Search

Plasmaspheric hiss plays an important role in the loss of radiation belt electrons via cyclotron resonant interactions. The cold plasma approximation is widely used in the evaluation of hiss‐driven electron losses, which however can break down during disturbed periods of geomagnetic storms and substorms. The kappa particle velocity distribution, characterized by a pronounced high‐energy tail, is well‐established to model the profile of superthermal plasma under disturbed geomagnetic conditions. In the present study, by calculating the electron bounce‐averaged pitch angle diffusion coefficients with kappa plasma dispersion relations, we investigate the sensitivity of hiss‐induced cyclotron‐resonant electron scattering loss to the spectral index κ under a variety of superthermal plasma conditions. Our results demonstrate that, with increasing κ, the diffusion coefficients of ∼20–100 keV radiation belt electrons significantly decrease at lower pitch angles and increase at higher pitch angles. In contrast, for electrons at higher energies, the diffusion coefficients tend to increase at lower pitch angles and decrease at relatively higher pitch angles. We also find that decrease of L‐shell and increase of α* and temperature anisotropy tend to weaken the hiss‐driven pitch angle scattering efficiency of electrons at energies from tens to hundreds of keV with a dip at ∼30–50 keV, while the scattering of higher energy electrons can be enhanced. This study confirms the important role of superthermal plasmas in the hiss‐driven electron loss processes and should be carefully incorporated in future modeling of radiation belt electron dynamics. Key Points: We calculate electron pitch angle diffusion coefficients due to plasmaspheric hiss in a kappa‐distribution plasmaAn increase in κ causes significant decreases in hiss‐induced diffusion coefficients of ∼20–100 keV electrons at low pitch anglesIncreasing α*, temperature anisotropy and decreasing L‐shell weaken the scattering efficiency of tens to hundreds of keV electrons [ABSTRACT FROM AUTHOR]

The 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. Plain Language Summary: Charged particles present different pitch angle distributions (PADs) due to different physical mechanisms in Saturn's magnetosphere. Butterfly PADs have local minimum at ∼90° pitch angle, which are commonly observed in Saturn's magnetosphere for electrons over a wide energy range from ∼100 eV to several MeV. Previous studies proposed that the adiabatic transport dominates the transformation from the electron field‐aligned PADs of ∼100 eV–several keV to the butterfly shapes in Saturn's magnetosphere. In our study, we report a simultaneous electrostatic electron cyclotron harmonic (ECH) wave event with the butterfly PADs of electrons at L ∼ 7.6–9 observed by the Cassini spacecraft. Via calculating the bounce‐averaged electron diffusion rates of ECH waves and simulating the evolution of electron phase space densities (PSDs), we find that the formation of ∼100 eV to ∼1 keV electron butterfly PADs is mainly caused by the scattering of electrons induced by ECH waves. Our simulation results successfully reproduce the variation of peak pitch angle (αp) and PSD ratios between α90° and αp with L‐shell. These results would improve the current understanding of the electron dynamics in Saturn's magnetosphere and shed light on understanding the plasma environment of the outer planets and exoplanets. Key Points: We report a simultaneous equatorial electrostatic electron cyclotron harmonic (ECH) wave event with the butterfly PADs of electronsThe Saturnian ECH waves can resonate with ∼10 eV to several keV electrons at <60° pitch anglesThe formation of ∼100 eV to ∼1 keV electron butterfly PADs is attributed to the scattering of electrons induced by ECH waves [ABSTRACT FROM AUTHOR]

