Showing papers on "Whistler published in 2010"

Wave and particle characteristics of earthward electron injections associated with dipolarization fronts

Abstract: [1] A comprehensive examination of particle and wave data from multiple Thermal Emission Imaging System (THEMIS) satellites has been made of an electron injection structure in the magnetotail as it propagated earthward from −20 RE to −11 RE on 27 February 2009. The electron injection, which was closely associated with a dipolarization front and bursty bulk flows, occurred within a thin plasma boundary layer and had both perpendicular and parallel energization, with very little energy dispersion. The thin plasma boundary layer had a thickness comparable to the ion inertial length and displayed different plasma characteristics at different locations. Strong electromagnetic waves between the lower hybrid frequency and the electron gyrofrequency, as well as electrostatic waves up to the electron plasma frequency, were observed within the thin plasma boundary layers. The two outermost spacecraft at X = −20.1 RE and X = −16.7 RE detected intense whistler waves, most likely driven by an observed electron temperature anisotropy with T⊥/T∥ > 1. Closer to Earth at X = −11.1 RE, whistlers were not seen, consistent with the observed electron distribution having T⊥/T∥ < 1. Near the electron injection region, nonlinear electrostatic structures such as electrostatic solitary waves and double layers were also observed. These nonlinear electrostatic structures can interact with the electron distribution and accelerate electrons; high energy distributions could be generated if the electrons encountered a large number of these structures. The observations show that nonideal MHD, nonlinear, and kinetic behavior is intrinsic to the electron injections with multiscale coupling.

Time of flight analysis of pulsating aurora electrons, considering wave-particle interactions with propagating whistler mode waves

Abstract: [1] We propose a model for the energy dispersion of electron precipitation associated with pulsating auroras, considering the wave-particle interactions with propagating whistler mode waves from the equator. Since the resonant energy depends on the magnetic latitude, the pitch angle scattering of different energy electrons can occur continuously along the field line. Considering the energy-dependent path length and the precipitation start time of the precipitating electrons, the transit time of whistler mode waves, and the frequency drift, we calculated the precipitation of electrons observed at the topside ionosphere. Note that higher energy electrons precipitate into the ionosphere of the opposite hemisphere earlier than lower energy electrons. As a result, an energy dispersion of precipitating electrons is observed at the topside ionosphere, even though the modulation of low energy electrons occurs prior to that of high energy electrons. Using the model, we conducted a time-of-flight (TOF) analysis of precipitating electrons observed by the REIMEI satellite, assuming an interaction with the whistler mode chorus rising tone. Our TOF analysis suggests that the modulation region of the pitch angle scattering is near the magnetic equator, whereas previous models expected that the modulation region is far from the magnetic equator. The estimated parameters, such as wave-frequency and latitudinal distribution of the modulation region, are consistent with previous statistical studies of whistler waves at the magnetosphere.

Electron Injection by Whistler Waves in Non-relativistic Shocks

Abstract: Electron acceleration to non-thermal, ultra-relativistic energies (~ 10-100 TeV) is revealed by radio and X-ray observations of shocks in young supernova remnants (SNRs). The diffusive shock acceleration (DSA) mechanism is usually invoked to explain this acceleration, but the way in which electrons are initially energized or 'injected' into this acceleration process starting from thermal energies is an unresolved problem. In this paper we study the initial acceleration of electrons in non-relativistic shocks from first principles, using two- and three-dimensional particle-in-cell (PIC) plasma simulations. We systematically explore the space of shock parameters (the Alfv\'enic Mach number, M_A, the shock velocity, v_{sh}, the angle between the upstream magnetic field and the shock normal, theta_{Bn}, and the ion to electron mass ratio, m_i/m_e). We find that significant non-thermal acceleration occurs due to the growth of oblique whistler waves in the foot of quasi-perpendicular shocks. The obtained electron energy distributions show power law tails with spectral indices up to alpha ~ 3-4. The maximum energies of the accelerated particles are consistent with the electron Larmor radii being comparable to that of the ions, indicating potential injection into the subsequent DSA process. This injection mechanism, however, requires the shock waves to have fairly low Alf\'enic Mach numbers, M_A <~ 20, which is consistent with the theoretical conditions for the growth of whistler waves in the shock foot (M_A <~ (m_i/m_e)^{1/2}). Thus, if the whistler mechanism is the only robust electron injection process at work in SNR shocks, then SNRs that display non-thermal emission must have significantly amplified upstream magnetic fields. Such field amplification is likely achieved by the escaping cosmic rays, so electron and proton acceleration in SNR shocks must be interconnected.

