Sub‐MeV Electron Precipitation Driven by EMIC Waves Through Nonlinear Fractional Resonances

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 order n = 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.

Improved Energy Resolution Measurements of Electron Precipitation Observed During an IPDP‐Type EMIC Event

“…This area of study has been investigated extensively by Denton et al (2019) through numerical simulations, although no definitive mechanism for the generation of peak energies <1 MeV has been identified to date. Hanzelka et al (2023Hanzelka et al ( , 2024 used test particle simulations of fractional sub-cyclotron resonant interactions with EMIC waves to generate sub-MeV electron precipitation consistent with some of the ELFIN cubesat observations described in Capannolo et al (2023). The presence of lower energy precipitation is particularly important when considering the impact of observed EMIC-induced losses on radiation belt populations (e.g., Usanova et al, 2014) and resultant atmospheric ozone decreases (Hendry, Seppälä, et al, 2021).…”

Inclusion of Nonresonant Effects Into Quasi‐Linear Diffusion Rates for Electron Scattering by Electromagnetic Ion Cyclotron Waves

“…While these two mechanisms offer promising perspectives, they do not always work (e.g., Capannolo et al, 2019). The third and fourth mechanisms are more sophisticated: electrons can be scattered below the minimum resonance energy due to (c) nonlinear fractional resonances (Hanzelka et al, 2023(Hanzelka et al, , 2024 or (d) due to nonresonant interactions with EMIC wave packets (An et al, 2022;Chen et al, 2016;Grach & Demekhov, 2023), the primary topic of this paper. To be effective, fractional resonances require high wave intensity.…”