Showing papers on "Whistler published in 2009"

Relativistic electron loss timescales in the slot region

Abstract: [1] Recent observations show that the decay rate of relativistic electrons measured at low altitudes in the slot region at L = 2 is an order of magnitude shorter than theoretical estimates based on CRRES wave data. Here we compare the decay rates of 2–6 MeV electrons measured at low altitudes by the SAMPEX spacecraft with those derived from CRRES wave observations. We show that pitch angle scattering by plasmaspheric hiss (0.1 < f < 2 kHz) is the dominant process responsible for electron loss in the outer slot region (2.4 < L < 3.0), but hiss alone cannot account for the observed loss timescales at lower L. Although SAMPEX samples small equatorial pitch angles (αeq ≈ 18°), this is not the dominant reason for the different timescales. We find that the decay of 2–6 MeV electrons measured by SAMPEX in the inner slot region (2.0 < L < 2.4) is most likely due to the combined effects of hiss and guided whistlers propagating with small wave normal angles. Unguided whistlers have little or no effect on the loss timescales. Magnetosonic waves may be as important as guided whistlers for electron loss under active conditions. Guided whistlers and fast magnetosonic waves increase the diffusion rates in a “bottleneck region” near αeq = 75°, enabling electrons with larger pitch angles to diffuse into the loss cone more effectively and hence the entire distribution function decays more rapidly. Even though the power of guided whistlers and magnetosonic waves may be two orders of magnitude less than hiss, they play a very important role in electron loss in the inner slot region.

Short‐wavelength turbulence in the solar wind: Linear theory of whistler and kinetic Alfvén fluctuations

Abstract: [1] There is a debate as to the identity of the fluctuations which constitute the relatively high-frequency plasma turbulence observed in the solar wind. One school holds that these modes are kinetic Alfven waves, whereas another opinion is that they are whistler modes. Here linear kinetic theory for electromagnetic fluctuations in homogeneous, collisionless, magnetized plasmas is used to compute two dimensionless transport ratios, the electron compressibility Ce and the magnetic compressibility C for these two modes. The former is a measure of the amplitude of density fluctuations, and the latter indicates the relative energy in magnetic fluctuations in the component parallel to the background magnetic field Bo. For βe ≪ 1, [C]Alfven ≪ [C]whistler, and the latter quantity is of order 0.5 at whistler propagation strongly oblique to Bo. Such values of C are sometimes measured at relatively high frequencies and βe ≪ 1 in the solar wind; thus, it is concluded that such observations correspond to whistler mode turbulence. But other solar wind observations indicate that kinetic Alfven fluctuations also contribute to relatively high-frequency solar wind turbulence.

Low‐frequency whistler waves and shocklets observed at quasi‐perpendicular interplanetary shocks

Abstract: [1] We present observations of low-frequency waves (0.25 Hz < f < 10 Hz) at five quasi-perpendicular interplanetary (IP) shocks observed by the Wind spacecraft. Four of the five IP shocks had oblique precursor whistler waves propagating at angles with respect to the magnetic field of 20–50 and large propagation angles with respect to the shock normal; thus they do not appear to be phase standing. One event, the strongest in our study and likely supercritical, had low-frequency waves consistent with steepened magnetosonic waves called shocklets. The shocklets are seen in association with diffuse ion distributions. Both the shocklets and precursor whistlers are often seen simultaneously with anisotropic electron distributions unstable to the whistler heat flux instability. The IP shock with upstream shocklets showed much stronger electron heating across the shock ramp than the four events without upstream shocklets. These results may offer new insights into collisionless shock dissipation and wave-particle interactions in the solar wind.