Based 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. Plain Language Summary: Saturn possesses a huge magnetosphere formed by the interaction between the solar wind flows and the internal magnetic field. Electrostatic electron cyclotron harmonic (ECH) waves are electrostatic emissions frequently observed in Saturn's magnetosphere, which occur near the half‐integral harmonics of electron cyclotron frequency (fce), that is, (n + 1/2) fce. ECH waves are closely related to the injection events driven by the centrifugal force. Due to the gradient and curvature drift, the injected hot electrons produce dispersion signatures on the energy‐time spectrogram, which is a typical characteristic in the remote injections. In this work, we report typical ECH wave events in the remote injections and investigate the wave generation mechanism. Our results show that the excitation of ECH waves in the remote injections is caused by loss cone instability. The wave growth is dominated by injected hot electrons (∼100 eV–several keV). In addition, we find that ECH wave amplitudes increase as the hot electron fluxes increase and decrease with increasing ages of the injection events. Our results help us to deeply understand the generation of ECH waves in the Saturn's magnetosphere. Key Points: The loss cone instability of electrons excites the Saturnian electron cyclotron harmonic (ECH) waves within remote plasma injectionsThe free energy for Saturnian ECH wave growth is mainly provided by remotely injected, hot electrons at energies of ∼100 eV to several keVSaturnian ECH waves intensify with increased fluxes of injected hot electrons but weakening with increased evolution time of the flux tubes [ABSTRACT FROM AUTHOR]

It 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. Key Points: Electron cyclotron harmonic (ECH) waves can accelerate electrons at intermediate pitch angles, resulting in the formation of butterfly pitch angle distributionThe scattering rates decrease as magnetic field intensity and hot electron abundance increase but increase as electron density increasesECH wave‐induced electron scattering is parametric dependent at energies <1 keV but tend to be insensitive above 1 keV [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron (EMIC) waves play an important role in the loss of radiation belt electrons and ring current protons. After EMIC waves are excited near the magnetic equator, they will propagate to high latitudes and suffer a polarization reversal from left‐handed to right‐handed polarized at the crossover frequency. To improve the current understanding of the EMIC wave‐induced loss of radiation belt electrons and ring current protons, we perform a detailed analysis of the sensitivity of polarization reversal effects to L‐shell, electron number density, and wave frequency spectrum. Our results show that polarization reversal contributes significantly to the pitch angle diffusion coefficients at low pitch angles extending to the loss cone angle for various parameter sets. Compared with the diffusion coefficients only using L‐mode dispersion relation, inclusion of polarization reversal mainly enhances the electron scattering loss at several MeV and weaken the proton scattering loss over a broad energy range from a few keV to 1 MeV. While the increase of electron diffusion coefficients can be up to a factor of 5, the decrease of proton diffusion coefficients can be larger than an order of magnitude. As L‐shell or electron density increases, the affected resonant region by polarization reversal for both electrons and protons tends to expand to lower energies, while an increase of peak wave frequency will shift the resonant region to higher energies. Our results confirm the vital importance of including polarization reversal in understanding the EMIC wave‐induced scattering loss of radiation belt electrons and ring current protons. Key Points: Electromagnetic ion cyclotron wave polarization reversal strongly affects the pitch angle scattering of both electrons and protons for various parameter setsAs L‐shell or electron density increases, polarization reversal can modify particle diffusion coefficients over a broader energy rangeFor increasing peak wave frequency, the affected resonant region by polarization reversal shrinks to higher particle energies [ABSTRACT FROM AUTHOR]

Based on Van Allen Probes observations, in this study we perform a statistical analysis of the spectral intensities of plasmaspheric hiss at L‐shells of 1.8–3.0 in the slot region. Our results show that slot region hiss power intensifies with a strong day‐night asymmetry as the level of substorm activity or L‐shell increases. Using the statistical spectral profiles of plasmaspheric hiss, we calculate the drift‐ and bounce‐averaged electron pitch angle diffusion coefficients and subsequently obtain the resultant electron loss timescales through 1‐D Fokker‐Planck simulations. We find that slot region electron loss timescales vary significantly from <1 day to several years, showing a strong dependence on electron energy, L‐shell and substorm activity. We also construct an empirical model of slot region electron loss timescales due to scattering by plasmaspheric hiss, which agrees well with the 1‐D simulation results and can be readily used in modeling the dynamics of slot region electrons.Key Points: We perform a statistical analysis of the global distribution of hiss wave spectral intensities in the slot regionThe drift‐ and bounce‐averaged pitch angle diffusion coefficients of slot region electrons are calculatedWe develop an empirical model of slot region electron loss timescales due to plasmaspheric hiss [ABSTRACT FROM AUTHOR]

