Electrostatic Electron Cyclotron Harmonic Waves Research Articles

Pitch-angle diffusion coefficients from resonant interactions with electrostatic electron cyclotron harmonic waves in planetary magnetospheres

Abstract. Pitch-angle diffusion coefficients have been calculated for resonant interaction with electrostatic electron cyclotron harmonic (ECH) waves in the magnetospheres of Earth, Jupiter, Saturn, Uranus and Neptune. Calculations have been performed at two radial distances of each planet. It is found that observed wave electric field amplitudes in the magnetospheres of Earth and Jupiter are sufficient to put electrons on strong diffusion in the energy range of less than 100 eV. However, for Saturn, Uranus and Neptune, the observed ECH wave amplitude are insufficient to put electrons on strong diffusion at any radial distance.

Scattering by chorus waves as the dominant cause of diffuse auroral precipitation

Earth's diffuse aurora occurs over a broad latitude range and is primarily caused by the precipitation of low-energy (0.1-30-keV) electrons originating in the central plasma sheet, which is the source region for hot electrons in the nightside outer magnetosphere. Although generally not visible, the diffuse auroral precipitation provides the main source of energy for the high-latitude nightside upper atmosphere, leading to enhanced ionization and chemical changes. Previous theoretical studies have indicated that two distinct classes of magnetospheric plasma wave, electrostatic electron cyclotron harmonic waves and whistler-mode chorus waves, could be responsible for the electron scattering that leads to diffuse auroral precipitation, but it has hitherto not been possible to determine which is the more important. Here we report an analysis of satellite wave data and Fokker-Planck diffusion calculations which reveals that scattering by chorus is the dominant cause of the most intense diffuse auroral precipitation. This resolves a long-standing controversy. Furthermore, scattering by chorus can remove most electrons as they drift around Earth's magnetosphere, leading to the development of observed pancake distributions, and can account for the global morphology of the diffuse aurora.

Chorus keeps the diffuse aurora humming

The origin of the diffuse aurora, whose beauty and intensity pale beside those of the famous aurora borealis, has remained controversial. A convincing explanation for this auroral display is now at hand. See Letter p.943 Earth's diffuse aurora, almost invisible from the ground, was discovered in images from the Isis-2 satellite in 1971 as a broad ring of light around the auroral oval. The diffuse aurora is the main source of energy for the high-latitude night-side upper atmosphere, and for many years there has been controversy over the mechanism causing the scattering loss of plasma-sheet electrons that powers it. Two distinct classes of magnetospheric plasma waves have been suggested: electrostatic electron cyclotron harmonic waves, and the more structured whistler-mode 'chorus' waves generated by electrons travelling through Earth's magnetic field. Thorne et al. report an analysis of satellite wave data that reveals that scattering by chorus is the dominant cause of the most intense diffuse auroral precipitation.

Reimei observation of highly structured auroras caused by nonaccelerated electrons

We present observations of highly structured, thin auroras cased by precipitation of nonaccelerated electrons on the basis of optical and particle measurements performed by the Reimei satellite near the equatorward edge of the main auroral oval. The aurora has the following characteristics: (1) A full width at half maximum (FWHM) value is as low as only ∼1.8 km from optical measurements, and ∼0.6 km from particle measurements at the ionospheric altitude, which is much smaller than previously determined. (2) The FWHM value of 0.6 km corresponds to 9 km in the equatorial plane, which is ∼10 times smaller than the gyroradius of typical protons trapped in the near‐Earth plasma sheet. (3) At high energies greater than ∼1 keV, the velocity distribution function of precipitating electrons is comparable to that of the trapped ones and does not demonstrate any plateau or positive gradient in the distribution. (4) The aurora was observed in geomagnetically quiet condition. (5) A geosynchronous satellite observed a significant increase in the plasma pressure of hot electrons in comparison with that of hot ions. The structuring of the aurora may be attributed to scattering processes of hot electrons. If this were the case, hot electrons would be scattered by whistler mode chorus, or electrostatic electron cyclotron harmonic waves, and the structured aurora would be a visual manifestation of the highly structured, cold plasma that determines the growth of the waves scattering the hot electrons.