Contrasting the efficiency of radiation belt losses caused by ducted and nonducted whistler-mode waves from ground-based transmitters

Abstract: It has long been recognized that whistler-mode waves can be trapped in plasmaspheric whistler ducts which guide the waves. For nonguided cases these waves are said to be "nonducted", which is dominant for L < 1.6. Wave-particle interactions are affected by the wave being ducted or nonducted. In the field-aligned ducted case, first-order cyclotron resonance is dominant, whereas nonducted interactions open up a much wider range of energies through equatorial and off-equatorial resonance. There is conflicting information as to whether the most significant particle loss processes are driven by ducted or nonducted waves. In this study we use loss cone observations from the DEMETER and POES low-altitude satellites to focus on electron losses driven by powerful VLF communications transmitters. Both satellites confirm that there are well-defined enhancements in the flux of electrons in the drift loss cone due to ducted transmissions from the powerful transmitter with call sign NWC. Typically, ∼80% of DEMETER nighttime orbits to the east of NWC show electron flux enhancements in the drift loss cone, spanning a L range consistent with first-order cyclotron theory, and inconsistent with nonducted resonances. In contrast, ∼1% or less of nonducted transmissions originate from NPM-generated electron flux enhancements. While the waves originating from these two transmitters have been predicted to lead to similar levels of pitch angle scattering, we find that the enhancements from NPM are at least 50 times smaller than those from NWC. This suggests that lower-latitude, nonducted VLF waves are much less effective in driving radiation belt pitch angle scattering.

Electron trapping and charge transport by large amplitude whistlers

Abstract: Trapping of electrons by magnetospheric whistlers is investigated using data from the Waves experiment on Wind and the S/WAVES experiment on STEREO. Waveforms often show a characteristic distortion which is shown to be due to electrons trapped in the potential of the electrostatic part of oblique whistlers. The density of trapped electrons is significant, comparable to that of the unperturbed whistler. Transport of these trapped electrons to new regions can generate potentials of several kilovolts, Trapping and the associated potentials may play an important role in the acceleration of Earth's radiation belt electrons.

Observations of large-amplitude, narrowband whistlers at stream interaction regions

Abstract: [1] We present the first solar wind observations of large-amplitude, narrowband waveforms in the frequency range 10–100 Hz, consistent with the whistler mode. These whistlers are only observable in high time resolution electric field waveform data provided by the Time Domain Sampler (TDS) instrument on STEREO. Amplitudes range from a few to >40 mV/m peak-to-peak, one to three orders of magnitude larger than any previous observations of whistler mode waves in the solar wind. The whistlers are obliquely propagating with a large electrostatic component and are right-hand elliptically polarized in the spacecraft frame. The whistlers occur in groups that are strongly correlated with stream interaction regions (SIRs). The groups persist from a few seconds to minutes and are observed at 88% of SIRs and 17% of shocks from available data. A more detailed look shows that the whistler groups are observed near sudden disturbances of the solar wind magnetic field and plasma. We suggest that, owing to the oblique and narrowband nature of these waves, an electron or ion beam instability may be responsible for their creation. Test particle simulations show that the waves can interact strongly with halo (>60 eV) electrons. Test electrons were scattered by tens of degrees and energized/deenergized by up to 50% in a few tens of milliseconds. Thus these whistlers may play an important role in the dynamics of solar wind electrons within SIRs and near some shocks.

