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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]

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]

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]

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]

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]