Electron Cyclotron Harmonic Waves Research Articles

Interplanetary shock induced intensification of electron cyclotron harmonic waves in the Earth’s inner magnetosphere

Electron cyclotron harmonic (ECH) waves are electrostatic emissions frequently observed in the Earth’s magnetosphere. By precipitating magnetospheric hot electrons into the ionosphere, ECH waves play a critical role in the formation of diffuse aurora. Previous research has extensively investigated the strong dependence of ECH waves on the geomagnetic activities. In this study, we present the first report of the prompt response of ECH waves to an interplanetary shock on the basis of WIND and Van Allen Probes observations. Our observations and analyses demonstrate that the interplanetary shock compression can increase >0.1 keV hot electron fluxes in the dayside inner magnetosphere, consequently leading to the prompt intensification of ECH waves by promoting the wave instability. These findings expand our comprehension of the impacts of solar wind disturbances on magnetospheric plasma waves and offer fresh insights into solar wind-magnetosphere-ionosphere coupling.

Variation of Electron Lifetime Due To Scattering by Electrostatic Electron Cyclotron Harmonic Waves in the Inner Magnetosphere With Electron Distribution Parameters

AbstractThrough resonant wave‐particle interactions with electrostatic electron cyclotron harmonic (ECH) waves, low‐energy plasma sheet electrons can be scattered into the atmospheric loss cone and precipitate. This process can be investigated by calculating bounce‐averaged quasi‐linear diffusion coefficients. However, the numerical calculation of ECH wave‐induced scattering rates requires the specification of several parameters, including the properties of the hot plasma sheet electrons responsible for the wave excitation. The dependence of the diffusion coefficients on these parameters and the exact contribution of scattering by ECH waves to diffuse auroral precipitation is still poorly understood and has not been carefully quantified yet. In this study, we calculate bounce‐averaged quasi‐linear scattering rates and compare the resulting lifetimes to the strong diffusion limit. We vary the temperature, plasma density, and loss cone parameters of the hot component in the electron loss cone distribution over a large range based on previous studies and observations. By adopting a new model that derives the wave normal angle from the midpoint and the angular width of the growth rate profile, our findings suggest that the electron lifetime near the loss cone is not significantly affected by the hot electron temperature and loss cone parameters. However, there is an increase in electron lifetimes with increasing plasma density. For an electric field amplitude of 1 mV/m, the electron lifetime is below or equal to the lifetime calculated using the strong diffusion limit up to E = 0.25 keV when n = 1.5 cm−3, and up to E = 0.22 keV when n = 9 cm−3.

Electron cyclotron harmonic waves in Jovian magnetosphere as seen by Juno

Electron cyclotron harmonic (ECH) waves along with whistler mode waves are suggested to be responsible for causing the persistent diffuse aurora in Jupiter. In this work, for the first time we systematically analyze the ECH waves in the Jovian inner magnetosphere, which was surveyed by Juno during the later orbits (>25). We find that in the Jovian inner magnetosphere, ECH waves occur in two specific regions—one equatorial and the other off-equatorial, just outside the Io torus. Equatorial ECH waves have higher intensity compared to their off-equatorial counterpart. We also notice an overlap between the region of mid-latitude hot injections and the region of off-equatorial ECH wave occurrence. Finally, we show an event to describe the complex nature of ECH wave growth/damping varying with particle density structures of the injection region at mid-latitude.

Conjugate Observation of Whistler Mode Chorus, ECH Waves and Dayside Diffuse Aurora by MMS and Ground‐Based Yellow River Station