Diffusion coefficients from resonant interactions with electrostatic electron cyclotron harmonic waves

Pitch-angle diffusion coefficients have been calculated for resonant interaction with electrostatic electron cyclotron harmonic (ECH) waves using quasilinear diffusion theory. Unlike previous calculations, the parallel group velocity has been included in this study. Further, ECH wave intensity is expressed as a function of wave frequency and wave normal angle with respect to ambient magnetic field. It is found that observed wave electric field amplitudes in Earth’s magnetosphere are sufficient to set electrons on strong diffusion in the energy ranges of a few hundred eV. However, the required amplitudes are larger than the observed values for keV electrons and higher by about a factor of 3 compared to past calculations. Required electric field amplitudes are smaller at larger radial distances. It is concluded that ECH waves are responsible for diffuse auroral precipitation of electrons with energies less than about 500 eV.

THEMIS observation of multiple dipolarization fronts and associated wave characteristics in the near‐Earth magnetotail

Multiple dipolarization fronts were observed by THEMIS spacecraft in the near‐Earth magnetotail during a substorm. The dipolarization fronts were located at the leading edge of earthward propagating plasma bubbles. Major energetic electron flux enhancements were observed at the dipolarization fronts, which were also associated with large wave fluctuations extending from below the lower hybrid frequency to above the electron cyclotron frequency. Intense electric field wave packets, primarily contributed by the Hall electric field and lower hybrid drift (LHD) wave, were observed right at the front, which was a thin current layer with size of the order of the ion inertial length. The LHD wave was believed to be generated by a diamagnetic current in the presence of density and temperature gradients. Electrostatic electron cyclotron harmonic (ECH) waves were detected slightly after the front. The ECH waves were probably generated by the positive slope of the electron perpendicular velocity distribution. Both of these waves are suggested to be able to heat electrons. The observation of these waves at the dipolarization front could be important for the understanding of electron energization during substorm injection, as well as the mechanism of current disruption.

Numerical study of electrostatic electron cyclotron harmonic waves due to Maxwellian ring velocity distributions

Abstract Excitation of electrostatic electron cyclotron harmonic (ECH) waves is studied by performing linear dispersion analysis and particle-in-cell computer simulation. The ECH wave emissions can be excited by a positive slope in a velocity distribution function perpendicular to the ambient magnetic field, such as that due to a loss cone or ring velocity distribution. However, there exists no analytic expression for integration of Maxwellian ring velocity distribution functions. Here we present a method to solve the linear dispersion relations of Maxwellian ring velocity distribution functions with numerical integration. The obtained dispersion relations are confirmed by particle-in-cell simulation.

Electrostatic electron cyclotron harmonic waves observed by the Akebono satellite near the equatorial region of the plasmasphere

Abstract Analysis of the plasma wave observation data provided by the plasma waves and sounder experiment (PWS) on board the Akebono satellite frequently reveals the presence of electrostatic electron cyclotron harmonic (ESCH) waves in the low-latitude region (MLAT < 45°) of the plasmasphere within an altitude range from about 3000 km to the apogee of the satellite (initial apogee was 10,500 km). Even at moderate or low geomagnetic activity, intense ESCH waves often appear near the equatorial region of the plasmasphere above the upper hybrid resonance (UHR) frequency at the lowest harmonic number branch of the f Qn ESCH waves. We identified these plasma waves as the equatorial plasmasphere f Qn waves (EP-f Qn). The spectra of the EP-f Qn waves are characterized by a narrow band structure and by a strong nature, with a wave intensity that ranges from 3.46 × 10-8 to 3.31 × 10-4 V/m. The maximum intensity is nearly coincident with the upper limit of the PWS receiver in the low-gain mode. Statistical analysis results reveal that the EP-f Qn waves are observable in all the local time sectors; however, the occurrence probability shows a clear enhancement in the early morning sector of 01–03 MLT in the plasmasphere. The EP-f Qn wave activities are suppressed within a period of strong magnetic disturbances as well as solar minimum phase. The linear dispersion relation analysis using a two-component plasma model reveals that supra-thermal plasma with the energy of about 750 eV and with a large temperature anisotropy (A = T-perp/T-parallel–1 > 40) must be present in order to realize an appearance of a positive growth rate at the observed frequency and propagation angle of the ESCH waves. Since the hot plasma with such a high anisotropy has not been detected, the validity of the present two-component plasma model remains an open question. The occurrence feature of the ESCH waves showed that there is a constant activation or a constant flow-in of free energy to generate the strong plasma instability of ESCH waves near the post-midnight sector of the plasmasphere. The existence of ESCH waves revealed that the nature of the plasmaspheric plasma is more turbulent and active than has been believed.