Three dimensional character of whistler turbulence

Abstract: It is shown that the dominant nonlinear effect makes the evolution of whistler turbulence essentially three dimensional in character. Induced nonlinear scattering due to slow density perturbation resulting from ponderomotive force triggers energy flux toward lower frequency. Anisotropic wave vector spectrum is generated by large angle scatterings from thermal plasma particles, in which the wave propagation angle is substantially altered but the frequency spectrum changes a little. As a consequence, the wave vector spectrum does not indicate the trajectory of the energy flux. There can be conversion of quasielectrostatic waves into electromagnetic waves with large group velocity, enabling convection of energy away from the region. We use a two-dimensional electromagnetic particle-in-cell model with the ambient magnetic field out of the simulation plane to generate the essential three-dimensional nonlinear effects.

Where Does Fluid-like Turbulence Break Down in the Solar Wind?

Abstract: Power spectra of the magnetic field in solar wind display a Kolmogorov law f –5/3 at intermediate range of frequencies f, say within the inertial range. Two spectral breaks are also observed: one separating the inertial range from an f –1 spectrum at lower frequencies, and another one between the inertial range and an f –7/3 spectrum at higher frequencies. The breaking of fluid-like turbulence at high frequencies has been attributed to either the occurrence of kinetic Alfven wave fluctuations above the ion-cyclotron frequency or to whistler turbulence above the frequency corresponding to the proton gyroradius. Using solar wind data, we show that the observed high-frequency spectral break seems to be independent of the distance from the Sun, and then of both the ion-cyclotron frequency and the proton gyroradius. We suppose that the observed high-frequency break could be either caused by a combination of different physical processes or associated with a remnant signature of coronal turbulence.

Wavenumber spectrum of whistler turbulence: Particle-in-cell simulation

Abstract: The forward cascade of decaying whistler turbulence is studied in low beta plasma to understand essential properties of the energy spectrum at electron scales, by using a two-dimensional electromagnetic particle-in-cell (PIC) simulation. This simulation demonstrates turbulence in which the energy cascade rate is greater than the dissipation rate at the electron inertial length. The PIC simulation shows that the magnetic energy spectrum of forward-cascaded whistler turbulence at electron inertial scales is anisotropic and develops a very steep power-law spectrum which is consistent with recent solar wind observations. A comparison of the simulated spectrum with that predicted by a phenomenological turbulence scaling model suggests that the energy cascade at the electron inertial scale depends on both magnetic fluctuations and electron velocity fluctuations, as well as on the whistler dispersion relation. Thus, not only kinetic Alfven turbulence but also whistler turbulence may explain recent solar wind observations of very steep magnetic spectra at short scales.

Whistler intensities above thunderstorms

Abstract: . We report a study of penetration of the VLF electromagnetic waves induced by lightning to the ionosphere. We compare the fractional hop whistlers recorded by the ICE experiment onboard the DEMETER satellite with lightning detected by the EUCLID detection network. To identify the fractional hop whistlers, we have developed software for automatic detection of the fractional-hop whistlers in the VLF spectrograms. This software provides the detection times of the fractional hop whistlers and the average amplitudes of these whistlers. Matching the lightning and whistler data, we find the pairs of causative lightning and corresponding whistler. Processing data from ~200 DEMETER passes over the European region we obtain a map of mean amplitudes of whistler electric field as a function of latitudinal and longitudinal difference between the location of the causative lightning and satellite magnetic footprint. We find that mean whistler amplitude monotonically decreases with horizontal distance up to ~1000 km from the lightning source. At larger distances, the mean whistler amplitude usually merges into the background noise and the whistlers become undetectable. The maximum of whistler intensities is shifted from the satellite magnetic footprint ~1° owing to the oblique propagation. The average amplitude of whistlers increases with the lightning current. At nighttime (late evening), the average amplitude of whistlers is about three times higher than during the daytime (late morning) for the same lightning current.

