Electrostatic Electron Cyclotron Waves Research Articles

Broadband Electrostatic Waves Behind Dipolarization Front: Observations and Analyses

AbstractUsing high‐resolution MMS data, we report the observations of broadband electrostatic waves associated with parallel electron temperature anisotropy () behind a dipolarization front (DF). These electrostatic waves include electrostatic solitary waves and electron cyclotron waves. To quantify electron anisotropy, we define the parallel flux anisotropy parameter , where is phase space densities at each energy. By performing correlation analyses between parallel flux anisotropies () and power spectral densities (PSDs) of these waves, we find that: (a) in 0.8–20 keV are positively correlated with the parallel electric field fluctuations in 50–3,000 Hz; (b) in 0.8–20 keV are positively correlated with perpendicular electric field fluctuations in all frequency ranges; (c) the correlation between parallel flux anisotropies and perpendicular electric field fluctuations is more significant in this case. We also discuss the possibility of electron firehose instability and wave–particle interactions. Our study promotes the understanding of the properties and dynamics of DFs.

How Magnetically Conjugate Atmospheres and the Magnetosphere Participate in the Formation of Low‐Energy Electron Precipitation in the Region of Diffuse Aurora

AbstractThe electron precipitation in the region of the diffuse aurora should be considered as a two‐step process (Khazanov et al., 2017, https://doi.org/10.1002/2016GL072063). The first one is the interaction of plasma sheet electrons with electrostatic electron cyclotron and/or whistler waves, moving those electrons into the loss cone to precipitate in both magnetically conjugate atmospheres. The second step is the interaction of these electrons with the ionosphere and atmosphere via their elastic and nonelastic collisions and reflection (backscatter) of degraded electrons back to magnetosphere and conjugate ionospheres. This paper presents the results of a newly developed scenario of nonsteady‐state electron precipitation dynamics that accounts for magnetosphere‐ionosphere‐atmosphere energy interplay over the entire energy range of the plasma sheet electron population and their affiliated secondary electrons. It also studies how both magnetically conjugate auroral regions work together with the magnetosphere in the formation of electron precipitation in the region of the diffuse aurora with the energy range coverage from 1 eV up to 10 keV.

First Measurements of Electrons and Waves inside an Electrostatic Solitary Wave.

Electrostatic solitary wave (ESW)-a Debye-scale structure in space plasmas-was believed to accelerate electrons. However, such a belief is still unverified in spacecraft observations, because the ESW usually moves fast in spacecraft frame and its interior has never been directly explored. Here, we report the first measurements of an ESW's interior, by the Magnetospheric Multiscale mission located in a magnetotail reconnection jet. We find that this ESW has a parallel scale of 5λ_{De} (Debye length), a superslow speed (99 km/s) in spacecraft frame, a longtime duration (250ms), and a potential drop eφ_{0}/kT_{e}∼5%. Inside the ESW, surprisingly, there is no electron acceleration, no clear change of electron distribution functions, but there exist strong electrostatic electron cyclotron waves. Our observations challenge the conventional belief that ESWs are efficient at particle acceleration.

Linear analysis of electrostatic waves in the lunar wake plasma

Propagation characteristics of electrostatic electron and ion cyclotron waves, ion- and electron- acoustic waves in a four-component magnetized plasma comprising of protons, doubly charged Helium ions, beam electrons and superthermal electrons following a kappa distribution are presented. The model supports 12 plasma modes: two electron cyclotron (modes 1 and 12), two electron acoustic (modes 2 and 11), two fast ion acoustic (modes 3 and 10), two slow ion acoustic (modes 4 and 9), two proton cyclotron (modes 5 and 8) and two Helium cyclotron (modes 6 and 7). At parallel propagation, with increase in electron beam speed, mode 11 first merges with slow ion acoustic mode 4 and then with fast ion acoustic mode 3 and drives them unstable. For oblique propagation and without the electron streaming, coupling of various plasma modes occurs and it weakens with increase in the angle of propagation. Further, for oblique propagation with finite electron beam velocity, merging as well as coupling of various plasma modes are observed. Growth rates as well as wave numbers of the excited slow and fast ion acoustic modes are much smaller in magnetized plasma than in an unmagnetized one. The results are relevant to observations of electrostatic waves in the lunar wake.