Although electrostatic electron cyclotron harmonic (ECH) waves are frequently observed in the dayside magnetosphere, their effects on the formation of dayside diffuse aurora remain poorly understood. In this study, we quantitatively evaluate the efficiency of dayside ECH wave scattering in producing the electron diffuse aurora precipitation during a typical ECH wave event by calculating the quasi‐linear bounce‐averaged scattering rates. We find that dayside ECH waves can efficiently pitch angle scatter diffuse auroral electrons on timescales of a few hours to ∼1 day over a broad range of electron energy and equatorial pitch angle αeq, that is, from ∼300 eV to 10 keV with αeq from the loss cone to 45°. Especially for ∼300 eV–2 keV electrons, the scattering rates can even approach the strong diffusion limit, resulting in an almost fully filled loss cone. Our results confirm the significant role of ECH waves in driving the dayside diffuse aurora. Plain Language Summary: Electrostatic electron cyclotron harmonic (ECH) waves are electrostatic emissions observed between the harmonics of electron cyclotron frequency with wave normal near perpendicular to the ambient magnetic field. ECH waves are known to be excited by the loss cone instability of hot electron velocity distribution. In this study, we perform a detailed survey of a typical dayside ECH wave event observed by MMS1 spacecraft on 11 Dec 2015. Using a realistic magnetic field model and a latitudinal varying wave normal angle model based on well‐fitted electron velocity distribution, we calculate the quasi‐linear bounce‐averaged scattering rates of electrons. Our results demonstrate that ECH waves can efficiently pitch angle scatter hundreds of eV to tens of keV electrons, which are the source of diffuse aurora. ECH wave induced scattering of ~300 eV to 2 keV electrons at low pitch angles can even approach the strong diffusion limit, resulting in an almost fully filled loss cone. These electrons can be subsequently precipitated into the atmosphere and collide with neutral molecule, forming the dayside diffuse aurora. Our results demonstrate that ECH waves play an important role in the dayside diffuse auroral precipitation. Key Points: We quantitatively evaluate the scattering effect of dayside electron cyclotron harmonic (ECH) waves on diffuse auroral electronsECH waves can efficiently pitch angle scatter electrons at energies from 300 eV to 10 keV on timescales from a few hours to ∼1 dayScattering of ∼300 eV–2 keV electrons can even approach the strong diffusion limit, resulting in an almost fully filled loss cone [ABSTRACT FROM AUTHOR]

In this study, we investigate the effects of polarization reversal of electromagnetic ion cyclotron (EMIC) waves at the crossover frequencies on computations of bounce‐averaged pitch angle diffusion coefficients of radiation belt electrons and ring current protons. We find that inclusion of polarization reversal can cause significant changes of H+ band‐induced particle diffusion coefficients, while scattering by the He+ band is almost unaffected. Our results show that the pure L‐mode approach, which has been widely implemented in previous studies, tends to underestimate the diffusion coefficients of ultrarelativistic (>4 MeV) electrons and overestimate those of 10–50 and >100 keV protons caused by H+ band EMIC waves. Especially for >100 keV protons, the differences in diffusion coefficients can be larger by an order of magnitude. We confirm that the polarization reversal can contribute importantly to the scattering loss of radiation belt electrons and ring current protons by H+ band EMIC waves. Key Points: The effects of polarization reversal at the crossover frequencies on the particle scattering by EMIC waves are investigatedWhile polarization reversal causes obvious changes in H+ band‐induced scattering rates, scattering by the He+ band is almost unaffectedPolarization reversal of H+ band increases the scattering rates of >4 MeV electrons and decreases those of 10–50 and >100 keV protons [ABSTRACT FROM AUTHOR]

