Wave Normal Angle Research Articles

Observation of Whistler Mode Waves Inside Mirror Mode Structures in the Earth's Outer Magnetosphere

AbstractWhistler mode waves are commonly observed inside mirror mode structures (MM‐Ss) in the magnetosheath. Although MM‐Ss have also been detected in the magnetosphere, there is no statistical study investigating the properties of whistler mode waves inside MM‐Ss. Using Magnetospheric Multiscale (MMS) satellites, our present study identifies and statistically analyzes whistler mode waves detected inside MM‐Ss in the Earth's magnetosphere. Both the observational evidence (bidirectional propagation) and theoretical analyses suggest that whistler mode waves are excited in the low‐magnetic‐field regions of MM‐Ss. Statistical results indicate that whistler mode waves are frequently observed inside the MM‐Ss in the dusk sector. Most of these waves (>70%) are in the frequency range of 0.1–0.4 fce and have amplitudes less than ∼50 pT. Moreover, most of them are observed to propagate in the direction both parallel and antiparallel to the background magnetic field, and their wave normal angles are typically less than ∼40°. Our study analyzes the properties and generation of whistler mode waves inside MM‐Ss and reveals that MM‐Ss are a possible source region in the Earth's magnetosphere. Therefore, our study provides new insight into the properties of whistler mode waves inside MM‐Ss which are pervasive in space plasmas.

Full-wave modeling of EMIC wave packets: ducted propagation and reflected waves

Electromagnetic ion cyclotron (EMIC) waves can scatter radiation belt electrons with energies of a few hundred keV and higher. To accurately predict this scattering and the resulting precipitation of these relativistic electrons on short time scales, we need detailed knowledge of the wave field’s spatio-temporal evolution, which cannot be obtained from single spacecraft measurements. Our study presents EMIC wave models obtained from two-dimensional (2D) finite-difference time-domain (FDTD) simulations in the Earth’s dipole magnetic field. We study cases of hydrogen band and helium band wave propagation, rising-tone emissions, packets with amplitude modulations, and ducted waves. We analyze the wave propagation properties in the time domain, enabling comparison with in situ observations. We show that cold plasma density gradients can keep the wave vector quasiparallel, guide the wave energy efficiently, and have a profound effect on mode conversion and reflections. The wave normal angle of unducted waves increases rapidly with latitude, resulting in reflection on the ion hybrid frequency, which prohibits propagation to low altitudes. The modeled wave fields can serve as an input for test-particle analysis of scattering and precipitation of relativistic electrons and energetic ions.

Resolution of a Few Problems in the Application of Quasilinear Theory to Calculating Diffusion Coefficients in Heliophysics

AbstractA theory of quasilinear diffusion for obliquely propagating electromagnetic waves was developed in the 1960's and applied in the 1970's to model scattering of relativistic electrons by a prescribed distribution of waves. In the latter work, a transformation of variables, from wavevector space to the temporal frequency and tangent of the wave normal angle, was used so that simple Gaussian functions of frequency and tangent of the wave normal angle could be multiplied together to define the distribution of wave power, although arbitrary distributions of power in these two variables is also permitted. Finally, in 2005, previous work was consolidated and has been widely used in heliophysics studies that require computation of quasilinear diffusion coefficients. Here it is shown that this transformation is inppropriate when the precise wave vector distribution is known. The correct transformation is derived and used to produce diffusion coefficients that can differ by orders of magnitude from those computed using the inappropriate transformation. The differences are largest when the distribution of wave power extends to wave normal angles near the resonance cone. When the ratio of the plasma frequency to the gyrofrequency is large, only low energies (keV) are affected, but as the ratio decreases higher energies (MeV) also show differences. It is also shown that the derivation from the 1960's uses a notation that results in the diffusion coefficients depending on the distribution of wave power with respect to the wave azimuthal angle whereas there should be no such dependence.

