    In the past decade, studies have shown that non-volcanic tremor in southwest Japan can be explained by swarms of low frequency earthquakes (LFEs). More recently LFEs have been identified and documented in southern Vancouver Island. Here we extend study of LFEs to Washington state by exploiting data from the IRIS-PASCAL CAFE experiment. We use network autocorrelation to detect LFEs within 4 different tremor episodes (2007,2008,2010 and 2011). We separate the dataset into 2 components, one involving stations between the Olympic Peninsula and Puget Sound (North Group) and the other involving stations to the south of Puget Sound (South Group). For the South Group, only the 2007 and 2008 tremor episodes are available. From an initial 4915 (North Group) and 3306 (South Group) detections of LFEs using a set of 7 3-components stations, we use iterative network cross-correlation to register further detections and stack to improve signal-to-noise ratio of LFE family templates. On the basis of progress to date, we anticipate the assembly of ~150 and ~50 LFE family templates for the North and South Groups, respectively. We plan to locate these LFE family templates, determine representative focal mechanisms and investigate LFE occurrence relative to regular seismicity for comparison with previous results from southern Vancouver Island and southwest Japan.

  6. Extremely low frequency electromagnetic fields and cancer: the epidemiologic evidence.

    PubMed Central

    Bates, M N

    1991-01-01

    This paper reviews the epidemiologic evidence that low frequency electromagnetic fields generated by alternating current may be a cause of cancer. Studies examining residential exposures of children and adults and studies of electrical and electronics workers are reviewed. Using conventional epidemiologic criteria for inferring causal associations, including strength and consistency of the relationship, biological plausibility, and the possibility of bias as an explanation, it is concluded that the evidence is strongly suggestive that such radiation is carcinogenic. The evidence is strongest for brain and central nervous system cancers in electrical workers and children. Weaker evidence supports an association with leukemia in electrical workers. Some evidence also exists for an association with melanoma in electrical workers. Failure to find consistent evidence of a link between residential exposures and adult cancers may be attributable to exposure misclassification. Studies so far have used imperfect surrogates for any true biologically effective magnetic field exposure. The resulting exposure misclassification has produced relative risk estimates that understate any true risk. PMID:1821368

  7. Energy harvesting from low frequency applications using piezoelectric materials

    SciTech Connect

    Li, Huidong; Tian, Chuan; Deng, Z. Daniel

    2014-12-15

    In an effort to eliminate the replacement of the batteries of electronic devices that are difficult or impractical to service once deployed, harvesting energy from mechanical vibrations or impacts using piezoelectric materials has been researched over the last several decades. However, a majority of these applications have very low input frequencies. This presents a challenge for the researchers to optimize the energy output of piezoelectric energy harvesters, due to the relatively high elastic moduli of piezoelectric materials used to date. This paper reviews the current state of research on piezoelectric energy harvesting devices for low frequency (0–100 Hz) applications and the methods that have been developed to improve the power outputs of the piezoelectric energy harvesters. Various key aspects that contribute to the overall performance of a piezoelectric energy harvester are discussed, including geometries of the piezoelectric element, types of piezoelectric material used, techniques employed to match the resonance frequency of the piezoelectric element to input frequency of the host structure, and electronic circuits specifically designed for energy harvesters.

  8. Endogenous modulation of low frequency oscillations by temporal expectations

    PubMed Central

    Cravo, Andre M.; Rohenkohl, Gustavo; Wyart, Valentin

    2011-01-01

    Recent studies have associated increasing temporal expectations with synchronization of higher frequency oscillations and suppression of lower frequencies. In this experiment, we explore a proposal that low-frequency oscillations provide a mechanism for regulating temporal expectations. We used a speeded Go/No-go task and manipulated temporal expectations by changing the probability of target presentation after certain intervals. Across two conditions, the temporal conditional probability of target events differed substantially at the first of three possible intervals. We found that reactions times differed significantly at this first interval across conditions, decreasing with higher temporal expectations. Interestingly, the power of theta activity (4–8 Hz), distributed over central midline sites, also differed significantly across conditions at this first interval. Furthermore, we found a transient coupling between theta phase and beta power after the first interval in the condition with high temporal expectation for targets at this time point. Our results suggest that the adjustments in theta power and the phase-power coupling between theta and beta contribute to a central mechanism for controlling neural excitability according to temporal expectations. PMID:21900508

  9. Planck 2013 results. II. Low Frequency Instrument data processing

    NASA Astrophysics Data System (ADS)

    Planck Collaboration; Aghanim, N.; Armitage-Caplan, C.; Arnaud, M.; Ashdown, M.; Atrio-Barandela, F.; Aumont, J.; Baccigalupi, C.; Banday, A. J.; Barreiro, R. B.; Battaner, E.; Benabed, K.; Benoît, A.; Benoit-Lévy, A.; Bernard, J.-P.; Bersanelli, M.; Bielewicz, P.; Bobin, J.; Bock, J. J.; Bonaldi, A.; Bonavera, L.; Bond, J. R.; Borrill, J.; Bouchet, F. R.; Bridges, M.; Bucher, M.; Burigana, C.; Butler, R. C.; Cappellini, B.; Cardoso, J.-F.; Catalano, A.; Chamballu, A.; Chen, X.; Chiang, L.-Y.; Christensen, P. R.; Church, S.; Colombi, S.; Colombo, L. P. L.; Crill, B. P.; Cruz, M.; Curto, A.; Cuttaia, F.; Danese, L.; Davies, R. D.; Davis, R. J.; de Bernardis, P.; de Rosa, A.; de Zotti, G.; Delabrouille, J.; Dickinson, C.; Diego, J. M.; Dole, H.; Donzelli, S.; Doré, O.; Douspis, M.; Dupac, X.; Efstathiou, G.; Enßlin, T. A.; Eriksen, H. K.; Falvella, M. C.; Finelli, F.; Forni, O.; Frailis, M.; Franceschi, E.; Gaier, T. C.; Galeotta, S.; Ganga, K.; Giard, M.; Giardino, G.; Giraud-Héraud, Y.; Gjerløw, E.; González-Nuevo, J.; Górski, K. M.; Gratton, S.; Gregorio, A.; Gruppuso, A.; Hansen, F. K.; Hanson, D.; Harrison, D.; Henrot-Versillé, S.; Hernández-Monteagudo, C.; Herranz, D.; Hildebrandt, S. R.; Hivon, E.; Hobson, M.; Holmes, W. A.; Hornstrup, A.; Hovest, W.; Huffenberger, K. M.; Jaffe, A. H.; Jaffe, T. R.; Jewell, J.; Jones, W. C.; Juvela, M.; Kangaslahti, P.; Keihänen, E.; Keskitalo, R.; Kiiveri, K.; Kisner, T. S.; Knoche, J.; Knox, L.; Kunz, M.; Kurki-Suonio, H.; Lagache, G.; Lähteenmäki, A.; Lamarre, J.-M.; Lasenby, A.; Lattanzi, M.; Laureijs, R. J.; Lawrence, C. R.; Leach, S.; Leahy, J. P.; Leonardi, R.; Lesgourgues, J.; Liguori, M.; Lilje, P. B.; Linden-Vørnle, M.; Lindholm, V.; López-Caniego, M.; Lubin, P. M.; Macías-Pérez, J. F.; Maggio, G.; Maino, D.; Mandolesi, N.; Maris, M.; Marshall, D. J.; Martin, P. G.; Martínez-González, E.; Masi, S.; Massardi, M.; Matarrese, S.; Matthai, F.; Mazzotta, P.; Meinhold, P. R.; Melchiorri, A.; Mendes, L.; Mennella, A.; Migliaccio, M.; Mitra, S.; Moneti, A.; Montier, L.; Morgante, G.; Morisset, N.; Mortlock, D.; Moss, A.; Munshi, D.; Naselsky, P.; Natoli, P.; Netterfield, C. B.; Nørgaard-Nielsen, H. U.; Novikov, D.; Novikov, I.; O'Dwyer, I. J.; Osborne, S.; Paci, F.; Pagano, L.; Paladini, R.; Paoletti, D.; Partridge, B.; Pasian, F.; Patanchon, G.; Peel, M.; Perdereau, O.; Perotto, L.; Perrotta, F.; Pierpaoli, E.; Pietrobon, D.; Plaszczynski, S.; Platania, P.; Pointecouteau, E.; Polenta, G.; Ponthieu, N.; Popa, L.; Poutanen, T.; Pratt, G. W.; Prézeau, G.; Prunet, S.; Puget, J.-L.; Rachen, J. P.; Reach, W. T.; Rebolo, R.; Reinecke, M.; Remazeilles, M.; Ricciardi, S.; Riller, T.; Robbers, G.; Rocha, G.; Rosset, C.; Rossetti, M.; Roudier, G.; Rubiño-Martín, J. A.; Rusholme, B.; Salerno, E.; Sandri, M.; Santos, D.; Scott, D.; Seiffert, M. D.; Shellard, E. P. S.; Spencer, L. D.; Starck, J.-L.; Stolyarov, V.; Stompor, R.; Sureau, F.; Sutton, D.; Suur-Uski, A.-S.; Sygnet, J.-F.; Tauber, J. A.; Tavagnacco, D.; Terenzi, L.; Toffolatti, L.; Tomasi, M.; Tristram, M.; Tucci, M.; Tuovinen, J.; Türler, M.; Umana, G.; Valenziano, L.; Valiviita, J.; Van Tent, B.; Varis, J.; Vielva, P.; Villa, F.; Vittorio, N.; Wade, L. A.; Wandelt, B. D.; Watson, R.; Wehus, I. K.; White, S. D. M.; Wilkinson, A.; Yvon, D.; Zacchei, A.; Zonca, A.

    2014-11-01

    We describe the data processing pipeline of the Planck Low Frequency Instrument (LFI) data processing centre (DPC) to create and characterize full-sky maps based on the first 15.5 months of operations at 30, 44, and 70 GHz. In particular, we discuss the various steps involved in reducing the data, from telemetry packets through to the production of cleaned, calibrated timelines and calibrated frequency maps. Data are continuously calibrated using the modulation induced on the mean temperature of the cosmic microwave background radiation by the proper motion of the spacecraft. Sky signals other than the dipole are removed by an iterative procedure based on simultaneous fitting of calibration parameters and sky maps. Noise properties are estimated from time-ordered data after the sky signal has been removed, using a generalized least squares map-making algorithm. A destriping code (Madam) is employed to combine radiometric data and pointing information into sky maps, minimizing the variance of correlated noise. Noise covariance matrices, required to compute statistical uncertainties on LFI and Planck products, are also produced. Main beams are estimated down to the ≈- 20 dB level using Jupiter transits, which are also used for the geometrical calibration of the focal plane.

  10. Very Low Frequency Remote Sensing of the Ionosphere and Magnetosphere

    NASA Astrophysics Data System (ADS)

    Cohen, M.

    2013-05-01

    This review talk will explore the technique of Very Low Frequency (VLF, 3-30 kHz) remote sensing of the ionosphere and magnetosphere, in which ground-based transmitter beacons (nominally for submarine communications) are used as a probe wave to study the D-region of the ionosphere (60-90 km), a layer is too low for satellites and too high for balloons. Guided efficiently by the Earth-ionosphere waveguide, VLF waves can be used on a global level, to sensitively quantify any ionospheric disturbance in the D-region. These include the impacts of solar flares, lightning heating (both the EMP and the quasi-static field changes), electron precipitation from lightning, and cosmic gamma-ray bursts. We will review many experimental and modeling efforts that have been made over the past several decades, including recent work on the transionospheric absorption of VLF waves from transmitters and lightning radio emissions. We will also review recent international efforts to build a global network of VLF receivers under the umbrella of the United Nations Basic Space Science Initiative.

  11. Low-frequency sound level in the Southern Indian Ocean.

    PubMed

    Tsang-Hin-Sun, Eve; Royer, Jean-Yves; Leroy, Emmanuelle C

    2015-12-01

    This study presents long-term statistics on the ambient sound in the Southern Indian Ocean basin based on 2 years of data collected on six widely distributed autonomous hydrophones from 47°S to 4°S and 53°E to 83°E. Daily mean power spectra (10-100 Hz) were analyzed in order to identify the main sound sources and their space and time variability. Periodic signals are principally associated with the seasonal presence of three types of blue whales and fin whales whose signatures are easily identified at specific frequencies. In the low frequencies, occurrence of winter lows and summer highs in the ambient noise levels are well correlated with iceberg volume variations at the southern latitudes, suggesting that icebergs are a major sound source, seasonally contributing to the ambient noise, even at tropical latitudes (26°S). The anthropogenic contribution to the noise spectrum is limited. Shipping sounds are only present north and west of the study area in the vicinity of major traffic lanes. Acoustic recordings from the southern sites may thus be representative of the pristine ambient noise in the Indian Ocean. PMID:26723301

  12. The VLA Low Frequency Sky Survey Redux (VLSSr)

    NASA Astrophysics Data System (ADS)

    Peters, Wendy M.; Cotton, W. D.; Kassim, N. E.

    2014-01-01

    We present the Very Large Array (VLA) Low-frequency Sky Survey Redux (VLSSr), which covers the sky above declinations δ > -30 degrees at a frequency of 74 MHz with 75" resolution and an average RMS noise of 0.1 Jy/beam. The theoretical largest angular size imaged is 36', and there are approximately 95,000 cataloged sources. We have completely re-imaged all data from the original VLSS survey leading to improvements in a number of areas. These include the application of a more accurate primary beam correction which removes substantial radially dependent flux errors present in the VLSS, and smart-windowing to reduce the clean bias by half. We look ahead to the possibility of an expanded, "VLSS generation 2", made by piggybacking observations of the planned VLA Sky Survey (VLASS) using a proposed 24/7 commensal system, called the LOw Band Observatory (LOBO). Catalogs and images for the VLSSr are available at .

  13. Suppression of Leidenfrost effect via low frequency vibrations

    NASA Astrophysics Data System (ADS)

    Ng, Boon Thiam; Hung, Yew Mun; Tan, Ming Kwang

    2015-11-01

    Leidenfrost effect occurs when vapor layer forms in between the coolant and the hot surface above Leidenfrost point, which dramatically reduces the cooling efficiency due to low thermal conductivity of the vapor layer. To prevent surface overheating, there have been number of reported methods to suppress the Leidenfrost effect that were mainly based on functionalization of the substrate surface and application of electric field across the droplet and substrate. In this work, we induce low frequency vibrations (f ~ 100 Hz) to the heated substrate to suppress the Leidenfrost effect. Three distinct impact dynamics are observed based on different magnitudes of surface acceleration and surface temperature. In gentle film boiling regime, formation of thin spreading lamella around the periphery of the impinged droplet is observed; in film boiling regime, due to thicker vapor cushion, rebound of the impinged droplet is observed; in contact boiling regime, due to the direct contact between the impinged droplet and heated substrate, ejection of the tiny droplet is observed. Also, estimated cooling enhancement ratio for contact boiling regime shows an improvement from 95% to 105%.

  14. A perceived low-frequency sound in Taos, New Mexico

    SciTech Connect

    Mullins, J.H. ); Poteet, H. )

    1994-11-01

    Persistent complaints of an annoying low-frequency sound in Northern New Mexico, particularly in the vicinity of Taos, led to a request by members of the Congressional delegation of NM for an investigation. During the summer of 1993, in Taos, extensive simultaneous measurements were carried out of acoustic, seismic, electric, magnetic, and electromagnetic signals by a team from Sandia and Los Alamos National Laboratories, the Air Force Phillips Laboratory, and the University of New Mexico. Since anecdotal evidence and signal matching tests by the hearers implicated the frequencies between 30 to 100 Hz, special attention was given to that range. However, no signals were found matching the description, and in particular no airborne audio signals in this range were found other than background, even though the acoustical detector was capable of measuring signals less than [minus]50 dB SPL. Subsequent complaints of similar sounds from widely distributed areas in the U.S., and a long history of these in the U.K. [R. N. Vasudevan and C. G. Gordon, Appl. Acoust. [bold 10], 57--69 (1977)] have focused attention on human hearing in the 20--100 Hz range. New instruments are being developed and controlled clinical tests are planned with hearers and nonhearers in the Taos area.

  15. Low-frequency electromagnetic exploration for groundwater on Mars

    NASA Astrophysics Data System (ADS)

    Grimm, Robert E.

    2002-02-01

    Water with even a small amount of dissolved solidshas an electrical conductivity orders of magnitude higher than dry rock andis therefore a near-ideal exploration target on Mars for low-frequency, diffusiveelectromagnetic methods. Models of the temperature- and frequency-dependentelectrical properties of rock-ice-water mixtures are used to predict the electromagneticresponse of the Martian subsurface. Detection of ice is difficult unless itis massively segregated. In contrast, liquid water profoundly affects soundings,and even a small amount of adsorbed water in the cryosphere can be detected.Subcryospheric water is readily distinguishable at frequencies as low as 100Hz for fresh water to 10 mHz for brines. These responses can be measured usingeither natural or artificial sources. ULF signals from solar wind and diurnal-heatingperturbations of the ionosphere are likely, and disturbances of regional crustalmagnetic fields may also be observable. Spherics, or ELF-VLF signals fromlightning discharge, would provide optimal soundings; however, lightning maybe the least likely of the possible natural sources. Among the active techniques,only the time-domain electromagnetic (TDEM) method can accommodate a closelyspaced transmitter and receiver and sound to depths of hundreds of metersor more. A ground- or aircraft-based TDEM system of several kilograms candetect water to a depth of several hundred meters, and a system of tens ofkilograms featuring a large, fixed, rover- or ballistically deployed loopcan detect water to several kilometers depth.

  16. Office worker exposure to extremely low frequency magnetic fields

    SciTech Connect

    Hiebert, D.G.

    1994-05-01

    A study of office worker exposure to extremely low frequency magnetic fields (ELF MF) was conducted at Los Alamos National Laboratory. The main purpose of this study was to quantitatively assess ELF MF exposures. A secondary objective was to determine whether or not exposures to ELF MF can be reduced by implementing administrative controls and educating workers on the sources of such fields. EMDEX dosimeters were used to determine full shift personal exposures for 12 volunteers from two personnel sections and one training section. In addition, using the EMDEX meter in survey mode, office area evaluations were conducted. Administrative controls and training were implemented in an attempt to reduce exposures. Post control monitoring was conducted to determine if a reduction in ELF MF occurred among the workers. On average, baseline office worker exposures to ELF MF were 2.3 mG, ranging from 0.6 to 9.7 mG. The post control exposures averaged 1.1 mG with a range from 0.5 to 2.2 mG. A reduction of 53% overall was seen after implementation of administrative controls and training. The office area survey indicated that many sources of ELF MF influence exposure and that magnetic field strengths vary not only from one type of equipment to another, but also vary between two similar pieces of equipment.

  17. Peak luminosity correlated low-frequency oscillations in black holes

    NASA Astrophysics Data System (ADS)

    Li, Z. B.; Gao, H. Q.; Zhang, Z.; Zhang, S.; Qu, J. L.; Zhang, C. M.; Song, L. M.

    2014-05-01

    Based on Rossi X-ray Timing Explorer (RXTE) observational data, we study the timing and spectral properties of some peculiar low-frequency (LF) quasi-periodic oscillations (QPOs), which have been found at the peak luminosity of the outburst of some transient black hole (BH) binaries: the 2005 outburst of GRO J1655-40, the 2003 outburst of H1743-322 and the 1998 outburst of XTE J1550-564. Appearing in the ultraluminous state, these QPOs from different sources show some common properties. The amplitude is very weak (less than 1 per cent) and the quality factor is larger than 6. Moreover, these QPOs (about several Hz) sometimes show up simultaneously with another QPO (about 10 Hz), but their frequencies are not harmonically related. We also find that the frequencies of these QPOs are inversely correlated with the mass of the BH, which implies that these QPOs might be correlated with the innermost stable circular orbit. The QPO frequency is also negative correlated with the inner disc radius among BHs. However, its frequency is too low to ascribe it to the Keperlian orbit frequency. Moreover, we discuss the physical origin of these QPOs and we suggest that they are not produced by the viscous variability of the inner disc either.

  18. Collagen and component polypeptides: Low frequency and amide vibrations

    NASA Astrophysics Data System (ADS)

    Fontaine-Vive, F.; Merzel, F.; Johnson, M. R.; Kearley, G. J.

    2009-01-01

    Collagen is a fibrous protein, which exists widely in the human body. The biomechanical properties of collagen depend on its triple helix structure and the corresponding low frequency vibrations. We use first-principles, density functional theory methods and analytical force fields to investigate the molecular vibrations of a model collagen compound, the results being validated by comparison with published, inelastic neutron scattering data. The results from these atomistic simulations are used at higher frequency to study the Amide I and V vibrations and therefore the vibrational signature of secondary and tertiary structure formation. In addition to collagen, its component homopolymers, poly-glycine and poly-proline are also studied. The Amide V vibration of glycine is strongly modified in going from the single helix of poly-glycine II to the triple helix of collagen. The collagen models are hydrated and this work allows us to discuss the relative merits of density functional theory and force field methods when tackling complex, partially crystalline systems.

