Electron Cyclotron Harmonic Waves Research Articles

Global distribution of electrostatic electron cyclotron harmonic waves observed on THEMIS

[1] A global, statistical analysis of electrostatic electron cyclotron harmonic (ECH) waves is performed using THEMIS wave data. Our results confirm the high occurrence of 5). The strongest (≥1 mV/m) ECH waves are enhanced during geomagnetically disturbed periods, and are mainly confined close to the magnetic equator (∣λ∣ 8) magnetosphere where chorus emissions are statistically weak.

Evolution of electron pitch angle distributions following injection from the plasma sheet

[1] The temporal evolution of the phase space density of plasma sheet electrons (100 eV–30 keV) injected into the nightside at L = 6 during moderate geomagnetic activity is investigated using a quasi-linear diffusion formulation. Scattering in energy and pitch angle during interactions with both whistler mode chorus waves and electron cyclotron harmonic waves are included using an improved wave model recently obtained using CRRES spacecraft data. We compare our simulation results with observations from the THEMIS spacecraft and demonstrate that the formation of the observed electron pitch angle distributions is mainly due to resonant interactions with a combination of upper and lower band chorus waves. The pancake distributions at low energies (E < 2 keV), the flattened pitch angle distributions at medium energies (between 2–3 keV), and the distributions with enhanced pitch angle anisotropy at high energies (E > 3 keV) are explained using the banded chorus wave structure with a power minimum at half the electron cyclotron frequency. Results of the current work can be used to model the dynamical evolution and resultant global distribution of plasma sheet electrons.

Resonant scattering of plasma sheet electrons leading to diffuse auroral precipitation: 1. Evaluation for electrostatic electron cyclotron harmonic waves

[1] Using statistical wave power spectral profiles obtained from CRRES and the latitudinal distributions of wave propagation modeled by the HOTRAY code, a quantitative analysis has been performed on the scattering of plasma sheet electrons into the diffuse auroral zone by multiband electrostatic electron cyclotron harmonic (ECH) emissions near L = 6 within the 0000–0600 MLT sector. The results show that ECH wave scattering of plasma sheet electrons varies from near the strong diffusion rate (timescale of an hour or less) during active times with peak wave amplitudes of an order of 1 mV/m to very weak scattering (on the timescale of >1 day) during quiet conditions with typical wave amplitudes of tenths of mV/m. However, for the low-energy (∼100 eV to below 2 keV) electron population mainly associated with the diffuse auroral emission, ECH waves are only responsible for rapid pitch angle diffusion (occasionally near the limit of strong diffusion) for a small portion of the electron population with pitch angles αeq 70°. Computations of the bounce-averaged coefficients of momentum diffusion and (pitch angle, momentum) mixed diffusion indicate that both mixed diffusion and energy diffusion of plasma sheet electrons due to ECH waves are very small compared to pitch angle diffusion and that ECH waves have little effect on local electron acceleration. Consequently, the multiple harmonic ECH emissions cannot play a dominant role in the occurrence of diffuse auroral precipitation near L = 6, and other wave-particle interaction mechanisms, such as whistler mode chorus-driven resonant scattering, are required to explain the global distribution of diffuse auroral precipitation and the formation of the pancake distribution in the inner magnetosphere.

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.

Excitation of electron cyclotron harmonic waves in the inner Saturn magnetosphere within local plasma injections

Strong electron cyclotron harmonic (ECH) waves have been observed by Cassini associated with local plasma injection regions in the Saturnian magnetosphere. Using measured electron distributions from the Cassini plasma spectrometer, we calculated local growth rates and path‐integrated wave gain of ECH waves inside an injection event using the HOTRAY code. We showed that electrons with energy near a few eV have a collision frequency comparable to their bounce frequency; thus, they cannot have an empty loss cone. We then demonstrated that the growth of ECH waves inside the injection event can be driven by electron phase space density gradients associated with the loss cone distribution of injected electrons at energies between a few hundred eV and a few keV. This conclusion is contrary to previous results that the source of free energy for growth of ECH waves is provided by electrons near a few eV. Results in this work are helpful for understanding the generation of ECH waves and their roles in electron dynamics in the Saturnian magnetosphere.

THEMIS observations of electron cyclotron harmonic emissions, ULF waves, and pulsating auroras

We present multiprobe, multi‐instrument observations of the electron cyclotron harmonic (ECH) emissions and ultralow‐frequency (ULF) waves from Time History of Events and Macroscale Interactions during Substorms (THEMIS) and explore their potential linkage to the concurrent ground‐observed pulsating auroras (PsA) on 4 January 2009. The ECH emissions were observed as discrete packets modulated by the ULF flapping motion of the neutral sheet around the probes. Combining different data sets of the ECH observations, we infer that the ECH emission intensities were strongly fluctuating and contained multi‐time scale fine structures. The distribution of PsA patches featured longitudinal “wavelength” in concert with the in situ ULF wave characteristics inferred from a cross‐phase analysis. The overall activeness of the PsA correlated with the in situ‐measured energetic electron fluxes and ECH wave intensities. We suggest that ECH waves played the key role in the pitch angle diffusion of the plasma sheet electrons that led to the PsA, while the ULF waves structured the plasma sheet and imposed a macroscopic effect over the spatial distribution of the PsA.

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.