Global Empirical Models of Wave Amplitudes and Normal Angles of Banded Plasmaspheric Hiss Obtained From Van Allen Probes Observations

AbstractThe unique banded structure of plasmaspheric hiss with an upper band above ∼200 Hz and a lower band below ∼150 Hz has been recently reported. Based on observations from the twin Van Allen Probes from 15 September 2012 to 30 June 2019, we statistically investigate the global distributions of amplitude and power‐weighted wave normal angle (WNA) of banded hiss waves with respect to L, MLT, MLAT, and AE*. The results indicate that amplitudes of dayside banded hiss significantly depend on the substorm level (AE* index), while the variation of WNAs on the AE* index is slight. The WNAs of upper band hiss show a peak at L ∼ 3, while the WNAs of lower band hiss are relatively uniform at L < 4. The WNAs of both bands decrease with increasing L values at L > 4. We further develop the global empirical models of banded hiss wave amplitudes and WNAs to fit the statistical results, which show good agreement with satellite observations. These results can be essentially useful to evaluate the role of banded plasmaspheric hiss in the dynamic variations of Earth's radiation belt electrons.

Modeling the Propagation of Fast Magnetosonic Waves and Their Conversion to Electromagnetic Ion Cyclotron Waves at Low L Shells

AbstractThe propagation of fast magnetosonic (MS) waves from high to extremely low L shells and their conversion into electromagnetic ion cyclotron (EMIC) waves is investigated with a ray tracing model and a full‐wave model. The ray tracing simulations show that MS waves in the vicinity of the local crossover frequency at low L shells have a latitudinal range from to with wave normal angles from to , both of which provide the opportunity for their mode conversion to EMIC waves. Results from ray tracing are fed into the full‐wave model as initial conditions. The full‐wave simulations show that the incoming MS waves with small () and intermediate () wave normal angles may be efficiently converted to right‐handedly polarized band EMIC waves, which may be further converted to right‐handedly polarized band EMIC waves or to guided left‐handedly polarized band EMIC waves through bi‐ion resonance reflection and polarization reversal. With intermediate () or larger wave normal angles, via polarization reversal and left‐handed polarization cut‐off reflection, the incoming MS waves may be efficiently converted to guided left‐handedly polarized band EMIC waves and the optimal wave normal angles corresponding to the maximum conversion efficiencies decrease with magnetic latitudes. Our study verifies the mode conversion mechanism of MS waves to EMIC waves proposed with a previous ray tracing study and examines the dependence of the efficiency of the wave energy transfer on the magnetic latitudes and wave normal angles, which may help to understand the wave properties and distribution of MS and EMIC waves observed at extremely low L shells.

Effects of Cold Plasma on the Mode Conversion From Fast Magnetosonic Wave to Electromagnetic Ion Cyclotron Wave in the Inner Plasmasphere

AbstractPrevious studies show that fast magnetosonic (MS) wave with frequency of tens of Hz may propagate to low L‐shells (L < 2.0) in the inner plasmasphere and mode convert to electromagnetic ion cyclotron (EMIC) wave where its wave frequency is close to the local crossover‐cutoff‐resonance frequency triplet through different propagation paths. In this study, we investigate how this mode conversion process may depend on the ion composition and electron density by full‐wave simulation with various plasma density profiles and initial wave normal angles (WNAs) of incoming MS wave from magnetic equator and a higher latitude of 15°. We find that critical fraction of minority ion (He+ or M/Q = 2 ion − D+ or He++) exists during the mode conversion, over which the conversion from MS wave to H+ band right‐handedly polarized EMIC wave starts to become negligible. This critical fraction decreases with the incident WNA of MS wave from equator. The efficiency of this conversion process also decreases with the electron density. As incoming MS is from higher latitudes off equator, optimal fraction of minority ion exists, with which the efficiency of conversion from MS wave to H+ band left‐handedly polarized EMIC wave reaches a maximum. Moreover, this maximum conversion efficiency varies with the incident WNA and may approach 100% with WNA close to 90° at the magnetic latitude of 15°. We also find that localized wave structure may be formed when the converted H+ band EMIC wave is trapped between local H+ − D+(He++) bi‐ion resonance frequency and D+(He++) − He+ left‐handed polarization cut‐off frequency with a small M/Q = 2 ion fraction and a relatively larger He+ fraction as incoming MS wave is near equator.