  19. Mechanical monolithic horizontal sensor for low frequency seismic noise measurement.

    PubMed

    Acernese, Fausto; Giordano, Gerardo; Romano, Rocco; De Rosa, Rosario; Barone, Fabrizio

    2008-07-01

    This paper describes a mechanical monolithic horizontal sensor for geophysical applications developed at the University of Salerno. The instrument is basically a monolithic tunable folded pendulum, shaped with precision machining and electric discharge machining, that can be used both as seismometer and, in a force-feedback configuration, as accelerometer. The monolithic mechanical design and the introduction of laser interferometric techniques for the readout implementation makes it a very compact instrument, very sensitive in the low frequency seismic noise band, with a very good immunity to environmental noises. Many changes have been produced since last version (2007), mainly aimed to the improvement of the mechanics and of the optical readout of the instrument. In fact, we have developed and tested a prototype with elliptical hinges and mechanical tuning of the resonance frequency together with a laser optical lever and a new laser interferometer readout system. The theoretical sensitivity curve for both laser optical lever and laser interferometric readouts, evaluated on the basis of suitable theoretical models, shows a very good agreement with the experimental measurements. Very interesting scientific result is the measured natural resonance frequency of the instrument of 70 mHz with a Q=140 in air without thermal stabilization. This result demonstrates the feasibility of a monolithic folded pendulum sensor with a natural resonance frequency of the order of millihertz with a more refined mechanical tuning. PMID:18681722

  20. Low-frequency earthquakes in the Mexican Sweet Spot

    NASA Astrophysics Data System (ADS)

    Frank, William B.; Shapiro, Nikolaï M.; Kostoglodov, Vladimir; Husker, Allen L.; Campillo, Michel; Payero, Juan S.; Prieto, GermáN. A.

    2013-06-01

    We use data from the Meso-America Subduction Experiment to detect and locate low-frequency earthquakes (LFEs) in the Mexican subduction zone. We use visually-identified templates to perform a network waveform correlation search that produced ~17,000 robustly detected LFEs that form 15 distinct families. Stacking an LFE family's corresponding detections results in seismograms with high signal-to-noise ratios and clear P and S wave arrivals; we use these travel times to locate the sources. The resulting locations superpose a previously identified region of permanent non-volcanic tremor (NVT) activity. Husker et al. (2012) called this region a Sweet Spot, suggesting that the local conditions are adequate to continuously generate NVT. The LFE hypocenters have been located at a depth of 40-45 km in an area that is surrounding the upper slab-plate interface. We characterize their focal mechanisms by comparing their stacked seismograms to synthetic seismograms. This analysis reveals a common low-dipping focal mechanism.

  1. DOUBLE COMPACT OBJECTS AS LOW-FREQUENCY GRAVITATIONAL WAVE SOURCES

    SciTech Connect

    Belczynski, Krzysztof; Bulik, Tomasz; Benacquista, Matthew

    2010-12-10

    We study the Galactic field population of double compact objects (DCOs; NS-NS, BH-NS, BH-BH binaries) to investigate the number (if any) of these systems that can potentially be detected with the Laser Interferometer Space Antenna (LISA) at low gravitational wave frequencies. We calculate the Galactic numbers and physical properties of these binaries and show their relative contributions from the disk, bulge, and halo. Although the Galaxy hosts {approx}10{sup 5} DCO binaries emitting low-frequency gravitational waves, only a handful of these objects in the disk will be detectable with LISA, but none from the halo or bulge. This is because the bulk of these binaries are NS-NS systems with high eccentricities and long orbital periods (weeks/months) causing inefficient signal accumulation (a small number of signal bursts at periastron passage in one year of LISA observations) and rendering them undetectable in the majority of these cases. We adopt two evolutionary models that differ in their treatment of the common envelope (CE) phase that is a major (and still mostly unknown) process in the formation of close DCOs. Depending on the evolutionary model adopted, our calculations indicate the likely detection of about four NS-NS binaries and two BH-BH systems (model A; likely survival of progenitors through CE) or only a couple of NS-NS binaries (model B; suppression of the DCO formation due to CE mergers).

  2. Proud elastic target discrimination using low-frequency sonar signatures

    NASA Astrophysics Data System (ADS)

    Mallen, Brenton

    This thesis presents a comparative analysis of various low-frequency sonar signature representations and their ability to discriminate between proud targets of varying physical parameters. The signature representations used include: synthetic aperture sonar (SAS) beamformed images, acoustic color plot images, and bispectral images. A relative Mean-Square Error (rMSE) performance metric and an effective Signal-to-Noise Ratio (SNReff) performance metric have been developed and implemented to quantify the target differentiation. The analysis is performed on a subset of the synthetic sonar stave data provided by the Naval Surface Warfare Center -- Panama City Division (NSWC-PCD). The subset is limited to aluminum and stainless steel, thin-shell, spherical targets in contact with the seafloor (proud). It is determined that the SAS signature representation provides the best, least ambiguous, target differentiation with a minimum mismatch difference of 14.5802 dB. The acoustic color plot and bispectrum representations resulted in a minimum difference of 9.1139 dB and 1.8829 dB, respectively.

  3. Model rotor low frequency broadband noise at moderate tip speeds

    NASA Technical Reports Server (NTRS)

    Humbad, N. G.; Harris, W. L.

    1980-01-01

    The results of an experimental investigation of low frequency broadband noise (LFBN) radiated from model helicopter rotors are presented. The results are for a range of tip Mach numbers (Mt) up to 0.50. The effect of rotor blade loading, advance ratio, tip speed, number of blades and free stream turbulence on the sound pressure level (SPL) and the spectrum of LFBN have been investigated. The peak SPL of LFBN appears to follow an M(4) law if the effect of rms turbulence velocity is removed. The peak SPL of LFBN seems to saturate with increases in advance ratio and with blade loading, and is proportional to the square of the turbulence integral scale when the effect of rms turbulence velocity and Mt are removed. Also, a simple peak SPL scaling law for noise from a helicopter rotor in forward flight due to convected sinusoidal gust is developed. The trend predicted by this scaling law is found to be satisfactory for the variation of the peak SPL of LFBN with tip speed.

  4. Modeling low frequency vibrational modes of large biomolecules

    NASA Astrophysics Data System (ADS)

    Sankey, Otto; Dykeman, Eric

    2008-03-01

    Mechanical oscillations of proteins in their native state are relevant to understanding the flexibility of the protein assembly, the binding of substrates, the mechanical action involved in enzymatic activity, and the vibrational response to light scattering. Often, only the low frequency modes are of interest and coarse grained methods or other approximations are used due to the large size of the dynamical matrix. We introduce a computational approach, which exploits the methodology from electronic structure Order N methods, to find the vibrational modes below some frequency threshold (analogous to a Fermi-level in electronic structure theory). The approach allows systems to be described in atomistic detail. We use a generalized Born force field to model the interactions. Examples of normal modes for icosahedral viruses (e.g. satellite tobacco necrosis virus), tubular viruses (e.g. M13), and enzymes (e.g. lysozyme, HIV-protease, alpha-lytic protease) will be discussed. This effort is motivated by recent experimental work to produce high amplitude vibrations of viruses from impulsive stimulated Raman scattering.

  5. Extremely low frequency fields and cancer: laboratory studies

    SciTech Connect

    Anderson, Larry E.)

    1998-10-01

    There is now convincing evidence from a large number of laboratories, that exposure to extremely low frequency (ELF) magnetic and electric fields produces biological responses in animals. However, no animal studies clearly demonstrate deleterious effects of ELF fields, although several are suggestive of potential health impacts. A major current emphasis in laboratory research is to determine whether or not the reported epidemiological studies that suggest an association between EMF exposure and risk of cancer are supported in studies using animal models. Several approaches are outlined in the experimental approach to this question. With specific reference to the radiofrequencies (RF) associated with wireless technology, even less research has been carried out than with ELF. Particularly, in regard to research on carcinogenesis and RF exposure in animals, little is known This section addresses laboratory studies in animals exposed to extremely low-power-frequency EMF, the relevance of which, to RF, is unknown. However, the approaches used with ELF may be useful in guiding laboratory research on the issue of RF exposure and cancer. From the perspective of laboratory animal studies, this paper will discuss studies investigating the potential relationship between ELF magnetic and/or electric field exposure and the risk of cancer.

  6. Low-Frequency Envelope Sensitivity Produces Asymmetric Binaural Tuning Curves

    PubMed Central

    Agapiou, John P.; McAlpine, David

    2008-01-01

    Neurons in the auditory midbrain are sensitive to differences in the timing of sounds at the two ears—an important sound localization cue. We used broadband noise stimuli to investigate the interaural-delay sensitivity of low-frequency neurons in two midbrain nuclei: the inferior colliculus (IC) and the dorsal nucleus of the lateral lemniscus. Noise-delay functions showed asymmetries not predicted from a linear dependence on interaural correlation: a stretching along the firing-rate dimension (rate asymmetry), and a skewing along the interaural-delay dimension (delay asymmetry). These asymmetries were produced by an envelope-sensitive component to the response that could not entirely be accounted for by monaural or binaural nonlinearities, instead indicating an enhancement of envelope sensitivity at or after the level of the superior olivary complex. In IC, the skew-like asymmetry was consistent with intermediate-type responses produced by the convergence of ipsilateral peak-type inputs and contralateral trough-type inputs. This suggests a stereotyped pattern of input to the IC. In the course of this analysis, we were also able to determine the contribution of time and phase components to neurons' internal delays. These findings have important consequences for the neural representation of interaural timing differences and interaural correlation—cues critical to the perception of acoustic space. PMID:18753329

  7. Imaging the Subduction Plate Interface Using Low-Frequency Earthquakes

    NASA Astrophysics Data System (ADS)

    Plourde, A. P.; Bostock, M. G.

    2015-12-01

    Low-frequency Earthquakes (LFEs) in subduction zones are commonly thought to represent slip on the plate interface. They have also been observed to lie near or within a zone of low shear-wave velocity, which is modelled as fluid-rich upper oceanic crust. Due to relatively large depth uncertainties in absolute hypocenters of most LFE families, their location relative to an independently imaged subucting plate and, consequently, the nature of the plate boundary at depths between 30-45 km have not been precisely determined. For a selection of LFE families in northern Washington, we measure variations in arrival time of individual LFE detections using multi-channel cross-correlation incorporating both arrivals at the same station and different events (cross-detection data), and the same event but different stations (cross-station data). Employing HypoDD, these times are used to generate relative locations for individual LFE detections. After creating templates from spatial subgroups of detections, network cross-correlation techniques will be used to search for new detections in neighbouring areas, thereby expanding the local catalogue and enabling further subdivision. By combining the source ``arrays'' and the receiver arrays from the Array of Arrays experiment we plan to interrogate plate boundary structure using migration of scattered waves from the subduction complex as previously documented beneath southern Vancouver Island.

  8. Transient Modeling of Hybrid Rocket Low Frequency Instabilities

    NASA Technical Reports Server (NTRS)

    Karabeyoglu, M. Arif; DeZilwa, Shane; Cantwell, Brian; Zilliac, Greg

    2003-01-01

    A comprehensive dynamic model of a hybrid rocket has been developed in order to understand and predict the transient behavior including instabilities. A linearized version of the transient model predicted the low-frequency chamber pressure oscillations that are commonly observed in hybrids. The source of the instabilities is based on a complex coupling of thermal transients in the solid fuel, wall heat transfer blocking due to fuel regression rate and the transients in the boundary layer that forms on the fuel surface. The oscillation frequencies predicted by the linearized theory are in very good agreement with 43 motor test results obtained from the hybrid propulsion literature. The motor test results used in the comparison cover a very wide spectrum of parameters including: 1) four separate research and development programs, 2) three different oxidizers (LOX, GOX, N2O), 3) a wide range of motor dimensions (i.e. from 5 inch diameter to 72 inch diameter) and operating conditions and 4) several fuel formulations. A simple universal scaling formula for the frequency of the primary oscillation mode is suggested.

  9. Recent developments in thermoacoustically-driven low-frequency projectors

    SciTech Connect

    Ward, W.C.; Merrigan, M.A.

    1992-01-01

    Thermoacoustic engines are a recent class of devices that can efficiently convert heat to acoustic energy without moving parts or intervening mechanisms. These engines have a natural potential for powering low-frequency sonar projectors with high reliability and efficiencies that cannot be matched by conventional technologies. A recent design study has produced thermoacoustic projector configurations that can execute standard projector performance requirements such as FM sweep and velocity magnitude and phase control in array environments for a wide range of positive and negative radiation resistances. The thermoacoustic driver is a vertically oriented, helium-filled resonator that contains a movable tuning element to vary the resonator frequency. It is coupled to a variable length water column that is tunable by a similar means to adjust the effective source impedance of the device. Modeling results indicate a sweep range of at least an octave for a single device, and maximum overall (heat-to-acoustic) conversion efficiencies of 25% at 50 Hz. Efficiency increases slightly at lower frequencies, and the lowest operational frequency is limited only by the size of the projector. Output power increases linearly with mean pressure, and at depths of 200 m or more, power densities in excess of 500 kW/m{sup 2} are achievable. Control aspects have been investigated, including rapid startup and shutdown that can be performed by manipulating the water tuning column. Future trends and development prospects are discussed. 4 refs.

  10. Recent developments in thermoacoustically-driven low-frequency projectors

    SciTech Connect

    Ward, W.C.; Merrigan, M.A.

    1992-05-01

    Thermoacoustic engines are a recent class of devices that can efficiently convert heat to acoustic energy without moving parts or intervening mechanisms. These engines have a natural potential for powering low-frequency sonar projectors with high reliability and efficiencies that cannot be matched by conventional technologies. A recent design study has produced thermoacoustic projector configurations that can execute standard projector performance requirements such as FM sweep and velocity magnitude and phase control in array environments for a wide range of positive and negative radiation resistances. The thermoacoustic driver is a vertically oriented, helium-filled resonator that contains a movable tuning element to vary the resonator frequency. It is coupled to a variable length water column that is tunable by a similar means to adjust the effective source impedance of the device. Modeling results indicate a sweep range of at least an octave for a single device, and maximum overall (heat-to-acoustic) conversion efficiencies of 25% at 50 Hz. Efficiency increases slightly at lower frequencies, and the lowest operational frequency is limited only by the size of the projector. Output power increases linearly with mean pressure, and at depths of 200 m or more, power densities in excess of 500 kW/m{sup 2} are achievable. Control aspects have been investigated, including rapid startup and shutdown that can be performed by manipulating the water tuning column. Future trends and development prospects are discussed. 4 refs.

  11. Progress on the Low Frequency All Sky Monitor

    NASA Astrophysics Data System (ADS)

    Murray, James; Jenet, F.; Craig, J.; Creighton, T. D.; Dartez, L. P.; Ford, A. J.; Hicks, B.; Hinojosa, J.; Jaramillo, R.; Kassim, N. E.; Lunsford, G.; Miller, R. B.; Ray, P. S.; Rivera, J.; Taylor, G. B.

    2014-01-01

    The Low Frequency All Sky Monitor is a system of geographically separated radio arrays dedicated to the study of radio transients. LoFASM consists of four stations, each comprised of 12 crossed dipole antennas designed to operate between 10 - 88 MHz. The antennas and front end electronics for LoFASM were designed by the Naval Research Laboratory for the Long Wavelength Array project. Over the past year undergraduate students from the University of Texas at Brownsville have established LoFASM stations in Port Mansfield, Texas, at the LWA1 North Arm site in New Mexico, at the National Radio Astronomy Observatory in Green Bank, West Virginia, and NASA’s Goldstone tracking complex in California. In combination with the establishment of these stations was the development of the analog hardware, which consists of custom RF power dividers/combiners, and a new custom amplifier and filter receiving system, which was developed and built in house. This poster will expound on progress in site installation and development of the analog signal chain, specifically the redesigned analog receiving system.

  12. Low-frequency losses at high fields in multifilamentary superconductors

    SciTech Connect

    Zaleski, A.J.; Orlando, T.P.; Zieba, A.; Schwartz, B.B.; Foner, S.

    1984-12-01

    Low-frequency (approx.1 Hz) ac loss measurements were made at applied fields up to 20 T with field modulation amplitudes of up to 1 T. Results for the alternative in situ and powder metallurgy (P/M) processed Nb/sub 3/Sn and Nb--Al wires are presented for single-strand and multistrand geometries. Loss measurements for commercial continuous-fiber Nb/sub 3/Sn wires manufactured by Holec and Supercon also are presented and show that the effective fiber size determined by these ac-loss measurements corresponds to that measured optically. The effective fiber size for alternative processed wires was intermediate between the wire size and the fiber size, and the losses (and effective fiber size) were reduced by twisting. Multistrand geometries showed further reductions in losses. The smallest effective fiber size at high field was measured for P/M processed Nb--Al. The losses were fully hysteretic and the ac losses were used to determine the critical-current density at low fields where it was difficult to measure directly.

  13. Low-frequency fluctuations in the Indonesian throughflow through Lombok Strait

    NASA Astrophysics Data System (ADS)

    Arief, Dharma; Murray, Stephen P.

    1996-05-01

    A significant component of the Indonesian throughflow, apparently about 25%, passes through the Lombok Strait. Direct observations in 1985 reported a ˜2 Sv annual average, with an annual cycle of amplitude ˜2 Sv. There are also significant fluctuations in this transport in the 0.01-0.1 cpd frequency band. Shallow pressure gauge data (sea level) inside the strait during the current meter observations were of limited use in explaining the large fluctuations in currents. Sea level data at Cilacap, 720 km west (upcoast), however, overlap the current observations for 5 months and correlate exceedingly well (r = 0.87) with the observations inside the strait at these frequencies. These sea level oscillations in the Indian Ocean force fluctuations in the Lombok throughflow that reach 50-70 cm/s, equivalent to 2-3 Sv. Lagged regression analysis indicates Cilacap sea level leads Lombok currents by 1-2 days, suggesting a low-frequency, progressive wave. Simultaneous data in 1989 from four stations extending from the near-equatorial station at Padang at 1°S to Benoa in the Lombok Strait (2000 km downcoast) clearly show the persistent propagation of low-frequency waves of 20- to 40-cm range along this coast. Lagged correlation on station pairs indicates a phase speed consistent with coastally trapped internal Kelvin waves. We speculate that further eastward progression of these waves to the Timor Passage of Ombai Strait will further modulate the throughflow. The forcing of these waves is not yet identified, but it appears likely that intraseasonal oscillations in the equatorial Indian Ocean winds, as demonstrated by Enfield [1987] for the Pacific, are a probable mechanism. Improved wind data quality in 1991 due to the assimilation of satellite data (special sensor microwave/imager) will allow investigation of remote forcing on more recent data sets.