AbstractDayside diffuse auroras (DDAs) originate from the precipitation of energetic electrons in the central plasma sheet, induced by wave‐particle interactions. This study examines a specific case characterized by the detection of whistler mode chorus and electron cyclotron harmonic (ECH) waves using the Magnetospheric Multiscale (MMS) satellites, along with the conjugate observation of DDA at the Yellow River (YR) station. ECH waves are observed shortly after the satellite enters the magnetosphere from the magnetosheath and persist throughout the magnetosphere region. Chorus waves are also observed within the magnetosphere, and probably generated near the B minima, followed by the propagation along magnetic field lines. Analysis of MMS particle data reveals an increase in low‐energy electrons and scattering of high‐energy electrons during periods of enhanced whistler mode chorus waves. These precipitated electrons manifest as DDA phenomena. The observed evidence demonstrates the simultaneous occurrence of ECH, chorus waves, electron pitch angle scattering, and DDA. Moreover, the concurrent presence of these waves in the outer magnetosphere during the dayside raises important considerations regarding the relative roles of different plasma waves, particle energization, precipitation dynamics, and the overall development of DDA phenomena.

Formation of Electron Butterfly Pitch Angle Distributions in Saturn's Magnetosphere Due To Scattering by Equatorial ECH Waves

AbstractThe features of electron pitch angle distributions (PADs) often imply different physical mechanisms in planetary magnetospheres. We report a simultaneous equatorial electrostatic electron cyclotron harmonic (ECH) wave event with butterfly PADs of electrons at L ∼ 7.6–9 observed by the Cassini spacecraft. Via calculating the bounce‐averaged electron diffusion rates, we found that Saturnian ECH waves can resonate with ∼10 eV to several keV electrons at <60° pitch angles at time scales from ∼10−8 to 10−4 s−1. Simulations show that the formation of ∼100 eV to ∼1 keV electron butterfly PADs are mainly caused by the pitch angle scattering of electrons at low pitch angles (<30°) and the momentum scattering at intermediate pitch angles of ∼30°–45°, though previous studies suggested the adiabatic transport is the dominated mechanism. Additionally, our results successfully reproduce the variation of peak pitch angles (αp) and phase space density ratios between α90° and αp with L‐shell.

Electrostatic Waves Around a Magnetopause Reconnection Secondary Electron Diffusion Region Modulated by Whistler and Lower‐Hybrid Waves

AbstractWe investigate electrostatic waves in a magnetopause reconnection event around a secondary electron diffusion region. Near the current sheet mid‐plane, parallel electron beam‐mode waves are modulated by whistler waves. We conclude that the anisotropy of energized electrons in the reconnection exhaust excites whistler waves, which produce spatially modulated electron beams through nonlinear Landau resonance, and these beams excite beam‐mode electrostatic waves. In the separatrix region, parallel propagating electrostatic waves associated with field‐aligned electron beams and perpendicular propagating electron cyclotron harmonic waves with loss cone distributions exhibit modulation frequencies in the lower‐hybrid wave (LHW) frequency range. We infer that LHWs scatter electrons to produce beams and alter loss cones to modulate electrostatic waves. The results advance our understanding about the regimes and mechanisms of electrostatic waves in reconnection, with an emphasis on their coupling with lower‐frequency electromagnetic waves.

Plasmaspheric High‐Frequency Whistlers as a Candidate Cause of Shock Aurora at Earth

AbstractAuroral brightening driven by interplanetary shocks on Earth's closed magnetic field lines is commonly attributed to the 0.1–10 keV electron precipitations by electron cyclotron harmonic waves and whistler‐mode chorus waves in the low‐density region. In contrast, we propose here a new candidate driver, high‐frequency whistlers in the high‐density plasmasphere. Theoretical calculations predict the comparable abilities of whistler‐mode waves with sufficiently high frequencies inside the plasmasphere and chorus waves outside to produce auroral electron precipitations. Such expected waves are further verified to exist following an interplanetary shock. The shock perpendicularly accelerated source electrons inside the plasmasphere and then produced strong high‐frequency whistler‐mode waves. These waves appeared as a hybrid of chorus and hiss in the frequency‐time spectrogram and probably resulted from a combination of linear and nonlinear instabilities. Our findings demonstrate that interplanetary shocks can promptly promote energy transfer toward the ionosphere from the inner high‐density magnetosphere.