Quasi-parallel whistler mode waves observed by THEMIS during near-earth dipolarizations

Abstract: . We report on quasi-parallel whistler emissions detected by the near-earth satellites of the THEMIS mission before, during, and after local dipolarization. These emissions are associated with an electron temperature anisotropy α=T⊥e/T||e>1 consistent with the linear theory of whistler mode anisotropy instability. When the whistler mode emissions are observed the measured electron anisotropy varies inversely with β||e (the ratio of the electron parallel pressure to the magnetic pressure) as predicted by Gary and Wang (1996). Narrow band whistler emissions correspond to the small α existing before dipolarization whereas the broad band emissions correspond to large α observed during and after dipolarization. The energy in the whistler mode is leaving the current sheet and is propagating along the background magnetic field, towards the Earth. A simple time-independent description based on the Liouville's theorem indicates that the electron temperature anisotropy decreases with the distance along the magnetic field from the equator. Once this variation of α is taken into account, the linear theory predicts an equatorial origin for the whistler mode. The linear theory is also consistent with the observed bandwidth of wave emissions. Yet, the anisotropy required to be fully consistent with the observations is somewhat larger than the measured one. Although the discrepancy remains within the instrumental error bars, this could be due to time-dependent effects which have been neglected. The possible role of the whistler waves in the substorm process is discussed.

Potential waves for pitch-angle scattering of near-equatorially mirroring energetic electrons due to the violation of the second adiabatic invariant

Abstract: [1] Bounce and MLT averaged diffusion rates due to gyroresonance scattering by plasmaspheric hiss, lightning generated whistlers, and whistlers due to VLF transmitters inside the plasmasphere; and chorus waves outside the plasmasphere; are computed in high resolution in pitch-angle. The computed rates show that another scattering mechanism is required to provide pitch-angle scatting for nearly equatorially mirroring particles at low energies outside the plasmasphere and at a wide range of energies in the slot region. We show that magnetosonic waves and ElectroMagnetic Ion Cyclotron (EMIC) waves can be in bounce resonance with electrons for a wide range of energies and pitch-angles, and may be responsible for the scattering of electrons with equatorial pitch-angels close to 90°. Bounce resonance may also result in an in-situ acceleration of equatorially mirroring particles.

Nonstationarity of a two-dimensional perpendicular shock: Competing mechanisms

Abstract: Two-dimensional particle-in-cell (PIC) simulations are used for analyzing in detail different nonstationary behaviors of a perpendicular supercritical shock. A recent study by Hellinger et al. (2007) has shown that the front of a supercritical shock can be dominated by the emission of large-amplitude whistler waves. These waves inhibit the self-reformation driven by the reflected ions; then, the shock front appears almost “quasi-stationary.” The present study stresses new complementary results. First, for a fixed βi value, the whistler waves emission (WWE) persists for high MA above a critical Mach number (i.e., MA ≥ MAWWE). The quasi-stationarity is only apparent and disappears when considering the full 3-D field profiles. Second, for lower MA, the self-reformation is retrieved and becomes dominant as the amplitude of the whistler waves becomes negligible. Third, there exists a transition regime in MA within which both processes compete each other. Fourth, these results are observed for a strictly perpendicular shock only as B0 is within the simulation plane. When B0 is out of the simulation plane, no whistler waves emission is evidenced and only self-reformation is recovered. Fifth, the occurrence and disappearance of the nonlinear whistler waves are well recovered in both 2-D PIC and 2-D hybrid simulations. The impacts on the results of the mass ratio (2-D PIC simulations), of the resistivity and spatial resolution (2-D hybrid simulations), and of the size of the simulation box along the shock front are analyzed in detail.

Full-wave modeling of transionospheric propagation of VLF waves

Abstract: [1] The full-wave method (FWM) of N. G. Lehtinen and U. S. Inan (2008) is used to model trans-ionospheric propagation of VLF electromagnetic waves from ground-based transmitters up to satellite altitudes. Direct comparison with satellite observations indicates that the VLF wave intensities measured at satellite altitudes are substantially smaller than predicted. The apparent reduction in amplitude is attributed to the presence of irregularities in the ionosphere, which the waves encounter during their traversal of the lower ionosphere. Linear mode scattering from the irregularities convert the whistler waves into quasi-electrostatic whistler mode (QEWM) waves with wave normal angles near the resonance cone. Recent enhancements to the FWM are also described, which allow the minimization of an aliasing error encountered in taking the inverse Fourier transform.