The Radial Distribution of Ion-scale Waves in the Inner Heliosphere

Determining the mechanism responsible for plasma heating and particle acceleration is a fundamental problem in the study of the heliosphere. Due to efficient wave–particle interactions of ion-scale waves with charged particles, these waves are widely believed to be a major contributor to ion energization, and their contribution considerably depends on the wave occurrence rate. By analyzing the radial distribution of quasi-monochromatic ion-scale waves observed by the Parker Solar Probe, this work shows that the wave occurrence rate is significantly enhanced in the near-Sun solar wind, specifically 21%–29% below 0.3 au, in comparison to 6%–14% beyond 0.3 au. The radial decrease of the wave occurrence rate is not only induced by the sampling effect of a single spacecraft detection, but also by the physics relating to the wave excitation, such as the enhanced ion beam instability in the near-Sun solar wind. This work also shows that the wave normal angle θ, the absolute value of ellipticity ϵ, the wave frequency f normalized by the proton cyclotron frequency f cp, and the wave amplitude δ B normalized by the local background magnetic field B 0 slightly vary with the radial distance. The median values of θ, ∣ϵ∣, f, and δ B are about 9°, 0.73, 3f cp, and 0.01B 0, respectively. Furthermore, this study proposes that the wave mode natures of the observed left-handed and right-handed polarized waves correspond to the Alfvén ion cyclotron mode wave and the fast magnetosonic whistler mode wave, respectively.

The effect of heavy ions on the dispersion properties of kinetic Alfvén waves in astrophysical plasmas

Context.Spacecraft measurements have shown Kinetic Alfvén Waves propagating in the terrestrial magnetosphere at lower wave-normal angles than predicted by linear Vlasov theory of electron-proton plasmas. To explain these observations, it has been suggested that the abundant heavy ion populations in this region may have strong, non-trivial effects that allow Alfvénic waves to acquire right-handed polarization at lower angles with respect to the background magnetic field, as in the case of typical electron-proton plasma.Aims.We study the dispersion properties of Alfvénic waves in plasmas with stationary phase-space distribution functions with different heavy ion populations. Our extensive numerical analysis has allowed us to quantify the role of the heavy ion components on the transition from the left-hand polarized electromagnetic ion-cyclotron (EMIC) mode to the right-hand polarized kinetic Alfvén wave (KAW) mode.Methods.We used linear Vlasov-Maxwell theory to obtain the dispersion relation for oblique electromagnetic waves. The dispersion relation of Alfvén waves was obtained numerically by considering four different oxygen ion concentrations ranging between 0.0 and 0.2 for all propagation angles, as a function of both the wavenumber and the plasma beta parameter.Results.The inclusion of the heavy O+ions is found to considerably reduce the transition angle from EMIC to KAW both as a function of the wave number and plasma beta. With increasing O+concentrations, waves become more damped in specific wavenumber regions. However, the inclusion of oxygen ions may allow weakly damped KAW to effectively propagate at smaller wave-normal angles than in the electron-proton case, as suggested by observations.

Amplitude‐Dependent Properties and Excitation Mechanisms of EMIC Waves in the Earth's Inner Magnetosphere

AbstractLarge‐amplitude (Bw > 1 nT) electromagnetic ion cyclotron (EMIC) waves can cause the rapid loss of >1 MeV electrons, greatly impacting radiation belt dynamics. With long‐term Van Allen Probe B observations from 2013 to 2018, we conducted a statistical survey to reveal the amplitude‐dependent EMIC wave properties and excitation mechanisms in the Earth's inner magnetosphere. Statistical results show that large‐amplitude EMIC waves prefer to occur in the afternoon‐dusk sector in the northern hemisphere and tend to be more left‐hand polarized with smaller wave normal angles. In addition, the high proton beta parallel conditions also favor the generation of larger‐amplitude EMIC waves. From the variations of EMIC wave occurrence rate as a function of SuperMAG electrojet (SME) index and solar wind dynamic pressure, we find that the small‐amplitude EMIC waves are generally triggered by high solar wind dynamic pressure, while large‐amplitude EMIC wave generation is both affected by substorm activity and solar wind dynamic pressure. The normalized magnetic field perturbations during EMIC wave appearance, which enable us to distinguish the relative roles of magnetospheric compression and substorm injection in the excitation of different‐amplitude EMIC waves, provide further evidence that as wave amplitude increases, substorm injection plays a more important role in EMIC wave excitation, and magnetospheric compression is also an indispensable trigger.