Statistical Evidence for Off‐Equatorial Minimum‐B‐Pocket as a Source Region of Electron Cyclotron Harmonic Waves in the Dayside Outer Magnetosphere

AbstractThe off‐equatorial minimum‐B‐pocket (OEMBP) at high latitudes in the dayside outer magnetosphere is expected as another source region of the electrostatic electron cyclotron harmonic (ECH) waves, in addition to the magnetic equator. However, its crucial role in the excitation of the dayside ECH waves has never been directly investigated due to its spatiotemporal‐varying and difficulty to be located. Based on the observations from Magnetospheric Multiscale satellites, we statistically analyze the properties of dayside ECH waves by organizing them according to the latitude distance from the OEMBP located by the spatiotemporal‐varying geomagnetic field model. Such mapping confirms larger occurrence rates and electric field amplitudes of dayside ECH waves in the vicinity of the OEMBP, which gradually fall down nearby with the latitude distance increasing. Our results provide statistical evidence for the OEMBP as another source region of ECH waves besides the magnetic equator.

Diffuse auroral intensities produced by whistler mode and electron cyclotron harmonic waves

Diffuse auroral intensities produced by whistler mode and electron cyclotron harmonic waves

Plasmapause Surface Waves Triggered by Substorms

AbstractIn this study, we present in‐situ measurement on board Van Allen Probes showing that surface wave can cause oscillations of the plasmapause and modulate hiss and electron cyclotron harmonic (ECH) waves. Plasmapause surface wave (PSW) in Ps6 band was observed during an intense substorm on 16 July 2017, accompanied with quasi‐periodic emissions of hiss (positively correlated with plasma density) and ECH waves (anticorrelated with density). Phase relation between magnetic field and velocity perturbations indicate that the PSW was an eigenmode between southern and northern ionosphere. Measurements from ground‐based magnetometers suggest that such PSW propagates sunward at the dusk flank and were excited around the expansion phase of an intense substorm. Additional cases of PSWs are also presented to demonstrate that such waves are commonly observed near plasmapause and are likely to be related to the substorm activity.

Statistical Analysis of Concurrent Electron Cyclotron Harmonic and Whistler Waves Driven by ULF Waves

AbstractPlasma sheet electron precipitation to the diffuse aurora is one of the primary mechanisms of magnetospheric energy transport to the Earth’s atmosphere. Such precipitation is provided by electron resonant interactions with electron cyclotron harmonics (ECH) and whistler‐mode waves in the near‐Earth plasma sheet. ECH and whistler‐mode wave generation is traditionally attributed to two main sources of free energy: the loss‐cone anisotropy and the temperature anisotropy due to transverse electron heating during earthward convection or plasma sheet injections. This paper considers a third free energy source for ECH and whistler‐mode wave generation—the electron distribution modulation by compressional ultra‐low‐frequency (ULF) waves. We conduct statistics of ECH and whistler‐mode wave bursts associated with ULF waves and show that: (a) such bursts are mostly observed on the dawn flank, (b) 83% of events show a correlation between ECH and whistler‐mode bursts, and the rest 17% show anti‐correlation, (c) ECH and whistler‐mode wave intensities are comparable with those observed around the night‐side injection region. We investigate the main characteristics of ECH and whistler‐mode waves modulated by ULF waves and provide an empirical model for these characteristics. This model can be directly incorporated into simulation frameworks, so as to include these ULF‐modulated wave bursts in simulations of the magnetosphere‐ionosphere coupling and radiation belt dynamics.

Global Occurrence of Higher‐Bands ECH Waves in Radiation Belts Based on a Novel Noise Reduction Algorithm (NORA)

AbstractElectrostatic electron cyclotron harmonic (ECH) waves which can efficiently produce scattering loss of electrons, are frequently observed in the first band. Here we present new results on the global occurrence of higher‐band ECH waves in the radiation belts from Van Allen Probes data. We develop a novel noise reduction algorithm which can be widely applied to various plasma waves to better identify naturally occurring waves during periods of instrument interference. The occurrence rate of ECH waves in higher harmonic bands is comparable to that in the first harmonic band, except in the noon‐to‐dusk sector. As the level of geomagnetic activity increases, ECH waves in all bands can be observed at lower L‐shells. The occurrence rates of weak (moderate, strong) waves in all four bands increase 3–4 (4–6, 5–10) times during disturbed periods compared to those during quiet periods. The current results may provide better understanding of ECH‐driven magnetosphere‐ionosphere coupling.