  14. Mobilization of colloidal particles by low-frequency dynamic stress stimulation

    SciTech Connect

    Beckham, Richard Edward; Amr, Abdel - Fattah I; Peter, Roberts M; Reem, Ibrahim; Tarimala, Sowmitri

    2009-01-01

    Naturally occurring seismic events and artificially generated low-frequency (1 to 500 Hertz) elastic waves have been observed to alter the production rates of oil and water wells, sometimes increasing and sometimes decreasing production, and to influence the turbidity of surface and well water. The decreases in production are of particular concern - especially when artificially generated elastic waves are applied as a method for enhanced oil recovery. The exact conditions that result in a decrease in production remain unknown. While the underlying environment is certainly complex, the observed increase in water well turbidity after natural seismic events suggests the existence of a mechanism that can affect both the subsurface flow paths and mobilization of in-situ colloidal particles. This paper explores the macroscopic and microscopic effects of low-frequency dynamic stress stimulations on the release of colloidal particles from an analog core representing an infinitesimal section along the propagation paths of an elastic wave. Experiments on a column packed with 1-mm borosilicate beads and loaded with polystyrene microspheres demonstrate that axial mechanical stress oscillations enhance the mobilization of captured microspheres. Increasing the amplitude of the oscillations increases the number of microspheres released and can also result in cyclical spikes in effluent microsphere concentration during stimulation. Under a prolonged period of stimulation, the cyclical effluent spikes coincided with fluctuations in the column pressure data, and continue at a diminished level after stimulation. This behavior can be attributed to rearrangements of the beads in the column, resulting in possible changes to the void space and/or tortuosity of the packing. Optical microscopy observations of the beads during low frequency oscillations reveal that individual beads rotate, thereby rubbing against each other and scraping away portions of the adsorbed microspheres. These

  15. Mobilization of colloidal particles by low-frequency dynamic stress stimulation.

    PubMed

    Beckham, Richard E; Abdel-Fattah, Amr I; Roberts, Peter M; Ibrahim, Reem; Tarimala, Sowmitri

    2010-01-01

    Naturally occurring seismic events and artificially generated low-frequency (1 to 500 Hz) elastic waves have been observed to alter the production rates of oil and water wells, sometimes increasing and sometimes decreasing production, and to influence the turbidity of surface and well water. The decreases in production are of particular concern, especially when artificially generated elastic waves are applied as a method for enhanced oil recovery. The exact conditions that result in a decrease in production remain unknown. Although the underlying environment is certainly complex, the observed increase in water well turbidity after natural seismic events suggests the existence of a mechanism that can affect both the subsurface flow paths and the mobilization of in situ colloidal particles. This article explores the macroscopic and microscopic effects of low-frequency dynamic stress stimulations on the release of colloidal particles from an analog core representing an infinitesimal section along the propagation paths of an elastic wave. Experiments on a column packed with 1 mm borosilicate beads and loaded with polystyrene microparticles demonstrate that axial mechanical stress oscillations enhance the mobilization of captured microparticles. Increasing the amplitude of the oscillations increases the number of microparticles released and can also result in cyclical spikes in effluent microparticle concentration during stimulation. Under a prolonged period of stimulation, the cyclical effluent spikes coincided with fluctuations in the column pressure data and continued at a diminished level after stimulation. This behavior can be attributed to rearrangements of the beads in the column, resulting in possible changes in the void space and/or tortuosity of the packing. Optical microscopy observations of the beads during low-frequency oscillations reveal that individual beads rotate, thereby rubbing against each other and scraping away portions of the adsorbed

  16. Analysis of waves in the plasma guided by a periodical vane-type slow wave structure

    SciTech Connect

    Wu, T.J.; Kou, C.S.

    2005-10-01

    In this study, the dispersion relation has been derived to characterize the propagation of the waves in the plasma guided by a periodical vane-type slow wave structure. The plasma is confined by a quartz plate. Results indicate that there are two different waves in this structure. One is the plasma mode that originates from the plasma surface wave propagating along the interface between the plasma and the quartz plate, and the other is the guide mode that originally travels along the vane-type slow wave structure. In contrast to its original slow wave characteristics, the guide mode becomes a fast wave in the low-frequency portion of the passband, and there exists a cut-off frequency for the guide mode. The vane-type guiding structure has been shown to limit the upper frequency of the passband of the plasma mode, compared with that of the plasma surface wave. In addition, the passband of the plasma mode increases with the plasma density while it becomes narrower for the guide mode. The influences of the parameters of the guiding structure and plasma density on the propagation of waves are also presented.

  17. Generation of low-frequency nonlinear currents in plasma by an ultrashort pulse of high-frequency radiation

    SciTech Connect

    Grishkov, V. E.; Uryupin, S. A.

    2015-07-15

    A kinetic theory of low-frequency currents induced in plasma by an ultrashort high-frequency radiation pulse is developed. General expressions for the currents flowing along the propagation direction of the pulse and along the gradient of the field energy density are analyzed both analytically and numerically for pulse durations longer or shorter than or comparable with the electron collision time in plasma. It is demonstrated that the nonlinear current flowing along the gradient of the field energy density can be described correctly only when the modification of the isotropic part of the electron distribution function is taken into account.

  18. The low-frequency continuum as observed in the solar wind from ISEE 3 - Thermal electrostatic noise

    NASA Technical Reports Server (NTRS)

    Hoang, S.; Steinberg, J.-L.; Epstein, G.; Tilloles, P.; Fainberg, J.; Stone, R. G.

    1980-01-01

    The low frequency continuum (LFC) noise between 30 and 200 kHz has been investigated from the ISEE 3 spacecraft in the solar wind by means of a radio astronomy experiment more sensitive than previously available. It is demonstrated that the LFC radiation observed in the solar wind is in the form of longitudinal plasma waves rather than transverse electromagnetic waves. The observed spectral characteristics are found to be a function of antenna length. In addition, both the absence of antenna spin modulation and the fact that these plasma waves do not propagate to large distances imply a local origin for the LFC.

  19. Low-frequency resonances of the refractive index in weakly ionized plasma with an admixture of dust

    SciTech Connect

    Prudskikh, V. V.

    2013-12-15

    The propagation of low-frequency electromagnetic waves along the magnetic field in weakly ionized plasma with an admixture of dust is studied in the framework of the Hall magnetohydrodynamics. Explicit expressions for the coefficients of magnetic field diffusion in plasma are derived. The resonance of the refractive index is found to occur for either right- or left-hand polarized waves. A quantitative criterion is obtained that allows one to determine the polarization of waves that experience resonance at given plasma parameters. The physical mechanism of the resonance is discussed, and the obtained results are compared with the available literature data.

  20. Low-frequency intraseasonal variability in a zonally symmetric aquaplanet model

    NASA Astrophysics Data System (ADS)

    Das, Surajit; Sengupta, Debasis; Chakraborty, A.; Sukhatme, Jai; Murtugudde, Raghu

    2016-04-01

    We use the aquaplanet version of the community atmospheric model, with perpetual spring equinox forcing and zonally symmetric sea surface temperature (SST), to study tropical intraseasonal oscillations (ISOs). In the first two experiments, we specify zonally symmetric SST profiles that mimic observed climatological July and January SSTs as surface boundary conditions. In the January SST simulation, we find a zonal wavenumber 1 mode with dominant period of 60 days, moving east at about 6 m s-1. This mode, which resembles the Madden-Julian oscillation (MJO), is absent in the July SST case, although convectively coupled Kelvin waves are prominent in both experiments. To further investigate the influence of tropical SST on ISO and convectively coupled equatorial waves, we conduct experiments with idealised symmetric SST profiles having different widths of warm ocean centered at the equator. In the narrowest SST experiment, the variance of moist activity is predominantly in weather-scale Kelvin waves. When the latitudinal extent of warm SST is comparable to or larger than the equatorial Rossby radius, we find a dominant low frequency (50-80 days) eastward mode that resembles the MJO, as in the January SST experiment. We also find westward propagating waves with intraseasonal (30-120 days) periods and zonal wavenumber 1-3; the structure of these signals projects onto equatorially trapped Rossby waves with meridional mode numbers 1, 3 and 5, associated with convection that is symmetric about the equator. In addition, the model generates 30-80 days westward moving signals with zonal wavenumber 4-7, particularly in the narrow SST experiment. Although these waves are seen in the wavenumber-frequency spectra in the equatorial region, they have largest amplitude in the middle and high latitudes. Thus, our study shows that wider, meridionally symmetric SST profiles support a strong MJO-like eastward propagation, and even in an aquaplanet setting, westward propagating Rossby

  1. Global model of low-frequency chorus (fLHR

    NASA Astrophysics Data System (ADS)

    Meredith, Nigel P.; Horne, Richard B.; Li, Wen; Thorne, Richard M.; Sicard-Piet, Angélica

    2014-01-01

    Whistler mode chorus is an important magnetospheric emission, playing a dual role in the acceleration and loss of relativistic electrons in the Earth's outer radiation belt. Chorus is typically generated in the equatorial region in the frequency range 0.1-0.8 fce, where fce is the local electron gyrofrequency. However, as the waves propagate to higher latitudes, significant wave power can occur at frequencies below 0.1fce. Since this wave power is largely omitted in current radiation belt models, we construct a global model of low-frequency chorus, fLHRlow-frequency chorus is strongest, with an average intensity of 200 pT2, in the prenoon sector during active conditions at midlatitudes (20°<|λm|<50°) from 4low-frequency chorus wave power will contribute to the acceleration and loss of relativistic electrons and should be taken into account in radiation belt models.

  2. Intense low-frequency chorus waves observed by Van Allen Probes: Fine structures and potential effect on radiation belt electrons

    NASA Astrophysics Data System (ADS)

    Gao, Zhonglei; Su, Zhenpeng; Zhu, Hui; Xiao, Fuliang; Zheng, Huinan; Wang, Yuming; Shen, Chao; Wang, Shui

    2016-02-01

    Frequency distribution is a vital factor in determining the contribution of whistler mode chorus to radiation belt electron dynamics. Chorus is usually considered to occur in the frequency range 0.1-0.8fce_eq (with the equatorial electron gyrofrequency fce_eq). We here report an event of intense low-frequency chorus with nearly half of wave power distributed below 0.1fce_eq observed by Van Allen Probe A on 27 August 2014. This emission propagated quasi-parallel to the magnetic field and exhibited hiss-like signatures most of the time. The low-frequency chorus can produce the rapid loss of low-energy (˜0.1 MeV) electrons, different from the normal chorus. For high-energy (≥0.5 MeV) electrons, the low-frequency chorus can yield comparable momentum diffusion to that of the normal chorus but much stronger (up to 2 orders of magnitude) pitch angle diffusion near the loss cone.

  3. Plasma filamentation and shock wave enhancement in microwave rockets by combining low-frequency microwaves with external magnetic field

    NASA Astrophysics Data System (ADS)

    Takahashi, Masayuki; Ohnishi, Naofumi

    2016-08-01

    A filamentary plasma is reproduced based on a fully kinetic model of electron and ion transports coupled with electromagnetic wave propagation. The discharge plasma transits from discrete to diffusive patterns at a 110-GHz breakdown, with decrease in the ambient pressure, because of the rapid electron diffusion that occurs during an increase in the propagation speed of the ionization front. A discrete plasma is obtained at low pressures when a low-frequency microwave is irradiated because the ionization process becomes more dominant than the electron diffusion, when the electrons are effectively heated by the low-frequency microwave. The propagation speed of the plasma increases with decrease in the incident microwave frequency because of the higher ionization frequency and faster plasma diffusion resulting from the increase in the energy-absorption rate. An external magnetic field is applied to the breakdown volume, which induces plasma filamentation at lower pressures because the electron diffusion is suppressed by the magnetic field. The thrust performance of a microwave rocket is improved by the magnetic fields corresponding to the electron cyclotron resonance (ECR) and its higher-harmonic heating, because slower propagation of the ionization front and larger energy-absorption rates are obtained at lower pressures. It would be advantageous if the fundamental mode of ECR heating is coupled with a lower frequency microwave instead of combining the higher-harmonic ECR heating with the higher frequency microwave. This can improve the thrust performance with smaller magnetic fields even if the propagation speed increases because of the decrease in the incident microwave frequency.

  4. The interference of electronic implants in low frequency electromagnetic fields.

    PubMed

    Silny, J

    2003-04-01

    Electronic implants such as cardiac pacemakers or nerve stimulators can be impaired in different ways by amplitude-modulated and even continuous electric or magnetic fields of strong field intensities. For the implant bearer, possible consequences of a temporary electromagnetic interference may range from a harmless impairment of his well-being to a perilous predicament. Electromagnetic interferences in all types of implants cannot be covered here due to their various locations in the body and their different sensing systems. Therefore, this presentation focuses exemplarily on the most frequently used implant, the cardiac pacemaker. In case of an electromagnetic interference the cardiac pacemaker reacts by switching to inhibition mode or to fast asynchronous pacing. At a higher disturbance voltage on the input of the pacemaker, a regular asynchronous pacing is likely to arise. In particular, the first-named interference could be highly dangerous for the pacemaker patient. The interference threshold of cardiac pacemakers depends in a complex way on a number of different factors such as: electromagnetic immunity and adjustment of the pacemaker, the composition of the applied low-frequency fields (only electric or magnetic fields or combinations of both), their frequencies and modulations, the type of pacemaker system (bipolar, unipolar) and its location in the body, as well as the body size and orientation in the field, and last but not least, certain physiological conditions of the patient (e.g. inhalation, exhalation). In extensive laboratory studies we have investigated the interference mechanisms in more than 100 cardiac pacemakers (older types as well as current models) and the resulting worst-case conditions for pacemaker patients in low-frequency electric and magnetic fields. The verification of these results in different practical everyday-life situations, e.g. in the fields of high-voltage overhead lines or those of electronic article surveillance systems is

  5. Natural very-low-frequency sferics and headache

    NASA Astrophysics Data System (ADS)

    Vaitl, D.; Propson, N.; Stark, R.; Schienle, A.

      Very-low-frequency (VLF) atmospherics or sferics are pulse-shaped alternating electric and magnetic fields which originate from atmospheric discharges (lightning). The objective of the study was threefold: (i) to analyse numerous parameters characterizing the sferics activity with regard to their suitability for field studies, (ii) to identify meteorological processes related to the sferics activity and (iii) to investigate the possible association of sferics with pain processes in patients suffering from migraine- and tension-type headaches. Over a period of 6 months (July through December) the sferics activity in the area of Giessen (Germany) was recorded. Three sferics parameters were chosen. The number of sferics impulses per day, the variability of the impulse rate during a day and the variability in comparison to the preceding day were correlated with weather processes (thunderstorm, temperature, vapour pressure, barometric pressure, humidity, wind velocity, warm sector). Significant correlations were obtained during the summer months (July, August) but not during the autumn months (October, November, December). During autumn, however, the sferics activity was correlated with the occurrence of migraine-type headaches (r=0.33, P<0.01) recorded by 37 women who had filled out a headache diary over a period of 6 months (July-December). While the thunderstorm activity was very intense during July and August, no relationship between sferics and migraine was found. In summer, tension-type headaches were associated with meteorological parameters such as temperature (r=0.42, P<0.01) and vapour pressure (r=0.28, P<0.05). Although the sferics activity can explain a small percentage of the variation in migraine occurrence, a direct influence was more likely exerted by visible or otherwise perceptible weather conditions (thunderstorms, humidity, vapour pressure, warm sector, etc.) than by the sferics activity itself.

  6. Natural very-low-frequency sferics and headache.

    PubMed

    Vaitl, D; Propson, N; Stark, R; Schienle, A

    2001-09-01

    Very-low-frequency (VLF) atmospherics or sferics are pulse-shaped alternating electric and magnetic fields which originate from atmospheric discharges (lightning). The objective of the study was threefold: (i) to analyse numerous parameters characterizing the sferics activity with regard to their suitability for field studies, (ii) to identify meteorological processes related to the sferics activity and (iii) to investigate the possible association of sferics with pain processes in patients suffering from migraine- and tension-type headaches. Over a period of 6 months (July through December) the sferics activity in the area of Giessen (Germany) was recorded. Three sferics parameters were chosen. The number of sferics impulses per day, the variability of the impulse rate during a day and the variability in comparison to the preceding day were correlated with weather processes (thunderstorm, temperature, vapour pressure, barometric pressure, humidity, wind velocity, warm sector). Significant correlations were obtained during the summer months (July, August) but not during the autumn months (October, November, December). During autumn, however, the sferics activity was correlated with the occurrence of migraine-type headaches (r=0.33, P<0.01) recorded by 37 women who had filled out a headache diary over a period of 6 months (July-December). While the thunderstorm activity was very intense during July and August, no relationship between sferics and migraine was found. In summer, tension-type headaches were associated with meteorological parameters such as temperature (r=0.42, P<0.01) and vapour pressure (r=0.28, P<0.05). Although the sferics activity can explain a small percentage of the variation in migraine occurrence, a direct influence was more likely exerted by visible or otherwise perceptible weather conditions (thunderstorms, humidity, vapour pressure, warm sector, etc.) than by the sferics activity itself. PMID:11594631

  7. Low-frequency electrical response to microbial induced sulfide precipitation

    NASA Astrophysics Data System (ADS)

    Ntarlagiannis, Dimitrios; Williams, Kenneth Hurst; Slater, Lee; Hubbard, Susan

    2005-12-01

    We investigated the sensitivity of low-frequency electrical measurements to microbe-induced metal sulfide precipitation. Three identical sand-packed monitoring columns were used; a geochemical column, an electrical column and a control column. In the first experiment, continuous upward flow of nutrients and metals in solution was established in each column. Cells of Desulfovibrio vulgaris (D. vulgaris) were injected into the center of the geochemical and electrical columns. Geochemical sampling and post-experiment destructive analysis showed that microbial induced sulfate reduction led to metal precipitation on bacteria cells, forming motile biominerals. Precipitation initially occurred in the injection zone, followed by chemotactic migration of D. vulgaris and ultimate accumulation around the nutrient source at the column base. Results from this experiment conducted with metals show (1) polarization anomalies, up to 14 mrad, develop at the bacteria injection and final accumulation areas, (2) the onset of polarization increase occurs concurrently with the onset of lactate consumption, (3) polarization profiles are similar to calculated profiles of the rate of lactate consumption, and (4) temporal changes in polarization and conduction correlate with a geometrical rearrangement of metal-coated bacterial cells. In a second experiment, the same biogeochemical conditions were established except that no metals were added to the flow solution. Polarization anomalies were absent when the experiment was replicated without metals in solution. We therefore attribute the polarization increase observed in the first experiment to a metal-fluid interfacial mechanism that develops as metal sulfides precipitate onto microbial cells and form biominerals. Temporal changes in polarization and conductivity reflect changes in (1) the amount of metal-fluid interfacial area, and (2) the amount of electronic conduction resulting from microbial growth, chemotactic movement and final

  8. Low-frequency fluctuations in plasma magnetic fields

    SciTech Connect

    Cable, S.; Tajima, T.

    1992-02-01

    It is shown that even a non-magnetized plasma with temperature T sustains zero-frequency magnetic fluctuations in thermal equilibrium. Fluctuations in electric and magnetic fields, as well as in densities, are computed. Four cases are studied: a cold, gaseous, isotropic, non-magnetized plasma; a cold, gaseous plasma in a uniform magnetic field; a warm, gaseous plasma described by kinetic theory; and a degenerate electron plasma. For the simple gaseous plasma, the fluctuation strength of the magnetic field as a function of frequency and wavenumber is calculated with the aid of the fluctuation-dissipation theorem. This calculation is done for both collisional and collisionless plasmas. The magnetic field fluctuation spectrum of each plasma has a large zero-frequency peak. The peak is a Dirac {delta}-function in the collisionless plasma; it is broadened into a Lorentzian curve in the collisional plasma. The plasma causes a low frequency cutoff in the typical black-body radiation spectrum, and the energy under the discovered peak approximates the energy lost in this cutoff. When the imposed magnetic field is weak, the magnetic field were vector fluctuation spectra of the two lowest modes are independent of the strength of the imposed field. Further, these modes contain finite energy even when the imposed field is zero. It is the energy of these modes which forms the non-magnetized zero-frequency peak of the isotropic plasma. In deriving these results, a simple relationship between the dispersion relation and the fluctuation power spectrum of electromagnetic waves if found. The warm plasma is shown, by kinetic theory, to exhibit a zero-frequency peak in its magnetic field fluctuation spectrum as well. For the degenerate plasma, we find that electric field fluctuations and number density fluctuations vanish at zero frequency; however, the magnetic field power spectrum diverges at zero frequency.

  9. Low frequency paternal transmission of plastid genes in Brassicaceae.

    PubMed

    Schneider, Anja; Stelljes, Christian; Adams, Caroline; Kirchner, Stefan; Burkhard, Gabi; Jarzombski, Sabine; Broer, Inge; Horn, Patricia; Elsayed, Ashraf; Hagl, Peter; Leister, Dario; Koop, Hans-Ulrich

    2015-04-01

    Plastid-encoded genes are maternally inherited in most plant species. Transgenes located on the plastid genome are thus within a natural confinement system, preventing their distribution via pollen. However, a low-frequency leakage of plastids via pollen seems to be universal in plants. Here we report that a very low-level paternal inheritance in Arabidopsis thaliana occurs under field conditions. As pollen donor an Arabidopsis accession (Ler-Ely) was used, which carried a plastid-localized atrazine resistance due to a point mutation in the psbA gene. The frequency of pollen transmission into F1 plants, based on their ability to express the atrazine resistance was 1.9 × 10(-5). We extended our analysis to another cruciferous species, the world-wide cultivated crop Brassica napus. First, we isolated a fertile and stable plastid transformant (T36) in a commercial cultivar of B. napus (cv Drakkar). In T36 the aadA and the bar genes were integrated in the inverted repeat region of the B. napus plastid DNA following particle bombardment of hypocotyl segments. Southern blot analysis confirmed transgene integration and homoplasmy of plastid DNA. Line T36 expressed Basta resistance from the inserted bar gene and this trait was used to estimate the frequency of pollen transmission into F1 plants. A frequency of <2.6 × 10(-5) was determined in the greenhouse. Taken together, our data show a very low rate of paternal plastid transmission in Brassicacea. Moreover, the establishment of plastid transformation in B. napus facilitates a safe use of this important crop plant for plant biotechnology. PMID:25343875

  10. Low frequency spin dynamics in a quantum Hall canted antiferromagnet

    NASA Astrophysics Data System (ADS)

    Muraki, Koji

    2007-03-01

    In quantum Hall (QH) systems, Coulomb interactions combined with the macroscopic degeneracy of Landau levels (LLs) drive the electron system into strongly correlated phases as illustrated by the series of fractional QH effects and may also lead to various forms of broken symmetry dictated by the LL filing factor ν. When two layers of such electron systems are closely separated by a thin tunnel barrier, the addition of interlayer interactions and the layer degree of freedom brings about even richer electronic phases, opening up possibilities for different classes of symmetry breaking. In particular, at total filling factor νT = 2, where the two of the four lowest LLs split by the Zeeman and interlayer tunnel couplings are occupied, the competing degrees of freedom due to the layer and spin are predicted to lead to rich magnetic phases. Here we present results of resistively detected nuclear spin relaxation measurements in closely separated electron systems that reveal strong low-frequency spin fluctuations in the QH regime at νT = 2 [1]. As the temperature is decreased, the spin fluctuations, manifested by a sharp enhancement of the nuclear spin-lattice relaxation rate 1/T1, continue to grow down to the lowest temperature of 66 mK. The observed divergent behavior of 1/T1 signals a gapless spin excitation mode (i.e., a Goldstone mode) and is a hallmark of the theoretically predicted canted antiferromagnetic order. Our data demonstrate the realization of a two-dimensional system with broken planar spin rotational symmetry, in which fluctuations do not freeze out when approaching the zero temperature limit. [1] N. Kumada, K. Muraki, and Y. Hirayama, Science 313, 329 (2006).