Particle‐in‐cell simulation of Maxwellian ring velocity distribution

Electrostatic electron cyclotron harmonic (ECH) and electromagnetic whistler wave emissions are sometimes observed simultaneously in the near‐Earth nightside equatorial magnetotail region. The excitation mechanism for these two wave emissions, however, is quite different with ECH waves excited by a positive slope in the velocity distribution function perpendicular to the ambient magnetic field (such as due to a loss cone or ring velocity distribution), while whistler waves are excited by a temperature anisotropy whereby the perpendicular temperature is larger than the parallel temperature. Here we examine, using a two‐dimensional electromagnetic particle‐in‐cell computer simulation, the excitation of both waves as a consequence of a warm electron ring distribution in the presence of cold background electrons. Initially, ECH waves are excited by the ring distribution in the linear stage, which results in heating of the cold electrons and smearing of the ring distribution. During a nonlinear consequence of the ECH excitation and ensuing electron wave‐particle interactions, whistler waves are also excited by the different instability. Thus both waves are excited during the same simulation run, which may account for the magnetospheric observations of both ECH and whistler waves at the same location where electron loss cone distributions form in the equatorial magnetotail at about 6–9 RE from the Earth.

Plasmasphere electron temperature structures

Inspection of electron temperature ( T e) profiles obtained by the thermal electron detector instrument along 5676 individual orbitsof the Akebono satellite reveal the existence of localized structures with a latitude extent of 1–3θ. The analysis was constrained to the range 2 < L < 3.2 in the northern hemisphere within the period 1991–1997. Eighty-seven well-defined structures were selected, containing a marked increase of T e at least 20% above the background level. The electron density ( N e) and the wave intensity at the upper hybrid resonance frequency, recorded by the plasma wave and sounder instrument on the same satellite, were also used in the analysis. It was found that Ne troughs and an increase of the electrostatic electron cyclotron harmonic waves perfectly co-located with the T e structures. The structures were observed at altitudes above 3000 km, as 70% of them are found between 03 and 08 h magnetic local time. While the plasma inside the structures is always warmer, the plasma pressure can be either higher or lower than outside.

Electron pitch angle diffusion by electrostatic electron cyclotron harmonic waves: The origin of pancake distributions

It has been suggested that highly anisotropic electron pancake distributions are the result of pitch angle diffusion by electrostatic electron cyclotron harmonic (ECH) and whistler mode waves in the equatorial region. Here we present pitch angle diffusion rates for ECH wave spectra centered at different frequencies with respect to the electron gyrofrequency Ωe corresponding to spacecraft observations. The wave spectra are carefully mapped to the correct resonant electron velocities. We show that previous diffusion calculations of ECH waves at 1.5Ωe, driven by the loss cone instability, result in large diffusion rates confined to a small range of pitch angles near the loss cone and therefore cannot account for pancake distributions. However, when the wave spectrum is centered at higher frequencies in the band (&gt; 1.6Ωe), the diffusion rates become very small inside the loss cone, peak just outside, and remain large over a wide range of pitch angles up to 60° or more. When the upper hybrid resonance frequency ωUHR is several times Ωe, ECH waves excited in higher bands also contribute significantly to pitch angle diffusion outside the loss cone up to very large pitch angles. We suggest that ECH waves driven by a loss cone could form pancake distributions as they grow if the wave spectrum extends from the middle to the upper part of the first (and higher) gyroharmonic bands. Alternatively, we suggest that pancake distributions can be formed by outward propagation in a nonhomogeneous medium, so that resonant absorption occurs at higher frequencies between (n + ½) and (n + 1)Ωe in regions where waves are also growing locally at &lt; 1.5Ωe. The calculated diffusion rates suggest that ECH waves with amplitudes of the order of l mV m−1 can form pancake distributions from an initially isotropic distribution on a timescale of a few hours. This is consistent with recent CRRES observations of ECH wave amplitudes following substorm injections near geostationary orbit and the timescales for pancake formation. Persistent but much weaker ECH waves can further intensify and maintain pancake distributions during magnetically quiet periods.