Excitation of Saturnian ECH Waves Within Remote Plasma Injections: Cassini Observations

AbstractBased on Cassini observations, we report representative electrostatic electron cyclotron harmonic (ECH) wave events observed in Saturn's magnetosphere within remote plasma injections. Unlike local injections, remote injections are “older” injection events that have evolved to form a dispersed signature in particle energy spectrum. We show that Saturnian ECH waves within remote injections present a strong fundamental band and much weaker high harmonic bands. By calculating the linear wave growth rates based on the measured electron velocity distributions, we indicate that Saturnian ECH waves can be excited by the loss cone distribution of remotely injected, hot electrons of ∼100 eV to several keV. We find that ECH waves tend to intensify with increased fluxes of injected hot electrons but weakening with increased evolution time of the flux tubes, which is consistent with the results of wave growth rates and improves the current understanding of the generation of ECH waves at Saturn.

Electron distribution-controlled electron cyclotron harmonic wave generation in the magnetosphere

Electron cyclotron harmonic (ECH) waves, contributing significantly to magnetospheric dynamics, usually appear as a series of harmonics between the multiples of electron gyrofrequency. Previous studies have demonstrated that ECH waves are the electron Bernstein mode excited by the electron loss cone distribution. To investigate how the electron distribution controls the frequency and wave normal angle of excited ECH waves, we derive a concise analytic formula for ECH growth rates using the bi-Maxwellian distribution. Parametric studies show that the expansion of the loss cone size could effectively enlarge ECH growth rates and move the peak growth rates to a smaller wave number k (higher frequency) region, while the deepening of the loss cone can only increase the growth rates. It is also found that the growth rate pattern in the k space exhibits different trends on the two sides of 1.5 fce because the dominant resonance orders on the two sides are different. This study further develops our understanding of the generation mechanism of ECH waves.

Dipolarization Fronts in Cold‐Dense and Hot‐Tenuous Plasma Sheet Conditions: A Comparative Study

AbstractWe present the first comparative observation of two dipolarization fronts (DFs) in cold‐dense plasma sheet (CDPS) and hot‐tenuous plasma sheet (HTPS) conditions. We find that: (a) the CDPS and HTPS are formed under distinct solar wind conditions. (b) The DF in HTPS has a higher Bz increase than the DF in CDPS; (c) DF in CDPS can transfer energy from magnetic field to particles more efficiently than the DF in HTPS; (d) the >2 keV (<2 keV) electron phase space density of flux pileup region (FPR) in HTPS is higher (lower) than that of FPR in CDPS; (e) the electron cyclotron harmonic waves are excited at DF in HTPS instead of the DF in CDPS. (f) The responses in the ionospheric currents are weak for both DFs in CDPS and HTPS. These results reveal the differences in properties and physical processes between the DFs in CDPS and HTPS. Our findings can significantly improve the understanding of physical mechanisms underlying the formation and evolution processes of DFs.

A missing dusk-side loss process in the terrestrial electron ring current

The Earth’s magnetic field traps charged particles which are transported longitudinally around Earth, generating a near-circular current, known as the ring current. While the ring current has been measured on the ground and space for many decades, the enhancement of the ring current during geomagnetic storms is still not well understood, due to many processes contributing to its dynamics on different time scales. Here, we show that existing ring current models systematically overestimate electron flux observations of 10–50 keV on the nightside during storm onset. By analyzing electron drift trajectories, we show that this systematic overestimation of flux can be explained through a missing loss process which operates in the pre-midnight sector. Quantifying this loss reveals that the theoretical upper limit of loss has to be reached over a broad region of space in order to reproduce the observations. This missing loss may be attributed to inaccuracies in the parameterization of the loss due to chorus wave interactions, combined with the scattering by electrostatic electron cyclotron harmonic waves which is currently not included in ring current models.

