Whistler Wave Intensity Research Articles

Electron Heat Fluxes Generated by Intense Whistler Waves at the Upper Ionospheric Altitudes

AbstractMagnetospheric whistler waves, chorus and hiss, can't provide the resonance heating of the core electron plasma population. However, these whistler‐mode branches, can implicitly participate in the heating processes of the core plasma thermal electron population by triggering the electron precipitation over a broad energy range from the magnetosphere and subsequent atmospheric ionization processes leading to the production of superthermal electron population. These superthermal electrons play a large role in the magnetosphere‐ionosphere‐atmosphere energy interplay with participation of both magnetically conjugate hemispheres, and their Coulomb interaction with background magnetospheric thermal electrons. Using strong hiss and chorus wave events measured by the Van Allen Probes and SuperThermal Electron Transport code, we evaluate the formation of electron heat fluxes at the upper ionospheric altitudes and discuss their consequences on the formation of electron temperature. It is found that chorus and hiss waves that initiate the precipitation of magnetospheric electrons with energies below 30 keV and the follow‐up production of secondary electrons play an important role in the energy balance of ionosphere‐magnetosphere system.

Whistler waves generated by nongyrotropic and gyrotropic electron beams during asymmetric guide field reconnection

Using a two-dimensional particle-in-cell simulation of asymmetric reconnection with a guide field whose strength is 0.3 times the reconnecting magnetic field, we study electron distribution functions and wave intensities in the diffusion region, focusing on the electron diffusion region (EDR). Wave activities with frequencies below the electron cyclotron frequency are observed, and these are whistler waves propagating almost anti-parallel to the magnetic field. The waves are concentrated near the magnetospheric separatrix away from the X line, but the wave activity also spreads through the EDR near the X line. The reconnection outflows are asymmetric in the outflow direction in the magnetospheric side, and the wave intensity is stronger in the side of the faster electron outflow. We study the whistler waves using the fast Fourier transform, analyses of electron velocity distribution functions, and the dispersion solver calculation. Along the magnetospheric separatrix in the stronger outflow side, highly anisotropic electron beams exist with super-Alfvénic drift speeds. The dispersion analysis shows that there are two modes: a temperature anisotropy mode and a beam mode. Outside the EDR, the whistler wave intensity is highest near the separatrix, but the wave intensity decreases if we move away from the separatrix toward the magnetic neutral line because of the increase in the electron population near zero parallel velocity. In the EDR, in the velocity plane perpendicular to the magnetic field, ring/crescent electron distribution functions are observed. Near the X-line, the wave power is enhanced where nongyrotropic electrons contribute to increase the perpendicular temperature anisotropy.

Excitation of electrostatic standing wave in the superposition of two counter propagating relativistic whistler waves

The problem of standing wave formation by superposing two counter-propagating whistler waves in an overdense plasma, studied recently by Sano et al (2019 Phys. Rev. E 100, 053 205 and 2020 Phys. Rev. E 101, 013 206), has been revisited in the relativistic limit. A detailed theory along with simulation has been performed to study the standing wave formation in the interaction of two counter propagating relativistically intense whistler waves. The relativistic theory explains such interaction process more precisely and predicts correct field amplitudes of the standing wave for a much wider range of physical parameters of the problem as compared to its non-relativistic counterpart. The analytical results are compared with 1-D Particle-in-Cell (PIC) simulation results, performed using OSIRIS 4.0. The results are of relevance to ion heating and fast ignition scheme of inertial confinement fusion.

Conversion efficiency of even harmonics of whistler pulse in quantum magnetoplasma

AbstractStudy of even harmonic generation resulting from propagation of whistler pulse in homogeneous high-density quantum plasma immersed in an externally applied magnetic field, using the recently developed quantum hydrodynamic model is presented. The effects of quantum Bohm potential, quantum statistical pressure, and electron spin have been taken into account. The field amplitude of even harmonic of the whistler with respect to fundamental wave and the conversion efficiency for phase-mismatch has been analyzed. The conversion efficiency of harmonic radiation depends on the plasma electron density, magnetic field strength as well as the intensity of whistler pulse. The efficiency increases significantly with an increase in plasma density, magnetic field and whistler wave intensity. Higher conversion efficiency is observed in degenerate plasma for lower values of the static magnetic field as compared with classical plasma.