  11. LOW-FREQUENCY OSCILLATIONS IN XTE J1550-564

    SciTech Connect

    Rao Fengyun; Belloni, Tomaso; Stella, Luigi; Zhang Shuangnan; Li Tipei E-mail: tomaso.belloni@brera.inaf.i

    2010-05-10

    We present the results of a timing analysis of the low-frequency quasi-periodic oscillation (QPO) in the Rossi X-Ray Timing Explorer data of the black hole binary XTE J1550-564 during its 1998 outburst. The QPO frequency is observed to vary on timescales between {approx}100 s and days, correlated with the count rate contribution from the optically thick accretion disk: we studied this correlation and discuss its influence on the QPO width. In all observations, the quality factors ({nu}{sub 0}/FWHM) of the fundamental and second harmonic peaks were observed to be consistent, suggesting that the quasi-periodic nature of the oscillation is due to frequency modulation. In addition to the QPO and its harmonic peaks, a new 1.5{nu} component was detected in the power spectra. This component is broad, with a quality factor of {approx}0.6. From this, we argue that the peak observed at half the QPO frequency, usually referred to as 'sub-harmonic', could be the fundamental frequency, leading to the sequence 1:2:3:4. We also studied the energy dependence of the timing features and conclude that the two continuum components observed in the power spectrum, although both more intense at high energies, show a different dependence on energy. At low energies, the lowest-frequency component dominates, while at high energies the higher-frequency one has a higher fractional rms. An interplay between these two components was also observed as a function of their characteristic frequency. In this source, the transition between the low/hard state and the hard-intermediate state appears to be a smooth process.

  12. Abnormality of low frequency cerebral hemodynamics oscillations in TBI population.

    PubMed

    Chernomordik, Victor; Amyot, Franck; Kenney, Kimbra; Wassermann, Eric; Diaz-Arrastia, Ramon; Gandjbakhche, Amir

    2016-05-15

    Functional Near Infrared Spectroscopy (fNIRS) can non-invasively capture dynamic cognitive activation and underlying physiological processes by measuring changes in oxy- and deoxy-hemoglobin levels, correlated to brain activation. It is a portable, inexpensive and user-friendly device which is easily adapted to the outpatient setting for the assessment of cognitive functions after Traumatic Brain Injury (TBI). Low frequency oscillations in hemodynamic signal, attributed in the literature to cerebral autoregulation, were assessed using recently introduced metrics, Oxygenation Variability (OV Index), obtained from oxy/deoxy-hemoglobin variations in response to mental tasks for a group of healthy control (HC, n=14) and TBI (n=29). Participants responded to an action complexity judgment task (evaluating the complexity of daily life activities by classifying the number of steps as "few" or "many") with a varying degree of cognitive load to produce brain activation. During the task, we measured blood variations with fNIRS and analyzed OV Index changes. Mean OV indices, corresponding to high complexity tasks, are higher than that of low complexity tasks in the HC group, revealing strong parametric effect (0.039±0.017 for low, 0.057±0.036 for high, p-value=0.069). However, no significant difference has been recorded for the OV indexes for two different loads in the TBI group (0.055±0.033 for low, 0.054±0.035 for high, p=0.9). OV index metrics proves to be sensitive to chronic TBI and can potentially be used to separate subpopulations TBI vs. HC. Noticeable differences in OV index spatial distributions between subpopulations have been observed. PMID:26996413

  13. Design Concepts for a Sky Noise Limited Low Frequency Array

    NASA Astrophysics Data System (ADS)

    Bregman, Jaap D.

    The LOw Frequency ARray is an aperture synthesis instrument for the frequency range from10 MHz up to over 300 MHz. There are order thirty stations spanning baselines up to 300 km and distributed such that bandwidth synthesis in the semi-octave observing bands provides sufficient visibility sampling for reliable wide-field imaging. Each antenna station has a few hundred active dipole receptors arranged in a sparse fractal like configuration of about two hundred meter diameter. With appropriate weighting an almost frequency independent station beamwidth is obtained. The effective collecting area scales proportional with wavelength squared approaching at the lowest frequency a square kilometre for all stations together. The digital beamformers at the stations not only perform the spatio-spectral nulling to adaptively remove any interference, but also form the spectrometer part of the array correlator. We propose the F3X correlator architecture where the efficient Fourier butterfly algorithm is not only used to transform a time sequence into a frequency sequence but also to transform a set of station sub-apertures into a set of beams before cross-correlation between antenna stations is performed. Commercial digital signal processing elements form by 2006 a cost effective implementation solution creating a "processing window of opportunity" where the signal processing power of beam-formers and correlators turns out to be matched to the data processing power of a battery of a few hundred general purpose image processing engines. We are entering the era where the performance of processing electronics in phased array antennas can cost effectively meet the performance requirements of a synthesis array.

  14. Tremor entrainment by patterned low-frequency stimulation.

    PubMed

    Barnikol, Utako B; Popovych, Oleksandr V; Hauptmann, Christian; Sturm, Volker; Freund, Hans-Joachim; Tass, Peter A

    2008-10-13

    High-frequency test stimulation for tremor suppression is a standard procedure for functional target localization during deep brain stimulation. This method does not work in cases where tremor vanishes intraoperatively, for example, due to general anaesthesia or due to an insertional effect. To overcome this difficulty, we developed a stimulation technique that effectively evokes tremor in a well-defined and quantifiable manner. For this, we used patterned low-frequency stimulation (PLFS), i.e. brief high-frequency pulse trains administered at pulse rates similar to neurons' preferred burst frequency. Unlike periodic single-pulse stimulation, PLFS enables one to convey effective and considerably greater integral charge densities without violation of safety requirements. In a computational investigation of an oscillatory neuronal network temporarily rendered inactive, we found that PLFS evokes synchronized activity, phase locked to the stimulus. While a stronger increase in the amount of synchrony in the neuronal population requires higher stimulus intensities, the portion of synchronously active neurons nevertheless becomes strongly phase locked to PLFS already at weak stimulus intensities. The phase entrainment effect of PLFS turned out to be robust against variations in the stimulation frequency, whereas enhancement of synchrony required precisely tuned stimulation frequencies. We applied PLFS to a patient with spinocerebellar ataxia type 2 (SCA2) with pronounced tremor that disappeared intraoperatively under general anaesthesia. In accordance with our computational results, PLFS evoked tremor, phase locked to the stimulus. In particular, weak PLFS caused low-amplitude, but strongly phase-locked tremor. PLFS test stimulations provided the only functional information about target localization. Optimal target point selection was confirmed by excellent post-operative tremor suppression. PMID:18632457

  15. A low-frequency directional flextensional transducer and line array

    SciTech Connect

    Butler, S.C.; Butler, J.L.; Butler, A.L.; Cavanagh, G.H.

    1997-07-01

    A unique low-frequency (900 Hz) class IV flextensional transducer that produces an enhanced far-field pressure on one side and canceled far-field pressure on the other side has been developed. The transducer radiating surface consists of a thick-walled elliptical aluminum shell and a U.S. Navy type III piezoelectric stack along its major axis with two active sections and one inactive section. The directionality is achieved by simultaneously exciting the shell into an omnidirectional and dipole operation by driving stack into both extensional and bending modes. Both measurements and modeling on this device show a front to back pressure ratio of more than 30 dB, producing cardioid-type radiation patterns over an octave band, for a single transducer element. The transducers measured mechanical Q is 8, coupling coefficient is 0.25, and electroacoustic efficiency is 80{percent} and produced a source level of 215 dB {ital re:} 1 {mu}Pa at 1 m when driven at a field limit of 394 kV/m (10 kV/in.) at resonance. The uniqueness of this transducer is its directional beam patterns (directivity index=3.4 dB) and high acoustic output power from a small (less than a third of a wavelength) single element. Six of these transducers were placed in a closely packed line array two-wavelengths long. The array successfully produced narrow directional sound beams (directivity index=8.7 dB) with a front to back ratio greater than 30 dB and a source level of 225 dB {ital re:} 1 {mu}Pa at 1 m. {copyright} {ital 1997 Acoustical Society of America.}

  16. Heating of the nighttime D region by very low frequency transmitters

    NASA Technical Reports Server (NTRS)

    Rodriguez, Juan V.; Inan, Umran S.; Bell, Timothy F.

    1994-01-01

    VLF signals propagating in the Earth-ionosphere waveguide are used to probe the heated nighttime D region over three U.S. Navy very low frequency (VLF,3-30 kHz) transmitters. Ionospheric cooling and heating are observed when a transmitter turns off and on in the course of normal operations. Heating by the 24.0-kHz NAA transmitter in Cutler, Maine, (1000 kW radiated power) was observed by this method in 41 of 52 off/on episodes during December 1992, increasing the amplitude and retarding the phase of the 21.4-kHz NSS probe wave propagating from Annapolis, Maryland, to Gander, Newfoundland, by as much as 0.84 dB and 5.3 deg, respectively. In 6 of these 41 episodes, the amplitude of the 28.5-kHz NAU probe wave propagating from Puerto Rico to Gander was also perturbed by as much as 0.29 dB. The latter observations were unexpected due to the greater than 770 km distance between NAA and the NAU-Gander great circle path. Heating by the NSS (21.4 kHz, 265 kW) and NLK (24.8 kHz, 850 kW) transmitters was observed serendipitously in data from earlier measurements of the amplitudes of VLF signals propagating in the Earth-ionosphere waveguide. A three-dimensional model of wave absorption and electron heating in a magnetized, weakly ionized plasma is used to calculate the extent nad shape of the collision frequency (i.e., electron temperature) enhancement above a VLF transmitter. The enhancements are annular, with a geomagnetic north-south asymmetry and a radius at the outer half-maximum of the collision frequency enhancement of about 150 km. Heating by the NAA transmitter is predicted to increase the nighttime D region electron temperature by as much as a factor of 3. The calculated changes in the D region conductivity are used in a three-dimensional model of propagation in the Earth-ionosphere wavelength to predict the effect of the heated patch on a subionospheric VLF probe wave. The range of predicted scattered field amplitudes is in general consistent with the observed

  17. Low-frequency velocity correlation spectrum of fluid in a porous media by modulated gradient spin echo.

    PubMed

    Stepisnik, J; Callaghan, P T

    2001-01-01

    In addition to the fast correlation for local stochastic motion, the molecular velocity correlation function in a fluid enclosed within the pore boundaries features a slow long time-tail decay. Here we present its study by the NMR modulated gradient spin-echo method (MGSE) [1] on a system of water trapped in the space between the closely packed polystyrene beads. With MGSE pulse sequence, a repetitive train of RF pulses with interspersed gradient pulses periodically modulates the spin phase. It gives the spin echo attenuation proportional to a value of the molecular velocity correlation spectrum at the modulation frequency. Covering the frequency range between Hz and MHz, it is a complement to the quasi-elastic neutron scattering, and so a suitable technique for the investigation of low frequency molecular dynamics in fluids. In our experiment, it enables to extract the low frequency correlation spectrum of water molecules confined in porous media. The function exhibits a negative long time-tail characteristic (a low frequency decay of the spectrum), which can be interpreted as a molecular back scattering on boundaries. The results can be well fitted with the spectrum calculated from the solution of the Langevin equation for restricted diffusion (which exhibits an exponential decay) [2] as well as with the spectrum obtained when simulating the hydrodynamics of molecular motion constrained by capillary walls (which gives an algebraic decay) [3]. Despite much work on theories and simulation, which predict slow negative long time tail of molecular velocity correlation dynamics in confined fluids, the obtained velocity correlation spectrum is the first experimental evidence to confirm these effects. The obtained dependence of spin echo attenuation on time, gradient strength and modulation frequency is also the first experimental verification of the recently developed approach to the spin echo in porous media, that uses the spin phase average with the cumulant expansion

  18. Low-frequency predictability of the Dynamical Extended-Range Forecast Experiment

    NASA Technical Reports Server (NTRS)

    Nogues-Peagle, Julia; Rodgers, Dennis A.; Mo, Kingtse C.

    1992-01-01

    The objective of the study was to analyze data from the Dynamical Extended-Range Forecast Experiment conducted from January 1986 to March 1987, and to evaluate differences between analysis and forecasts with emphasis on the tropical 30-50-day oscillation. The diagnostic toll used is the projection of analysis and forecast data onto the normal modes of a primitive equation model. Examination of zonal-wind anomalies in the tropics shows that the forecast model predicts propagation of intraseasonal variations more accurately for slow propagation rates. The forecast amplitude is generally weaker than the analyzed amplitude. Analyzed kinetic energy and error fields exhibit similar horizontal scales for internal and external modes. External Rossby-mode components maximize in the extratropics while Rossby internal modes exhibit patterns that extend over the entire globe.

  19. A Population Based Study of the Genetic Association between Catecholamine Gene Variants and Spontaneous Low-Frequency Fluctuations in Reaction Time

    PubMed Central

    Bastiaansen, Jojanneke A.; Cummins, Tarrant D. R.; Riese, Harriëtte; van Roon, Arie M.; Nolte, Ilja M.; Oldehinkel, Albertine J.; Bellgrove, Mark A.

    2015-01-01

    The catecholamines dopamine and noradrenaline have been implicated in spontaneous low-frequency fluctuations in reaction time, which are associated with attention deficit hyperactivity disorder (ADHD) and subclinical attentional problems. The molecular genetic substrates of these behavioral phenotypes, which reflect frequency ranges of intrinsic neuronal oscillations (Slow-4: 0.027-0.073 Hz; Slow-5: 0.010-0.027 Hz), have not yet been investigated. In this study, we performed regression analyses with an additive model to examine associations between low-frequency fluctuations in reaction time during a sustained attention task and genetic markers across 23 autosomal catecholamine genes in a large young adult population cohort (n = 964), which yielded greater than 80% power to detect a small effect size (f2 = 0.02) and 100% power to detect a small/medium effect size (f2 = 0.15). At significance levels corrected for multiple comparisons, none of the gene variants were associated with the magnitude of low-frequency fluctuations. Given the study’s strong statistical power and dense coverage of the catecholamine genes, this either indicates that associations between low-frequency fluctuation measures and catecholamine gene variants are absent or that they are of very small effect size. Nominally significant associations were observed between variations in the alpha-2A adrenergic receptor gene (ADRA2A) and the Slow-5 band. This is in line with previous reports of an association between ADRA2A gene variants and general reaction time variability during response selection tasks, but the specific association of these gene variants and low-frequency fluctuations requires further confirmation. Pharmacological challenge studies could in the future provide convergent evidence for the noradrenergic modulation of both general and time sensitive measures of intra-individual variability in reaction time. PMID:25978426

  20. Characterization of Indoor Extremely Low Frequency and Low Frequency Electromagnetic Fields in the INMA-Granada Cohort

    PubMed Central

    Calvente, Irene; Dávila-Arias, Cristina; Ocón-Hernández, Olga; Pérez-Lobato, Rocío; Ramos, Rosa; Artacho-Cordón, Francisco; Olea, Nicolás; Núñez, María Isabel; Fernández, Mariana F.

    2014-01-01

    Objective To characterize the exposure to electric fields and magnetic fields of non-ionizing radiation in the electromagnetic spectrum (15 Hz to 100 kHz) in the dwellings of children from the Spanish Environment and Childhood-“INMA” population-based birth cohort. Methodology The study sample was drawn from the INMA-Granada cohort. Out of 300 boys participating in the 9–10 year follow-up, 123 families agreed to the exposure assessment at home and completed a specific ad hoc questionnaire gathering information on sources of non-ionizing radiation electric and magnetic fields inside the homes and on patterns of use. Long-term indoor measurements were carried out in the living room and bedroom. Results Survey data showed a low exposure in the children's homes according to reference levels of the International Commission on Non-Ionizing Radiation Protection but with large differences among homes in mean and maximum values. Daytime electrostatic and magnetic fields were below the quantification limit in 78.6% (92 dwellings) and 92.3% (108 dwellings) of houses, with an arithmetic mean value (± standard deviation) of 7.31±9.32 V/m and 162.30±91.16 nT, respectively. Mean magnetic field values were 1.6 lower during the night than the day. Nocturnal electrostatic values were not measured. Exposure levels were influenced by the area of residence (higher values in urban/semi-urban versus rural areas), type of dwelling, age of dwelling, floor of the dwelling, and season. Conclusion Given the greater sensitivity to extremely low-frequency electromagnetic fields of children and following the precautionary principle, preventive measures are warranted to reduce their exposure. PMID:25192253

  1. PageRank for low frequency earthquake detection

    NASA Astrophysics Data System (ADS)

    Aguiar, A. C.; Beroza, G. C.

    2013-12-01

    We have analyzed Hi-Net seismic waveform data during the April 2006 tremor episode in the Nankai Trough in SW Japan using the autocorrelation approach of Brown et al. (2008), which detects low frequency earthquakes (LFEs) based on pair-wise waveform matching. We have generalized this to exploit the fact that waveforms may repeat multiple times, on more than just a pair-wise basis. We are working towards developing a sound statistical basis for event detection, but that is complicated by two factors. First, the statistical behavior of the autocorrelations varies between stations. Analyzing one station at a time assures that the detection threshold will only depend on the station being analyzed. Second, the positive detections do not satisfy "closure." That is, if window A correlates with window B, and window B correlates with window C, then window A and window C do not necessarily correlate with one another. We want to evaluate whether or not a linked set of windows are correlated due to chance. To do this, we map our problem on to one that has previously been solved for web search, and apply Google's PageRank algorithm. PageRank is the probability of a 'random surfer' to visit a particular web page; it assigns a ranking for a webpage based on the amount of links associated with that page. For windows of seismic data instead of webpages, the windows with high probabilities suggest likely LFE signals. Once identified, we stack the matched windows to improve the snr and use these stacks as template signals to find other LFEs within continuous data. We compare the results among stations and declare a detection if they are found in a statistically significant number of stations, based on multinomial statistics. We compare our detections using the single-station method to detections found by Shelly et al. (2007) for the April 2006 tremor sequence in Shikoku, Japan. We find strong similarity between the results, as well as many new detections that were not found using

  2. A Sub-Hertz, Low-Frequency Vibration Isolation Platform

    NASA Technical Reports Server (NTRS)

    Ortiz, Gerardo, G.; Farr, William H.; Sannibale, Virginio

    2011-01-01

    One of the major technical problems deep-space optical communication (DSOC) systems need to solve is the isolation of the optical terminal from vibrations produced by the spacecraft navigational control system and by the moving parts of onboard instruments. Even under these vibration perturbations, the DSOC transceivers (telescopes) need to be pointed l000 fs of times more accurately than an RF communication system (parabolic antennas). Mechanical resonators have been extensively used to provide vibration isolation for groundbased, airborne, and spaceborne payloads. The effectiveness of these isolation systems is determined mainly by the ability of designing a mechanical oscillator with the lowest possible resonant frequency. The Low-Frequency Vibration Isolation Platform (LFVIP), developed during this effort, aims to reduce the resonant frequency of the mechanical oscillators into the sub-Hertz region in order to maximize the passive isolation afforded by the 40 dB/decade roll-off response of the resonator. The LFVIP also provides tip/tilt functionality for acquisition and tracking of a beacon signal. An active control system is used for platform positioning and for dampening of the mechanical oscillator. The basic idea in the design of the isolation platform is to use a passive isolation strut with an approximately equal to 100-mHz resonance frequency. This will extend the isolation range to lower frequencies. The harmonic oscillator is a second-order lowpass filter for mechanical disturbances. The resonance quality depends on the dissipation mechanisms, which are mainly hysteretic because of the low resonant frequency and the absence of any viscous medium. The LFVIP system is configured using the well-established Stewart Platform, which consists of a top platform connected to a base with six extensible struts (see figure). The struts are attached to the base and to the platform via universal joints, which permit the extension and contraction of the struts. The

  3. Low Frequency Radio-wave System for subsurface investigation

    NASA Astrophysics Data System (ADS)

    Soldovieri, Francesco; Gennarelli, Gianluca; Kudelya, Anatoliy; Denisov, Alexander

    2015-04-01

    Low frequency radio-wave methods (RWM) allow subsurface investigations in terms of lithological structure characterization, detection of filtration flows of ground water, anthropogenic and natural cavities. In this contribution, we present a RWM that exploits two coils working at frequencies of few MHz as transmitting and receiving antennas. The basic principle of this inductive method is as follows. The primary alternating electromagnetic field radiated by the transmitting coil induces eddy currents in the subsurface mainly due to the conductivity anomalies. These eddy currents generate a secondary (scattered) magnetic field which overlaps to the incident magnetic field and is detected by the receiving coil. Despite the simple operation of the system, the complexity of the electromagnetic scattering phenomenon at hand must be properly modeled to achieve adequate performance. Therefore, an advanced data processing technique, belonging to the class of the inverse scattering approaches, has been developed by the authors in a full 3D geometry. The proposed method allows to deal with data collected on a scanning surface under a dipole inductive profiling (DIP) modality, where the transmitting/receiving coils are moved simultaneously with fixed offset (multi-bistatic configuration). The hardware, called Dipole Inductive Radio-wave System (DIRS), is composed by an electronic unit and transmitting and receiving loop antennas radiating at frequencies of few MHz (2-4 MHz), which are installed on theodolite supports. The compactness of DIRS and its robustness to external electromagnetic interference offers the possibility to perform geophysical research up to the depth of some tens of meters and under several types of ground and water surfaces, vegetation, and weather conditions. The light weight and small size of system (the single antenna with support weights about 5 kg and has a diameter of 0.5m) allows two operators to perform geophysical research without disturbing the

  4. Low-Frequency Earthquakes in Cascadia Using Texan Array

    NASA Astrophysics Data System (ADS)

    Sweet, J. R.; Creager, K. C.; Vidale, J. E.; Ghosh, A.; Nichols, M. L.; Pratt, T. L.