Ion observations from geosynchronous orbit as a proxy for ion cyclotron wave growth during storm times

Abstract: [1] There is still much to be understood about the processes contributing to relativistic electron enhancements and losses in the radiation belts. Wave particle interactions with both whistler and electromagnetic ion cyclotron (EMIC) waves may precipitate or accelerate these electrons. This study examines the relation between EMIC waves and resulting relativistic electron flux levels after geomagnetic storms. A proxy for enhanced EMIC waves is developed using Los Alamos National Laboratory Magnetospheric Plasma Analyzer plasma data from geosynchronous orbit in conjunction with linear theory. In a statistical study using superposed epoch analysis, it is found that for storms resulting in net relativistic electron losses, there is a greater occurrence of enhanced EMIC waves. This is consistent with the hypothesis that EMIC waves are a primary mechanism for the scattering of relativistic electrons and thus cause losses of such particles from the magnetosphere.

Propagation of unducted whistlers from their source lightning: A case study

Abstract: (1) We analyze nightside measurements of the DEMETER spacecraft related to lightning activity. At the 707 km altitude of DEMETER, we observe 3-D electric and magnetic field waveforms of fractional-hop whistlers. At the same time, the corresponding atmospherics are recorded by a very low frequency (VLF) ground-based station located in Nancay (France). The source lightning strokes are identified by the METEORAGE lightning detection network. We perform multidimensional analysis of the DEMETER measurements and obtain detailed information on wave polarization characteristics and propagation directions. This allows us for the first time to combine these measurements with ray-tracing simulation in order to directly characterize how the radiation penetrates upward through the ionosphere. We find that penetration into the ionosphere occurs at nearly vertical wave vector angles (as was expected from coupling conditions) at distances of 100-900 km from the source lightning. The same distance is traveled by the simultaneously observed atmospherics to the VLF ground station. The measured dispersion of fractional-hop whistlers, combined with the ionosonde measurements at the Ebro observatory in Spain, allows us to derive the density profile in the topside ionosphere.

Electromagnetic wave propagation through an overdense magnetized collisional plasma layer

Abstract: The results of investigations into the feasibility of using a magnetic window to propagate electromagnetic waves through a finite-sized overdense plasma slab are described. We theoretically calculate the transmission coefficients for right- and left-handed circularly polarized plane waves through a uniform magnetized plasma slab. Using reasonable estimates for the plasma properties expected to be found in the ionized shock layer surrounding a hypersonic aircraft traveling in the earth’s upper atmosphere (radio blackout conditions), and assuming a 1 GHz carrier frequency for the radio communications channel, we find that the required magnetic field for propagation of right-handed circularly polarized, or whistler, waves is on the order of a few hundred gauss. Transmission coefficients are calculated as a function of sheath thickness and are shown to be quite sensitive to the electron collision frequency. One-dimensional particle-in-cell simulations are shown to be in good agreement with the theory. These s...

Limitations of Hall MHD as a model for turbulence in weakly collisional plasmas

Abstract: . The limitations of Hall MHD as a model for turbulence in weakly collisional plasmas are explored using quantitative comparisons to Vlasov-Maxwell kinetic theory over a wide range of parameter space. The validity of Hall MHD in the cold ion limit is shown, but spurious undamped wave modes exist in Hall MHD when the ion temperature is finite. It is argued that turbulence in the dissipation range of the solar wind must be one, or a mixture, of three electromagnetic wave modes: the parallel whistler, oblique whistler, or kinetic Alfven waves. These modes are generally well described by Hall MHD. Determining the applicability of linear kinetic damping rates in turbulent plasmas requires a suite of fluid and kinetic nonlinear numerical simulations. Contrasting fluid and kinetic simulations will also shed light on whether the presence of spurious wave modes alters the nonlinear couplings inherent in turbulence and will illuminate the turbulent dynamics and energy transfer in the regime of the characteristic ion kinetic scales.