Observations show that particle velocity distributions in space plasmas generally exhibit a non-Maxwellian high-energy tail that can be well fitted with kappa distributions. To better understand the correlation between realistic particle velocity distributions and plasma wave excitation, we investigate the linear cyclotron instability of multiband electromagnetic ion cyclotron (EMIC) waves in a kappa plasma containing hot anisotropic protons, which provides the free energy for the wave growth. We find that the effects of superthermal plasmas on EMIC wave instability have a strong dependence on the emission band, temperature anisotropy Ahp, and parallel beta βhp of hot protons. For H+ and He+ band EMIC waves, the maximum growth rates exhibit distinct behaviors with the variation of the spectral index κ of kappa distributions for different Ahp values. The maximum growth rates decrease with increasing κ-value for low Ahp and increase with increasing κ-value for high Ahp. For O+ band waves, the effects of superthermal plasmas on the maximum growth rate are strongly controlled by βhp. For low βhp, the growth rate decreases monotonically with increasing κ-value for all Ahp. For high βhp, increase of κ-value tends to enhance the wave growth for intermediate Ahp and to suppress the wave growth otherwise. Our results also indicate that the presence of a high-energy tail tends to decrease the real frequency corresponding to the maximum growth rate for all three bands. While the minimum electron resonant energy for O+ band EMIC waves decreases as the κ-value increases, the minimum electron resonant energies for H+ and He+ band waves remain unaffected. [ABSTRACT FROM AUTHOR]

Whistler mode hiss acts as an important loss mechanism contributing to the radiation belt electron dynamics inside the plasmasphere and plasmaspheric plumes. Based on Van Allen Probes observations from September 2012 to December 2015, we conduct a detailed analysis of hiss properties in plasmaspheric plumes and illustrate that corresponding to the highest occurrence probability of plumes at L = 5.0–6.0 and MLT = 18–21, hiss emissions occur concurrently with a rate of >~80%. Plume hiss can efficiently scatter ~10‐ to 100‐keV electrons at rates up to ~10−4 s−1 near the loss cone, and the resultant electron loss timescales vary largely with energy, that is, from less than an hour for tens of kiloelectron volt electrons to several days for hundreds of kiloelectron volt electrons and to >100 days for >5‐MeV electrons. These newly obtained statistical properties of plume hiss and associated electron scattering effects are useful to future modeling efforts of radiation belt electron dynamics. Key Points: Plume hiss occurs concurrently with a rate of >~80%, corresponding to the highest occurrence rate of plumes at L = 5.0‐6.0 and MLT = 18‐21Plume hiss is efficient for pitch angle scattering ~10‐ to 100‐keV electrons near the loss conePlume hiss can drive more efficient scattering of >500‐keV electrons at higher L‐shells than normal frequency hiss at lower L‐shells [ABSTRACT FROM AUTHOR]

We develop a general 3-D relativistic test particle (TP) simulation code to examine the trajectories and pitch angle variations of test electrons. We investigate the conditions when the electrons transit broadband, parallel propagating H+ band electromagnetic ion cyclotron (EMIC) waves to find whether the classical quasilinear theory (QLT) can be applied to interactions between H+ band EMIC waves and radiation belt relativistic (and ultra-relativistic) electrons. We find that to reach good consistency between TP scattering coefficients and QLT calculations, the wave-particle resonant interaction time (which indicates the time of particles traveling in the wave fields and being resonant with the wave fields) must be carefully calculated to ensure the validity of the linear diffusion process. This wave-particle resonant interaction time depends largely on the wave amplitude, the bandwidth, and the initial parameters of electrons for TP simulations; once the time of electrons traveling in the wave fields becomes large enough when the pitch angles of electrons scattered by wave fields reach the maximal value limited by the resonance condition and cannot be diffused any longer, the linear diffusion process of particles defined by QLT will not be valid any longer. When the resonant electrons start to travel through the wave field, they undergo the linear diffusion process resulting from the pitch angle scattering by the H+ band EMIC wave field. As the time of traveling in the wave fields increases, the test electrons will suffer effective pitch angle scattering and the pitch angles of these electrons will reach the maximum value limited by the resonance condition, which is determined by the wave and electron parameters. These electrons are then reflected by the wave fields and the linear wave-particle interactions defined by QLT will not be valid anymore. This kind of wave-particle interaction subsequently introduces considerable deviation of TP diffusion coefficients from those predicted by QLT calculations. Our results demonstrate that the general validity of QLT in describing how broadband EMIC waves affect radiation belt relativistic electrons is highly related to the time of electron traveling in the wave fields, which tends to increase for smaller wave power and broader wave frequency spectra. [ABSTRACT FROM AUTHOR]