Precipitation Rates of Electrons Interacting With Lower‐Band Chorus Emissions in the Inner Magnetosphere

AbstractElectrons trapped in the Earth's magnetic field can be scattered by whistler mode chorus emissions and precipitate into the Earth's upper atmosphere. Whistler mode chorus waves propagating in the Earth's inner magnetic field are usually observed with oblique wave normal angles (WNAs). In this study, we apply 12 chorus wave models with four various WNA sets (the maximum WNA are 0°, 20°, 60°, and 90% of resonance cone angles) and three wave amplitude sets (the maximum wave magnetic fields are 2.1 nT, 307 pT, and 49.4 pT) at L = 4.5. We use test‐particle simulations to trace electrons interacting with the waves and create Green's function sets for electrons initially at kinetic energies (K) 10–6,000 keV and equatorial pitch angles (α) 5°–89°. The simulation results show that in the 2.1 nT cases, the very oblique chorus waves contribute to more electron precipitation than the other three chorus wave models, especially at energies 50–100 keV. Checking the highest initial equatorial pitch angle of the precipitated electrons, we find that the very oblique chorus waves can precipitate electrons with α > 45°. In contrast, the other chorus waves can only precipitate electrons with α < 30°. Furthermore, the precipitation rates reveal that the anomalous trapping effect, which moves low equatorial pitch angle electrons away from the loss cone, in the oblique cases is much weaker than in the parallel case, resulting in higher precipitation rates. Finally, we derive the pitch angle scattering rates and verify the precipitation by nth cyclotron resonances with oblique chorus.

Characteristics of Boomerang Whistler‐Mode Waves Emitted From the DSX Spacecraft

AbstractThe Air Force Research Laboratory's Demonstration and Science Experiments (DSX) spacecraft carried a high‐voltage very low frequency transmitter and a sensitive broadband receiver to medium Earth orbit in 2019. During many pulsed transmission experiments, DSX detected apparent “boomerang” echoes when its emitted waves refracted in the magnetosphere and returned to the spacecraft. We simulated a series of these detected pulses using cold plasma ray tracing to characterize their likely wavelengths, indices of refraction, and initial wave normal angles. The waves were shown to remain relatively local to DSX, to be lightly damped, and to have a wide variety of wavelengths and indices of refraction, but they were all emitted with very oblique wave normal angles tightly clustered about half a degree from the Gendrin angle, which theoretical antenna models predict is preferentially excited. Our results are remarkably consistent with this prediction but are statistically biased closer to the resonance cone, possibly because of limitations in the ray tracing technique. The result is robust to perturbations of the simulation and confirms a very narrow beam of oblique radiation quite unlike the behavior of a dipole in vacuo.

The upward propagating ionospheric hiss waves during the seismic time observed by the China seismo-electromagnetic satellite

The ionospheric hiss wave is a broadband incoherent and structureless electromagnetic emission. They appear in a relatively narrower frequency range between −0.1 and 1.5 kHz. However, according to previous observations, abnormal electromagnetic emissions during seismic activities also preferentially appear in the same frequency range of ionospheric hiss. This work studies the propagation features of the ionospheric hiss during seismic time based on the observations from the CSES (China Seismo-Electromagnetic Satellite). The wave vector analysis shows that during seismic activities, except for the downward propagating ionospheric hiss which is a common phenomenon in the ionosphere, there are upward propagating emissions mixed with the downward propagating ionospheric hiss. We made a statistical analysis of the shallow strong earthquakes (M ≥ 6.0, depth below 30 km) that occurred in mainland China from 2019 to 2022. We selected the ionospheric hiss events recorded by orbits passing over the epicenters within a time window (the 1-month prior to and 1-week after the main shock). We found that although most of the events are the typical downward propagating ionospheric hiss waves, however, there are certain events mixed with the upward propagating emissions. The statistical distribution analysis of wave propagation parameters shows that the major part of wave normal angles vary from 40 to 60, the azimuthal angles predominately attain below 40, and the ellipticity shows a more complicated feature varying around ± 0.5, and the planarity values predominate at values between 0.6 and 1. The frequency band of the upward propagating ionospheric hiss mostly varies between 300 Hz and 800 Hz. To further study the behavior of such upward propagating ionospheric hiss wave during the seismic time, we compared the wave activities under non-seismic activity and quiet space weather conditions, and the results confirm that the occurrence rate of the upward propagating emissions under quiet conditions is far less than that in the seismic time. We suggest that there is a link between the upward propagating ionospheric hiss and the seismic activity, but the physical reason behind it still remains a puzzle to us.