Upper hybrid waves and energetic electrons in the radiation belt

AbstractVan Allen radiation belt is characterized by energetic electrons and ions trapped in the Earth's dipolar magnetic field lines and persisting for long periods. It is also permeated by high‐frequency electrostatic fluctuations whose peak intensity occurs near the upper hybrid frequency. Such a phenomenon can be understood in terms of spontaneous emission of electrostatic multiple harmonic electron cyclotron waves by thermal plasmas. In the literature, the upper hybrid fluctuations are used as a proxy for determining the electron number density, but they also contain important information concerning the energetic electrons in the radiation belt and possibly the ring current electrons. The companion paper analyzes sample quiet time events and demonstrates that the upper hybrid fluctuations are predominantly emitted by tenuous population of energetic electrons. The present paper supplements detailed formalism of spontaneous thermal emission of multiple‐harmonic cyclotron waves that include upper hybrid fluctuations.

Stability of Electrostatic Electron Cyclotron Waves in a Multi-Ion Plasma

We have studied the stability of the electrostatic electron cyclotron wave in a plasma composed of hydrogen, oxygen and electrons. To conform to satellite observations in the low latitude boundary layer we model both the ionic components as drifting perpendicular to the magnetic field. Expressions for the frequency and the growth rate of the wave have been derived. We find that the plasma can support electron cyclotron waves with a frequency slightly greater than the electron cyclotron frequency ω ce ; these waves can be driven unstable when the drift velocities of both the ions are greater than the phase velocity of the wave. We thus introduce another source of instability for these waves namely multiple ion beams drifting perpendicular to the magnetic field.

THEMIS observations of the magnetopause electron diffusion region: Large amplitude waves and heated electrons

AbstractWe present the first observations of large amplitude waves in a well‐defined electron diffusion region based on the criteria described by Scudder et al. [2012] at the subsolar magnetopause using data from one Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite. These waves identified as whistler mode waves, electrostatic solitary waves, lower hybrid waves, and electrostatic electron cyclotron waves, are observed in the same 12 s waveform capture and in association with signatures of active magnetic reconnection. The large amplitude waves in the electron diffusion region are coincident with abrupt increases in electron parallel temperature suggesting strong wave heating. The whistler mode waves, which are at the electron scale and which enable us to probe electron dynamics in the diffusion region were analyzed in detail. The energetic electrons (~30 keV) within the electron diffusion region have anisotropic distributions with Te ⊥/Te ∥ > 1 that may provide the free energy for the whistler mode waves. The energetic anisotropic electrons may be produced during the reconnection process. The whistler mode waves propagate away from the center of the “X‐line” along magnetic field lines, suggesting that the electron diffusion region is a possible source region of the whistler mode waves.

Characteristics of electron cyclotron harmonic waves observed in an active two‐point propagation experiment in the ionosphere

Electrostatic electron cyclotron waves (ECWs), also‐called electron Bernstein waves, observed at harmonics nfc of the electron cyclotron frequency fc were transmitted over field‐aligned emitter‐receiver separations of hundreds of meters in the active rocket experiment OEDIPUS‐C. Although the 300‐μs rectangular current pulses into the emitting antenna were smoothly maintained during the experiment, the resulting ECW pulses at the receiver exhibited considerable variety in both the time and frequency domains. A full hot‐plasma dispersion relation has been applied to ray‐tracing investigations to identify the rays that could connect the emitter and receiver in a smoothly varying model of the auroral ionospheric magnetoplasma. Theoretical connecting rays were limited to frequencies extending from 1 to a few kilohertz above 2fc, which was about 2.6 MHz. But the observed pulse spectra extended over a much broader bandwidth, from several kilohertz below nfc to several kilohertz above, for n = 2, 3, and 4. The broadening is interpreted as a consequence of Doppler effect caused by payload motion and backscatter of the ECWs. Field‐aligned density irregularities typical of the auroral topside ionosphere or waves nonlinearly induced by the intense near fields of the HEX antenna may act as the scatterers.