Parallel whistler instability in a plasma with an anisotropic bi-kappa distribution

Abstract: [1] The parallel-propagating whistler instability in a magnetized plasma of electrons and positive ions having bi-kappa velocity distributions is investigated for a wide range of parameters. The threshold condition for instability does not depend on the index, κe, of the electron bi-kappa distribution, but the maximum growth rate depends strongly on this parameter. The functional dependence of the growth rate on κe is strongly influenced by the value of the electron temperature anisotropy. For small anisotropies the maximum growth rate is enhanced by the presence of a low-κe power-law tail, as is well known. At larger temperature anisotropies, however, the reverse applies. A hard tail on the electron velocity distribution is deleterious to the whistler instability in this parameter region. For electron temperature anisotropies intermediate between those described, the (maximum with respect to wave number) growth rate maximizes for a particular κe value. The dependence of the maximum growth rate on κe in this electron temperature anisotropy regime is non-monotonic. For a fixed value of the electron temperature anisotropy, the growth rate is strongly controlled by the parallel electron beta. Larger values of this parameter increase the growth rate, and conversely. The parallel electron beta also governs the value of the electron temperature anisotropy that separates the different types of monotonic behavior of the growth rate described above. The instability is not very sensitive to the electron temperature, provided cases with similar parallel electron beta values are compared. It is pointed out that all whistler dispersion relations, regardless of the value of κe used, pass through a common (ω, k) point. This point is closely related to the instability threshold condition. A novel application is suggested.

Localized whistlers in magnetized spin quantum plasmas

Abstract: The nonlinear propagation of electromagnetic (EM) electron-cyclotron waves (whistlers) along an external magnetic field, and their modulation by electrostatic small but finite amplitude ion-acoustic density perturbations are investigated in a uniform quantum plasma with intrinsic spin of electrons. The effects of the quantum force associated with the Bohm potential and the combined effects of the classical as well as the spin-induced ponderomotive forces (CPF and SPF, respectively) are taken into consideration. The latter modify the local plasma density in a self-consistent manner. The coupled modes of wave propagation is shown to be governed by a modified set of nonlinear Schrodinger-Boussinesq-like equations which admit exact solutions in form of stationary localized envelopes. Numerical simulation reveals the existence of large-scale density fluctuations that are self-consistently created by the localized whistlers in a strongly magnetized high density plasma. The conditions for the modulational instability (MI) and the value of its growth rate are obtained. Possible applications of our results, e.g., in strongly magnetized dense plasmas and in the next generation laser-solid density plasma interaction experiments are discussed.

Particle‐in‐cell simulations of whistler waves excited by an electron κ distribution in space plasma

Abstract: [1] Satellite observations clearly reveal that superthermal electrons in space plasma generally possess a pronounced non-Maxwellian distribution that can be well modeled by a κ distribution. In this paper, one-dimensional (1-D) particle-in-cell simulations are performed to investigate the evolution of whistler waves driven by superthermal electrons with a typical κ distribution in the presence of a cold plasma population. The results obtained from the linear theory are first confirmed: with the increase of the spectral index κ for the κ distribution, the linear growth rate of the excited waves increases and instability threshold for the temperature anisotropy (A = T ⊥ /T ∥ - 1) decreases. Then we further find that with the increase of κ, the fluctuating magnetic field energy density at the saturation stage also increases. Therefore, from both the linear growth rate and the fluctuating magnetic field energy density at the saturation stage, we can find that a bi-Maxwellian distribution (κ → oo) overestimates the importance of whistler waves, since the observed value of κ lies in the range 2 < κ < 6. We also find that the κ values of the electron distributions become smaller with the excitation of the whistler waves.