Wave form analysis of the continuum radiation observed by GEOTAIL

The Plasma Wave Instrument (PWI) onboard the GEOTAIL spacecraft has frequently observed Continuum Radiation (CR) throughout the geomagnetic tail region, including the Magnetosheath (MS), the Lobe, the Plasma Sheet Boundary Layer (PSBL), and the Plasma Sheet (PS). In addition to the usual Escaping CR (from 10 kHz to 30 kHz) and the Trapped CR (from about 5 kHz to 10 kHz), CR with the frequency structure extending down to 1 kHz (the local plasma frequency) has often been observed in the Lobe region. Waveforms of the electric field in the frequency range less than 4 kHz, which is acquired by Wave Form Capture in the PWI, are used to analyze in detail the frequency‐time structure of such low‐frequency CR near the lower cutoff. Two distinct cutoff frequencies modulated by the antenna spinning indicates that the CR in the Lobe region propagates both in the O mode and in the X mode almost perpendicular to the earthward geomagnetic field line. The CR in the Lobe region, especially in the vicinity of the PSBL, is sometimes accompanied by intense electrostatic electron‐cyclotron‐harmonic (ECH) (n + 1/2) waves. These suggest that such a low‐frequency CR in the distant tail region is most likely to be generated from the ECH waves near the PSBL, and trapped within the Lobe between the PSBL and the MS.

Periodic bursts of electrostatic electron cyclotron harmonic (ESCH) waves in the boundary plasmasheet region of the magnetosphere tail

Periodic bursts of very intense electrostatic electron cyclotron harmonic waves have been discovered by the PWP (Plasma Wave Spectra Probe) onboard the Sakigake spacecraft while the spacecraft was crossing the plasma sheet boundary layer within the distance range from 40 to 50 earth radii. The observed pulsive bursts of the plasma waves show very intense nature which is characterized by BEN (broadband electrostatic noise) with the maximum intensity of about 200mV/m and the bursts axe enhanced periodically with the period around 1.5 minutes suggesting large volume (within the inferred size from 100km to about 1000 km) of plasma cloud periodically flowing toward tail region. There were also periodic variation of ESCH wave spectra suggesting the velocity space modulation in the plasma flow with period of ranging from 300 to 400sec; the magnetic field intensity shows the periodic variation with the same characteristic periods from 300 to 400sec.

Electrostatic wave excitation in planetary magnetospheres: Application to Neptune

Voyager 2 observations of electrostatic electron and ion cyclotron harmonic waves in Neptune's magnetosphere are addressed. A model of electron Bernstein modes generated by a loss cone distribution of superthermal electrons is scaled to Neptune parameters and a comparison of theory with the observed electron flux shows good agreement. A model of proton Bernstein modes generated by a ring distribution of Tritonogenic nitrogen ions is also investigated and satisfactory agreement with the data are obtained compatible with known properties of the magnetosphere. The success of the model in accounting for electrostatic emissions observed by Voyager over a wide range of sampled parameters recommends its general applicability to planetary magnetospheres.