Resonant scattering of plasma sheet electrons by whistler‐mode chorus: Contribution to diffuse auroral precipitation

A quantitative analysis is presented of the resonant scattering of plasma sheet electrons (∼100 eV–20 keV) at L = 6 due to resonant interactions with whistler‐mode chorus. Using wave parameters modeled from observations under geomagnetically disturbed conditions, it is demonstrated that the rate of pitch angle scattering can exceed the level of strong diffusion over a broad energy range (200 eV–10 keV) containing the bulk of the injected plasma sheet population. Scattering by chorus appears to be more effective than previous analyses of resonant interaction with electrostatic electron cyclotron waves. Chorus scattering is therefore a major contributor to the origin of the diffuse aurora and should also control the MLT distribution of injected plasma sheet electrons. The rates of scattering are sensitive to the distributions of wave power with respect to wave normal direction and wave frequency spectrum. Upper‐band chorus (ω/Ωe > 0.5) is the dominant scattering process for electrons below ≈5 keV while lower‐band chorus (0.1 ≤ ω/Ωe ≤ 0.5) is more effective at higher energies especially near the loss cone.

Polar observations of plasma waves in and near the dayside magnetopause/magnetosheath

The plasma wave instrument (PWI) on board the Polar spacecraft made numerous passages of the dayside magnetopause and several probable encounters with the magnetosheath during the years 1996 and 1997. During periods of relatively high density, the PWI antenna–receiver system is coupled to the plasma and oscillates. The oscillations have been shown (cf. Radio Sci. 36 (2001) 203) to be indicative of periods of higher plasma density and plasma flows, possibly associated with magnetic reconnection. We have studied the plasma waves observed on three distinct magnetopause passes distinguished by the presence of these oscillations of the PWI receivers, and we report on the data obtained near, but not during, the times of the oscillations and the possible role of these waves in magnetic reconnection. Sweep-frequency receiver and high-resolution waveform data for some of these times are presented. The plasma wave measurements on each of the passes are characterized by turbulence. The most stable waves are whistler mode emissions typically of several hundred hertz that are seen intermittently in these regions. The data indicate the presence of impulsive solitary-like wave structures with strong electric fields both parallel and perpendicular to the magnetic field near, but not always within, suspected reconnection sites. The solitary waves show the highest occurrence when observed with electrostatic electron cyclotron waves. These latter waves have been observed in the past in the cusp, polar magnetosphere, and auroral regions and therefore may represent excursions into the cusp, but also indicate the presence of low-energy electron beams. Turbulence near the lower hybrid frequency, low-frequency EM waves, and impulsive monopolar electrostatic pulses are seen throughout the magnetopause and particularly near regions of large decrease in the local magnetic field and enhanced field-aligned flows, the suspected reconnection sites. The absence of significant solitary wave signatures within suspected reconnection sites may require modifications to some reconnection models.

Correction to “Electrostatic electron cyclotron waves generated by low‐energy electron beams” by J. D. Menietti et al.

Correction to “Electrostatic electron cyclotron waves generated by low‐energy electron beams” by J. D. Menietti et al.

Electrostatic electron cyclotron waves generated by low‐energy electron beams

We report the results of an investigation of waves observed by the Polar spacecraft at high altitudes and latitudes and at frequencies just above the cyclotron frequency. These observations are made frequently when the spacecraft is over the polar cap as well as near the dayside cusp and the nightside auroral region and for ratios of gyrofrequency to plasma frequency, fp/fg ∼ 1. We investigate the role of electron beams with E≲1 keV in the generation of these waves. Observed plasma parameters are used as input to a modification of the Waves in Homogeneous, Anisotropic Multi‐component Plasmas (WHAMP) computer code to place constraints on the free energy source and growth of these waves. We conclude these waves are an indicator of the presence of low‐energy electron beams and a cold electron component (E≲0.2 eV) in regions with fp/fg≲1.

Electrostatic electron cyclotron waves observed by the plasma wave instrument on board Polar

We report the results of an investigation of waves observed by the Polar spacecraft at high altitudes and latitudes and at frequencies just above the cyclotron frequency. These observations are made frequently when the spacecraft is over the polar cap as well as near the dayside cusp and near the nightside auroral region, and observations are made for ratios of plasma frequency to cyclotron frequency, ƒp/ƒc ∼ 1. Using the six‐channel high‐frequency waveform receiver (HFWR) on board the spacecraft, which can provide three‐axis electric and three‐axis magnetic field measurements, we attempt to identify the wavemode of these emissions and investigate possible source mechanisms including low‐energy electron beams. We further observe electromagnetic emission associated with upper hybrid waves near and within the plasmasphere. This emission is consistent with both Z and O modes.

