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Abstract Nonlinear interactions between electrons and whistler‐mode chorus waves play an important role in driving electron precipitation in Earth's radiation belts. In this letter, we employ the second fundamental model of the Hamiltonian approach to derive the inhomogeneity ratio, assessing nonlinear resonant interactions between nearly field‐aligned electrons and parallel propagating chorus waves. We perform test particle simulations by launching electrons from a high latitude to the equator, encountering counter‐streaming chorus waves. Our simulations reveal that anomalous scattering, encompassing anomalous trapping and positive bunching, extends the resonant location to the downstream of electrons. The comparison with test particle results demonstrates the efficacy of the inhomogeneity ratio in characterizing nonlinear interactions at small pitch angles. We emphasize the importance of applying this ratio specifically for small pitch angle electrons, as the previously provided inhomogeneity ratio significantly underestimates the impact of nonlinear interactions on electron precipitation. 

Abstract Electromagnetic Ion Cyclotron (EMIC) wave scattering has been proved to be responsible for the fast loss of both radiation belt (RB) electrons and ring current (RC) protons. However, its role in the concurrent dropout of these two co‐located populations remains to be quantified. In this work, we study the effect of EMIC wave scattering on both populations during the 27 February 2014 storm by employing the global physics‐based RAM‐SCB model. Throughout this storm event, MeV RB electrons and 100s keV RC protons experienced simultaneous dropout following the occurrence of intense EMIC waves. By implementing data‐driven initial and boundary conditions, we perform simulations for both populations through the interplay with EMIC waves and compare them against Van Allen Probes observations. The results indicate that by including EMIC wave scattering loss, especially by the He‐band EMIC waves, the model aligns closely with data for both populations. Additionally, we investigate the simulated pitch angle distributions (PADs) for both populations. Including EMIC wave scattering in our model predicts a 90° peaked PAD for electrons with stronger losses at lower pitch angles, while protons exhibit an isotropic PAD with enhanced losses at pitch angles above 40°. Furthermore, our model predicts considerable precipitation of both particle populations, predominantly confined to the afternoon to midnight sector (12 hr < MLT < 24 hr) during the storm's main phase, corresponding closely with the presence of EMIC waves. 

Abstract Electromagnetic ion cyclotron waves in the Earth's outer radiation belt drive rapid electron losses through wave‐particle interactions. The precipitating electron flux can be high in the hundreds of keV energy range, well below the typical minimum resonance energy. One of the proposed explanations relies on nonresonant scattering, which causes pitch‐angle diffusion away from the fundamental cyclotron resonance. Here we propose the fractional sub‐cyclotron resonance, a second‐order nonlinear effect that scatters particles at resonance ordern = 1/2, as an alternate explanation. Using test‐particle simulations, we evaluate the precipitation ratios of sub‐MeV electrons for wave packets with various shapes, amplitudes, and wave normal angles. We show that the nonlinear sub‐cyclotron scattering produces larger ratios than the nonresonant scattering when the wave amplitude reaches sufficiently large values. The ELFIN CubeSats detected several events with precipitation ratio patterns matching our simulation, demonstrating the importance of sub‐cyclotron resonances during intense precipitation events. 

Abstract Magnetopause shadowing (MPS) effect could drive a concurrent dropout of radiation belt electrons and ring current protons. However, its relative role in the dropout of both plasma populations has not been well quantified. In this work, we study the simultaneous dropout of MeV electrons and 100s keV protons during an intense geomagnetic storm in May 2017. A radial diffusion model with an event‐specific last closed drift shell is used to simulate the MPS loss of both populations. The model well captures the fast shadowing loss of both populations atL* > 4.6, while the loss atL* < 4.6, possibly due to the electromagnetic ion cyclotron wave scattering, is not captured. The observed butterfly pitch angle distributions of electron fluxes in the initial loss phase are well reproduced by the model. The initial proton losses at low pitch angles are underestimated, potentially also contributed by other mechanisms such as field line curvature scattering. 

Abstract In the inner magnetosphere, fast magnetosonic waves (MS waves) are known to resonantly interact with ring current protons, causing these protons to gain energy preferentially in the direction perpendicular to the background magnetic field. An anisotropic distribution of enhanced ring current protons is a necessary condition to excite electromagnetic ion cyclotron (EMIC) waves which are known to facilitate a rapid depletion of ultra‐relativistic electrons in the outer radiation belt. So, when a simultaneous observation of high‐frequency EMIC (HFEMIC) waves, anisotropic low‐energy protons, and MS waves was first reported, a chain of energy flow from MS waves to HFEMIC waves through proton heating was naturally proposed. In this study, we carry out a statistical analysis using Van Allen Probes data to provide deeper insights into this energy pathway. Our results show that the occurrence of HFEMIC waves exhibits good correlation with the enhanced flux and anisotropy of low‐energy protons, but the correlation between the low‐energy protons and the concurrent MS waves is rather poor. The latter result is given support by quasilinear diffusion analysis, indicating negligible momentum diffusion rates at sub‐keV energies, unless MS wave frequency gets very close to the proton cyclotron frequency (which constitutes only a small number of the cases). The fact that the first chain of the coupling is statistically inconclusive calls for an alternative explanation for the major source of the low‐energy anisotropic proton population in the inner magnetosphere. 

Abstract Electromagnetic ion cyclotron (EMIC) waves can drive radiation belt depletion and Low‐Earth Orbit satellites can detect the resulting electron and proton precipitation. The ELFIN (Electron Losses and Fields InvestigatioN) CubeSats provide an excellent opportunity to study the properties of EMIC‐driven electron precipitation with much higher energy and pitch‐angle resolution than previously allowed. We collect EMIC‐driven electron precipitation events from ELFIN observations and use POES (Polar Orbiting Environmental Satellites) to search for 10s–100s keV proton precipitation nearby as a proxy of EMIC wave activity. Electron precipitation mainly occurs on localized radial scales (∼0.3 L), over 15–24 MLT and 5–8 L shells, stronger at ∼MeV energies and weaker down to ∼100–200 keV. Additionally, the observed loss cone pitch‐angle distribution agrees with quasilinear predictions at ≳250 keV (more filled loss cone with increasing energy), while additional mechanisms are needed to explain the observed low‐energy precipitation. 

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 within situobservations. 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. 

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.