Electromagnetic ion cyclotron waves have long been recognized to play a crucial role in the dynamic loss of ring current protons. While the field‐aligned propagation approximation of electromagnetic ion cyclotron waves was widely used to quantify the scattering loss of ring current protons, in this study, we find that the wave normal distribution strongly affects the pitch angle scattering efficiency of protons. Increase of peak normal angle or angular width can considerably reduce the scattering rates of ≤10 keV protons. For >10 keV protons, the field‐aligned propagation approximation results in a pronounced underestimate of the scattering of intermediate equatorial pitch angle protons and overestimates the scattering of high equatorial pitch angle protons by orders of magnitude. Our results suggest that the wave normal distribution of electromagnetic ion cyclotron waves plays an important role in the pitch angle evolution and scattering loss of ring current protons and should be incorporated in future global modeling of ring current dynamics. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) wave is a class of electromagnetic wave that is frequently observed in the Earths' magnetosphere. EMIC waves can cause the loss of ring current protons by scattering them into the atmosphere. Previous attempts to quantify the loss of ring current protons by EMIC waves are mostly limited to the assumption that the waves are propagating exactly along the direction of geomagnetic field line. In this study, we show that this assumption will seriously break down once the waves are obliquely propagating. The obliquity of EMIC waves not only influences the pitch angle evolution of ring current protons but also affects their loss time scales. Our study confirms the importance of including the obliquity of EMIC waves in the modeling efforts of ring current dynamics. Key Points: The wave normal angle distribution strongly affects the EMIC wave‐driven scattering loss of ring current protonsQuantitative differences in the diffusion coefficients between field‐aligned and oblique waves are presentedPitch‐angle scattering of higher‐energy ring current protons is more sensitive to the variation of wave normal angle distribution [ABSTRACT FROM AUTHOR]

Abstract: Plasmaspheric hiss is known to play an important role in radiation belt electron dynamics in high plasma density regions. We present observations of two crossings of a plasmaspheric plume by the Van Allen Probes on 26 December 2012, which occurred unusually at the post‐midnight‐to‐dawn sector between L ~ 4–6 during a geomagnetically quiet period. This plume exhibited pronounced electron densities higher than those of the average plume level. Moderate hiss emissions accompanied the two plume crossings with the peak power at about 100 Hz. Quantification of quasi‐linear bounce‐averaged electron scattering rates by hiss in the plume demonstrates that the waves are efficient to pitch angle scatter ~10–100 keV electrons at rates up to ~10−4 s−1 near the loss cone but become gradually insignificant to scatter the higher energy electron population. The resultant timescales of electron loss due to hiss in the nightside plume vary largely with electron kinetic energy over 3 orders of magnitude, that is, from several hours for tens of keV electrons to a few days for hundreds of keV electrons to well above 100 days for >1 MeV electrons. Changing slightly with L‐shell and the multiquartile profile of hiss spectral intensity, these electron loss timescales suggest that hiss emissions in the nightside plume act as a viable candidate for the fast loss of the ≲100 keV electrons and the slow decay of higher energy electrons. [ABSTRACT FROM AUTHOR]