    2008-12-01

    Low-frequency earthquakes (LFEs) were first reported in Japan and have been observed to occur coincidently with non-volcanic tremor in both space and time. Compared to similarly-sized ordinary earthquakes, LFEs are deficient in frequencies above 5 Hz. The frequency spectrum of LFEs mirrors the spectrum of tremor. Indeed Shelly et al. (2006, 2007, Nature) have suggested that tremor is simply the superposition of many individual LFEs. Accordingly, LFEs have been used to constrain the location of tremor. In Japan, LFEs are routinely identified by their S-waves, while their P-waves are typically below noise levels. In March 2008 we deployed a 1km aperture array on the Olympic Peninsula of Washington State that recorded a small tremor swarm prior to the main episodic tremor and slip event in May. Our array consisted of approximately 80 geophones paired with Texan recorders and was laid out on a 100m spaced grid. Initial analysis of one hour of data reveals many LFE-like events with similar spectra to locally observed tremor. Unlike LFEs in Japan, P-waves are clearly seen on many individual stations. Using a clear LFE as a template event, nearly 100 matching events have been found with S minus P times that differ by less than a few hundredths of a second from event to event suggesting that they are all within a few hundred meters of each other. Preliminary locations of this cluster indicate that the LFEs are near the plate interface east of our array. For many of the LFEs, stacks of the P-wave are very similar to stacks of the S-wave, suggesting that these stacks are reasonably good approximations of source time functions. These source time functions vary in complexity from one LFE to another. Synthetic waveform modeling suggests a source focal mechanism consistent with thrust faulting on the plate interface. The location and focal mechanism of these LFEs support the notion that tremor is associated with slip on the plate interface. This LFE cluster provides a promising

  5. Frequency-dependent moment release of very low frequency earthquakes in the Cascadia subduction zone

    NASA Astrophysics Data System (ADS)

    Takeo, A.; Houston, H.

    2014-12-01

    Episodic tremor and slip (ETS) has been observed in Cascadia subduction zone at two different time scales: tremor at a high-frequency range of 2-8 Hz and slow slip events at a geodetic time-scale of days-months. The intermediate time scale is needed to understand the source spectrum of slow earthquakes. Ghosh et al. (2014, IRIS abs) recently reported the presence of very low frequency earthquakes (VLFEs) in Cascadia. In southwest Japan, VLFEs are usually observed at a period range around 20-50 s, and coincide with tremors (e.g., Ito et al. 2007). In this study, we analyzed VLFEs in and around the Olympic Peninsula to confirm their presence and estimate their moment release. We first detected VLFE events by using broadband seismograms with a band-pass filter of 20-50 s. The preliminary result shows that there are at least 16 VLFE events with moment magnitudes of 3.2-3.7 during the M6.8 2010 ETS. The focal mechanisms are consistent with the thrust earthquakes at the subducting plate interface. To detect signals of VLFEs below noise level, we further stacked long-period waveforms at the peak timings of tremor amplitudes for tremors within a 10-15 km radius by using tremor catalogs in 2006-2010, and estimated the focal mechanisms for each tremor source region as done in southwest Japan (Takeo et al. 2010 GRL). As a result, VLFEs could be detected for almost the entire tremor source region at a period range of 20-50 s with average moment magnitudes in each 5-min tremor window of 2.4-2.8. Although the region is limited, we could also detect VLFEs at a period range of 50-100 s with average moment magnitudes of 3.0-3.2. The moment release at 50-100 s is 4-8 times larger than that at 20-50 s, roughly consistent with an omega-squared spectral model. Further study including tremor, slow slip events and characteristic activities, such as rapid tremor reversal and tremor streaks, will reveal the source spectrum of slow earthquakes in a broader time scale from 0.1 s to days.

  6. Low-frequency transient dynamic clusters in simulated amorphous Ni0.5Zr0.5 around the glass temperature

    NASA Astrophysics Data System (ADS)

    Ladadwa, I.; Teichler, H.

    2008-10-01

    Molecular dynamics results are reported concerning cooperatively rearranging regions in simulated Ni0.5Zr0.5 melts down to 700K . Emphasis is laid on discriminating between clusters of mobile atoms (CMA) from low-frequency dynamics and the all-frequency case, where the former characterize fluctuations and relaxations on the scale of the late β regime and α decay, while the latter include, in addition, reversible high-frequency vibrations. Separation of the low-frequency part of the dynamics is carried out by low-pass filtering, exploiting the separation of time scales below the critical temperature Tc of the mode-coupling theory. With increasing temperature, the low-frequency and all-frequency dynamics merge in the range of Tc when the separation of time scales disappears. In the low-frequency CMA, the average size of correlated clusters of connected atoms turns out to be nearly one order of magnitude larger than in the all-frequency CMA. The low-frequency CMA appear as local clusters propagating extremely slowly in space with characteristic time scale of μs at 700K , the scale of the onset of α decay.

  7. Low-frequency connectivity is associated with mild traumatic brain injury.

    PubMed

    Dunkley, B T; Da Costa, L; Bethune, A; Jetly, R; Pang, E W; Taylor, M J; Doesburg, S M

    2015-01-01

    Mild traumatic brain injury (mTBI) occurs from a closed-head impact. Often referred to as concussion, about 20% of cases complain of secondary psychological sequelae, such as disorders of attention and memory. Known as post-concussive symptoms (PCS), these problems can severely disrupt the patient's quality of life. Changes in local spectral power, particularly low-frequency amplitude increases and/or peak alpha slowing have been reported in mTBI, but large-scale connectivity metrics based on inter-regional amplitude correlations relevant for integration and segregation in functional brain networks, and their association with disorders in cognition and behaviour, remain relatively unexplored. Here, we used non-invasive neuroimaging with magnetoencephalography to examine functional connectivity in a resting-state protocol in a group with mTBI (n = 20), and a control group (n = 21). We observed a trend for atypical slow-wave power changes in subcortical, temporal and parietal regions in mTBI, as well as significant long-range increases in amplitude envelope correlations among deep-source, temporal, and frontal regions in the delta, theta, and alpha bands. Subsequently, we conducted an exploratory analysis of patterns of connectivity most associated with variability in secondary symptoms of mTBI, including inattention, anxiety, and depression. Differential patterns of altered resting state neurophysiological network connectivity were found across frequency bands. This indicated that multiple network and frequency specific alterations in large scale brain connectivity may contribute to overlapping cognitive sequelae in mTBI. In conclusion, we show that local spectral power content can be supplemented with measures of correlations in amplitude to define general networks that are atypical in mTBI, and suggest that certain cognitive difficulties are mediated by disturbances in a variety of alterations in network interactions which are differentially expressed across

  8. Low-frequency connectivity is associated with mild traumatic brain injury

    PubMed Central

    Dunkley, B.T.; Da Costa, L.; Bethune, A.; Jetly, R.; Pang, E.W.; Taylor, M.J.; Doesburg, S.M.

    2015-01-01

    Mild traumatic brain injury (mTBI) occurs from a closed-head impact. Often referred to as concussion, about 20% of cases complain of secondary psychological sequelae, such as disorders of attention and memory. Known as post-concussive symptoms (PCS), these problems can severely disrupt the patient's quality of life. Changes in local spectral power, particularly low-frequency amplitude increases and/or peak alpha slowing have been reported in mTBI, but large-scale connectivity metrics based on inter-regional amplitude correlations relevant for integration and segregation in functional brain networks, and their association with disorders in cognition and behaviour, remain relatively unexplored. Here, we used non-invasive neuroimaging with magnetoencephalography to examine functional connectivity in a resting-state protocol in a group with mTBI (n = 20), and a control group (n = 21). We observed a trend for atypical slow-wave power changes in subcortical, temporal and parietal regions in mTBI, as well as significant long-range increases in amplitude envelope correlations among deep-source, temporal, and frontal regions in the delta, theta, and alpha bands. Subsequently, we conducted an exploratory analysis of patterns of connectivity most associated with variability in secondary symptoms of mTBI, including inattention, anxiety, and depression. Differential patterns of altered resting state neurophysiological network connectivity were found across frequency bands. This indicated that multiple network and frequency specific alterations in large scale brain connectivity may contribute to overlapping cognitive sequelae in mTBI. In conclusion, we show that local spectral power content can be supplemented with measures of correlations in amplitude to define general networks that are atypical in mTBI, and suggest that certain cognitive difficulties are mediated by disturbances in a variety of alterations in network interactions which are differentially expressed across

  9. Monitoring the Low Frequency Sky with the LWA1 and the Prototype All-Sky Imager

    NASA Astrophysics Data System (ADS)

    Obenberger, Kenneth Steven; LWA Collaboration

    2015-01-01

    We present findings from the Prototype All-Sky Imager (PASI), a backend correlator of the first station of the Long Wavelength Array (LWA1). PASI cross-correlates a live stream of all 260 dual-polarization dipole antennas of the LWA1, creates all-sky images, and uploads them to the LWA-TV website in near real-time. PASI has recorded over 14,000 hours of all-sky images at frequencies between 10 and 88 MHz. These data have resulted in the discovery of radio emission from large meteors (Fireballs), and has been used to set improved limits on slow transients at 38, 52, and 74 MHz. PASI is also being used to characterize how the ionosphere affects low frequency transient astronomy. Construction of the LWA has been supported by the Office of Naval Research under Contract N00014-07-C-0147. Support for operations and continuing development of the LWA1 is provided by the National Science Foundation under grants AST-1139963 and AST-1139974 of the University Radio Observatory program.

  10. Using systematically characterized low-frequency earthquakes as a fault probe in Guerrero, Mexico

    NASA Astrophysics Data System (ADS)

    Frank, William B.; Shapiro, Nikolaï M.; Husker, Allen L.; Kostoglodov, Vladimir; Romanenko, Alexey; Campillo, Michel

    2014-10-01

    Studies of low-frequency earthquakes (LFEs) have focused on detecting events within previously identified tectonic tremor. However, the principal LFE detection tools of matched-filter searches are intrinsically incapable of detecting events that have not already been characterized previously as a template event. In this study, we therefore focus on generating the largest number possible of LFE templates by uniformly applying a recently developed LFE template detection method to a 2.5 yearlong data set in Guerrero, Mexico. Using each of the detected templates in a matched-filter search, we then form event families that each represents a single source. We finally develop simple, empirical statistics to select the event families that represent LFEs. Our resulting catalog contains 1120 unique LFE sources and a total of 1,849,486 detected LFEs over the 2.5 yearlong data set. The locations of the LFE sources are then divided into subcatalogs based on their distance from the subduction trench. Considering each LFE as a small unit of slip along the subduction interface, we observe discrete episodes of LFE activity in the region associated with large slow-slip events; this is in direct contrast to the near-continuous activity observed 35 km farther downdip within the previously identified LFE/tremor sweet spot.

  11. Respiratory calcium fluctuations in low-frequency oscillating astrocytes in the pre-Bötzinger complex.

    PubMed

    Oku, Yoshitaka; Fresemann, Jens; Miwakeichi, Fumikazu; Hülsmann, Swen

    2016-06-01

    Astrocytes have been found to modulate neuronal activity through calcium-dependent signaling in various brain regions. However, whether astrocytes of the pre-Bötzinger complex (preBötC) exhibit respiratory rhythmic fluctuations is still controversial. Here we evaluated calcium-imaging experiments within preBötC in rhythmically active medullary slices from TgN(hGFAP-EGFP) mice using advanced analyses. 13.8% of EGFP-negative cells, putative neurons, showed rhythmic fluorescent changes that were highly correlated to the respiratory rhythmic fluctuation (cross-correlation coefficient>0.5 and dF/F>0.2%). In contrast, a considerable number of astrocyte somata exhibited synchronized low-frequency (<0.03Hz) calcium oscillations. After band-pass filtering, signals that irregularly preceded the calcium signal of EGFP-negative cells were observed in 10.2% of astrocytes, indicating a functional coupling between astrocytes and neurons in preBötC. A model simulation confirmed that such preinspiratory astrocytic signals can arise from coupled neuronal and astrocytic oscillators, supporting a concept that slow oscillatory changes of astrocytic functions modulate neighboring neuronal activity to add variability in respiratory rhythm. PMID:25747384

  12. Investigating the low-frequency earthquake and non-volcanic tremor system

    NASA Astrophysics Data System (ADS)

    Frank, W.; Shapiro, N.; Kostoglodov, V.; Husker, A. L.; Daub, E. G.; Radiguet, M.; Guyer, R. A.; Nadeau, R. M.; Campillo, M.; Payero, J. S.; Prieto, G. A.; Shelly, D. R.; Johnson, P. A.

    2013-12-01

    The current prevailing theory in the literature is that non-volcanic tremor (NVT) is made up of a swarm of low-frequency earthquakes (LFEs) (Shelly et al. [2007]), suggesting a burst-like behavior of LFEs. We attempt to study this relationship using LFE and NVT catalogs from Guerrero, Mexico and Parkfield, California. In Mexico, we use the NVT catalog of Husker et al. (2012) and the modified LFE catalog of Frank et al. (2013) which is generated by using a new automatic detection method. In Parkfield, we use the LFE catalog of Shelly et al. (2010) and the NVT catalog of Guilhem and Nadeau (2012). Both of the LFE catalogs exhibit a clear burst-like, or episodic, behavior. We develop an algorithm to generate catalogs of observed bursts, regrouping only the events that participate in a burst-like behavior. This opens the possibility to quantify the relationship between NVT and LFEs, as we can directly compare the activity of the LFE bursts, which are potentially the main constituents of tremor, to the NVT catalogs. This relationship is key to understanding how slip and deformation manifest in slow earthquake source regions and could provide a better picture of the continuum of processes that lies between traditional stick-slip and stable-sliding behavior.

  13. Effect of preionization in Aton-type Hall thruster on low frequency oscillation

    SciTech Connect

    Yu Daren; Wei Liqiu; Zhao Zuoyang; Han Ke; Yan Guojun

    2008-04-15

    It was found through the experiments made with an Aton-type Hall thruster that some of the propellant was ionized in the buffer chamber by 'quick electrons'. This ionization is called 'preionization' to discriminate it from the ionization in the discharge channel. The effect of preionization on low frequency oscillation was experimentally studied by changing the electric field intensity in the buffer chamber. The relationship between low frequency oscillation and preionization ratio was investigated through numerical simulation using a one-dimensional quasineutrality hydrodynamic model. The results obtained indicate that the amplitude of low frequency oscillation decreases as the preionization ratio increases. It was found through the analysis and numerical simulation of the physical process of low frequency oscillation that the positive feedback of electron density was the main cause of low frequency oscillation. The increase of preionization ratio decreases the amplitude of the feedback variation thereby reducing the amplitude of low frequency oscillation.

  14. Mechanical and Acoustic Signature of Slow Earthquakes on Laboratory Faults

    NASA Astrophysics Data System (ADS)

    Scuderi, Marco Maria; Marone, Chris; Tinti, Elisa; Scognamiglio, Laura; Di Stefano, Giuseppe; Collettini, Cristiano

    2015-04-01

    Recent seismic and geodetic observations show that fault slip occurs via a spectrum of behaviors that range from seismic (fast dynamic) to aseismic (creep). Indeed faults can slip via a variety of quasi-dynamic processes such as Slow-Slip, Low Frequency Earthquakes (LFE), and Tremor. These transient modes of slip represent slow, but self-propagating acceleration of slip along fault zones. These phenomena have been observed worldwide in a variety of active tectonic environments, however the physics of quasi-dynamic rupture and the underlying fault zone processes are still poorly understood. Rate- and State- frictional constitutive equations predict that fast dynamic slip will occur when the stiffness of the loading system (k) is less than a critical stiffness (kc) characterizing the fault gouge. In order to investigate quasi-dynamic transients, we performed laboratory experiments on simulated fault gouge (silica powders) in the double direct shear configuration with a compliant central block allowing boundary conditions where k≈kc. In addition, PZTs were used to measure acoustical properties of the gouge layers during shear. We document an evolution of the fault mechanical properties as the σn is increased. For σn < 10 MPa we observe a steady state frictional type of shear. When σn ≥ 15 MPa we observe emergent slow-slip events from steady state shear with accumulated shear displacement of about 10 mm. The typical values of stress drop (Δτ) vary between 0.2 and 0.8 MPa, and have typical duration from 0.5 up to 3 seconds giving the characteristics of slow stick-slip. As σn is varied we observe different characteristics of slow slip. For σn = 15MPa a repetitive double period oscillation is observed with slow slip growing until a maximum stress drop and then self attenuating. When σn is increased to 20 and 25 MPa slow slip are characterized by larger Δτ with constant τmax and τmin, however still showing a co-seismic duration of ~2 seconds. Our results

  15. Low-frequency Slivan states in the outer main belt?

    NASA Astrophysics Data System (ADS)

    Vraštil, J.; Vokrouhlický, D.

    2016-02-01

    Context. Spin states of several main belt asteroids have been recently found to reside in what is called the Slivan state, namely a secular spin-orbit resonance with the s6 mode of their orbital precession in space. Aims: We examine a possibility of the Slivan states of asteroids with other than orbital s6 precession frequency. Methods: The asteroids' orbital and spin states are numerically propagated using well-tested computer codes. We select parameter space favorable for the Slivan-state capture with the s7 frequency mode of the orbital precession. The stability of these states is numerically verified. Results: We find that asteroid (184) Dejopeja has a spin state captured in (or very nearly) the Slivan state with the s7 orbital frequency. In general, such a situation may favorably occur for low-inclination orbits in the outermost part of the main asteroid belt. We expect these states to be common among the Themis family members.

  16. Radial evolution of the high/low frequency breakpoint in magnetic field spectra

    NASA Technical Reports Server (NTRS)

    Feynman, J.; Ruzmaikin, A.; Smith, E. J.