Electron dynamics and wave activities associated with mirror mode structures in the near-Earth magnetotail

We report the observation of mirror mode structures by Cluster spacecraft at around X∼-16 RE in the Earth’s magnetotail. The wavelength of the mirror structure is larger than 7000 km, corresponding to tens of ion gyroradii. Features of the mirror structures are similar to those detected in the magnetosheath: the anti-correlation between the magnetic field strength and plasma density, zero phase velocity in the plasma rest frame and linear polarization. The structures were observed in a region bounded by two dipolarizations during a substorm intensification. Thus, the dipolarization process may provide a plasma condition facilitating the growth of the mirror mode structures. Another interesting feature is the electron dynamics within the mirror structures. Thermal electron energy flux has an enhancement at 0° and 180° pitch angles inside the magnetic dips of the first three mirror structures and an enhancement at 90° pitch angle inside the magnetic dip of the last structure. The different electron distribution inside the mirror structures might be a result of different evolution stages of the mirror wave. The last structure may be in the nonlinear stage of the mirror instability, whereas the three others with quasi-sinusoidal waveforms may be in the linear stage. In addition, we found that intense whistler waves were confined within the magnetic dips. We conjecture that whistler waves observed in the first three dips were generated in a remote region, then they were trapped in the mirror mode troughs and transported toward the spacecraft; while the whistler wave detected in the last dip was excited locally by the electron anisotropy instability.

A numerical study of chorus generation and the related variation of wave intensity using the DAWN code

AbstractChorus waves play an important role in energetic electron dynamics in the inner magnetosphere. In this work, we present a new hybrid code, DAWN, to simulate the generation of chorus waves. The DAWN code is unique in that it models cold electrons using linearized fluid equations and hot electrons using particle‐in‐cell techniques. The simplified fluid equations can be solved with robust and simple algorithms. We demonstrate that discrete chorus elements can be generated using the code. Waveforms of the generated element show amplitude modulation or “subpackets,” and the frequency sweep rate of the generated element is found to be consistent with that of observed chorus waves. Using the DAWN code, we then investigate the variation of wave intensity () with respect to linear growth rates on the equatorial plane. Previous observations showed that the change in linear growth rates of whistler waves modulated by external processes such as density modulations is usually small (), while the variation of the wave intensity is large (). Using a chosen set of background plasma parameters, we demonstrate that a small change () in linear growth rates can lead to significant variation () of wave intensity only in the transition from the broadband whistler wave generation regime to the chorus wave generation regime. Our results demonstrate the importance of including nonlinear dynamics of chorus generation in understanding the whistler wave intensity modulation process in the inner magnetosphere.

Multiscale whistler waves within Earth's perpendicular bow shock

We present observations of intense whistler waves made by Polar in the frequency range from a few Hz to 600 Hz within Earth's nearly perpendicular bow shock. The long duration burst data provided by Polar reveal the detailed properties of whistler waves in context with the macrostructure of the layer of this supercritical shock. We show that the pedestal and ramp have superposed quasiperiodic, large amplitude precursor substructure occurring at a cadence of ∼3 sec, which is near the ion cyclotron period. With increasing penetration into the shock front, the amplitude of this substructure increases and ultimately reaches downstream values. The nonlinear substructure is shown to be concentrated regions of intense whistler wave activity. Power spectra in the whistler range show strong enhancements in two distinct bands: a relatively broadband lower frequency component occurring near the lower hybrid frequency (a few tens of Hertz) and a higher frequency component at a few hundred Hertz. The lower frequency component is composed of right‐hand polarized whistler wave packets propagating quasiparallel to the magnetic coplanarity plane at oblique angles with respect to both the magnetic field and shock normal, with respective anglesθkb varying from 50°–70° and θkn ∼ 50°. These waves generally have relatively large amplitude (δB/B0∼ 0.1–0.4) magnetic fields ranging from a few nT to 15 nT. Given their preferential upstream propagation near the magnetic coplanarity plane, they are likely generated by a kinetic cross‐field streaming instability driven by the relative drift between the reflected ion beam and the electrons. The high‐frequency component appears to be the shock analog of whistler “lion roars” often observed in the magnetosheath. The lion roars occur within the foot and into the shock ramp in regions where sufficiently intense low‐frequency whistlers exist. These are right‐hand circularly polarized wave packets lasting up to ∼10 cycles, with amplitudes reaching 1 nT, and propagating nearly parallel or antiparallel with respect to the background magnetic field (θkb ⪅ 30° or θkb ⪆ 150°). These packets, which appear regularly with a cadence of a few tens of Hertz, occur within local magnetic field (and also electric field) minima of the lower frequency whistler waves. These waves are likely generated by the whistler anisotropy instability driven by preferential perpendicular anisotropies in the electron distribution function induced by the strong magnetic field of the ramp substructure. The growth rate for the instability is estimated at γ ∼ ωci, which is rather low. We suggest that the low‐frequency whistler waves play a key role in the growth and spatial distribution of the lion roar by forming a quasiperiodic series of magnetic field ducts (troughs) that confine their propagation nearly along the surface of the shock. This allows enough time for the lion roar to grow to observed amplitudes before they leave the region of growth. The low‐frequency waves may also be contributing to the growth of the lion roar by inducing enhancements in the electron anisotropy within magnetic troughs.