Abstract: To investigate the hot plasma effects on the cyclotron‐resonant interactions between electromagnetic ion cyclotron (EMIC) waves and radiation belt electrons in a realistic magnetospheric environment, calculations of the wave‐induced bounce‐averaged pitch angle diffusion coefficients are performed using both the cold and hot plasma dispersion relations. The results demonstrate that the hot plasma effects have a pronounced influence on the electron pitch angle scattering rates due to all three EMIC emission bands (H+, He+, and O+) when the hot plasma dispersion relation deviates significantly from the cold plasma approximation. For a given wave spectrum, the modification of the dispersion relation by hot anisotropic protons can strongly increase the minimum resonant energy for electrons interacting with O+ band EMIC waves, while the minimum resonant energies for H+ and He+ bands are not greatly affected. For H+ band EMIC waves, inclusion of hot protons tends to weaken the pitch angle scattering efficiency of >5 MeV electrons. The most crucial differences introduced by the hot plasma effects occur for >3 MeV electron scattering rates by He+ band EMIC waves. Mainly due to the changes of resonant frequency and wave group velocity when the hot protons are included, the difference in scattering rates can be up to an order of magnitude, showing a strong dependence on both electron energy and equatorial pitch angle. Our study confirms the importance of including hot plasma effects in modeling the scattering of ultra‐relativistic radiation belt electrons by EMIC waves. [ABSTRACT FROM AUTHOR]

Strong electrostatic electron cyclotron harmonic (ECH) waves on the dayside magnetosphere have been reported based on observations of the Magnetospheric Multiscale (MMS) spacecraft. In this study, we analyze high‐quality wave data from the four MMS satellites between 1 September 2015 and 30 August 2018 to investigate the statistical properties of dayside ECH emissions. The results show that dayside ECH waves are preferentially observed on the prenoon side in the outer magnetosphere (L = 8–12), with average wave amplitude Ew > 0.1 mV/m. In addition, besides the typical near‐equatorial (|MLAT| ≤ 15°) region, dayside ECH waves exhibit moderate occurrence rate and wave amplitude in higher latitudinal regions (i.e., 15 < |MLAT| ≤ 40°), possibly due to the off‐equatorial geomagnetic field minimum. Our reported double peaks of dayside ECH wave occurrence zone and considerable occurrence rates of prenoonside ECH waves suggest that dayside ECH waves can be a potentially important contributor to the formation of dayside diffuse aurora. Plain Language Summary: We use wave spectral data from the four Magnetospheric Multiscale (MMS) spacecraft to statistically analyze the distributions of wave amplitudes and occurrence rates as a function of L shell, MLT, geomagnetic activity (AE* and Kp indices), and MLAT for dayside first harmonic band electron cyclotron harmonic (ECH) waves. The conclusions are summarized as follows: (1) Dayside ECH waves preferentially occur on the prenoonside of the outer magnetosphere, with peak occurrence rates around 30%. (2) In higher latitudinal regions (|MLAT| = 15–25°,25–40°), contrary to previous conclusions regarding ECH wave occurrence patterns, dayside ECH waves occur with significant occurrence rates, comparable to that of dayside ECH waves in near‐equatorial areas. (3) Similar to the distribution of occurrence rates, dayside ECH waves have most intense electric field amplitudes over L = 8–12, MLT = 8–11, with an average wave strength >0.1 mV/m.Whistler mode chorus waves are considered as the primary mechanism to explain dayside diffuse aurora, but the role of ECH waves has not been systematically evaluated because of lack of dayside ECH observations. As shown by our results, moderate dayside ECH waves can be observed on the prenoonside of the outer magnetosphere. This observation could be of great importance in understanding the generation of dayside diffuse aurora. Key Points: Dayside ECH waves are preferentially observed with moderate average wave strength on the prenoonside to noonside in the outer magnetosphereIn higher latitudinal regions, dayside ECH waves have comparable occurrence rates and wave amplitude to that of the near‐equatorial regionsAlthough weak dayside ECH waves occur predominantly, the occurrence rates of moderate and strong dayside ECH waves are not negligible [ABSTRACT FROM AUTHOR]