Transient auroral features at Saturn: Signatures of energetic particle injections in the magnetosphere

Abstract: [1] We report for the first time transient isolated auroral spots at Saturn's southern polar region, based on Hubble Space Telescope (HST) FUV images. The spots last several minutes and appear distinct from the rest of the auroral emissions. We study two sets of HST and Cassini observations during which Cassini instrumentation detected signatures of energetic particle injections close to the region where, on the same day, HST observed transient auroral spots. On the basis of the simultaneous remote and in situ observations, we discuss the possibility that the transient features are associated with the dynamical processes taking place in the Kronian magnetosphere. Given the limitations in the available observations, we suggest the following possible explanations for the transient aurora. The injection region could directly be coupled to Saturn's ionosphere by pitch angle diffusion and electron scattering by whistler waves, or by the electric current flowing along the boundary of the injected cloud. The energy contained in the injection region indicates that electron scattering could account for the transient aurora process.

Magnetic field threshold for runaway generation in tokamak disruptions

Abstract: Experimental observations show that there is a magnetic field threshold for runaway electron generation in tokamak disruptions. In this work, two possible reasons for this threshold are studied. The first possible explanation for these observations is that the runaway beam excites whistler waves that scatter the electrons in velocity space prevents the beam from growing. The growth rates of the most unstable whistler waves are inversely proportional to the magnetic field strength. Taking into account the collisional and convective damping of the waves it is possible to derive a magnetic field threshold below which no runaways are expected. The second possible explanation is the magnetic field dependence of the criterion for substantial runaway production obtained by calculating how many runaway electrons can be produced before the induced toroidal electric field diffuses out of the plasma. It is shown, that even in rapidly cooling plasmas, where hot-tail generation is expected to give rise to substantial runaway population, the whistler waves can stop the runaway formation below a certain magnetic field unless the postdisruption temperature is very low.

Limitations of Hall MHD as a model for turbulence in weakly collisional plasmas

Abstract: The limitations of Hall MHD as a model for turbulence in weakly collisional plasmas are explored using quantitative comparisons to Vlasov-Maxwell kinetic theory over a wide range of parameter space. The validity of Hall MHD in the cold ion limit is shown, but spurious undamped wave modes exist in Hall MHD when the ion temperature is finite. It is argued that turbulence in the dissipation range of the solar wind must be one, or a mixture, of three electromagnetic wave modes: the parallel whistler, oblique whistler, or kinetic Alfven waves. These modes are generally well described by Hall MHD. Determining the applicability of linear kinetic damping rates in turbulent plasmas requires a suite of fluid and kinetic nonlinear numerical simulations. Contrasting fluid and kinetic simulations will also shed light on whether the presence of spurious wave modes alters the nonlinear couplings inherent in turbulence and will illuminate the turbulent dynamics and energy transfer in the regime of the characteristic ion kinetic scales.

Whistler Wave Cascade in Solar Wind Plasma

Abstract: Nonlinear three dimensional, time dependent, fluid simulations of whistler wave turbulence are performed to investigate role of whistler waves in solar wind plasma turbulence in which characteristic turbulent fluctuations are characterized typically by the frequency and length scales that are respectively bigger than ion gyro frequency and smaller than ion gyro radius. The electron inertial length is an intrinsic length scale in whistler wave turbulence that distinguishably divides the high frequency solar wind turbulent spectra into scales smaller and bigger than the electron inertial length. Our simulations find that the dispersive whistler modes evolve entirely differently in the two regimes. While the dispersive whistler wave effects are stronger in the large scale regime, they do not influence the spectral cascades which are describable by a Kolmogorov-like $k^{-7/3}$ spectrum. By contrast, the small scale turbulent fluctuations exhibit a Navier-Stokes like evolution where characteristic turbulent eddies exhibit a typical $k^{-5/3}$ hydrodynamic turbulent spectrum. By virtue of equipartition between the wave velocity and magnetic fields, we quantify the role of whistler waves in the solar wind plasma fluctuations.