Possibility of magnetospheric VLF response to atmospheric infrasonic waves

In this paper, we consider a model of the influence of atmospheric infrasonic waves on VLF magnetospheric whistler wave excitation. This excitation occurs as a result of a succession of processes: a modulation of the plasma density by acoustic-gravity waves in the ionosphere, a reflection of the whistlers by ionosphere modulation, and a modification of whistler wave generation in the magnetospheric resonator. A variation of the magnetospheric resonator Q-factor has an influence on the operation of the plasma magnetospheric maser, where the active substances are radiation belt particles, and the working modes are electromagnetic whistler waves. The magnetospheric maser is an oscillating system which can be responsible for the excitation of self-oscillations. These self-oscillations are frequently characterized by alternating stages of accumulation and precipitation of energetic particles into the ionosphere during a pulse of whistler emissions. Numerical and analytical investigations of the response of self-oscillations to harmonic oscillations of the whistler reflection coefficient shows that even a small modulation rate can significantly change magnetospheric VLF emissions. Our results can explain the causes of the modulation of energetic electron fluxes and whistler wave intensity with a time scale from 10 to 150 s in the day-side magnetosphere. Such quasi-periodic VLF emissions are often observed in the sub-auroral and auroral magnetosphere and have a noticeable effect on the formation of space weather phenomena.

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

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.

Whistler critical Mach number and electron acceleration at the bow shock: Geotail observation

The ‘whistler critical Mach number’, Mcritw, is one of the dimensionless parameters that characterizes collisionless shocks. Originally, it was introduced to indicate the critical point above which whistler waves do not propagate upstream. Indeed our analysis of Geotail data at the Earth's bow shock shows intense whistler waves in the sub‐critical regime, MA < Mcritw, but not in the super‐critical regime. In this paper, we further report that Mcritw seems to regulate the electron acceleration efficiency at the shocks. At the shock transition layer it is found that the spectral index Γ of electron energy spectra defined by f(E) ∝ E−Γ is distributed between 3.5 and 5.0 in the sub‐critical regime, while the hardest energy spectra with Γ = 3–3.5 are detected in the super‐critical regime. We discuss a possible relationship between Mcritw and the electron acceleration.

Synchronized oscillations in energetic electron fluxes and whistler wave intensity in Jupiter's middle magnetosphere

Synchronized oscillations in energetic electron fluxes and electromagnetic wave emissions at the planetary rotation period have been observed in and near Jupiter's magnetosphere. In this work we theoretically examine time‐dependent processes in Jupiter's middle magnetosphere energetic electron radiation belts which are relevant to these oscillations, where intervals of particle accumulation are followed by precipitation into the ionosphere during a whistler electromagnetic radiation pulse. These oscillations are described by a relativistic system of quasi‐linear equations, in which diffusion of particles in adiabatic invariant space and evolution of the electromagnetic radiation are taken into account. Linear analysis shows that the natural oscillation frequency of the Jovian radiation belt is almost independent of magnetic shell and is close in magnitude to the planet's rotation frequency, thus suggesting the possibility of a global resonance. We propose that such near‐resonant oscillations can be excited by periodic modulations of the whistler decay rate due to ionospheric absorption, which has components that depend both on local time in the magnetosphere and on asymmetries that rotate with the planet. These dependencies are shown to combine to produce a modulation at the planetary rotation period that is independent of local time and also independent of the angular velocity of rotation of the plasma. The response to this synchronized driving is investigated in both the linear and nonlinear regimes, and the conditions required for a synchronized response at the planetary period are examined.

