Capsule Electron Distributions Near the Diffusion Region of Magnetic Reconnection

Abstract

Understanding electron kinetic processes is crucial for elucidating the energy conversion mechanisms in magnetic reconnection. Non-Maxwellian electron distributions are strong indicators of kinetic-scale processes near the electron diffusion region, yet they remain incompletely understood. Using in-situ spacecraft data from 29 magnetopause reconnection events, we unambiguously identify a non-Maxwellian capsule electron distribution near the electron diffusion region. This distribution comprises an elongated component parallel with the magnetic field at lower energies and a butterfly component (with peaks at pitch angles near $45°$ and $135°$) at higher energies. We provide evidence that these distributions are partly linked to electron trapping and preferential heating along the direction of magnetic fields. The parallel electric potentials needed for the parallel heating may be linked to kinetic Alfvén waves. These capsule-like electron distributions are also found to generate whistler emissions. Our results suggest that these kinetic processes are prevalent in magnetic reconnection.