    1995-01-01

    The spectra of magnetic field variations in the solar wind show different behavior in two frequency regions; a high frequency region in which the spectral exponent is about -5/3 and a low frequency region in which it is typically -1. The two types of variations must arise from different processes and a clue to the relationship between the spectral regions lies in understanding the behavior of the breakpoint between the spectral regions. Studies of the average behavior of spectra have shown that the break point occurs at about 3.5 hours at 1 AU. It is also known that, on average, the breakpoint occurs at lower frequencies with larger heliocentric distances. Ideally however, instead of the average properties of the spectra, we would like to know how the breakpoint evolves in particular samples of the solar wind as they propagate to larger heliocentric distances. In the study reported here we take advantage of the fact that, in 1974, Pioneer 10 (4.4 AU) and Pioneer 11 (5.6 AU) were close to being co-aligned and being aligned with the Earth. Solar wind observed at Earth can be closely matched with solar wind later observed at P10 and P11. We here compare the breakpoint observed at Earth with that observed at Pioneers 10 and 11 for matched samples of the wind.

  17. Low Frequency Phased Array Techniques for Crack Detection in Cast Austenitic Piping Welds: A Feasibility Study

    SciTech Connect

    Anderson, Michael T.; Cumblidge, Stephen E.; Doctor, Steven R.

    2007-01-01

    Studies conducted at the Pacific Northwest National Laboratory (PNNL) in Richland, Washington have focused on developing and evaluating the reliability of nondestructive testing (NDT) approaches for coarse-grained stainless steel reactor components. The objective of this work is to provide information to the United States Nuclear Regulatory Commission (NRC) on the utility, effectiveness and limitation of NDT techniques as related to inservice testing of primary system piping components in pressurized water reactors. We examined cast stainless steel pipe specimens containing thermal and mechanical fatigue cracks located close to the weld roots and having inner and outer diameter surface geometrical conditions that simulate several water reactor primary piping configurations. In addition, segments of vintage centrifugally cast piping were examined to characterize the inherent acoustic noise and scattering caused by grain structures and to determine the consistency of ultrasonic responses when propagating through differing microstructures. Advanced ultrasonic phased array techniques were applied from the outside surface of these specimens using automated scanning devices and water coupling. The phased array approach was implemented with a modified instrument operating at low frequencies, and composite volumetric images of the specimens were generated. Results from laboratory studies for assessing crack detection effectiveness in cast stainless steel as a function of frequency are discussed in this paper.

  18. Phase measurements of very-low-frequency signals from the magnetosphere

    SciTech Connect

    Paschal, E.V.

    1988-01-01

    The usual methods of spectrum analysis applied to analog tape recordings of very low frequency (VLF) signals extract only magnitude information and ignore phase information. A digital signal-processing system using a recorded constant-frequency pilot tone was developed that can correct tape errors due to wow and flutter, and reconstruct the signal phases. Frequency shifts are corrected during analysis by interpolating between spectral points in the windowed Fourier transform, and the output phases of the synthesized filters are corrected for timing errors. Having signal-component phases as well as magnitudes doubles the available information. Whistler-mode signals from the VLF transmitter at Siple Station, Antarctica, were analyzed as received at Roberval, Quebec. The phase of a non-growing signal is found to give a less-noisy measure of duct motion than Doppler frequency shift, with improved time resolution. Correlations are seen between variations in the whistler-mode phase delay and the earth's magnetic field component D. They are interpreted as Pc 2 micropulsation transients, short compared to the length of the field line, which propagate from equator to ground as Alfven waves.

  19. Low-frequency sound transmission through a gas-liquid interface.

    PubMed

    Godin, Oleg A

    2008-04-01

    Typically, sound speed in gases is smaller and mass density is much smaller than in liquids, resulting in a very strong acoustic impedance contrast at a gas-liquid interface. Sound transmission through a boundary with a strong impedance contrast is normally very weak. This paper studies the power output of localized sound sources and acoustic power fluxes through a plane gas-liquid interface in a layered medium. It is shown that, for low-frequency sound, a phenomenon of anomalous transparency can occur where most of the acoustic power generated by a source in a liquid half-space can be radiated into a gas half-space. The main physical mechanism responsible for anomalous transparency is found to be an acoustic power transfer by inhomogeneous (evanescent) waves in the plane-wave decomposition of the acoustic field in the liquid. The effects of a liquid's stratification and of guided sound propagation in the liquid on the anomalous transparency of the gas-liquid interface are considered. Geophysical and biological implications of anomalous transparency of water-air interface to infrasound are indicated. PMID:18396996

  20. Low-frequency ultrawideband synthetic aperture radar: frequency subbanding for targets obscured by the ground

    NASA Astrophysics Data System (ADS)

    Happ, Lynn; Le, Francis; Ressler, Marc A.; Kappra, Karl A.

    1996-06-01

    The Army Research Laboratory (ARL) has been investigating the potential of ultra-wideband synthetic aperture radar (UWB SAR) technology to detect and classify targets concealed by subsurface targets and foliage. Our investigative approach is to collect high-quality precision data to support phenomenological investigations of electromagnetic wave propagation through dielectric media. These investigations, in turn, support the development of algorithms for automatic target recognition. In order to achieve these goals, ARL designed and built an impulse (very short pulse) radar to collect data at a variety of test sites to measure and analyze the responses from targets, clutter, and targets embedded in clutter. The UWB BoomSAR, mounted on a 150-foot-high mobile boom lift, collects the high-quality, precision data sets needed for understanding UWB SAR system requirements and foliage penetration and ground penetration phenomenology. The BoomSAR operates with over 1 gigahertz of bandwidth covering a spectrum from 40 MHz to 1 GHz and is fully polarimetric. This bandwidth contains low frequencies needed for ground penetration while also maintaining higher frequency coverage for high resolution imagery. This paper shows a GPEN target area from data collected at Yuma Proving Grounds, AZ in low- and high- frequency subbands.

  1. Simulation study of the role of ion kinetics in low-frequency wave train evolution

    NASA Technical Reports Server (NTRS)

    Vasquez, Bernard J.

    1995-01-01

    The evolution of uniform, parallel propagating, low-frequency (less than or approx. ion cyclotron) wave trains is followed with a one-dimensional hybrid numerical code with fluid electrons and particle ions. We show that moderate amplitude (delta B/B less than 1/2) wave trains give instabilities and saturated states which differ completely from pure fluid evolution. This is most clearly seen when beta greater than 1 and instability exists for wavenumbers both below and above the wavenumber of an inital, left-handed wave train or pump wave. For corresponding parameters a fluid theory gives only a narrow range of instability above the pump wavenumber where decay and beat instabilities can occur. in simulations wave energy inverse cascades to smaller wavenumbers and into a greater number of forward than backward going waves. In fluids energy by decay goes mostly to backward ones of smaller wavenumber, and energy by beat goes mostly to forward ones of larger wavenumber. Neither fluid instability explains simulation results. The instability is saturated by thermalizing ions and sometimes exciting small wavenumber electrostatic or acoustic modes. In contrast, saturation in fluids first occurs by generating the harmonics of the growing linear modes. Harmonic generation is mostly absent in simulations. Simulations are carried out to long times and mostly reach a limit beyond which no further significant evolution can occur. Application to Alfvenic fluctuations in the solar wind is discussed.

  2. Maximization of the effective impulse delivered by a high-frequency/low-frequency planetary drill tool.

    PubMed

    Harkness, Patrick; Lucas, Margaret; Cardoni, Andrea

    2011-11-01

    Ultrasonic tools are used for a variety of cutting applications in surgery and the food industry, but when they are applied to harder materials, such as rock, their cutting performance declines because of the low effective impulse delivered by each vibration cycle. To overcome this problem, a technique known as high-frequency/low-frequency (or alternatively, ultrasonic/sonic) drilling is employed. In this approach, an ultrasonic step-horn is used to deliver an impulse to a free mass which subsequently moves toward a drilling bit, delivering the impulse on contact. The free mass then rebounds to complete the cycle. The horn has time between impacts to build significant vibration amplitude and thus delivers a much larger impulse to the free mass than could be delivered if it were applied directly to the target. To maximize the impulse delivered to the target by the cutting bit, both the momentum transfer from the ultrasonic horn to the free mass and the dynamics of the horn/free mass/cutting bit stack must be optimized. This paper uses finite element techniques to optimize the ultrasonic horns and numerical propagation of the stack dynamics to maximize the delivered effective impulse, validated in both cases by extensive experimental analysis. PMID:22083772

  3. Extraction of low frequency signals from cross-correlations of the infrasonic ambient noise

    NASA Astrophysics Data System (ADS)

    Landès, Matthieu; Shapiro, Nikolaï; Le Pichon, Alexis

    2015-04-01

    Cross-correlation of ambient noise are widely used in seismology for imaging and monitoring purposes. The underlying result is the possibility to extract the Green Function between two locations on the Earth by correlating the noise recorded at these two points during long period of time. However, the applicability of this approach in atmospheric infrasound is not yet well established. We present cross-correlations of the infrasonic dataset of the USArray for the year 2012 filtered between 3 and 330 seconds. All cross-correlations were computed daily with a moving window of 3 hours. Only the amplitude normalization has been applied. We observe clear signals on the stacked cross-correlations for inter-station distances smaller than 400 km. The dominant period of this signal is around 60-100 s and its propagation velocity is approximately 320 m/s. We then use the daily cross-correlations to get information on the location of corresponding noise sources. Daily cross-correlations are asymmetric and show seasonal variations. These observations are due to the inhomogeneous noise sources distribution that can be inferred from a beamforming analysis. Our results show the seasonal variations of the back-azimuth of dominant infrasound noise sources generating this low frequency signal. This is a new opportunity to characterize the composition of the infrasonic ambient noise and to promote the application of passive approaches in atmospheric infrasound.

  4. The Study of Electromagnetic Field Response Using Very Low Frequency Methods in Geothermal Area, Sabang

    SciTech Connect

    Isa, M.; Lim, H. S.; Jafri, M. Z. Mat

    2011-03-30

    Electromagnetic field measurements have been performed using the method of Very Low Frequency tilt angle pattern in Jaboi, Sabang. A site survey was conducted and carried out to identify the anomalies of location, depth and geometry of hydrothermal based on tilt angle measurement and ellipse. Objective of this study is to see the response of the magnetic and electric field in relation to location, depth and hydrothermal geometry. The devices of T-VLF-R IRIS was used in this study, while the (NWC) station was selected as main station and (JJF4) station was selected as comparison station. The electromagnetic field survey was recorded for a line spanned as long as 900 meters at the interval of 10 meter depth. The results of this study show the greatest anomaly occurred at line interval between 400-600 meters with a depth of 10-140 meters. Anomaly pattern of each depth shows that the propagation pattern of hydrothermal is in the form of vertical pipe flow.

  5. Correlated low-frequency electric and magnetic noise along the auroral field lines

    NASA Technical Reports Server (NTRS)

    Gurnett, D. A.; Huff, R. L.; Menietti, J. D.; Burch, J. L.; Winningham, J. D.; Shawhan, S. D.

    1984-01-01

    Dynamics Explorer 1 measurements of intense low-frequency electric and magnetic noise observed at low altitudes over the auroral zone are described. The intensity of both the electric and magnetic fields decreases rapidly with increasing frequency. Most of the energy is at frequencies below the O(+) cyclotron frequency, and some evidence is found for a cutoff or change in spectral slope near that frequency. The magnetic to electric field ratio decreases rapidly with increasing radial distance and also decreases with increasing frequency. The polarization of the electric field in a plane perpendicular to the earth's magnetic field is essentially random. The transverse electric and magnetic fields are closely correlated, with the average Poynting flux directed toward the earth. The total electromagnetic power flow associated with the noise is substantial. Two general models are discussed to interpret these observations, one based on static electric and magnetic fields imbedded in the ionosphere and the other based on Alfven waves propagating along the auroral field lines.

  6. Analysis of the Low-Frequency Radio Noise Environment at Satellite Heights from Terrestrial Sources

    NASA Astrophysics Data System (ADS)

    Taylor, M. F.; Basart, J. P.; McCoy, M.; Rios, E.

    1996-05-01

    We have investigated the propagation of terrestrial radio sources from 1 to 30 MHz (HF spectral region) through the ionosphere for the purpose of characterizing the interference spectrum on potential space-based, low-frequency-radio telescopes. A recent survey of the HF noise environment at satellite heights from 1 to 14 MHz has been conducted using the WIND spacecraft. Radio frequencies for which the interference appears to be sufficiently low for radio telescopes are 1.3, 2.9, 3.1, 8.2, and 11.4 MHz. A model was developed to predict the HF noise environment. Our current model includes a source model, an ionospheric model, and a ray tracing model. The source model was developed using known commercial broadcast stations found in the World Radio TV Handbook. The ICED ionospheric model was used to generate a model ionosphere. By ray tracing a terrestrially based broadcast source through the model ionosphere, an ionospheric transfer function (ITF) was developed. By modifying the source model using the ITF, we were able to simulate the expected noise environment at satellite heights. Comparison of modeled and measured spectra show the majority of the noise environment is due to known commercial broadcasters. Improved modeling is necessary because the slopes of the simulated spectra above the plasma frequency are too shallow, and the plasma cutoff frequencies are too high compared to the measured data.

  7. Low frequency noise impact from road traffic according to different noise prediction methods.

    PubMed

    Ascari, Elena; Licitra, Gaetano; Teti, Luca; Cerchiai, Mauro

    2015-02-01

    The European Noise Directive 2002/49/EC requires to draw up noise action plans. Most of the implemented solutions consist in using barriers, even if some studies evidenced that annoyance could increase after their installation. This action dumps the high frequencies, decreasing the masking effect on low ones. Therefore, people annoyance and complaints may increase despite the mitigation. This can happen even in pedestrian zones near main roads due to the screening effect of first buildings row. In this paper, the authors analyze the post-operam screening effects in terms of low frequency noise. The difference between C- and A-weighted levels is calculated as annoyance indicator (LC-A). Different methods able to map noise with octave bands detail are tested in order to establish differences in the estimates of annoyance exposure. In particular, a comparison is carried out between data from interim method NMPB 96, its updated version 2008, NORD 2000 and those provided by a customized procedure through ISO 9613 propagation and Statistical Pass By measurements. Test sites are simulated in order to validate each model results through measurements. Results are discussed for real locations in Pisa city center and virtual scenarios in a rising scale of complexity. PMID:25461069

  8. Does Cooling Magma Drive Deep Low-Frequency Earthquakes?

    NASA Astrophysics Data System (ADS)

    Aso, N.; Ide, S.; Tsai, V. C.

    2013-12-01

    = Introduction = One major type of deep low-frequency earthquakes (LFEs) is tectonic LFEs that locate around plate boundaries, and are thought to be slip events. Another type of LFEs is volcanic LFEs that locate around the Moho mostly beneath active or Quaternary volcanoes, and their physical mechanism is not well established. We recently suggested that the volcanic LFEs in eastern Shimane can be interpreted as a resonant oscillation within an old magma conduit of 1600 m in length, which produces a focal mechanism that is equivalent to a compensated linear vector dipole (CLVD) oriented in the direction of the conduit [Aso et al., in prep. for re-submission]. However, a remaining question is what excites the resonance. In the present study, we suggest that cooling magma acts as a trigger for the resonant oscillation of volcanic LFEs, with the idea that thermal contraction of magma produces an anisotropic strain rate and the resulting brittle failure would trigger oscillation. To verify this, we calculated strain rates produced by this effect both analytically and numerically. = Model Setting and Method = We set the initial extent of magma as a 1600-m-long 400-m-wide pipe as is inferred from the distribution of hypocenters in eastern Shimane. The initial perturbation is set to be 400 K uniformly within the pipe. Thermal strain rate is calculated using the thermal stress potential of Timoshenko and Goodier [1970]. First, we estimated the strain rate without the effect of latent heat release. Assuming a cuboid pipe, it can be solved easily analytically. Next, to account for latent heat release, we use the enthalpy method [Eyres et al., 1946] to numerically solve the problem, assuming a cylindrical pipe. = Results = For the case without latent heat release, the strain rate remains higher than than 10-14/s for 600 years at the center of the pipe. For the case with latent heat release, although the latent heat delays the thermal evolution and decreases the strain rate, it

  9. Modulating action of low frequency oscillations on high frequency instabilities in Hall thrusters

    SciTech Connect

    Liqiu, Wei E-mail: weiliqiu@hit.edu.cn; Liang, Han; Ziyi, Yang; Jing, Li; Yong, Cao; Daren, Yu; Jianhua, Du

    2015-02-07

    It is found that the low frequency oscillations have modulating action on high frequency instabilities in Hall thrusters. The physical mechanism of this modulation is discussed and verified by numerical simulations. Theoretical analyses indicate that the wide-range fluctuations of plasma density and electric field associated with the low frequency oscillations affect the electron drift velocity and anomalous electron transport across the magnetic field. The amplitude and frequency of high frequency oscillations are modulated by low frequency oscillations, which show the periodic variation in the time scale of low frequency oscillations.

  10. Modulating action of low frequency oscillations on high frequency instabilities in Hall thrusters

    NASA Astrophysics Data System (ADS)

    Liqiu, Wei; Liang, Han; Ziyi, Yang; Jing, Li; Yong, Cao; Daren, Yu; Jianhua, Du

    2015-02-01

    It is found that the low frequency oscillations have modulating action on high frequency instabilities in Hall thrusters. The physical mechanism of this modulation is discussed and verified by numerical simulations. Theoretical analyses indicate that the wide-range fluctuations of plasma density and electric field associated with the low frequency oscillations affect the electron drift velocity and anomalous electron transport across the magnetic field. The amplitude and frequency of high frequency oscillations are modulated by low frequency oscillations, which show the periodic variation in the time scale of low frequency oscillations.

  11. Low-frequency variability of Western Boundary Currents in the turbulent ocean: intrinsic modes and atmospheric forcing

    NASA Astrophysics Data System (ADS)

    Sérazin, Guillaume; Penduff, Thierry; Terray, Laurent; Grégorio, Sandy; Barnier, Bernard; Molines, Jean-Marc

    2015-04-01

    Ocean-atmosphere heat fluxes are particularly strong in Western Boundary Current (WBC) regions where SST front variations influence basin-scale climate variability. Observed low-frequency fluctuations in latitude and strength of these oceanic jets are classically thought to be essentially atmospherically-driven by wind stress curl variability via the oceanic Rossby wave adjustment. Yet academic eddy-resolving process-oriented models with double-gyre configurations have revealed that an idealized WBC may exhibit low-frequency intrinsic fluctuations without low-frequency external forcing (e.g. Berloff et al., 2007, Dijkstra and Ghil, 2005, etc). Experiments with eddying Ocean General Circulation Models (OGCMs) have also shown that the amount of low-frequency Sea Level Anomaly (SLA) variability is largely intrinsic in WBCs (Penduff et al. 2011; Sérazin et al 2014) and that the frontal-scale (<10°) pattern of the Kuroshio Extension (KE) variability is similar to intrinsic modes (Taguchi et al. 2010). Based on a pair of atmospherically-forced 1/12° OGCM experiments that simulate with accuracy either the intrinsic variability (seasonally-forced) or the observed total variability (forced with the full range of atmospheric timescales), Empirical Orthogonal Function analysis is performed on zonally-averaged SLA fields of four main WBCs (e.g. Gulf Stream, Kuroshio Extension, Agulhas Current and East Australian Current). The first two modes of the KE and GS exhibit a similar spatial structure that is shaped by oceanic intrinsic processes. The frequency content is however different between the intrinsic and total Principal Components, the former containing a wide range of timescales similar to a red noise and the latter being more autocorrelated at interannual-to-decadal timescales. These modes are compared with those obtained from the 20 years of altimetry observation and relationships with low-frequency westward propagative features in the respective oceanic basin are

  12. SCALAR AND VECTOR NONLINEAR DECAYS OF LOW-FREQUENCY ALFVÉN WAVES

    SciTech Connect

    Zhao, J. S.; Wu, D. J.; Voitenko, Y.; De Keyser, J.

    2015-02-01

    We found several efficient nonlinear decays for Alfvén waves in the solar wind conditions. Depending on the wavelength, the dominant decay is controlled by the nonlinearities proportional to either scalar or vector products of wavevectors. The two-mode decays of the pump MHD Alfvén wave into co- and counter-propagating product Alfvén and slow waves are controlled by the scalar nonlinearities at long wavelengths ρ{sub i}{sup 2}k{sub 0⊥}{sup 2}<ω{sub 0}/ω{sub ci} (k {sub 0} is wavenumber perpendicular to the background magnetic field, ω{sub 0} is frequency of the pump Alfvén wave, ρ {sub i} is ion gyroradius, and ω {sub ci} is ion-cyclotron frequency). The scalar decays exhibit both local and nonlocal properties and can generate not only MHD-scale but also kinetic-scale Alfvén and slow waves, which can strongly accelerate spectral transport. All waves in the scalar decays propagate in the same plane, hence these decays are two-dimensional. At shorter wavelengths, ρ{sub i}{sup 2}k{sub 0⊥}{sup 2}>ω{sub 0}/ω{sub ci}, three-dimensional vector decays dominate generating out-of-plane product waves. The two-mode decays dominate from MHD up to ion scales ρ {sub i} k {sub 0} ≅ 0.3; at shorter scales the one-mode vector decays become stronger and generate only Alfvén product waves. In the solar wind the two-mode decays have high growth rates >0.1ω{sub 0} and can explain the origin of slow waves observed at kinetic scales.