Confirmation of quasi‐perpendicular shock reformation in two‐dimensional hybrid simulations

Abstract: [1] Shock reformation involves regions of a shock undergoing periodic collapse and redevelopment on a time scale close to the ion cyclotron period. Reformation is often observed in one-dimensional (1-D) hybrid and particle in cell (PIC) simulations of quasi-perpendicular collisionless shocks provided the Alfven Mach number MA and ion plasma beta βi are sufficiently high and low, respectively. Initial 2-D PIC simulations showed some evidence for shock reformation, with ion reflection providing the main energy dissipation mechanism, while recent spacecraft observations showed a reforming shock with large amplitude whistler waves in the foot region. While recent spacecraft observations showed an case with reforming shock crossing with whistler waves dominated in the foot region. However, recent 2-D hybrid and PIC simulations suggest that reformation does not occur in exactly perpendicular 2-D shocks. This paper re-examines shock reformation in quasi-perpendicular shocks using 1-D and 2-D hybrid simulations. We find that 2-D quasi-perpendicular shocks (θbn = 85°) indeed undergo cyclic reformation providing MA and βi are high and low enough, respectively. For low MA ≤ 4, 2-D quasi-perpendicular shocks are found to be quasi-stationary, despite 1-D simulations predicting reformation, confirming and extending recent work for perpendicular 2-D shocks. The dynamics of reformation are quite different in 2-D than in 1-D: in 2-D large amplitude whistler waves grow in the shock foot, have amplitudes of order the downstream magnetic field, and affect the reformation. The whistlers have almost zero phase speeds in the shock frame and oblique wave vectors with respect to the upstream magnetic field. The predicted reformation period increases in 2-D compared with 1-D and increases nonlinearly as MA decreases towards the reformation threshold.

Whistler wave cascades in solar wind plasma

Abstract: Non-linear, three-dimensional, time-dependent fluid simulations of whistler wave turbulence are performed to investigate role of whistler waves in solar wind plasma turbulence in which characteristic turbulent fluctuations are characterized typically by the frequency and length-scales that are, respectively, bigger than ion gyrofrequency and smaller than ion gyroradius. The electron inertial length is an intrinsic length-scale in whistler wave turbulence that distinguishably divides the high-frequency solar wind turbulent spectra into scales smaller and bigger than the electron inertial length. Our simulations find that the dispersive whistler modes evolve entirely differently in the two regimes. While the dispersive whistler wave effects are stronger in the large-scale regime, they do not influence the spectral cascades which are describable by a Kolmogorov-like k ―7/3 spectrum. By contrast, the small-scale turbulent fluctuations exhibit a Navier-Stokes-like evolution where characteristic turbulent eddies exhibit a typical k ―5/3 hydrodynamic turbulent spectrum. By virtue of equipartition between the wave velocity and magnetic fields, we quantify the role of whistler waves in the solar wind plasma fluctuations.

On the existence of Weibel instability in a magnetized plasma. I. Parallel wave propagation

Abstract: In a uniform static magnetic field (B0) the nonresonant mechanism of Weibel instability is certainly affected, because the free energy stored in the temperature anisotropy (e.g., T⊥>T∥, where ⊥ and ∥ denote directions relative to the background magnetic field) can drive the parallel whistler instability due to the cyclotron resonance with plasma particles. However, it has already been shown that the nonresonant mechanism should exist and drive the whistler instability, at least, for anisotropic distribution functions with no parallel thermal velocity spread. Here the investigation is extended to a magnetized plasma with a bi-Maxwellian temperature anisotropy. New frequency limits are introduced and the necessary physical conditions are provided: The nonresonant whistler instabilities have in general small frequencies, smaller than the growth rate, ωr 1 regime. These criteria restricts in general the ma...

Seasonal dependence of energetic electron precipitation: Evidence for a global role of lightning

Abstract: [1] Analysis of the DEMETER spacecraft particle data shows that energetic electron precipitation exhibits a seasonal dependence consistent with lighting-induced electron precipitation (LEP). Over the United States, energetic electron fluxes in the slot region (between L = 2 and 3) are significantly higher in the northern summer than in the winter, consistent with the seasonal variation of lightning activity in the Northern Hemisphere. The association of precipitating fluxes with lightning is explored using lightning location data from the National Lightning Detection Network (NLDN) and VLF wave data on DEMETER. The increased precipitation of particles into the drift loss cone over the Northern Hemisphere in summer is consistent with expected pitch-angle scattering by lightning-generated whistler waves, indicating that lightning is a significant contributor to the loss of slot region electrons.

