Relativistic electron scattering by high-frequency EMIC waves: Test-particle results using self-consistently generated wave fields

Theory and simulations predict that the resonant energy of radiation belt electrons interacting with electromagnetic ion cyclotron (EMIC) waves is typically $\gtrsim 1$ MeV, while experimental results suggest that EMIC waves should also be responsible for sub-MeV electron precipitation. The recent report of narrowband ($\Delta f\lesssim 0.1f_{\mathrm{cp}}$), high-frequency ($0.9f_{\mathrm{cp}}\lesssim f<f_{\mathrm{cp}}$) EMIC waves—HFEMIC waves for short—suggests the possibility of sub-MeV electron scattering by these waves owing to their large wave frequency ($f_{\mathrm{cp}}$ denoting the proton cyclotron frequency). To investigate this more quantitatively, we trace test electrons in the HFEMIC wave fields self-consistently generated from a hybrid simulation, and examine the advective and diffusive behaviors of the resultant interactions. Even though HFEMIC waves primarily occur in low density regions outside the plasmapause, the present results show that the wavenumber is large enough to reduce the resonant energy down to as low as several 100s keV. The resulting pitch angle scattering is primarily diffusive for the typical amplitude of HFEMIC waves, but the nonlinear effects also emerge when interacting with the largest-amplitude waves found in the magnetosphere. While the quasilinear theory correctly predicts the overall level of the pitch angle diffusion coefficient, the bounding energy is sensitively dependent on the dispersion relation used (and thus on the warm plasma effect). We expect that low-altitude measurements of energetic electron precipitation can distinguish its characteristic scattering signature, despite the low occurrence nature of HFEMIC waves.