  13. Scalar and Vector Nonlinear Decays of Low-frequency Alfvén Waves

    NASA Astrophysics Data System (ADS)

    Zhao, J. S.; Voitenko, Y.; De Keyser, J.; Wu, D. J.

    2015-02-01

    We found several efficient nonlinear decays for Alfvén waves in the solar wind conditions. Depending on the wavelength, the dominant decay is controlled by the nonlinearities proportional to either scalar or vector products of wavevectors. The two-mode decays of the pump MHD Alfvén wave into co- and counter-propagating product Alfvén and slow waves are controlled by the scalar nonlinearities at long wavelengths ρ i2k0\\perp 2<ω 0/ω ci (k 0 is wavenumber perpendicular to the background magnetic field, ω0 is frequency of the pump Alfvén wave, ρ i is ion gyroradius, and ω ci is ion-cyclotron frequency). The scalar decays exhibit both local and nonlocal properties and can generate not only MHD-scale but also kinetic-scale Alfvén and slow waves, which can strongly accelerate spectral transport. All waves in the scalar decays propagate in the same plane, hence these decays are two-dimensional. At shorter wavelengths, ρ i2k0\\perp 2\\gtω 0/ω ci, three-dimensional vector decays dominate generating out-of-plane product waves. The two-mode decays dominate from MHD up to ion scales ρ i k 0 ~= 0.3; at shorter scales the one-mode vector decays become stronger and generate only Alfvén product waves. In the solar wind the two-mode decays have high growth rates >0.1ω0 and can explain the origin of slow waves observed at kinetic scales.

  14. Low-frequency Intensity Variation of the South Asian High and its relationship to Boreal Summer Intraseasonal Oscillation

    NASA Astrophysics Data System (ADS)

    Shang, Wei; Ren, Xuejuan

    2016-04-01

    and northwest from day -12 to day 0 is associated with the Boreal Summer Intraseasonal Oscillation 2 (BSISO 2) of 10 - 30 days in the Asian monsoon areas. When BSISO2 propagates from subtropics to Asian continents , more anomalous rainfall appear in the Asian monsoon areas, which correspond more anomalous condensation heat release. The anomalous heating stimulate positive height anomalies with an anomalous anticyclonic circulation to its northwest in the upper troposphere, causing the strengthening of the SAH intensity. In addition, the strengthening of the west part of SAH may result from an anomalous low-frequency anticyclonic in the upper troposphere propagating northward from Arabian Sea to Iranian plateau.

  15. The Low-Frequency Encoding Disadvantage: Word Frequency Affects Processing Demands

    ERIC Educational Resources Information Center

    Diana, Rachel A.; Reder, Lynne M.

    2006-01-01

    Low-frequency words produce more hits and fewer false alarms than high-frequency words in a recognition task. The low-frequency hit rate advantage has sometimes been attributed to processes that operate during the recognition test (e.g., L. M. Reder et al., 2000). When tasks other than recognition, such as recall, cued recall, or associative…

  16. 77 FR 52317 - Record of Decision for Surveillance Towed Array Sensor System Low Frequency Active Sonar

    Federal Register 2010, 2011, 2012, 2013, 2014

    2012-08-29

    ... From the Federal Register Online via the Government Publishing Office DEPARTMENT OF DEFENSE Department of the Navy Record of Decision for Surveillance Towed Array Sensor System Low Frequency Active... Array Sensor System Low Frequency Active (SURTASS LFA) sonar systems with certain...

  17. THE USE OF LOW FREQUENCY RESIDUAL HEARING IN PROFOUNDLY DEAF CHILDREN.

    ERIC Educational Resources Information Center

    LING, DANIEL

    IN ORDER TO TEST THE HYPOTHESIS THAT ADDITIONAL LOW FREQUENCY AMPLIFICATION WOULD CONTRIBUTE SIGNIFICANTLY TO THE AUDITION OF SPEECH, 12 CHILDREN WITH LOW FREQUENCY RESIDUAL HEARING WERE STUDIED, USING TWO INDIVIDUAL HEARING AIDS--AN EXPERIMENTAL MODEL WITH A FREQUENCY RANGE OF 80-3500 CYCLES PER SECOND (CPS) AND A STANDARD MODEL WITH A FREQUENCY…

  18. Low Frequency High Amplitude Temperature Oscillations in Loop Heat Pipe Operation

    NASA Technical Reports Server (NTRS)

    Ku, Jentung; Rodriguez, Jose

    2003-01-01

    This paper presents viewgraphs on the low frequency high amplitude temperature oscillations observed in loop heat pipe operations. The topics include: 1) Proposed Theory; 2) Test Loop and Test Results; and 3) Effects of Various Parameters. The author also presents a short summary on the conditiions that must be met in order to sustain a low frequency high amplitude temperature oscillation.

  19. Slow Pseudotachylites

    NASA Astrophysics Data System (ADS)

    Pec, M.; Stunitz, H.; Heilbronner, R.

    2011-12-01

    Tectonic pseudotachylites as solidified, friction induced melts are believed to be the only unequivocal evidence for paleo-earthquakes. Earthquakes occur when fast slip (1 - 3 m/s) propagates on a localized failure plane and are always related with stress drops. The mechanical work expended, together with the rock composition and the efficiency of thermal dissipation, controls whether the temperature increase on a localized slip plane will be sufficient to induce fusion. We report the formation of pseudotachylites during steady-state plastic flow at slow bulk shear strain rates (~10^-3 to ~10^-5 /s corresponding to slip rates of ~10^-6 to ~10^-8 m/s) in experiments performed at high confining pressures (500 MPa) and temperatures (300°C) corresponding to a depth of ~15 km. Crushed granitioid rock (Verzasca gneiss), grain size ≤ 200 μm, with 0.2 wt% water added was placed between alumina forcing blocks pre-cut at 45°, weld-sealed in platinum jackets and deformed with a constant displacement rate in a solid medium deformation apparatus (modified Griggs rig). Microstructural observations show the development of a S-C-C' fabric with C' slip zones being the dominant feature. Strain hardening in the beginning of the experiment is accompanied with compaction which is achieved by closely spaced R1 shears pervasively cutting the whole gouge zone and containing fine-grained material (d < 100 nm). The peak strength is achieved at γ ~ 2 at shear stress levels of 1350-1450 MPa when compaction ceases. During further deformation, large local displacements (γ > 10) are localized in less densely spaced, ~10 μm thick C'-C slip zones which develop predominantly in feldspars and often contain micas. In TEM, they appear to have no porosity consisting of partly amorphous material and small crystalline fragments with the average grain size of 20 nm. After the peak strength, the samples weaken by ~20 MPa and continue deforming up to γ ~ 4 without any stress drops. Strain

  20. Effect of low frequency noise on the echocardiographic parameter E/A ratio.

    PubMed

    Chao, Pao-Chiang; Yeh, Ching-Ying; Juang, Yow-Jer; Hu, Ching-Yao; Chen, Chiou-Jong

    2012-01-01

    The hearing condition of the Taiwanese aerospace maintenance workers affected by the low frequency noise had not been reported. The purpose of this research is to clarify the maintenance workers' health effect when exposed to low frequency and/or general noises and to understand the relationship between the variations of the worker's echocardiographic E/A ratio and the low frequency noise. The low frequency noise monitoring and echocardiographic E/A ratio results obtained for 213 aerospace maintenance workers indicated that the workers' hearing loss was more serious at high frequency 4k and 6k when exposed to the low frequency noise and could be more than 40 dB. The abnormality of echocardiographic E/A ratio was also higher than that of control group. PMID:22918145

  1. Ponderomotive Force and Lower Hybrid Turbulence Effects in Space Plasmas Subjected to Large-Amplitude Low-Frequency Waves

    NASA Technical Reports Server (NTRS)

    Khazanov, G. V.; Khazanov, George; Liemohn, M. W.; Stone, N. H.; Coffey, V. N.

    1997-01-01

    In the auroral region, simultaneous occurrences of upward-flowing ions and field-aligned electrons have been observed by the Viking satellite. The occurrence is strongly correlated with large amplitude low frequency fluctuations of the electric field. Large-amplitude shear Alfven waves have also been observed by sounding rockets in the auroral ionosphere. When such LF waves are propagating in a plasma, a ponderomotive force and other types of waves are produced which may lead to significant effects on the plasma. This force is directed toward decreasing density, providing the electromagnetic lift of the background plasma and an increase of collisionless plasma expansion. We find that even for modest wave strengths, the influence on the outflowing oxygen ions can be dramatic, increasing the high-altitude density by orders of magnitude. It is also demonstrated that large-amplitude low-frequency waves (LFW) may generate lower hybrid waves (LHW) in the auroral zone. The excitation of LHW by a LF wave may lead to the appearance of an additional channel of energy transfer from, for example, Alfven or fast magnetosonic waves, to particles. This process then influences the formation of the plasma distribution function at the expense of acceleration in the tail of the distribution during the collapse of the LHW. The ion energization due to the LHW can be comparable with that produced by the ponderomotive force of the LFW. It is shown that the LH turbulence leads to equalization of the ponderomotive acceleration of the different ion species. The mechanism of LHW excitation due to the oxygen ion relative drift in a plasma subjected to low-frequency waves is used for analysis of Viking satellite data for events in the cusp/cleft region. It is found that, in some cases, such a mechanism leads to LHW energy densities and ion distribution functions close to those observed.

  2. The Low-Frequency Environment of the Murchison Widefield Array: Radio-Frequency Interference Analysis and Mitigation

    NASA Astrophysics Data System (ADS)

    Offringa, A. R.; Wayth, R. B.; Hurley-Walker, N.; Kaplan, D. L.; Barry, N.; Beardsley, A. P.; Bell, M. E.; Bernardi, G.; Bowman, J. D.; Briggs, F.; Callingham, J. R.; Cappallo, R. J.; Carroll, P.; Deshpande, A. A.; Dillon, J. S.; Dwarakanath, K. S.; Ewall-Wice, A.; Feng, L.; For, B.-Q.; Gaensler, B. M.; Greenhill, L. J.; Hancock, P.; Hazelton, B. J.; Hewitt, J. N.; Hindson, L.; Jacobs, D. C.; Johnston-Hollitt, M.; Kapińska, A. D.; Kim, H.-S.; Kittiwisit, P.; Lenc, E.; Line, J.; Loeb, A.; Lonsdale, C. J.; McKinley, B.; McWhirter, S. R.; Mitchell, D. A.; Morales, M. F.; Morgan, E.; Morgan, J.; Neben, A. R.; Oberoi, D.; Ord, S. M.; Paul, S.; Pindor, B.; Pober, J. C.; Prabu, T.; Procopio, P.; Riding, J.; Udaya Shankar, N.; Sethi, S.; Srivani, K. S.; Staveley-Smith, L.; Subrahmanyan, R.; Sullivan, I. S.; Tegmark, M.; Thyagarajan, N.; Tingay, S. J.; Trott, C. M.; Webster, R. L.; Williams, A.; Williams, C. L.; Wu, C.; Wyithe, J. S.; Zheng, Q.

    2015-03-01

    The Murchison Widefield Array is a new low-frequency interferometric radio telescope built in Western Australia at one of the locations of the future Square Kilometre Array. We describe the automated radio-frequency interference detection strategy implemented for the Murchison Widefield Array, which is based on the aoflagger platform, and present 72-231 MHz radio-frequency interference statistics from 10 observing nights. Radio-frequency interference detection removes 1.1% of the data. Radio-frequency interference from digital TV is observed 3% of the time due to occasional ionospheric or atmospheric propagation. After radio-frequency interference detection and excision, almost all data can be calibrated and imaged without further radio-frequency interference mitigation efforts, including observations within the FM and digital TV bands. The results are compared to a previously published Low-Frequency Array radio-frequency interference survey. The remote location of the Murchison Widefield Array results in a substantially cleaner radio-frequency interference environment compared to Low-Frequency Array's radio environment, but adequate detection of radio-frequency interference is still required before data can be analysed. We include specific recommendations designed to make the Square Kilometre Array more robust to radio-frequency interference, including: the availability of sufficient computing power for radio-frequency interference detection; accounting for radio-frequency interference in the receiver design; a smooth band-pass response; and the capability of radio-frequency interference detection at high time and frequency resolution (second and kHz-scale respectively).

  3. Assessment of Crack Detection in Heavy-Walled Cast Stainless Steel Piping Welds Using Advanced Low-Frequency Ultrasonic Methods

    SciTech Connect

    Anderson, Michael T.; Crawford, Susan L.; Cumblidge, Stephen E.; Denslow, Kayte M.; Diaz, Aaron A.; Doctor, Steven R.

    2007-03-01

    Studies conducted at the Pacific Northwest National Laboratory in Richland, Washington, have focused on assessing the effectiveness and reliability of novel approaches to nondestructive examination (NDE) for inspecting coarse-grained, cast stainless steel reactor components. The primary objective of this work is to provide information to the U.S. Nuclear Regulatory Commission on the effectiveness and reliability of advanced NDE methods as related to the inservice inspection of safety-related components in pressurized water reactors (PWRs). This report provides progress, recent developments, and results from an assessment of low frequency ultrasonic testing (UT) for detection of inside surface-breaking cracks in cast stainless steel reactor piping weldments as applied from the outside surface of the components. Vintage centrifugally cast stainless steel piping segments were examined to assess the capability of low-frequency UT to adequately penetrate challenging microstructures and determine acoustic propagation limitations or conditions that may interfere with reliable flaw detection. In addition, welded specimens containing mechanical and thermal fatigue cracks were examined. The specimens were fabricated using vintage centrifugally cast and statically cast stainless steel materials, which are typical of configurations installed in PWR primary coolant circuits. Ultrasonic studies on the vintage centrifugally cast stainless steel piping segments were conducted with a 400-kHz synthetic aperture focusing technique and phased array technology applied at 500 kHz, 750 kHz, and 1.0 MHz. Flaw detection and characterization on the welded specimens was performed with the phased array method operating at the frequencies stated above. This report documents the methodologies used and provides results from laboratory studies to assess baseline material noise, crack detection, and length-sizing capability for low-frequency UT in cast stainless steel piping.

  4. Source modeling sleep slow waves

    PubMed Central

    Murphy, Michael; Riedner, Brady A.; Huber, Reto; Massimini, Marcello; Ferrarelli, Fabio; Tononi, Giulio

    2009-01-01

    Slow waves are the most prominent electroencephalographic (EEG) feature of sleep. These waves arise from the synchronization of slow oscillations in the membrane potentials of millions of neurons. Scalp-level studies have indicated that slow waves are not instantaneous events, but rather they travel across the brain. Previous studies of EEG slow waves were limited by the poor spatial resolution of EEGs and by the difficulty of relating scalp potentials to the activity of the underlying cortex. Here we use high-density EEG (hd-EEG) source modeling to show that individual spontaneous slow waves have distinct cortical origins, propagate uniquely across the cortex, and involve unique subsets of cortical structures. However, when the waves are examined en masse, we find that there are diffuse hot spots of slow wave origins centered on the lateral sulci. Furthermore, slow wave propagation along the anterior−posterior axis of the brain is largely mediated by a cingulate highway. As a group, slow waves are associated with large currents in the medial frontal gyrus, the middle frontal gyrus, the inferior frontal gyrus, the anterior cingulate, the precuneus, and the posterior cingulate. These areas overlap with the major connectional backbone of the cortex and with many parts of the default network. PMID:19164756

  5. Magnitudes and moment-duration scaling of low-frequency earthquakes beneath southern Vancouver Island

    NASA Astrophysics Data System (ADS)

    Bostock, M. G.; Thomas, A. M.; Savard, G.; Chuang, L.; Rubin, A. M.

    2015-09-01

    We employ 130 low-frequency earthquake (LFE) templates representing tremor sources on the plate boundary below southern Vancouver Island to examine LFE magnitudes. Each template is assembled from hundreds to thousands of individual LFEs, representing over 269,000 independent detections from major episodic-tremor-and-slip (ETS) events between 2003 and 2013. Template displacement waveforms for direct P and S waves at near epicentral distances are remarkably simple at many stations, approaching the zero-phase, single pulse expected for a point dislocation source in a homogeneous medium. High spatiotemporal precision of template match-filtered detections facilitates precise alignment of individual LFE detections and analysis of waveforms. Upon correction for 1-D geometrical spreading, attenuation, free surface magnification and radiation pattern, we solve a large, sparse linear system for 3-D path corrections and LFE magnitudes for all detections corresponding to a single-ETS template. The spatiotemporal distribution of magnitudes indicates that typically half the total moment release occurs within the first 12-24 h of LFE activity during an ETS episode when tidal sensitivity is low. The remainder is released in bursts over several days, particularly as spatially extensive rapid tremor reversals (RTRs), during which tidal sensitivity is high. RTRs are characterized by large-magnitude LFEs and are most strongly expressed in the updip portions of the ETS transition zone and less organized at downdip levels. LFE magnitude-frequency relations are better described by power law than exponential distributions although they exhibit very high b values ≥˜5. We examine LFE moment-duration scaling by generating templates using detections for limiting magnitude ranges (MW<1.5, MW≥2.0). LFE duration displays a weaker dependence upon moment than expected for self-similarity, suggesting that LFE asperities are limited in fault dimension and that moment variation is dominated by

  6. Abnormal functional integration of thalamic low frequency oscillation in the BOLD signal after acute heroin treatment.

    PubMed

    Denier, Niklaus; Schmidt, André; Gerber, Hana; Vogel, Marc; Huber, Christian G; Lang, Undine E; Riecher-Rossler, Anita; Wiesbeck, Gerhard A; Radue, Ernst-Wilhelm; Walter, Marc; Borgwardt, Stefan

    2015-12-01

    Heroin addiction is a severe relapsing brain disorder associated with impaired cognitive control, including deficits in attention allocation. The thalamus has a high density of opiate receptors and is critically involved in orchestrating cortical activity during cognitive control. However, there have been no studies on how acute heroin treatment modulates thalamic activity. In a cross-over, double-blind, vehicle-controlled study, 29 heroin-maintained outpatients were studied after heroin and placebo administration, while 20 healthy controls were included for the placebo condition only. Resting-state functional magnetic resonance imaging was used to analyze functional integration of the thalamus by three different resting state analysis techniques. Thalamocortical functional connectivity (FC) was analyzed by seed-based correlation, while intrinsic thalamic oscillation was assessed by analysis of regional homogeneity (ReHo) and the fractional amplitude of low frequency fluctuations (fALFF). Relative to the placebo treatment and healthy controls, acute heroin administration reduced thalamocortical FC to cortical regions, including the frontal cortex, while the reductions in FC to the mediofrontal cortex, orbitofrontal cortex, and frontal pole were positively correlated with the plasma level of morphine, the main psychoactive metabolite of heroin. Furthermore, heroin treatment was associated with increased thalamic ReHo and fALFF values, whereas fALFF following heroin exposure correlated negatively with scores of attentional control. The heroin-associated increase in fALFF was mainly dominated by slow-4 (0.027-0.073 Hz) oscillations. Our findings show that there are acute effects of heroin within the thalamocortical system and may shed new light on the role of the thalamus in cognitive control in heroin addiction. Future research is needed to determine the underlying physiological mechanisms and their role in heroin addiction. PMID:26441146

  7. Low-frequency waves in the Martian magnetosphere and their response to upstream solar wind driving conditions

    NASA Astrophysics Data System (ADS)

    Ruhunusiri, Suranga; Halekas, J. S.; Connerney, J. E. P.; Espley, J. R.; McFadden, J. P.; Larson, D. E.; Mitchell, D. L.; Mazelle, C.; Jakosky, B. M.

    2015-11-01

    We characterize low-frequency plasma waves in the Martian magnetosphere and in the upstream region by using transport ratios. To compute the transport ratios, we use Mars Atmosphere and Volatile EvolutioN mission's (MAVEN) solar wind ion analyzer and suprathermal and thermal ion composition instrument measurements of the ion moments and the magnetometer measurements of the magnetic field. We find that the Alfvén waves are the most dominant wave mode in the upstream region and the magnetosheath. Fast waves are found frequently near the bow shock and the magnetic pileup boundary. Mirror and slow waves, on the other hand, occur much less frequently. We also find that the Alfvén and fast wave occurrences vary dominantly near the bow shock in response to the solar wind dynamic pressure.