On the generation of plasma waves in Saturn's inner magnetosphere

Voyager 1 plasma wave measurements of Saturn's inner magnetosphere are reviewed with regard to interpretative aspects of the wave spectrum. A comparison of the wave emission profile with the electron plasma frequency obtained from in situ measurements of the thermal ion density shows good agreement with various features in the wave data identified as electrostatic modes and electromagnetic radio waves. Theoretical calculations of the critical flux of superthermal electrons able to generate whistler‐mode waves and electrostatic electron cyclotron harmonic waves through a loss‐cone instability are presented. The comparison of model results with electron measurements shows excellent agreement lending support to the conclusion that a moderate perpendicular anisotropy in the hot electron distribution is present in the equatorial region of L = 5‐8.

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.

Plasma instabilities stimulated by HF transmitters in space

Diffuse incoherent signal returns are often observed on Alouette and ISIS topside ionograms in addition to coherent echoes of electromagnetic and electrostatic waves. These diffuse signals, which at times can be the dominant features on topside ionograms, have been attributed to sounder‐induced temperature anisotropies which drive the Harris instability. Previous theoretical investigations were based on the electrostatic approximation to the dispersion equation. The present paper will present calculations indicating that when the electromagnetic terms are retained in the dispersion equation and when the sounder‐stimulated perpendicular electron temperature approaches 1 keV, then the whistler mode can have a temporal growth rate larger than the electrostatic electron cyclotron harmonic wave mode central to the diffuse resonance problem. Present sounders lack the power and antenna lengths to generate whistler mode waves in this manner. In addition, such waves would have large group velocities and would quickly leave the vicinity of the sounder. Experiments to investigate the wave growth, propagation, and damping of such stimulated waves are planned for the 1990s using a highly flexible sounder on the space shuttle and a receiver on a subsatellite.

Electrostatic electron cyclotron harmonic (ESCH) echoes found near the plasmapause by the EXOS‐B (Jikiken) satellite

A stimulated plasma wave experiment by the EXOS‐B satellite showed the appearance of echoes of plasma waves with a delay time of several msec. We examined the frequency dependence of the echoes in comparison with the observed plasma resonances and the dispersion relation of plasma waves. The propagation mode of the echoes was identified as the electrostatic electron cyclotron harmonic (ESCH) waves. The appearance regions were examined with respect to the electron density profiles in the magnetosphere that have been determined by measuring the frequencies of stimulated plasma resonances. The ESCH echoes are frequently observed in a detached plasma region. Near the plasmapause the appearance of echoes is significantly frequent in regions where the plasma density is rapidly increasing in the plasmasphere due to a strong supply from the ionsphere. Because of their propagation mode and the state of the plasma density profiles in their appearance regions in the magnetosphere, the echoes are inferred to be produced by the reflection of propagating ESCH waves from small‐scale density irregularities of environmental plasma.

Beam instabilities induced by pulsed RF field: RF sounder experiment by L-3H-9 rocket.

RF sounder experiments on board L-3H-9 rocket were carried out in order to investigate the generation mechanism of plasma resonances produced by electrostatic electron cyclotron harmonic (ESCH) waves. The spectrum of the stimulated plasma resonances suggested that there are ESCH waves generated by the instability due to an electron beam, which is caused by antenna charging by an impressed RF field. The observed frequency range of the generated ESCH waves is in agreement with the range theoretically predicted for ESCH wave instabilities triggered by low energy electron beams. The generation of an electron beam is verified by a space simulation chamber experiment. When an RF pulse is applied from a sounder antenna in the chamber plasma, a clear signature of an electron burst was observed. The characteristics of the beam type instability are also verified by K-9M-61 sounding rocket experiment. In the space plasma, beam type instability generates the same branches of the ESCH waves as the plasma resonances by the L-3H-9 plasma sounder experiment.

Quasilinear Electron Heating by Electrostatic Electron Cyclotron Harmonic Waves in a Plasma

Electron heating via the interaction with linearly stable electrostatic electron cyclotron harmonic waves was observed. This heating is interpreted to be based on the cyclotron damping caused by the quasi-linear interaction for a weakly turbulent plasma.