Electron Precipitations from the Flow-Breaking Region: Can Whistler-Mode Waves Nonlinearly Resonate with Plasma Sheet Electrons?

Energetic electron precipitations from the flow-breaking region—the transition between the outer radiation belt and the plasma sheet—are typically characterized by dispersive signatures consistent with scattering via field-line curvature. However, during intervals of injections and magnetic field dipolarizations, low-altitude spacecraft have observed precipitation patterns more characteristic of whistler-mode scattering. In this study, we analyze examples of such precipitation patterns collected by the ELFIN CubeSat in conjunction with equatorial observations from the THEMIS mission. We show that whistler-mode-driven precipitation can include sub-second bursts, which are usually associated with nonlinear resonant scattering of electrons by chorus waves. Based on ELFIN and THEMIS observations, we discuss the possibility of nonlinear resonant electron scattering occurring in the flow-breaking region.