“Drifting tadpoles” in wavelet spectra of decimetric radio emission of fiber bursts

Abstract: Aims. The solar decimetric radio emission of fiber bursts was investigated searching for the “drifting tadpole” structures proposed by theoretical studies. Methods. Characteristic periods with the tadpole pattern were searched for in the radio flux time series by wavelet analysis methods. Results. For the first time, we have found drifting tadpoles in the wavelet spectra of the decimetric radio emission associated with the fiber bursts observed in July 11, 2005. These tadpoles were detected at all radio frequencies in the 1602-1780 MHz frequency range. The characteristic period of the wavelet tadpole patterns was found to be 81.4 s and the frequency drift of the tadpole heads is -6.8 MHz s -1 . These tadpoles are interpreted as a signature of the magnetoacoustic wave train moving along a dense flare waveguide and their frequency drift as a motion of the wave train modulating the radio emission produced by the plasma emission mechanism. Using the Aschwanden density model of the solar atmosphere, only low values of the Alfven speed and the magnetic field strength in the loop guiding this wave train were derived which indicates a neutral current sheet as the guiding structure. The present analysis supports the model of fiber bursts based on whistler waves.

A new whistler inversion method

Abstract: [1] A new whistler inversion method has been developed to obtain plasmaspheric electron densities and propagation paths deduced from measured whistler data. It is based on the exact Appleton-Hartree dispersion relation and recent experimental density distribution models, comprising the following components: a longitudinal whistler wave propagation model; an empirical electron density distributions model along the field lines based on Polar spacecraft data; and dipole and International Geomagnetic Reference Field approximation of the Earth's magnetic field. The new method predicts electron densities and propagation paths different from the earlier methods. The validation of the method is difficult owing to lack of in situ measurements. A multiple-path whistler group model was introduced in addition to the whistler inversion method assuming a simplified (logarithmic) dependence of equatorial electron density. A new, time-frequency domain transformation of multiple path groups led us to validate the components of the whistler inversion method, and it adds a major extension to the inversion method in the case of multiple-path whistler groups. Beside that, the method is capable of providing electron density profiles for the plasmasphere by the analysis of multiple-path propagation whistlers.

STEREO observations of upstream and downstream waves at low Mach number shocks

Abstract: [1] Early theories of upstream and downstream wave formation at laminar (low Mach number, low beta) shocks predicted that upstream waves would arise from phase-standing whistlers, propagating upstream along the shock normal. Downstream waves were attributed to nearly perpendicular shocks where waves had a different dispersion than the whistler mode, allowing them to stand downstream. Observations of low-Mach number shocks with STEREO reveal both upstream and downstream waves, but unlike the prediction of early theory, the downstream waves arise for a wide variety of shock conditions. These downstream waves appear to be compressional magnetosonic waves.

Penetration of lightning MF signals to the upper ionosphere over VLF ground-based transmitters

Abstract: [1] The MF data recorded by the low-altitude satellite DEMETER have been used to survey the MF waves around the Earth. A global map of the MF emissions indicates that there exists a wave activity in the frequency range 2–2.5 MHz above the main powerful VLF ground-based transmitters operating in the frequency range 18–50 kHz. It is shown that this is due to the high-frequency part of whistlers induced by the thunderstorm activity. They can penetrate trough the ionosphere at the locations of the transmitters because these transmitters induce large ionospheric perturbations. This means that an integrated map over several months is able to show these MF emissions above all main VLF transmitters. The discrepancy between intensities of the emissions in winter and summer (in the Northern Hemisphere) is explained by considering the geographic variations of the plasma frequency below the satellite. It is shown that the MF waves spread in longitude in the hemisphere opposite to the VLF transmitters.