  8. Coupled analysis of high and low frequency resonant ultrasound spectroscopy: Application to the detection of defects in ceramic balls

    SciTech Connect

    Deneuville, Francois; Duquennoy, Marc; Ouaftouh, Mohammadi; Jenot, Frederic; Ourak, Mohamed; Desvaux, Sebastien

    2009-05-15

    A coupled analysis of high and low frequency resonant ultrasound spectroscopy of spheroidal modes is presented in this paper. Experimentally, by using an ultrasonic probe for the excitation (piezoelectric transducer) and a heterodyne optic probe for the receiver (interferometer), it was possible to take spectroscopic measurements of spheroidal vibrations over a large frequency range of 100 kHz-45 MHz in a continuous regime. This wide analysis range enabled variations in velocity due to the presence of defects to be differentiated from the inherent characteristics of the balls and consequently, it offers the possibility of detecting cracks independently of production variations. This kind of defect is difficult to detect because the C-shaped surface crack is very small and narrow (500x5 {mu}m{sup 2}), and its depth does not exceed 50 {mu}m. The proposed methodology can excite spheroidal vibrations in the ceramic balls and detect such vibrations over a large frequency range. On the one hand, low frequency resonances are used in order to estimate the elastic coefficients of the balls according to various inspection depths. This method has the advantage of providing highly accurate evaluations of the elastic coefficients over a wide frequency range. On the other hand, high frequency vibrations are considered because they are similar to the surface waves propagating in the surface zone of the ceramic balls and consequently can be used to detect C-crack defects.

  9. Prediction of northern summer low-frequency circulation using a high-order vector auto-regressive model

    NASA Astrophysics Data System (ADS)

    Wang, Lei; Ting, Mingfang; Chapman, David; Lee, Dong Eun; Henderson, Naomi; Yuan, Xiaojun

    2016-02-01

    A data-driven, high-order vector auto-regressive (VAR) model is evaluated for predicting the Northern Hemisphere summer time (May through September) low frequency (>10 days or so) variability. The VAR model is suitable for linear stationary time series, similar to the commonly used linear inverse model (LIM), with additional temporal information incorporated to improve forecast skill. The intraseasonal forecast skill of the 250/750 hPa streamfunction is investigated using observational data since 1979, which shows significant improvements in high-order VAR models than the first-order model LIM. Furthermore, the tropical diabatic heating is found to significantly improve the forecast skill of the atmospheric low frequency circulation when included in the VAR model. The forecast skill of 250 hPa streamfunction at Arabian Peninsula is particularly enhanced for up to 5 weeks lead-time through circumglobal wave propagation associated with the persistent tropical eastern Pacific and equatorial Atlantic heating anomalies and the intraseasonal evolution of the tropical Indian Ocean and western Pacific heating anomalies.

  10. NONRESONANT INTERACTION OF CHARGED ENERGETIC PARTICLES WITH LOW-FREQUENCY NONCOMPRESSIVE TURBULENCE: NUMERICAL SIMULATION

    SciTech Connect

    Ragot, B. R.

    2012-10-20

    A new method for simulating the three-dimensional dynamics of charged energetic particles in very broadband noncompressive magnetic turbulence is introduced. All scales within the primary inertial range of the turbulence observed in the solar wind near 1 AU are now included for the independent computations of both the particle dynamics and the turbulent magnetic field lines (MFLs). While previous theories of resonant particle pitch-angle (PA) scattering and transport in interplanetary magnetic fields had favored interpreting the observed depletions in the electron PA distributions (PADs) around 90 Degree-Sign PA as evidence of poor scattering at low PA cosines, the computed particle dynamics reveal a very different reality. The MFL directions now vary on many scales, and the PADs are depleted around 90 Degree-Sign PA due to nonresonant filtering of the particles that propagate at too large an angle to the local magnetic field. Rather than being too weak, the scattering through 90 Degree-Sign PA is actually so strong that the particles (electrons and protons/ions) are reflected and trapped in the turbulent magnetic fields. While the low-frequency nonresonant turbulence produces ubiquitous magnetic traps that only let through particles with the most field-aligned velocities, higher-frequency near-gyroscale turbulence, when present, enhances particle transport by allowing the particles to navigate between magnetic traps. Finally, visualizing both particle trajectories and MFLs in the very same turbulence reveals a powerful tool for understanding the effects of the turbulent fields on the particle dynamics and cross-field transport. Some cross-field-line scattering, strongly amplified by MFL dispersal, results in a strong cross-field scattering of the particles. From this visualization, it also appears that near-gyroscale turbulence, previously known as gyroresonant turbulence, does not resonantly interact with the particles. The interaction between particles and

  11. Ultra-Narrow Bandwidth Optical Resonators for Integrated Low Frequency Noise Lasers

    NASA Astrophysics Data System (ADS)

    Spencer, Daryl T.

    The development of narrowband resonators has far reaching applications in integrated optics. As a precise reference of wavelength, filters can be used in sensors, metrology, nonlinear optics, microwave photonics, and laser stabilization. In this work, we develop record high quality factor (Q) Si 3N4 waveguide resonators, and utilize them to stabilize a heterogeneously integrated Si/III V laser. To increase the Q factor of waveguide resonators, particular attention is given to loss mechanisms. Propagation loss of <0.1 dB/m is demonstrated on the ultra low loss waveguide platform, a low index contrast, high aspect ratio Si3N4 waveguide geometry fabricated with high quality materials and high temperature anneals. Ideality in the directional couplers used for coupling to the resonators is studied and losses are reduced such that 81 million intrinsic Q factor is achieved. Additional results include 1x16 resonant splitters, low ? narrowband gratings, and a dual layer waveguide technology for low loss and low bend radius in separate regions of the same device layer. We then combine an ultra high Q resonator and a heterogeneous Si/III V laser in a Pound Drever Hall (PDH) frequency stabilization system to yield narrow linewidth characteristics for a stable on chip laser reference. The high frequency noise filtering is performed with Si resonant mirrors in the laser cavity. A 30 million Q factor Si3N4 resonator is used with electrical feedback to reduce close in noise and frequency walk off. The laser shows high frequency noise levels of 60x103 Hz2/Hz corresponding to 160 kHz linewidth, and the low frequency noise is suppressed 33 dB to 103 Hz2/Hz with the PDH system.

  12. Sapce based low frequency interferometric radioastronomy: the path towards the imaging of the inner heliosphere.

    NASA Astrophysics Data System (ADS)

    Cecconi, B.; Zarka, P. M.; Girard, J. N.; Klein Wolt, M.; Boonstra, A. J.; Baan, W.; Briand, C.; Maksimovic, M.; Segret, B.

    2014-12-01

    Low frequency radioastronomy observatories for the heliosphere have been using similar instrumentation for decades. The Cassini, STEREO, and the future Solar Orbiter mission are embarking goniopolarmetric radio receiver connected to 3 electric wire antennas. Such instrument provides the spectral matrix (or part of it) from which the wave parameters can be derived. They require a point source assumption (plane wave) to derive the direction of arrival of the wave, the polarization and the flux density. In case of a spatially extended source (disk shaped, with a given radial profile), the source centroid direction and the apparent source size can also be derived. This type of instrumentation cannot provide much more parameters, as there is a maximum of to 9 independent measurements for each time-frequency step (i.e. an instantaneous set of measurements). Radio maps can be produced a posteriori combining consecutive data at the cost of averaging out small scale temporal variations. Furthermore, these inversion do not allow solving for several sources, or for complex source geometry. We present a concept of radioastronomy instrumentation using a swarm of small satellites (possibly cubesats) with sensitive radio receivers measuring the wave front and phase of the radio waves on each spacecraft. This instrument will also provide 3-dimensional interferometric measurement from which real imaging capabilities will arise, as it is now occurring on ground at frequencies above 15 MHz, with the LOFAR interferometer in Europe, or the LWA in teh USA. The proposed concept will be very complementary to these instruments, as they will be orepating from a few kHz to a few 10 Mhz from space, and thus not affected by the ionospheric cutoff at 10 MHz. Such resolved imaging capabilities of the inner heliosphere would be a real step forward to better understand the radio emissions mechanisms and the energetic at the orgin of the radio sources, as well as the propagation processes. We will

  13. Deep level domain spectroscopy of low frequency oscillations in semi-insulating InP

    NASA Astrophysics Data System (ADS)

    Backhouse, C.; Young, L.

    1992-11-01

    It is known that low frequency current oscillations occur in semi-insulating GaAs due to the formation and transit of high field domains caused by enhanced trapping of hot electrons by deep levels and that power density spectra of the current show peaks whose temperature dependence gives information on deep levels. In the present work Fe-compensated InP was investigated. The peaks rose from an approximately {1}/{f}{3}/{2} background and by estimating and removing this and by averaging many spectra, no less than 14 frequency peaks were resolved which gave straight lines on an Arrhenius plot of log( {T 2}/{2f}) vs{1}/{T}. Although the amplitude of the current oscillations is not so large as to preclude multiple domain propagation, it seems more likely that the domains are caused by hot electron trapping by one level only, rather than that several traps should have the necessary characteristics to launch domains. The multiplicity of peaks could be partly due to harmonics of the basic high field domain oscillation and partly due to conductivity modulation by other levels whose occupancies are changed by the passage of the domains: the task, if so, is to determine which peaks are which. The activation energies from the Arrhenius plots fell into groups close to 0.30, 0.39, 0.41, 0.44 and 0.49 eV. The 14 peaks thus are believed to arise from 5 deep levels. Evidence was found that the 0.49 eV level is iron-related and is responsible for producing the high field domains and for drain current drift in InP metal-insulator-semiconductor field-effect transistors.

  14. A survey of low frequency waves at Jupiter: The Ulysses encounter

    NASA Technical Reports Server (NTRS)

    Tsurutani, Bruce T.; Southwood, David J.; Smith, Edward J.; Balogh, Andre

    1993-01-01

    We report the results of a survey of low-frequency (LF) plasma waves detected during the Ulysses Jupiter flyby. In the Jovian foreshock, two predominant wave periods are detected: 10(exp 2)-s and 5-s, as measured in the spacecraft frame. The 10(exp 2)-s waves are highly nonlinear propagate at large angles to vector-B(sub 0) (typically 50 deg), are steepened, and sometimes have attached whistler packets. For the interval analyzed the 10(exp 2)-s waves had mixed right-and left-hand polarizations. We argue that these are all consistent with being right-hand magnetosonic waves in the solar wind frame. The 10(exp 2)-s waves with attached whistler are similar to cometary waves. The trailing portions are linearly polaraized and the whistler portions circularly polarized with amplitudes decreasing linearly with time. The emissions are generated by approximately 2-keV protons flowing from the Jovian bow shock/magnetosheath into the upstream region. The instability is the ion beam instability. Higher Z ions were considered as a source of the waves but have been ruled out because of the low sunward velocities needed for their resonance. The 5-s waves have delta vector-B/B(sub 0 approximately = 0.5, are compressive and are left-hand polarized in the spacecraft frame. Local generation by three different resonant interactions were considered and have been ruled out. One possibility is that these waves are whistler mode by-products of the steepened lower-frequency magnetosonic waves. Mirror mode structures were detected throughout the outbound magnetosheath passes. For these structures, the theta(sub kB) values were consistently in the range of 80 deg to 90 deg, exceptionally high values.

  15. Low-frequency waves within isolated magnetic clouds and complex structures: STEREO observations

    NASA Astrophysics Data System (ADS)

    Siu-Tapia, A.; Blanco-Cano, X.; Kajdic, P.; Aguilar-Rodriguez, E.; Russell, C. T.; Jian, L. K.; Luhmann, J. G.

    2015-04-01

    Complex Structures (CSs) formed by the interaction of magnetic cloud (MC)-like structures with other transients (e.g., another MC, a stream interaction region, or a fast stream of solar wind) were frequently observed in the interplanetary space by STEREO spacecraft during the solar minimum 23 and the rising phase of the solar cycle 24. Here we report the presence of low-frequency waves (LFWs) inside some isolated MCs (IMCs) and inside the CSs observed by STEREO during such period (2007-2011). It is important to study in detail the properties of waves in space plasmas since particle distribution functions can be modified by wave-particle interactions. We compare wave characteristics within IMCs with those waves observed inside CSs. Both left-handed (LH) and right-handed (RH), near-circularly polarized, transverse and almost parallel-propagating LFWs (around the proton cyclotron frequency) were sporadically observed inside both IMCs and CSs. In contrast, compressive mirror-mode waves (MMs) were observed only within CSs. We studied local plasma conditions inside the IMCs and CSs to gain insight about wave origin: most of the MMs within CSs were observed in regions with enhanced plasma beta (β>1) the majority of the LH waves were found in low beta plasmas (β<1), and the RH waves were predominantly observed at moderate betas (0.4<β≤2). These observations are in agreement with linear kinetic theory predictions for the growth of the mirror, the LH ion cyclotron, and the RH ion firehose instability, respectively. It is possible that the waves were generated locally inside the IMCs and CSs via temperature anisotropies. The plasma beta enhancements that were frequently observed inside the CSs may be the result of compressions and heating taking place inside the interacting structures.

  16. Low Frequency Error Analysis and Calibration for High-Resolution Optical Satellite's Uncontrolled Geometric Positioning

    NASA Astrophysics Data System (ADS)

    Wang, Mi; Fang, Chengcheng; Yang, Bo; Cheng, Yufeng

    2016-06-01

    The low frequency error is a key factor which has affected uncontrolled geometry processing accuracy of the high-resolution optical image. To guarantee the geometric quality of imagery, this paper presents an on-orbit calibration method for the low frequency error based on geometric calibration field. Firstly, we introduce the overall flow of low frequency error on-orbit analysis and calibration, which includes optical axis angle variation detection of star sensor, relative calibration among star sensors, multi-star sensor information fusion, low frequency error model construction and verification. Secondly, we use optical axis angle change detection method to analyze the law of low frequency error variation. Thirdly, we respectively use the method of relative calibration and information fusion among star sensors to realize the datum unity and high precision attitude output. Finally, we realize the low frequency error model construction and optimal estimation of model parameters based on DEM/DOM of geometric calibration field. To evaluate the performance of the proposed calibration method, a certain type satellite's real data is used. Test results demonstrate that the calibration model in this paper can well describe the law of the low frequency error variation. The uncontrolled geometric positioning accuracy of the high-resolution optical image in the WGS-84 Coordinate Systems is obviously improved after the step-wise calibration.

  17. Understanding the low-frequency variability in hydroclimatic attributes over the southeastern US

    NASA Astrophysics Data System (ADS)

    Wang, Hui; Sankarasubramanian, A.; Ranjithan, R. S.

    2015-02-01

    Most studies on evaluating the potential in developing seasonal to interannual hydroclimatic forecasts have focused on associating low-frequency climatic conditions with basin-level precipitation/streamflow. The motivation of this study is to provide an understanding on how land surface characteristics modulate the low-frequency (interannual to decadal) variability in precipitation to develop low-frequency signal in streamflow. For this purpose, we consider basins with minimum anthropogenic impacts over southeastern United States and apply Singular Spectrum Analysis (SSA), a data-driven spectrum analysis tool, on annual precipitation and streamflow time series for detecting the dominant frequencies and for estimating the associated variability with them. Hypothesis test against an AR(1) process is carried out via Monte Carlo SSA for detecting significant (at 90% confidence level) low-frequency oscillations. Thus, the study investigates how the observed low-frequency oscillations in precipitation/streamflow vary over the southeastern United States and also their associations with climatic conditions. For most study basins, precipitation exhibits higher low-frequency oscillations than that of streamflow primarily due to reduction in variability by basin storage. Investigating this further, we found that the percentage variance accounted by low-frequency oscillations in streamflow being higher for larger basins which primarily indicates the increased role of climate and basin storage. To develop a fundamental understanding on how basin storage controls the low-frequency oscillations in streamflow, a simple annual hydrological model is employed to explore how the given low-frequency signal in precipitation being modified under different baseflow index conditions and groundwater residence time. Implications of these analyses relating to streamflow predictions and model calibration are also discussed.

  18. Effects on Performance and Work Quality due to Low Frequency Ventilation Noise

    NASA Astrophysics Data System (ADS)

    Persson Waye, K.; Rylander, R.; Benton, S.; Leventhall, H. G.

    1997-08-01

    A pilot study was carried out to assess method evaluating effects of low frequency noise on performance. Of special interest was to study objective and subjective effects over time. Two ventilation noises were used, one of a predominantly mid frequency character and the other of a predominantly low frequency character. Both had an NC value of 35. For the study, 50 students were recruited and 30 selected on the basis of subjective reports of pressure on the eardrum after exposure to a low frequency noise. Of these, 14 randomly selected subjects aged 21 and 34 took part. The subjects performed three computerized cognitive tests in the mid frequency or the low frequency noise condition alternatively. Tests I and II were performed together with a secondary task.Questionnaires were used to evaluate subjective symptoms, effects on mood and estimated interference with the test results due to temperature, light and noise. The results showed that the subjective estimations of noise interference with performance were higher for the low frequency noise (p<0·05). The exposure to low frequency noise resulted in lower social orientation (p<0·05) (more disagreeable, less co-operative, helpful) and a tendency to lower pleasantness (p=0·07) (more bothered, less content) as compared to the mid frequency noise exposure. Data from test III may indicate that the response time during the last part of the test was longer in the low frequency noise exposure. The effects seemed to appear over time. The hypothesis that cognitive demands are less well coped with under the low frequency noise condition, needs to be further studied. The results further indicate that the NC curves do not fully assess the negative effects of low frequency noise on work performance.

  19. Dynamics of barotropic low-frequency fluctuations in San Francisco Bay during upwelling

    NASA Astrophysics Data System (ADS)

    Sankaranarayanan, S.; Fringer, Oliver B.

    2013-08-01

    Observations of wind, surface elevations, and currents in San Francisco Bay during the 1999 upwelling season are analyzed to understand the dynamics of low-frequency currents in upwelling-dominated estuaries. Principal component analysis is carried out to distinguish the different uncorrelated components of the low-frequency fluctuations in the observations. Analyses of ADCP observations at two locations in the Bay show that barotropic currents flow in the direction of winds in the shallow parts of the cross section and flow against the wind in the deeper parts of the cross-section. We ran the SUNTANS model with three forcing functions: (i) winds, (ii) low-frequency surface elevations, and (iii) winds and low-frequency surface elevations, to determine the forcing functions that best reproduce the observed low-frequency fluctuations. Analyses of observations and model simulations show that wind-driven flow in the shallow areas and upwind in the deeper areas, consistent with linear theory. Model simulations also show that the low-frequency currents in the Bay generated due to local winds capture the mean low-frequency barotropic fluctuations seen in the observations during the upwelling season. Model simulations showed that the current generated due to the coastal sea level forcing at the mouth of the Bay is small because the coastal sea levels inside the Bay are in phase with that at the mouth and thus generate weak or negligible pressure gradients. We conclude that forcing of low-frequency sea level fluctuations along the offshore boundaries in the model simulations does not lead to improvement in the prediction of low-frequency currents in San Francisco Bay.

  20. Detecting Low-Frequency Seismic Signals From Surface Microseismic Monitoring of Hydraulic Fracturing of a Tight-Sand Gas Reservoir

    NASA Astrophysics Data System (ADS)

    Yu, H.; Zhang, H.; Zeng, X.

    2013-12-01

    For both surface and downhole microseismic monitoring, generally geophones with resonance frequency greater than 4.5 Hz are used. Therefore, useful information below 4.5 Hz may not be detected. In a recent experiment, we installed14 3-component broadband seismic sensors on the surface to monitor the process of hydraulic fracturing of tight sand gas reservoirs. The sensor has a broad frequency range of 30 s to 100 Hz with a very high sensitivity of 2400 m/v/s. The reservoirs are located around 1.5 km depth. There are two fracturing stages along a vertical well, lasting for about 2 hours. We recorded the data continuously during the fracturing process at a sampling rate of 50 Hz. From time-frequency analysis of continuous data, we found some high-energy signals at resonance frequencies between 10 and 20 Hz and a relatively weaker signal at a resonance frequency of ~27 Hz during the hydraulic fracturing. These signals with various resonance frequencies are likely caused by vibrations of high-pressure pipes. In addition to the resonance frequencies, the time-frequency analysis also showed consistent low frequency signals between 3 and 4 Hz at different time. The move-out analysis showed that these signals traveled at shear-wave speeds. We have detected 77 effective low frequency events during the 2-hour hydraulic fracturing process, among which 42 were located by a grid-search location method. The horizontal distribution of the events aligns with the maximum horizontal compressive stress direction. Because of the uncertainty in the velocity model, the low-frequency seismic events are not located in the fracturing depths. Recently, long-period, long-duration seismic events in the frequency band of 10 to 80 Hz were detected during hydraulic fracture stimulation of a shale gas reservoir, which may be caused by slow slip along faults/fractures (Das and Zoback, 2011). In the active volcanic areas, monochromatic events that are related to circulation of hydrothermal fluids