Experimental studies on the propagation of whistler-mode waves in a magnetized plasma structure with a non-uniform density

Propagation of whistler-mode waves in a magnetized plasma structure is investigated in the Keda linear magnetized plasma device. The magnetized plasma structure has its density peak in the center, and the background magnetic field is homogeneous along the axial direction. A whistler-mode wave with a frequency of 0.3 times of electron cyclotron frequency (f ce) is launched into the plasma structure. The wave normal angle (WNA) is about 25°, and the wavefront exhibits a wedge structure. During propagation of the whistler wave, both the propagating angle and WNA slowly approach zero, and then the wave is converged toward the center of the structure. Therefore, the wave tends to be trapped in the plasma structure. The results present observational evidence of the propagation of a whistler-mode wave trapped in the enhanced-density structure in a laboratory plasma. This trapping effect is consistent with satellite observations in the inner magnetosphere.

Parametric analysis of pitch angle scattering and losses of relativistic electrons by oblique EMIC waves

This study analyzes the effects of electromagnetic ion cyclotron (EMIC) waves on relativistic electron scattering and losses in the Earth’s outer radiation belt. EMIC emissions are commonly observed in the inner magnetosphere and are known to reach high amplitudes, causing significant pitch angle changes in primarily >1 MeV electrons via cyclotron resonance interactions. We run test-particle simulations of electrons streaming through helium band waves with different amplitudes and wave normal angles and assess the sensitivity of advective and diffusive scattering behaviors to these two parameters, including the possibility of very oblique propagation. The numerical analysis confirms the importance of harmonic resonances for oblique waves, and the very oblique waves are observed to efficiently scatter both co-streaming and counter-streaming electrons. However, strong finite Larmor radius effects limit the scattering efficiency at high pitch angles. Recently discussed force-bunching effects and associated strong positive advection at low pitch angles are, surprisingly, shown to cause no decrease in the phase space density of precipitating electrons, and it is demonstrated that the transport of electrons into the loss cone balances out the scattering out of the loss cone. In the case of high-amplitude obliquely propagating waves, weak but non-negligible losses are detected well below the minimum resonance energy, and we identify them as the result of non-linear fractional resonances. Simulations and theoretical analysis suggest that these resonances might contribute to subrelativistic electron precipitation but are likely to be overshadowed by non-resonant effects.

Whistler waves generated inside magnetic dips in the young solar wind: Observations of the search-coil magnetometer on board Parker Solar Probe

Context. Whistler waves are electromagnetic waves produced by electron-driven instabilities, which in turn can reshape the electron distributions via wave–particle interactions. In the solar wind they are one of the main candidates for explaining the scattering of the strahl electron population into the halo at increasing radial distances from the Sun and for subsequently regulating the solar wind heat flux. However, it is unclear what type of instability dominates to drive whistler waves in the solar wind. Aims. Our goal is to study whistler wave parameters in the young solar wind sampled by Parker Solar Probe (PSP). The wave normal angle (WNA) in particular is a key parameter to discriminate between the generation mechanisms of these waves. Methods. We analyzed the cross-spectral matrices of magnetic field fluctuations measured by the search-coil magnetometer (SCM) and processed by the Digital Fields Board (DFB) from the FIELDS suite during PSP’s first perihelion. Results. Among the 2701 wave packets detected in the cross-spectra, namely individual bins in time and frequency, most were quasi-parallel to the background magnetic field; however, a significant part (3%) of the observed waves had oblique (> 45°) WNA. The validation analysis conducted with the time series waveforms reveal that this percentage is a lower limit. Moreover, we find that about 64% of the whistler waves detected in the spectra are associated with at least one magnetic dip. Conclusions. We conclude that magnetic dips provide favorable conditions for the generation of whistler waves. We hypothesize that the whistlers detected in magnetic dips are locally generated by the thermal anisotropy as quasi-parallel and can gain obliqueness during their propagation. We finally discuss the implications of our results for the scattering of the strahl in the solar wind.

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.

