Relativistic Electron Precipitation by EMIC Waves: Importance of Nonlinear Resonant Effects

Relativistic electron losses in Earth's radiation belts are usually attributed to electron resonant scattering by electromagnetic waves. One of the most important wave modes for such scattering is the electromagnetic ion cyclotron (EMIC) mode. Within the quasi-linear diffusion framework, the cyclotron resonance of relativistic electrons with EMIC waves results in very fast electron precipitation to the atmosphere. However, wave intensities often exceed the threshold for nonlinear resonant interaction, and such intense EMIC waves have been shown to transport electrons away from the loss cone due to the force bunching effect. In this study we investigate if this transport can block electron precipitation. We combine test particle simulations, low-altitude observations of EMIC-driven electron precipitation by the Electron Losses and Fields Investigations mission, and ground-based EMIC observations. Comparing simulations and observations, we show that, despite the low pitch-angle electrons being transported away from the loss cone, the scattering at higher pitch angles results in the loss cone filling and electron precipitation.

Key Points
The Electron Losses and Fields Investigations observations of electromagnetic ion cyclotron-driven precipitation of relativistic electrons
Test-particle model reproduce precipitating electron energy range
Electron precipitation is caused by interplay of nonlinear resonant effects and the diffusive scattering

Plain Language Summary
Precipitation of relativistic electrons from the Earth's radiation belts to the atmosphere has long been attributed to electron resonant scattering by electromagnetic ion cyclotron (EMIC) waves. These relativistic electron losses significantly contribute to the radiation belt dynamics, whereas precipitating electron fluxes may alter the atmosphere chemical properties. Thus, electron resonant scattering by EMIC waves has been intensively studied, in both theory and spacecraft observations. Models of two main regimes of wave-particle interactions, diffusive scattering and nonlinear resonant transport, however, predict contradicting results for electron precipitation: diffusive scattering will lead to electron losses, whereas nonlinear resonant transport may block such losses. This study combines spacecraft observations and numerical simulations to demonstrate that in reality, these two regimes work together to cause strong electron precipitation that mainly come from higher pitch angles.