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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 We analyze the solar wind (SW) properties associated with relativistic electron precipitation (REP) observed at low‐Earth orbit. A statistical analysis is performed on the SW associated with ∼7,000 REP events likely driven by wave‐particle interactions. We analyze OMNI data to quantify the SW properties prior to REP and reveal temporal patterns that are favorable for electron precipitation. Compared to typical values, SW associated with REP typically exhibits a stronger North‐South interplanetary magnetic fieldB_zcomponent and higher plasma density, indicating dayside reconnection and compression. REP events observed at lowLshell (L ≲ 4), particularly from noon to post‐midnight, are triggered by enhancedB_zand density. Dayside REP is associated with slightly faster SW, whereas dawnside REP coincides with high plasma density without strongB_z. A typical SW trend leading to REP exhibits enhanced density, magnetic field increase, and aB_zminimum before the REP observation. Throughk‐means clustering, we further identify three distinct SW temporal profiles. Dayside REP at high L shells is associated with ∼500 km/s speeds, duskside REP is associated with enhanced SW density prior to REP, and REP over noon‐to‐dusk at low L shells is triggered by strong dayside reconnection. Furthermore, we found that REP events are more frequent during the declining phase of the solar cycle, with a 6‐month periodicity in occurrence rate. The findings in this study are critical to establishing a relationship between SW and REP, enabling future modeling efforts to predict REP from SW observations. 

Abstract We evaluate the diffusive and nonlinear scattering of ring current protons by electromagnetic ion cyclotron (EMIC) waves in the Earth's inner magnetosphere using test particle simulations. EMIC waves are commonly observed inside and outside the plasmasphere with wave amplitudes ranging from 100 pT to several nT. Field‐aligned EMIC waves can scatter 1 keV–1 MeV protons counter‐streaming with respect to the waves through first order cyclotron resonance. Through the analyses of the proton equatorial pitch angle variations along the field line, our simulations reveal the typical interaction features including quasilinear diffusion for small wave amplitudes, phase trapping and bunching at intermediate and large pitch angles, anomalous phase trapping and positive phase bunching at small pitch angles, and non‐resonant scattering at pitch angles and energies outside the resonance regime. Using different wave amplitudes from 100 pT to 5 nT, we compared the modeling results of proton equatorial pitch angle variations between quasilinear and test particle simulations, and between diffusive scattering and advective effects. For monochromatic He‐band EMIC waves atL = 5, the interaction between protons and EMIC waves with amplitudes below 500 pT could be described as a diffusive process and quantified by quasilinear theory; nonlinear interactions and advection effects become important for wave amplitudes larger than 1 nT. The interactions between EMIC waves and ring current protons are analogous to the interactions between whistler‐mode chorus waves and radiation belt electrons described in previous studies, despite the quantitative differences in the wave amplitude threshold of quasilinear diffusion applicability.