Generation of wakefields by whistlers in spin quantum magnetoplasmas

Abstract: The excitation of electrostatic wakefields in a magnetized spin quantum plasma by the classical and the spin-induced ponderomotive force (CPF and SPF, respectively) due to whistler waves is reported. The nonlinear dynamics of the whistlers and the wakefields is shown to be governed by a coupled set of nonlinear Schrodinger and driven Boussinesq-like equations. It is found that the quantum force associated with the Bohm potential introduces two characteristic length scales, which lead to the excitation of multiple wakefields in a strongly magnetized dense plasma (with a typical magnetic field strength B0≳109 T and particle density n0≳1036 m−3), where the SPF strongly dominates over the CPF. In other regimes, namely, B0≲108 T and n0≲1035 m−3, where the SPF is comparable to the CPF, a plasma wakefield can also be excited self-consistently with one characteristic length scale. Numerical results reveal that the wakefield amplitude is enhanced by the quantum tunneling effect; however, it is lowered by the exter...

Relationship between median intensities of electromagnetic emissions in the VLF range and lightning activity

Abstract: [1] We present results of a survey of VLF electromagnetic waves observed by the DEMETER spacecraft (altitude about 700 km, launched in June 2004, and still operating) The median value of the power spectral density of electric field fluctuations in the frequency range 1–10 kHz is evaluated as a function of the position of the spacecraft, frequency, magnetic local time, and season of the year It is shown that there are significant seasonal differences between the satellite observed wave intensities throughout the year and it is demonstrated that these are due to the lightning activity changes of the Earth The frequency spectrum at frequencies 0–20 kHz of electromagnetic emissions caused by the lightning activity is investigated as a function of geomagnetic latitude It is shown that the effect of the lightning activity is most pronounced at frequencies larger than about 2 kHz, forming a continuous band of emissions and being the strongest during the nighttime because of the better coupling efficiency of electromagnetic waves through the ionosphere Citation: Němec, F, O Santolik, M Parrot, and C J Rodger (2010), Relationship between median intensities of electromagnetic emissions in the VLF range and lightning activity,

Whistler turbulence wavevector anisotropies: particle-in-cell simulations

Abstract: Two-dimensional electromagnetic particle-in-cell simulations of whistler turbulence in a magnetized, homogeneous, collisionless plasma of electrons and protons are carried out. Enhanced magnetic fluctuation spectra are initially imposed at relatively long wavelengths, and, as in previous such simulations, the temporal evolution shows a forward cascade of magnetic fluctuation energy to shorter wavelengths, with more fluctuation energy at a given wavenumber perpendicular to B o than at the same wavenumber parallel to the background field. The new result here is that the wavevector anisotropy is very different for each of the three components of the fluctuating magnetic field. Here, ∥ denotes the direction parallel to the background magnetic field B o , ⊥ indicates the direction perpendicular to B o and in the simulation plane, and the symbol ⊥⊥ denotes the direction perpendicular to both B o and the simulation plane. The parallel magnetic fluctuations show more energy at than at , whereas the perpendicular in-plane magnetic fluctuations show more energy at than at . Finally, the out-of-plane magnetic fluctuations, , strongly prefer to propagate in the direction.

Beam-excited whistler waves at oblique propagation with relation to STEREO radiation belt observations

Abstract: . Isotropic electron beams are considered to explain the excitation of whistler waves which have been observed by the STEREO satellite in the Earth's radiation belt. Aside from their large amplitudes (~240 mV/m), another main signature is the strongly inclined propagation direction relative to the ambient magnetic field. Electron temperature anisotropy with Te⊥>Te||, which preferentially generates parallel propagating whistler waves, can be excluded as a free energy source. The instability arises due to the interaction of the Doppler-shifted cyclotron mode ω=−Ωe+kVbcosθ with the whistler mode in the wave number range of kc/ωe≤1 (θ is the propagation angle with respect to the background magnetic field direction, ωe is the electron plasma frequency and Ωe the electron cyclotron frequency). Fluid and kinetic dispersion analysis have been used to calculate the growth rate of the beam-excited whistlers including the most important parameter dependencies. One is the beam velocity (Vb) which, for instability, has to be larger than about 2VAe, where VAe is the electron Alfven speed. With increasing VAe the propagation angle (θ) of the fastest growing whistler waves shifts from θ~20° for Vb=2VAe to θ~80° for Vb=5VAe. The growth rate is reduced by finite electron temperatures and disappears if the electron plasma beta (βe) exceeds βe~0.2. In addition, Gendrin modes (kc/ωe≈1) are analyzed to determine the conditions under which stationary nonlinear waves (whistler oscillitons) can exist. The corresponding spatial wave profiles are calculated using the full nonlinear fluid approach. The results are compared with the STEREO satellite observations.