By investigating the resonant energy and latitudinal resonance region of Landau resonance between H+ band electromagnetic ion cyclotron (EMIC) waves and radiation belt electrons and performing calculations of quasi‐linear diffusion coefficients, we find that Landau resonance with EMIC waves, characterized by a very broad range of resonant energies from below 1 eV to above 10 MeV and the strikingly small extent of the latitudinal resonance region well below 0.1°, can be a feasible candidate accounting for the pitch angle scattering of near‐equatorially mirroring electrons. Compared to the dominant pitch angle scattering caused by the cyclotron resonance for greater than ~2‐MeV electrons at equatorial pitch angles less than ~80° with the rates well above 10−4 s−1, Landau resonance with H+ band EMIC waves has the potential to drive more efficient pitch angle scattering of radiation belt electrons from ~10 MeV down to tens of kiloelectron volts at equatorial pitch angles greater than ~ 85°. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are thought to be very important for pitch angle scattering of radiation belt electrons, which may play a crucial role for the rapid dropout of relativistic electrons fluxes in the outer radiation belt during a geomagnetic storm, or for the intense relativistic electron precipitation. Although cyclotron resonance with EMIC waves acts as an important loss process during geomagnetic storms, this mechanism is believed to solely affect trapped electrons with small pitch angles well away from 90°. In the present work, we introduce the Landau resonance between H+ band EMIC waves and radiation belt electrons. The crucial role for pitch angle scattering near‐equatorially mirroring electrons by Landau resonance is investigated by Quasilinear diffusion theory (QLT) calculations in our work. Key Points: H+ band EMIC waves drive the Landau resonance with radiation belt electrons over a very broad energy range from >10 MeV to tenths of an eVLandau resonance between H+ band EMIC waves and electrons at > ~60 degrees occurs in a very limited rangeEMIC scattering electrons due to the cyclotron resonance and Landau resonance occur at two distinct ranges of equatorial pitch angle [ABSTRACT FROM AUTHOR]

We report a typical event that fast magnetosonic (MS) waves, exohiss, and two‐band chorus waves occurred simultaneously on the dayside observed by Van Allen Probes on 25 December 2013. By combining calculations of electron diffusion coefficients and 2‐D Fokker‐Planck diffusion simulations, we quantitatively analyze the combined scattering effect of multiple waves to demonstrate that the net impact of combined scattering does not simply depend on the wave intensity dominance of various plasma waves. Although the observed MS waves are most intense, the electron butterfly distribution is inhibited by exohiss and chorus, and electrons are considerably accelerated by combined scattering of MS and chorus waves. The simulated electron pitch angle distributions exhibit the variation trend consistent with the observations. Our results strongly suggest that competition and cooperation between resonant interactions with concurrently occurring magnetospheric waves need to be carefully treated in modeling and comprehending the radiation belt electron dynamics. Plain Language Summary: It has been acknowledged that wave‐particle interactions play an important role in energy exchange between plasma waves and radiation belt electrons. Most previous studies focused on the wave‐particle interaction effect of individual wave modes, while only rather limited investigations look into the competition and cooperation between various kinds of plasma waves upon their modulation of the radiation belt electron dynamics. In the present study, we report a typical event that magnetosonic waves, exohiss, and two‐band chorus waves occurred simultaneously on the dayside. By computations of the electron diffusion coefficients and 2‐D Fokker‐Planck diffusion simulations, we have found that the competition and cooperation between these three wave modes are delicate in that the net combined scattering impact does not simply depend on the dominance of wave intensity but requires careful analyses via diffusion simulations. Our results demonstrate the importance of incorporating the combined scattering effect by various simultaneously occurring magnetospheric waves during modeling and comprehending the complex radiation belt electron dynamics. Key Points: A typical event that MS waves, exohiss, and two‐band chorus waves occurred simultaneously in the dayside plasmatrough is reportedThe combined scattering effect makes a big difference to the radiation belt electron dynamics compared to contributions of individual wavesNumerical simulations can approximately explain the variation trend of observed electron pitch angle distributions [ABSTRACT FROM AUTHOR]