Evidence for a magnetosphere at Ganymede from plasma-wave observations by the Galileo spacecraft

ON 27 June 1996 the Galileo spacecraft1,2 made the first of four planned close fly-bys of Ganymede, Jupiter's largest moon. Here we report measurements of plasma waves and radio emissions, over the frequency range 5 Hz to 5.6 MHz during the first encounter. Intense plasma waves were detected over a region of space nearly four times Ganymede's diameter, which is much larger than would be expected for a simple wake arising from Ganymede's passage through Jupiter's rapidly rotating magneto-sphere. The types of waves detected (whistler-mode emissions, upper hybrid waves, electrostatic electron cyclotron waves and escaping radio emission) strongly suggest that Ganymede has a large, extended magnetosphere of its own. The data indicate the presence of a strong (B > 400 nT) magnetic field, and show that Ganymede is surrounded by an ionosphere-like plasma with a maximum electron density of about 100 particles cm−3 and a scale height of about 1,000km.

Plasma wave observations at Neptune

During the Voyager 2 flyby of Neptune, the plasma wave instrument detected many familiar phenomena. These include radio emissions, electron plasma oscillations in the solar wind upstream of the bow shock, electrostatic turbulence at the bow shock, electrostatic electron cyclotron waves and upper hybrid resonance (UHR) waves, whistler mode noise, and dust impacts. The radio emissions occur in a broad continuum-like spectrum extending from about 5 kHz to above 50 kHz, and are emitted in a disk-like beam along the magnetic equatorial plane. The radio emissions are believed to be generated by mode conversion from UHR waves at the magnetic equator. The inner magnetosphere has relatively low plasma wave intensities, generally less than 100 μV/m. At the ring plane crossing, many small micron-sized dust particles were detected striking the spacecraft. The maximum impact rate was about 280 impacts per second at the inbound ring plane crossing, and about 110 impacts per second at the outbound ring plane crossing. Most of the particles are concentrated in a dense disk, about one thousand km thick, near the equatorial plane. A broader, more tenuous distribution of dust also extends along the entire trajectory inside of 6 R N including the northern polar region.

Simultaneous excitation of broadband electrostatic noise and electron cyclotron waves in the plasma sheet

Electron cyclotron harmonics and broadband electrostatic noise (BEN) are often observed in the Earth's outer plasma sheet. While it is well known that ion beams in the plasma sheet boundary layer can generate BEN, new two dimensional electrostatic simulations show that field‐aligned ion beams with a small perpendicular ring distribution can drive not only BEN, but also electron cyclotron harmonic (ECH) waves simultaneously. Simulation results are presented here using detailed diagnostics of wave properties, including dispersion relations of all wave modes.

Electrostatic electron and ion cyclotron harmonic waves in Neptune's magnetosphere

Voyager 2 observations of electrostatic electron and ion cyclotron waves detected in Neptune's magnetosphere are presented. Both types of emission appear in a frequency band above the electron and ion (proton) cyclotron frequencies, respectively, and are tightly confined to the magnetic equator occurring within a few degrees of it. The electron cyclotron waves are interpreted as electron Bernstein modes including an intense upper hybrid resonance emission excited by an unstable loss cone distribution of low‐density superthermal electrons. The ion cyclotron waves are interpreted as hydrogen Bernstein modes including an intense lower hybrid resonance emission excited by an unstable ring distribution of low‐density pickup N+ ions deriving from the satellite Triton.