Transverse instability and perpendicular electric field in two-dimensional electron phase-space holes

Abstract: [1] A multidimensional electron phase-space hole (electron hole) is considered to be unstable to the transverse instability. In this paper, we perform two-dimensional (2D) particle-in-cell (PIC) simulations to study the evolution of electron holes at different plasma conditions; we find that the evolution is determined by combined actions between the transverse instability and the stabilization by the background magnetic field. In very weakly magnetized plasma (Ωe ≪ ωpe, where Ωe and ωpe are the electron gyrofrequency and plasma frequency, respectively), the transverse instability dominates the evolution of the electron holes. The parallel cut of the perpendicular electric field (E⊥) has bipolar structures, accompanied by the kinking of the electron holes. Such structures last for only tens of electron plasma periods. With the increase of the background magnetic field, the evolution of the electron holes becomes slower. The bipolar structures of the parallel cut of E⊥ in the electron holes can evolve into unipolar structures. In very strongly magnetized plasma (Ωe ≫ ωpe), the unipolar structures of the parallel cut of E⊥ can last for thousands of electron plasma periods. At the same time, the perpendicular electric field (E⊥) in the electron holes can also influence electron trajectories passing through the electron holes, which results in variations of charge density along the direction perpendicular to the background magnetic field outside of the electron holes. When the amplitude of the electron hole is sufficiently strong, streaked structures of E⊥ can be formed outside of the electron holes, which then emit electrostatic whistler waves because of the interactions between the streaked structures of E⊥ and vibrations of the kinked electron holes.

Generation of whistler waves by a rotating magnetic field source

Abstract: The paper discusses the generation of polarized whistler waves radiated from a rotating magnetic field source created via a novel phased orthogonal two loop antenna. The results of linear three-dimensional electron magnetohydrodynamics simulations along with experiments on the generation whistler waves by the rotating magnetic field source performed in the large plasma device are presented. Comparison of the experimental results with the simulations and linear wave properties shows good agreement. The whistler wave dispersion relation with nonzero transverse wave number and the wave structure generated by the rotating magnetic field source are also discussed. The phase velocity of the whistler waves was found to be in good agreement with the theoretical dispersion relation. The exponential decay rate of the whistler wave propagating along the ambient magnetic field is determined by Coulomb collisions. In collisionless case the rotating magnetic field source was found to be a very efficient radiation sourc...

Inertial-range spectrum of whistler turbulence

Abstract: . We develop a theoretical model of an inertial-range energy spectrum for homogeneous whistler turbulence. The theory is a generalization of the Iroshnikov-Kraichnan concept of the inertial-range magnetohydrodynamic turbulence. In the model the dispersion relation is used to derive scaling laws for whistler waves at highly oblique propagation with respect to the mean magnetic field. The model predicts an energy spectrum for such whistler waves with a spectral index −2.5 in the perpendicular component of the wave vector and thus provides an interpretation about recent discoveries of the second inertial-range of magnetic energy spectra at high frequencies in the solar wind.