Magnetospheric VLF response to the atmospheric infrasonic waves

We consider the influence of atmospheric infrasonic waves on VLF whistler features in the Earth electron radiation belts. An infrasonic wave produces variations of plasma concentration in the ionosphere and VLF wave coefficient of reflection from the ionosphere. The coefficient of whistler reflection and its variation were calculated both analytically for simplified models and numerically for more realistic models of ionospheric altitude electron distributions. The frequency of infrasonic waves is close to the eigen-frequency of the wave - particle interaction processes in the radiation belts. Hence the magnetospheric resonator Q-factor modulation can switch the stationary VLF electromagnetic emission generation mode (hiss emissions) to a quasi-periodic or self-oscillation mode. The spatio-temporal processes in a disturbed magnetosphere were studied by an original code. The obtained results can explain the causes of the modulation of energetic electron precipitation fluxes and whistler wave intensity with a time scale of 10 to 100 sec in the morning and in the forenoon part of the radiation belts. Such periodical and quasiperiodical VLF emissions are often observed in the subauroral and auroral Earth's magnetosphere.

Extremely intense whistler mode waves near the bow shock: Geotail observations

Very intense whistler mode waves with amplitudes up to ∼1 nT were frequently recorded by the Waveform Capture on board the Geotail satellite in the bow shock. The intense whistler waves cover a frequency range from 0.02 to 0.56Ωe (10 ∼ 400 Hz) with durations from tens to hundreds of milliseconds. These intense whistler waves propagate in a direction quasi‐parallel to the ambient magnetic field with an average angle of 22°. Most (∼80%) of the intense whistler waves propagate in a downstream direction, while only 20% of them propagate toward solar wind. Quasi‐perpendicular shocks with shock normal angles from 66° to 86° are associated with the intense whistler waves. The quasi‐parallel propagation indicates that the electron cyclotron resonance is the dominant mechanism for the excitation of the intense whistler waves. It is found that loss cone electrons with temperature anisotropy at the foot of quasi‐perpendicular bow shocks are the major source of the intense whistler waves. The electron beams, formed by an electric field within the bow shock, are very likely the source of those whistlers propagating in an upstream direction. Trapping of electrons by these intense whistler waves may partially explain the flat‐topped electron distribution at the downstream edge of the bow shock.

Laboratory study of some lightning-induced effects in the ionospheric plasma

Laboratory experiments have been conducted at MIT, using the student-built Versatile Toroidal Facility (VTF), to investigate some ionospheric plasma effects produced by lightning-induced whistler waves. Lower hybrid waves, generated by the lightning-induced whistler waves, can cause a chain of extensive plasma effects, such as the acceleration of electrons and ions and the spectral broadening of plasma waves. Two mechanisms by which whistler waves generate lower hybrid waves can be important in the ionosphere. One is the simultaneous excitation of lower hybrid waves and low-frequency mode waves by intense whistler waves. The other is the nonlinear mode conversion of whistler waves into lower hybrid waves in the presence of short-scale field-aligned density striations. The effect of lower hybrid waves on the spectra of Langmuir waves has been investigated. The results of VTF laboratory experiments show that lower hybrid waves can beat with Langmuir waves to produce frequency-upshifted and frequency-downshifted Langmuir waves, broadening the spectra of Langmuir waves. The intensity of these beat waves, however, depends upon the angle of wave propagation with respect to the background magnetic field.

Wave rectification in plasma sheaths surrounding electric field antennas

Combined measurements of Langmuir or broadband whistler wave intensity and lower‐frequency electric field waveforms, all at 10‐µs time resolution, were made on several recent sounding rockets in the auroral ionosphere. It is found that Langmuir and whistler waves are partially rectified in the plasma sheaths surrounding the payload and the spheres used as antennas. This sheath rectification occurs whenever the HF potential across the sheath becomes of the same order as the electron temperature or higher, for wave frequencies near or above the ion plasma frequency. This rectification can introduce false low‐frequency waves into measurements of electric field spectra when strong high‐frequency waves are present. Second harmonic signals are also generated, although at much lower levels. The effect occurs in many different plasma conditions, primarily producing false waves at frequencies that are low enough for the antenna coupling to the plasma to be resistive.

