A Statistical Study of Magnetopause Boundary Layer Energetic Electron Enhancements Using MMS

The low-latitude boundary layer is the region at the outer edge of the magnetosphere that borders the magnetosheath and contains a mixture of magnetosphere and magnetosheath populations. In this study, we took a survey of boundary layer crossings by the Magnetospheric Multiscale (MMS) mission to analyze the statistical characteristics of the high-energy electron population found within. Out of 250 total crossings, about half showed enhancements of high-energy (>30 keV) electrons in the FEEPS sensor and a little less than half of those energetic electron events had whistler-mode waves present. For almost all of these events, the pitch angles of the FEEPS electrons were peaked at 900 or isotropic, not field-aligned, and almost all events had an elevated velocity moment within a few minutes of the whistler waves, suggesting reconnection nearby. Using test particle simulations, we found that phase trapping from non-linear whistler waves (typically invoked in radiation belt acceleration) could be a viable method of accelerating these electrons within the boundary layer. These results have implications for electron acceleration at other planetary systems.