Nonresonant Scattering of Energetic Electrons by Electromagnetic Ion Cyclotron Waves: Spacecraft Observations and Theoretical Framework.

Electromagnetic ion cyclotron (EMIC) waves lead to rapid scattering of relativistic electrons in Earth's radiation belts, due to their large amplitudes relative to other waves that interact with electrons of this energy range. A central feature of electron precipitation driven by EMIC waves is deeply elusive. That is, moderate precipitating fluxes at energies below the minimum resonance energy of EMIC waves occur concurrently with strong precipitating fluxes at resonance energies in low‐altitude spacecraft observations. This paper expands on a previously reported solution to this problem: nonresonant scattering due to wave packets. The quasi‐linear diffusion model is generalized to incorporate nonresonant scattering by a generic wave shape. The diffusion rate decays exponentially away from the resonance, where shorter packets lower decay rates and thus widen the energy range of significant scattering. Using realistic EMIC wave packets from δf particle‐in‐cell simulations, test particle simulations are performed to demonstrate that intense, short packets extend the energy of significant scattering well below the minimum resonance energy, consistent with our theoretical prediction. Finally, the calculated precipitating‐to‐trapped flux ratio of relativistic electrons is compared to ELFIN observations, and the wave power spectra is inferred based on the measured flux ratio. We demonstrate that even with a narrow wave spectrum, short EMIC wave packets can provide moderately intense precipitating fluxes well below the minimum resonance energy. Plain Language Summary: Electromagnetic ion cyclotron (EMIC) waves are one of the most important plasma emissions in the near‐Earth space. When electrons experience an approximately constant EMIC wave phase in gyration, they resonate with these waves and are scattered to precipitate to the Earth's upper atmosphere. Such cyclotron resonance between electrons and EMIC waves are typically above 1 MeV of electron energy. However, spacecraft at low Earth orbit often observe that electrons in the hundreds of keV range, which are not in resonance with EMIC waves, precipitate simultaneous with those >1 MeV. Strongly modulated EMIC wave packets are promising in precipitating the sub‐MeV electrons through nonresonant interactions. Here, the theoretical model of nonresonant scattering is verified for realistic EMIC wave packets from self‐consistent computer simulations. EMIC wave power spectra are inferred from electron precipitation measurements by ELFIN. Short EMIC wave packets are shown to give a better agreement between the theoretical and observed precipitating‐to‐trapped flux ratios. Key Points: The theoretical model of nonresonant scattering is verified for wave packets derived from self‐consistent simulationsShort EMIC wave packets extend the energies of efficient scattering well below the minimum resonance energy, consistent with the theoryEMIC wave power spectra are inferred from ELFIN observations of relativistic electron precipitation, including nonresonant scattering [ABSTRACT FROM AUTHOR]