Suprathermal electron 90° pitch angle depletions at reverse shocks in the solar wind

摘要

ACE/SWEPAM suprathermal electron pitch angle distributions often show depletions centered on and symmetric about 90 degree pitch angle. It has been suggested that these distributions can arise from a combination of focusing and mirroring of electrons associated with connection to magnetic field enhancements beyond the spacecraft. Assuming adiabatic particle motions, the width of the depletion in pitch angle is predicted to be a function of the strength of the magnetic field enhancement relative to the field at the measurement location. In this work, we test this model of depletion formation by examining depletions upstream from reverse shocks observed by ACE. Roughly symmetric depletions centered on 90 degree pitch angle were observed upstream from each of the 20 reverse shocks encountered by ACE from February 1998 to February 2003. In addition, the widths of the shock-associated depletions are consistent with formation by adiabatic focusing and mirroring of electrons, providing strong support for the validity of this model.