High‐frequency waves in the cusp/cleft regions

Because of the high time resolution of many of its scientific instruments, the Viking satellite has detected the presence of intense wave bursts in the cusp/cleft regions, at altitudes between 1 and 2 Earth radii. These bursts are closely related to the acceleration processes of charged particles and are characterized by a strong plasma turbulence illustrated by sporadic and intense broadband electrostatic noise detected in the 4‐ to 700‐kHz frequency range of the high‐frequency wave receiver. The duration of such bursts is small, typically a few hundreds of milliseconds, corresponding to a spatial width less than 1 km at the satellite altitude. The low‐frequency part of this noise is always the most intense, with electric field amplitudes of a few tens of millivolts per meter. This noise is correlated with 0.1‐ to 1‐keV electron beams and with small‐scale field‐aligned currents and is always associated with low‐frequency (∼1 Hz) electric field fluctuations. Perpendicular acceleration of ambient plasma ions occurs in connection with the bursts. The impulsive broadband waves are detected at the equatorward edge of the polar cusp and more generally in the entire cleft region. The central part of the cusp is characterized by an increase of the frequency cutoff of the hiss emissions, related to the increase of the electron density. In addition, electrostatic electron cyclotron waves are also recorded. Terrestrial kilometric radiation is often detected in the cusp/cleft regions usually in close correlation with particle acceleration processes. The Viking satellite tends to have been mostly above the generation regions of these electromagnetic waves in the dayside auroral oval, which makes it difficult to localize precisely their source.

ISEE observations of the plasma sheet boundary, plasma sheet, and neutral sheet: 2. Waves

The companion paper describes the dc electric and magnetic fields, E × B velocities, plasma flow, composition and ground magnetic activity for two periods in the magnetotail near 20 RE. The relationship of the plasma waves to the dc field characteristics is described herein. In addition, for two subintervals which had strong ground magnetic activity, we report comparisons of plasma waves in the magnetotail to characteristics of the dc fields and fluid parameters such as the plasma flow, β, density, temperature, and fpe/fce. Upper hybrid waves at multiples of the electron cyclotron frequency near and above the electron plasma frequency were observed for the first time in this region of the magnetosphere and occurred in regions of low β and temperature and high density. This contrasts with electrostatic electron cyclotron waves which occurred in high‐temperature, high‐density regions, as previously reported by Gurnett et al. (1976), and were suppressed when β ≲ 1. Both types of waves were associated with small dc electric fields, nonturbulent dc electric and magnetic fields, and plasma flows of ≲100 km/s. Broadband electrostatic noise (BEN) and magnetic noise bursts were observed in regions where the dc electric fields and the plasma flows were large, and the dc electric and magnetic fields were turbulent. The dependence of the amplitude and frequencies of BEN on the magnitude of the magnetic field, plasma flow, density, and β was very complex. Enhancements at the upper hybrid frequency and the electron cyclotron frequency were common. BEN was usually suppressed near the neutral sheet but the amplitude reduction varied. The fact that the magnetic noise was most intense during tailward flow and often was not suppressed near the neutral sheet suggests that it may provide the dissipation necessary for reconnection to occur.

Are equatorial electron cyclotron waves responsible for diffuse auroral electron precipitation?

On the basis of theoretical calculations of electron diffusion coefficients and of OGO 5 data, Lyons [1974] suggested that electrostatic electron cyclotron harmonic waves had amplitudes large enough to cause the strong pitch angle diffusion of plasma sheet keV electrons and to be responsible for diffuse auroral precipitation. However, recent measurements of the wave location and amplitude performed aboard the GEOS spacecraft have brought new pieces of information challenging these conclusions. Our calculations are based on the theoretical tool developed by Lyons [1974] but take into account the recently observed wave confinement within a few degrees from the magnetic equator. Under these conditions, we avoid the numerical averaging of the pitch angle diffusion coefficient over the whole line of force, and we can derive an analytical expression of the minimum wave amplitude required to cause strong pitch angle diffusion for plasma sheet electrons. This expression has the advantage to be easily tractable in further calculations, and permits us to evaluate the dependence of the results on parameters that are not reported by observations, such as the wave number spectrum. Our results are found to differ from Lyons's [1974] predictions by a factor of about 2.5, and typically, a wave amplitude of more than 2 mV m−1 is required to put 1‐keV electrons on strong diffusion. On the other hand, on the basis of a statistical analysis of electron cyclotron wave amplitudes measured in the nightside plasmasheet by the GEOS 2 spacecraft, we show that most of the time (>91 %), this typical value of 2 mV m−1 is not reached. This result appears inconsistent with the hypothesis that diffuse auroras, which are a permanent feature of the auroral zones, are due solely to electrostatic electron cyclotron waves. This leads us to the conclusion that these waves are not the only cause of diffuse electron precipitation. It is suggested that other mechanisms involving for instance the dynamics of the ions (such as field‐aligned currents) could play an important role.