Sub-MeV Electron Precipitation Driven by EMIC Waves in Plasmaspheric Plumes at High L Shells

Electromagnetic ion cyclotron (EMIC) waves are known to be efficient for precipitating >1 MeV electrons from the magnetosphere into the upper atmosphere. Despite considerable evidence showing that EMIC-driven electron precipitation can extend down to sub-MeV energies, the precise physical mechanism driving sub-MeV electron precipitation remains an active area of investigation. In this study, we present an electron precipitation event observed by ELFIN CubeSats on 11 January 2022, exclusively at sub-MeV energy at L ∼ 8–10.5, where trapped MeV electrons were nearly absent. The THEMIS satellites observed conjugate H-band and He-band EMIC waves and hiss waves in plasmaspheric plumes near the magnetic equator. Quasi-linear diffusion results demonstrate that the observed He-band EMIC waves, with a high ratio of plasma to electron cyclotron frequency, can drive electron precipitation down to ∼400 keV. Our findings suggest that exclusive sub-MeV precipitation (without concurrent MeV precipitation) can be associated with EMIC waves, especially in the plume region at high L shells.