Phase Difference of Electron Cyclotron Harmonic (ECH) Waves Observed by Using the Interferometry Observation Mode of the Arase Satellite

Phase Difference of Electron Cyclotron Harmonic (ECH) Waves Observed by Using the Interferometry Observation Mode of the Arase Satellite

Parametric Sensitivity of Electron Scattering Effects by Electrostatic Electron Cyclotron Harmonic Waves

AbstractIt is well known that electron cyclotron harmonic (ECH) waves can effectively precipitate hundreds of eV to tens of keV electrons into the upper atmosphere, resulting in the production of diffuse aurora. To deepen our outstanding of the contributions of ECH waves to diffuse aurora, in this study we perform a detailed analysis of the sensitivity of ECH wave‐induced electron scattering to ambient magnetic field intensity, total electron density and density ratio between hot and cold electrons. Our results show that for various parameter sets, ECH waves can cause the precipitation loss of diffuse auroral electrons at energies of 100 eV to several keV and are capable of accelerating 100 eV to keV electrons at intermediate pitch angle, therefore contributing to the formation of butterfly pitch angle distribution. We also find that the variations of background parameters significantly change the magnitude of diffusion coefficients and resonant pitch angle range of < ∼1 keV electrons, while the scattering efficiency of >∼1 keV electrons is almost unaffected. For <∼1 keV electrons, an increase of magnetic field intensity or density ratio between hot and cold electrons weakens the electron scattering efficiency and narrows the resonant region to lower pitch angles, while an increase of total electron density generally plays an opposite role. In addition, the momentum diffusion can be stronger than pitch angle diffusion at large pitch angles for <∼1 keV electrons, while it can be negligible at higher energies.

Wavelength Measurements of Electron Cyclotron Harmonic Waves in Earth's Magnetotail

AbstractElectron cyclotron harmonic (ECH) waves, potential drivers for diffuse aurora precipitation, have been extensively investigated for decades. The generation mechanism of ECH waves, however, remains an open question. Theoretical work in 1970s has demonstrated that ECH waves can be excited by loss cone distributions of hot plasma sheet electrons. Recent THEMIS spacecraft observations, however, indicate that the waves can also be excited by low energy electron beams. Utilizing interferometry techniques to analyze the phase difference between electric potentials measured by individual probes on Electric Field Instrument antenna pairs on THEMIS spacecraft, we compute the wavenumber of both beam‐driven ECH waves and loss‐cone‐driven ECH waves. These wavenumber measurements as well as other wave properties obtained from spacecraft measurements prove to be consistent with expectation from linear instability analysis. This provides us with independent verification of the generation mechanism and linear dispersion relation of beam‐driven and loss‐cone‐driven ECH waves. Our statistical results demonstrate that the median value of the wave vectors of beam‐driven ECH waves, characterized by wave normal angles () less than 80°, is 0.011 m−1; and that of loss‐cone‐driven ECH waves, characterized by wave normal angles larger than 85°, is 0.00765 m−1. Direct wavenumber measurements of ECH waves allow us to better understand the interaction between ECH waves and electrons in Earth's magnetosphere.

Ion‐Acoustic Waves in a Quasi‐Perpendicular Earth's Bow Shock

AbstractWe present analysis of electrostatic waves around the ramp of a quasi‐perpendicular Earth's bow shock observed by the Magnetospheric Multiscale spacecraft. The electrostatic waves have amplitudes up to 800 mV/m, which is the largest value ever reported in the Earth's bow shock. In contrast to previous studies, the electrostatic waves have large amplitudes of the electrostatic potential, up to 20 V or 20% of local electron temperature. The wavelengths are from 150 m to 3 km, that is from 15 to 300 Debye lengths and typically from 0.4 to 1.5 thermal electron gyroradii. Importantly, these waves can propagate not only quasi‐parallel or oblique, but also almost perpendicular to local magnetic field. The electrostatic waves are interpreted in terms of ion‐acoustic waves, although the presence of electron cyclotron harmonic waves cannot be entirely ruled out. These results suggest that electrostatic waves may strongly affect the dynamics of electrons in collisionless shocks.