A parametric study on the diffuse auroral precipitation by resonant interaction with whistler mode chorus

The diffuse aurora is generally believed to be the result of ∼keV electron precipitation from the magnetosphere, which presents an important sink of magnetospheric particles, a primary high‐latitude ionospheric energy source, and a strong magnetosphere‐ionosphere coupling mechanism. Resonant scattering of the plasma sheet electrons by electron cyclotron harmonic and whistler mode chorus waves is suggested to be responsible for the diffuse auroral precipitation. The dependence of quasi‐linear diffusion rates and consequent diffuse auroral precipitation on the spatial position L, normal angle, and frequency distribution of both upper‐band chorus (UBC) and lower‐band chorus (LBC) is investigated in detail. Whistler mode chorus waves are found to be more effective in the production of diffuse aurora at larger L shells. The normal angle and frequency distribution of UBC can significantly affect the diffusion rates for ∼keV electrons and the consequent diffuse aurora precipitation. Numerical results show that the strong diffusion limit for ∼keV electrons can be reached up to large pitch angles closer to 90°, and the diffuse auroral precipitation can be remarkably enhanced when the UBC waves possessing a relatively larger normal angle range or centering higher in the band. In contrast, the diffusion rates and diffuse auroral precipitation of electrons with energies Ek &lt; 2 keV are largely independent of the normal angle and frequency distribution of LBC waves. Enhanced precipitations of ∼5 keV electrons are found to occur when the LBC waves centering higher in the band. The current results suggest that the significant variability of wave characteristics and the variation with spatial position should be taken into account in order to identify the dominant mechanism for diffuse aurora.

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.

Survey of upper band chorus and ECH waves: Implications for the diffuse aurora

The origin of the diffuse aurora has been a source of controversy for many years. More recently, the question has taken a new significance in view of the associated changes in atmospheric chemistry which may affect the middle atmosphere. Here, we use CRRES data to assess the importance of upper band chorus and electron cyclotron harmonic (ECH) waves in the production of the diffuse aurora. Both wave modes increase with increasing geomagnetic activity, suggesting they are related to periods of enhanced convection and/or substorm activity. They are confined to the near‐equatorial region, which excludes the prenoon sector from the wave survey. During active conditions, intense ECH waves and upper band chorus, with amplitudes exceeding 1 mV m−1, are observed in the region 4 &lt; L&lt; 7 from 2100 to 0600 MLT approximately 20% and 6% of the time, respectively. This suggests that both wave modes can put electrons on strong diffusion, but only during active conditions and not at all local times. Scattering rates fall below the strong diffusion limit at other times when the wave amplitudes are weaker. Fluxes of low energy electrons (100 eV &lt; E &lt; 30 keV) also increase with increasing geomagnetic activity in approximately the same region of geospace as the waves, suggesting that these electrons are responsible for the generation of the waves. The patterns of the upper band chorus, ECH waves, and low‐energy electrons are similar to the global morphology of the diffuse aurora, suggesting that both wave modes play significant roles in the production of the diffuse aurora.

Comparison of parallel group velocity of ECH waves with electron resonant velocity: Implication for electron diffusion by ECH waves

We have investigated the role of group velocity in the calculation of pitch-angle diffusion coefficients by electron cyclotron harmonic (ECH) waves in planetary magnetospheres. The assumption which is generally made that the parallel group velocity can be neglected in comparison with particle parallel velocity is examined in detail. It is found that for lowest harmonic band this assumption is quite good. It is found that in general it is not possible to ignore the parallel group velocity. However, for lowest harmonic band this assumption is quite good at low electron temperatures.

A study of fine structure of diffuse aurora with ALIS-FAST measurements

Abstract. We present results of an investigation of the fine structure of the night sector diffuse auroral zone, observed simultaneously with optical instruments (ALIS) from the ground and the FAST electron spectrometer from space 16 February 1997. Both the optical and particle data show that the diffuse auroral zone consisted of two regions. The equatorward part of the diffuse aurora was occupied by a pattern of regular, parallel auroral stripes. The auroral stripes were significantly brighter than the background luminosity, had widths of approximately 5 km and moved southward with a velocity of about 100 m/s. The second region, located between the region with auroral stripes and the discrete auroral arcs to the north, was filled with weak and almost homogeneous luminosity, against which short-lived auroral rays and small patches appeared chaotically. From analysis of the electron differential fluxes corresponding to the different regions of the diffuse aurora and based on existing theories of the scattering process we conclude the following: Strong pitch angle diffusion by electron cyclotron harmonic waves (ECH) of plasma sheet electrons in the energy range from a few hundred eV to 3–4 keV was responsible for the electron precipitation, that produced the background luminosity within the whole diffuse zone. The fine structure, represented by the auroral stripes, was created by precipitation of electrons above 3–4 keV as a result of pitch angle diffusion into the loss cone by whistler mode waves. A so called "internal gravity wave" (Safargaleev and Maltsev, 1986) may explain the formation of the regular spatial pattern formed by the auroral stripes in the equatorward part of the diffuse auroral zone.

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.

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 &lt; 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 &gt; 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.

Experimental study of plasma breakdown by second harmonic electron cyclotron waves in Heliotron J

Second harmonic plasma breakdown using electron cyclotron (EC) waves has been studied experimentally in the helical-heliotron device Heliotron J. The magnetic field and the injected EC beam parameters such as power, polarization and injection angle are scanned. The experimental results show that the plasma starts up around the crossing point between the injected EC beam and the resonance layer. The initial plasma moves as the crossing point is shifted. The breakdown is earliest when the beam crosses the resonant layer around the magnetic axis with the X-mode polarization. These results indicate that the single pass absorption has the dominant role on the breakdown rather than the multi-reflection absorption, although the linear absorption is quite low. The bumpiness component in the magnetic field spectrum is also scanned from negative to positive values with the resonance located on-axis. The breakdown is most delayed when the bumpiness component is zero, suggesting that the confinement of accelerated electrons is important.