Linear Theory of Weakly Amplified, Parallel Propagating, Transverse Temperature-anisotropy Instabilities in Magnetized Thermal Plasmas

Abstract: A rigorous analytical study of the dispersion relations of weakly amplified transverse fluctuations with wavevectors () parallel to the uniform background magnetic field in an anisotropic bi-Maxwellian magnetized electron-proton plasma is presented. A general analytical instability condition is derived that holds for different values of the electron (Ae ) and proton (Ap ) temperature anisotropies. We determine the conditions for which the weakly amplified left-handed (LH) polarized Alfv?n proton cyclotron and right-handed (RH) polarized Alfv?n Whistler electron cyclotron branches can be excited. For different regimes of the electron plasma frequency phase speed w = ? p,e /(kc) these branches reduce to the RH- and LH-polarized Alfv?n waves, RH-polarized high and low phase speed Whistler, RH-polarized proton, and LH-polarized electron cyclotron modes. Analytic instability threshold conditions are derived in terms of the combined temperature anisotropy A = T ?/T ?, the parallel plasma beta ?? = 8 ? nekBT ?/B 2 and the electron plasma frequency phase speed w = ? p,e /(kc) for each mode. The results of our instability study are applied to the observed solar wind magnetic turbulence at values of 90 ? w ? 330. According to the existence conditions of the different instabilities, only the LH- and RH-polarized Alfv?n wave instabilities can operate here. Besides the electron-proton mass ratio ? = 1836, the Alfv?nic instability threshold conditions are controlled by the single observed plasma parameter w. The Alfv?nic instability diagram explains well the observed confinement limits at small parallel plasma beta values in the solar wind.

High-speed stream driven inferences of global wave distributions at geosynchronous orbit: relevance to radiation-belt dynamics

Abstract: Three superposed epoch analyses of plasma data from geosynchronous orbit are compared to infer relative distributions of electromagnetic ion cyclotron (EMIC)- and whistler-mode wave instabilities. Both local-time and storm-time behaviours are studied with respect to dynamics of relativistic electrons. Using LANL-GEO particle data and a quasi-linear approximation for the wave growth allows us to estimate the instability of the two wave modes. This simple technique can allow powerful insights into wave–particle interactions at geosynchronous orbit. Whistler-wave activity peaks on the dayside during the early recovery phase and can continue to be above normal levels for several days. The main phase of all storms exhibits the most EMIC-wave activity, whereas in the recovery phase of the most radiation-belt-effective storms, a significantly suppressed level of EMIC activity is inferred. These key results indicate new dynamics relating to plasma delivery, source and response, but support generally accepted views of whistlers as a source process and EMIC-mode waves as a major loss contributor at geosynchronous orbit.

Whistler Waves Driven by Anisotropic Strahl Velocity Distributions: Cluster Observations

Abstract: Observed properties of the strahl using high resolution 3D electron velocity distribution data obtained from the Cluster/PEACE experiment are used to investigate its linear stability. An automated method to isolate the strahl is used to allow its moments to be computed independent of the solar wind core+halo. Results show that the strahl can have a high temperature anisotropy (T(perpindicular)/T(parallell) approximately > 2). This anisotropy is shown to be an important free energy source for the excitation of high frequency whistler waves. The analysis suggests that the resultant whistler waves are strong enough to regulate the electron velocity distributions in the solar wind through pitch-angle scattering

Initial results from AWESOME VLF receivers: set up in low latitude Indian regions under IHY2007/UNBSSI program

Abstract: (ULF; 1–30 Hz), extremely low frequency (ELF; 30– 3000 Hz), very low frequency (VLF; 3–30 kHz), low frequency (LF; 30–300 kHz), medium frequency (MF; 0.3– 3 MHz), high frequency (HF; 3–30 MHz) and ultra high frequency (UHF; >30 MHz). However, maximum energy radiation is contained in the ELF/VLF band 1 . Continuous monitoring of these ELF/VLF waves provides a powerful remote sensing tool for understanding the processes in ionosphere and magnetosphere. Since both the Earth and the ionosphere are good reflectors at these frequencies, the lightning radiated impulses, commonly known as radio atmospheric or sferics (short form of atmospherics), travel thousands of kilometres in the so-called earthionosphere waveguide (EIWG) with little attenuation (~2–3 dB/1000 km). When this radiated energy is received at VLF/ELF bands, the received signals do not exhibit any dispersion, except near the cut-off frequency of the waveguide and are known as sferics/tweeks. Part of the radiated energy also penetrates the lower boundary of the ionosphere and is guided through the magnetosphere by field-aligned irregularities called ducts with no appreciable attenuation, before re-entering the EIWG to be received as whistlers, emissions, etc. This mode of propagation in magnetized plasma is called the whistlermode

