We report unusual Electromagnetic Ion Cyclotron (EMIC) waves with a very narrow frequency bandwidth, closely following and approaching the proton gyrofrequency. One interesting case analysis shows that magnetosonic waves, anisotropic suprathermal proton distributions, and high frequency EMIC waves are closely related. Magnetosonic waves potentially cause the resonant heating of suprathermal protons and the temperature anisotropy of suprathermal protons (10-100 eV) likely provides free energy for the excitation of high frequency EMIC waves. The statistical analysis shows that this type of EMIC waves has a typical wave amplitude of~100 pT, left-handed polarization, and small wave normal angles. Moreover, these low frequency EMIC waves typically occur near the equator in the low-density regions from dawn to dusk. These newly observed high frequency EMIC waves provide new insights into understanding the generation of EMIC waves and the energy transfer between magnetosonic waves and EMIC waves. Plain Language Summary Electromagnetic Ion Cyclotron (EMIC) waves are commonlyobserved in the Earth's magnetosphere and play an important role in causing the loss of ring current ions and relativistic electrons due to pitch angle scattering. In this study, we report unusual high frequency EMIC waves with frequency very close to the proton gyrofrequency. An interesting case study clearly shows the correlation between magnetosonic waves, the enhancement of suprathermal protons, and high frequency EMIC waves. The protons at suprathermal energies could be heated by magnetosonic waves and the anisotropic distribution of suprathermal protons is likely responsible for the excitation of high frequency EMIC waves. The statistical analysis shows that this type of EMIC waves has a typical wave amplitude of~100 pT, left-handed polarization, and small wave normal angles. These newly observed high frequency EMIC waves provide new insights into understanding the generation of EMIC waves and the energy transfer between magnetosonic waves and EMIC waves.

Properties of banded, no‐gap, lower band only, and upper band only whistler mode waves (0.1–0.8fce) outside the plasmasphere are investigated using Van Allen Probes data. Our analysis shows that no‐gap whistler waves have higher occurrence rate at morning side and dayside, while banded and lower band only waves have higher occurrence rate between midnight and dawn. We also find that the occurrence rate of no‐gap whistler waves peaks at magnetic latitude |MLAT|∼8–10°, while banded waves have higher occurrence rate near the equator for false|MLATfalse|≲6°. The wave normal angle distributions of these four groups of waves are similar to previous results. The distinct local time and latitudinal distribution of no‐gap and banded whistler mode waves is critical to further understand the formation mechanism of the power minimum at half electron gyrofrequency.

We evaluate the location, extent, and energy range of electron precipitation driven by ElectroMagnetic Ion Cyclotron (EMIC) waves using coordinated multisatellite observations from near‐equatorial and Low‐Earth‐Orbit (LEO) missions. Electron precipitation was analyzed using the Focused Investigations of Relativistic Electron Burst Intensity, Range and Dynamics (FIREBIRD‐II) CubeSats, in conjunction either with typical EMIC‐driven precipitation signatures observed by Polar Orbiting Environmental Satellites (POES) or with in situ EMIC wave observations from Van Allen Probes. The multievent analysis shows that electron precipitation occurred in a broad region near dusk (16–23 MLT), mostly confined to 3.5–7.5 L‐shells. Each precipitation event occurred on localized radial scales, on average ∼0.3 L. Most importantly, FIREBIRD‐II recorded electron precipitation from ∼200 to 300 keV to the expected ∼MeV energies for most cases, suggesting that EMIC waves can efficiently scatter a wide energy range of electrons.

Energetic electron dynamics is highly affected by plasma waves through quasilinear and/or nonlinear interactions in the Earth's inner magnetosphere. In this letter, we provide physical explanations for a previously reported intriguing event from the Van Allen Probes observations, where bursts of electron butterfly distributions at tens of keV exhibit remarkable correlations with chorus waves. Both test particle and quasilinear simulations are used to reveal the formation mechanism for the bursts of electron butterfly distribution. The test particle simulation results indicate that nonlinear phase trapping due to chorus waves is the key process to accelerate electrons to form the electron butterfly distribution withiñ 30 s, and reproduces the observed features. Quasilinear simulation results show that although the diffusion process alone also contributes to form the electron butterfly distribution, the timescale is slower. Our study demonstrates the importance of nonlinear interaction in rapid electron acceleration at tens of keV by chorus waves. Plain Language Summary In the Earth's magnetosphere, wave-particle interactions play an important role in changing energetic electron dynamics. In particular, whistler mode chorus waves are known to cause efficient electron acceleration. Electron butterfly distribution is a special type of electron pitch angle distribution with double flux peaks away from 90°pitch angle. In this letter, we provide physical explanations for a previously reported intriguing event from the Van Allen Probes observations, where bursts of electron butterfly distributions exhibit remarkable correlations with chorus waves. We use test particle and quasilinear simulations to evaluate the wave-particle interactions between the observed energetic electrons and chorus waves. The test particle simulation results indicate that nonlinear effects due to chorus waves are critical to form the electron butterfly distribution, which is consistent with the observation, while quasilinear results show that the diffusion process alone is insufficient to reproduce the observed electron butterfly distribution. Our study demonstrates the importance of nonlinear interaction in rapid acceleration of energetic electrons by chorus waves.

Radiation belt electrons are strongly affected by resonant interactions with cyclotron-resonant waves. For broad band, small amplitude waves the interactions are well described by quasi-linear diffusion in pitch angle and energy, e.g., Albert et al. (2016), but coherent, large amplitude waves such as strong whistler mode chorus call for a different treatment. A Hamiltonian formulation is often fruitful because resonance averaging reduces the dimensionality of the test particle motion to a single pair of action-angle variables (Ginet & Albert, 1991;Ginet & Heinemann, 1990). Then the phase portraits can be studied graphically, invoking general concepts of adiabatic invariants and their breaking. One such line of analysis was developed by Albert (1993Albert ( , 2000, which is drawn on here. A link between the Hamiltonian approach and quasi-linear theory was given by Albert (2010).The standard nonlinear framework reduces the problem to that of a classical pendulum, with an "island" bounded by a separatrix. Trajectories inside the island have a limited range of phase angle. While crossing the separatrix, resonant particles may become phase trapped within the island, or else phase bunch but circumvent the island. Either process leads to "breaking" an adiabatic invariant, with oppositely signed changes. The particle energy and pitch angle also change, as a direct result. The alternative taken depends sensitively on details including the gyrophase, so is often treated statistically.Though this picture has generally been confirmed by many numerical simulations, recent studies have uncovered additional, complex behavior, including anomalous phase trapping as characterized by Kitahara and Katoh (2019) and positive phase bunching as described by Gan et al. (2020). Both produce increases in pitch angle for values that are initially low. This might be expected on physical grounds, since the corresponding adiabatic invariant cannot become negative, but was not predicted by the standard treatment. Recent simulations involving electromagnetic ion cyclotron (EMIC) waves found preferential decrease of initially low pitch angle values, termed SLPA (scattering at low pitch angle) by Kubota and Omura (2017) and directed scattering by Grach and Demekhov (2020); this is similarly in the opposite direction from what is expected and has been seen at larger pitch angles (Albert & Bortnik, 2009). Kitahara and Katoh (2019) postulated that the effect was due to a common approximation made in deriving the pendulum-like Hamiltonian. Here, we show that eschewing that approximation leads to a more general but still tractable model Hamiltonian previously considered by Henrard andLemaitre (1983) andNeishtadt (1975). It involves not one but two regions of phase trapping, similar to the picture arrived at by