Electron Acceleration by Structured Parallel Electric Field Within a Reconnecting Current Sheet in the Turbulent Magnetosheath

Abstract

Turbulent energy dissipation and the resulting plasma heating are thought to be important in various space and astrophysical systems. In this study, we present in situ evidence of electron acceleration by a localized parallel electric field in the turbulent magnetosheath, the region downstream of the Earth's bow shock. The observed profile of the parallel electric field presents a near unipolar spike, with numbers of irregular bipolar signals superimposed. This structured parallel electric field is generated in an ion-scale reconnecting current sheet, concurrent with observations of field-aligned bidirectional electron beams, large energy dissipation, and significant electron parallel heating, indicating a link between them. The electrons are demonstrated to be trapped in this current sheet and accelerated directly by a parallel electric field. Our results reveal that flat-top electron distributions, which are commonly observed in the magnetosheath, do not have to originate from the shock acceleration but can also be generated by a localized acceleration potential. Such an acceleration potential is common in reconnecting current sheets, and its contribution to electron heating can be significant in a turbulent system populated by numerous current sheets.