Global lightning distribution and whistlers observed at Dunedin, New Zealand

Abstract: . Whistlers observed at Dunedin, New Zealand, are an enigma since they do not conform to the classical model of whistler production developed by Storey (1953). It is generally accepted that the causative lightning stroke for a whistler observed on the ground at a particular location was located in the neighbourhood of the conjugate point, and generated an electromagnetic signal which propagated in a plasmaspheric duct stretched along a magnetic field line linking the two hemispheres. The causative stroke is thought to have occurred within reasonable proximity of one footpoint of this field line, while the observer was located in the vicinity of the other footpoint. Support for this model has come from a number of previous studies of whistler-lightning observations and whistler-induced particle precipitation. However, as demonstrated here, this model does not always apply. Whistlers detected at Dunedin are nearly as common as those at Tihany, Hungary, despite there being at least 3 orders of magnitude more lightning in Tihany's conjugate region compared to that of Dunedin. Furthermore, whereas Tihany whistlers are generally observed at night, consistent with historical observations, Dunedin whistlers occur predominantly during the day. This paper aims to resolve two paradoxes regarding whistler occurrence at Dunedin: (i) an observation rate which is at variance with conjugate lightning activity, and (ii) a diurnal occurrence peak during daylight. The technique developed by Collier et al. (2009) is used to diagnose the location of the source lightning for Dunedin whistlers. It is found that the majority of the causative strokes occur within a region extending down the west coast of Central America.

Decrease of VLF transmitter signal and Chorus-whistler waves before l'Aquila earthquake occurrence

Abstract: We investigate the VLF emissions observed by the Instrument Champ Electrique (ICE) experiment onboard the DEMETER micro-satellite. We analyze intensity level variation 10 days before and after the occurrence of l'Aquila earthquake (EQ). We found a clear decrease of the VLF received signal related to ionospheric whistler mode (mainly Chorus emission) and to signal transmitted by the DFY VLF station in Germany, few days (more than one week) before the earthquake. The VLF power spectral density decreases of more than two orders of magnitude until the EQ, and it recovers to normal levels just after the EQ occurrence. The ge-omagnetic activity is principally weak four days before EQ and increases again one day before l'Aquila seismic event. Our results are discussed in the frame of short-and long-terms earthquakes prediction focusing on the crucial role of the magnetic field of the Earth.

Automatic Whistler Detector and Analyzer system: Implementation of the analyzer algorithm

Abstract: [1] The full potential of whistlers for monitoring plasmaspheric electron density variations has not yet been realized. The primary reason is the vast human effort required for the analysis of whistler traces. Recently, the first part of a complete whistler analysis procedure was successfully automated, i.e., the automatic detection of whistler traces from the raw broadband VLF signal was achieved. This study describes a new algorithm developed to determine plasmaspheric electron density measurements from whistler traces, based on a Virtual (Whistler) Trace Transformation, using a 2-D fast Fourier transform transformation. This algorithm can be automated and can thus form the final step to complete an Automatic Whistler Detector and Analyzer (AWDA) system. In this second AWDA paper, the practical implementation of the Automatic Whistler Analyzer (AWA) algorithm is discussed and a feasible solution is presented. The practical implementation of the algorithm is able to track the variations of plasmasphere in quasi real time on a PC cluster with 100 CPU cores. The electron densities obtained by the AWA method can be used in investigations such as plasmasphere dynamics, ionosphere-plasmasphere coupling, or in space weather models.