Statistical Analysis of Lunar 1 Hz Waves Using ARTEMIS Observations

Like 1 Hz waves occurring in the upstream of various celestial bodies in the solar system, 1 Hz narrowband whistler-mode waves are often observed around the Moon. However, wave properties have not been thoroughly investigated, which makes it difficult to proclaim the generation mechanism of the waves. Using 5.5 yr wave data from ARTEMIS, we perform a detailed investigation of 1 Hz waves in the near-lunar space. The amplitude of lunar 1 Hz waves is generally 0.05–0.1 nT. In the geocentric solar ecliptic coordinates, the waves show no significant regional differentiation pattern but show an absence inside the magnetosphere. Correspondingly, in the selenocentric solar ecliptic coordinates, the waves can occur extensively at ∼1.1–12 RL, while few events are observed in the lunar wake due to a lack of interaction with the solar wind. Furthermore, the wave distributions exhibit modest day–night and dawn–dusk asymmetries but less apparent north–south asymmetry. Compared with the nightside, more intense waves with lower peak wave frequency are present on the dayside. The preferential distribution of 1 Hz waves exhibits a moderate correlation with strong magnetic anomalies. The waves propagate primarily at wave normal angles <60° with an ellipticity of [−0.8, −0.3]. For stronger wave amplitudes and lower latitudes, 1 Hz waves generally have smaller wave normal angles and become more left-hand circularly polarized. Owing to the unique interaction between the Moon and solar wind, our statistical results might provide new insights into the generation mechanism(s) of 1 Hz waves in planetary plasma environments and promote the understanding of lunar plasma dynamics.

Very Low Frequency Whistler Mode Wave Events Observed Simultaneously by the Kannuslehto Station and Van Allen Probes

AbstractEvents characterized by substantial intensity enhancements in the frequency range between about 1.5 and 4 kHz in the measurements of the ground‐based Kannuslehto station, Finland are analyzed. Altogether, as many as 465 events are identified in the Kannuslehto data measured during the campaigns between December 2012 and October 2019. It is shown that the events usually last for several hours and they occur preferentially on the dawn side during geomagnetically active periods. Simultaneous measurements performed by the Van Allen Probes spacecraft are used to reveal the L‐shells and magnetic local times where a corresponding intensity increase occurs in space. A backward ray tracing analysis is further employed to investigate the wave propagation between the tentative source region and the ground. Wave normal angles of waves eventually detectable at Kannuslehto are determined and compared with those obtained from a detailed wave analysis. Either a wave ducting, propagation in the Earth‐ionosphere waveguide, or their combination seem to be needed for the waves to reach Kannuslehto.

Effects of Field‐Aligned Cold Plasma Density Filaments on the Fine Structure of Chorus

AbstractThe whistler‐mode chorus emission, a major driver of radiation belt electron energization and precipitation, exhibits significant amplitude modulations on millisecond timescales. These subpacket modulations are accompanied by fast changes in the wave normal angle. Understanding the evolution of wave propagation properties inside chorus elements is essential for modeling nonlinear chorus‐electron interactions, but the origin of these rapid changes is unclear. We propose that the variations come from propagation inside thin, field‐aligned cold plasma enhancements (density ducts), which produce differing modulations in parallel and perpendicular wave magnetic field components. We show that a full‐wave simulation on a filamented density background predicts wave vector and amplitude evolution similar to Van Allen Probes spacecraft observations. We further demonstrate that the commonly assumed wide density ducts, in which wave propagation can be studied with ray tracing methods, cannot explain the observed behavior. This indirectly proves the existence of wavelength‐scale field‐aligned density fluctuations.

Multi‐Event Analysis of the Correlation Between Chorus Waves and Electron Butterfly Distribution Using Van Allen Probes Observation

AbstractIn this study, using Van Allen Probes observations we identify 81 events of electron flux bursts with butterfly pitch angle distributions for tens of keV electrons with close correlations with chorus wave bursts in the Earth's magnetosphere. We use the high‐rate electron flux data from Magnetic Electron Ion Spectrometer available during 2013–2019 and the simultaneous whistler‐mode wave measurements from Electric and Magnetic Field Instrument Suite and Integrated Science to identify the correlated events. The events are categorized into 67 upper‐band chorus (0.5–0.8 fce) dominated events and 14 other events where lower‐band chorus (0.05–0.5 fce) has modest or strong amplitudes (fce represents electron cyclotron frequency). Each electron flux burst correlated with chorus has a short timescale of ∼1 min or less, suggesting potential nonlinear effects. The statistical distribution of selected electron burst events tends to occur in the post‐midnight sector at L &gt; 5 under disturbed geomagnetic conditions, and is associated with chorus waves with relatively strong magnetic wave amplitude and small wave normal angle. The frequency dependence of the electron flux peaks agrees with the cyclotron resonant condition, indicating the effects of chorus‐induced electron acceleration. Our study provides new insights into understanding the rapid nonlinear interactions between chorus and energetic electrons.