Non-neutral whistler envelope solitons

Whistler envelope solitons are reconsidered including the charge separation effects in the dynamics of quasistatic plasma slow response. The coupled nonlinear ordinary differential equations are solved with the help of a perturbation scheme in which the slow ambipolar potential is expanded in terms of the whistler wave intensity. The soliton profiles are obtained analytically.

On the correlation of whistlers and associated subionospheric VLF/LF perturbations

Several periods of whistler‐associated subionospheric signal perturbations (i.e., Trimpi events) observed at Palmer, Antarctica (L ∼2.4) have been studied. In a case study of the time signature of the signal perturbations during a 10‐min recording period on March 30, 1983, the time delay between the whistler‐producing spheric and the onset of the change was found to be in the range 0.52–0.62 s, independent of the amplitude of the change. Event amplitude, as expected from previous work, was found to be well correlated with the associated whistler wave intensity. Other temporal features such as the rise and decay times of the perturbations were also found to be independent of the event amplitude. These results are consistent with a recent theoretical model of gyroresonant particle scattering interaction in the magnetosphere. The amplitudes of simultaneous perturbations on signals of different frequency and arrival bearing were well correlated in several cases, but exhibited case to case differences that appear to depend upon the spatial distribution of precipitation regions. Whistler occurrence times during two recording periods were found to approximately obey a Poisson distribution, while the statistics of Trimpi event occurrence in those periods showed significant deviations from Poisson behavior. The probability of random alignment in time of the whistlers subionospheric and perturbations is estimated to be < 10−14, in agreement with previous inferences of a cause and effect relation between whistlers and VLF/LF perturbations. Although nearly all of the whistlers observed originated in northern hemisphere lightning, the first example was found of a Trimpi event associated with a southern hemisphere source.

Ducted whistler-mode propagation in the magnetosphere; A half-gyrofrequency upper intensity cutoff and some associated wave growth phenomena

A large class of ducted magnetospheric whistlers exhibits an upper intensity cutoff at which the wave amplitude decreases by > 10db within a fraction of a kHz. A preliminary study, based in large part on whistlers recorded at Eights, Antarctica (L ∼ 4), indicates that the ratio of cutoff frequency to whistler nose frequency ƒco/ƒn is invariant with respect to path equatorial radius, local time, and magnetic activity. Measurements on 541 whistler components propagating in the plasmasphere were such that ƒco/ƒn = 1.33 ± 0.08 for about 70% of the cases. The corresponding result for ƒco/ƒHo, the ratio of cutoff frequency to minimum path gyrofrequency, is ƒco/ƒHo = 0.51 ± 0.03. (To relate ƒn and ƒHo, the approximate relation ƒn = 0.38 ƒHo was used, from Angerami's work on the diffusive-equilibrium model of the distribution of thermal ionization along the field lines in the plasmasphere.) The observed behavior of ƒco/ƒHo is apparently controlled by a propagation effect and not by a resonant particle damping process, the propagation effect probably being the half-gyrofrequency upper limit on ducted whistler-mode propagation in tubes of enhanced ionization predicted by Smith. Spectrographic VLF records exhibit much evidence of wave growth in the 0.4–0.5 ƒHo band, an example being the stimulation of noise at ∼0.5 ƒHo by both whistlers and a low-power (∼100 watts) controlled source. A whistler-triggering event may include: an intensity maximum above the whistler nose, followed by a slight dispersion anomaly and the cutoff; a quasi-constant triggered noise tone with bandwidth 1 sec, and intensity comparable to that of the associated whistler. The noise tone itself does not usually exhibit dispersive whistler-mode echoing, nor does it exhibit an intensity cutoff as it rises in frequency through the 0.5 ƒHo level. Earlier observations by Kimura of triggering of magnetospheric noise by ∼1-sec, 10.2-kHz pulses from the Omega transmitter at Forest Port, New York, have been extended to show that the triggering is confined in space to a range of paths with minimum gyrofrequency within a few per cent of twice the transmitted frequency. A similar tendency is evident in noise produced by high-power (∼106 watts) transmitters. A crude ampligram analysis of the cutoff effect shows that in a typical example the whistler wave intensity drops roughly 20 db within a time interval <13 msec and a frequency interval <50 Hz.