Magnetospheric Multiscale Observations of the Source Region of Energetic Electron Microinjections Along the Duskside, High-Latitude Magnetopause Boundary Layer

The present paper demonstrates the first observations by the Magnetospheric Multiscale (MMS) mission of the counter-streaming energetic electrons and trapped energetic protons, localized in the magnetic field depressions between the mirror mode peaks, in the Earth's dusk sector high-latitude magnetosphere. This region is characterized by high plasma beta, strong ion temperature anisotropy, and intermediate plasma density between magnetospheric and magnetosheath plasma. We show that these plasma conditions are unstable for the drift mirror instability. The counter-streaming electron feature resembles those of the previously reported energetic electron microinjections, but without the energy-time dispersion signature. This suggests that MMS is passing through one of the potential microinjection source regions. The energetic ion data in the present study is mainly used to estimate the scale size of the mirror mode structures. Plain Language Summary Understanding the physical mechanisms that result in energetic electron acceleration and loss within the Van Allen radiation belts has been an active area of research for decades, and due to advances made possible by the Van Allen probe mission are now relatively well understood. However, the origin of the several 10-100s of keV seed population that can be accelerated to relativistic energies has remained more elusive. It is well known that magnetic reconnection and related secondary processes in the Earth's magnetotail during substorms can accelerate particles and inject them inward toward the radiation belts. In this paper we show the first observations of a possible source region of 10-100s of keV electrons and protons at the dayside of the Earth's high-latitude magnetosphere. Four MMS spacecraft periodically encountered high fluxes of energetic electrons at wide energy range which were streaming both parallel and anti-parallel to the magnetic field. Enhanced fluxes of counter-streaming energetic electrons and trapped protons were observed between magnetic field peaks of the ULF waves identified as mirror mode peaks. The source region of these electrons and protons are likely the large diamagnetic cavities created by magnetic reconnection. Key Points MMS detected dispersionless electron microinjections in ULF magnetic field wave depressions at the high-latitude magnetosphere ULF waves were consistent with mirror mode waves created by the drift mirror instability

Automated summary

This study demonstrates the first observations of counter-streaming energetic electrons and trapped energetic protons in the high-latitude magnetosphere, likely originating from large diamagnetic cavities created by magnetic reconnection.