Analysis of subprotonospheric whistlers observed by DEMETER: A case study

Abstract: [1] Subprotonospheric (SP) whistlers consist of a series of low-dispersion components that result from repeated reflections between the base of the ionosphere and altitudes up to ∼1000 km. We have used wave-normal angles and plasma characteristics measured by the DEMETER microsatellite as an input for a three-dimensional ray tracing technique. For several SP whistlers we have also succeeded in finding the causative lightning located at relatively large distances from the satellite footprint along the geomagnetic field line. We show that the reflections and formation of the SP whistlers take place owing to an oblique propagation, with respect to the magnetic field, in the waveguide formed by a profile of the increasing lower hybrid resonance frequency in the upper ionosphere and the base of the ionosphere. We have observed propagation across the magnetic meridian planes. We conclude that the individual components of the SP whistler propagate along different raypaths. The reflected components enter the ionosphere at relatively large distances from the satellite footprint and experience a spread of wave-normal angles during this entry. Depending on the initial wave-normal angle, these waves undergo a different number of reflections before reaching the satellite, thus arriving with different time delays. However, the first component observed of a SP whistler is formed by waves entering the ionosphere at relatively small distances from the satellite footprint and at relatively small wave-normal angles. These waves do not reflect above the satellite but propagate to the opposite hemisphere.

Correlation between global lightning and whistlers observed at Tihany, Hungary

Abstract: Although the generation and propagation mechanisms for whistlers are fairly well understood, the location and extent of the lightning source region for the whistlers observed at a given station are ...

Theory and simulations of whistler wave propagation

Abstract: A linear theory of whistler waves is developed within the paradigm of a two-dimensional incompressible electron magnetohydrodynamics model. Exact analytic wave solutions are obtained for small-amplitude whistler waves that exhibit magnetic field topological structures consistent with the observations and our simulations in a linear regime. In agreement with experiment, we find that the parallel group velocity of the wave is large compared to its perpendicular counterpart. Numerical simulations of collisional interactions demonstrate that the wave magnetic field either coalesces or repels depending upon the polarity of the associated current. In the nonlinear regime, our simulations demonstrate that the evolution of the wave magnetic field is governed essentially by the nonlinear Hall force.

Spin induced nonlinearities in the electron MHD regime

Abstract: We consider the influence of the electron spin on the nonlinear propagation of whistler waves. For this purpose a recently developed electron two-fluid model, where the spin up- and down populations are treated as different fluids, is adapted to the electron MHD regime. We then derive a nonlinear Schrodinger equation for whistler waves, and compare the coefficients of nonlinearity with and without spin effects. The relative importance of spin effects depend on the plasma density and temperature as well as the external magnetic field strength and the wave frequency. The significance of our results to various plasmas are discussed.

Origin of zebra pattern in type iv solar radio emission

Abstract: Strong and weak aspects of dierent theories of fine structure on solar radio emission dynamic spectra observed as several or numerous quasi-equidistant bands of enhanced and reduced radiation (zebra pattern) are discussed. Most of the works which propose zebra pattern interpretation are based on the plasma mechanism of radio emis- sion generation, which consists of excitation of plasma (electrostatic) waves and their subsequent transformation into electromagnetic emission. Plasma waves arise due to ki- netic or hydrodynamic instability at the upper hybrid frequencies at the levels of double plasma resonance in a distributed source. Some works are devoted to considering whistlers as the main reason for stripes in emission and absorption occurring in the dynamic spec- tra. An alternative theory of zebra pattern origin suggests that of a compact source with trapped plasma waves is present in the corona. Another interpretation is based on special eects that may occur when radio waves propagate through some periodic structure in the corona. All suggested mechanisms are analyzed with relation to their capability to give the best fit for the observed fine structure features in the framework of the source model with reasonable physical parameters. It is shown that the theory based on the eect of double plasma resonance in a nonhomogeneous coronal loop is the best-developed theory for the origin of zebra pattern at the meter-decimeter wavelengths at the present time.