On the relationship between Electrostatic Solitary Waves and Electron-Scale Current Sheets within the Central Plasma Sheet of the Earth's Magnetotail

We report MMS observations of typically unipolar Electrostatic Solitary Waves (ESWs) with amplitudes up to 100 mV/m in the Central Plasma Sheet (CPS) ($|B_x|<5$ nT) of the Earth’s magnetotail, outside the primary magnetic reconnection region during the intervals of Bursty Bulk Flows (BBFs). The generation of ESWs leads to a parallel electron temperature drop of up to 15\%, which is of the order of electric potential variations of the ESW. The observed ESWs are responsible for extreme energy conversion rates, up to 2.5 nW/m$^3$. We revealed that ESWs are typically observed within strong, field-aligned Electron-scale Current Sheets (ECSs) generated by suprathermal electron beams accelerated in remote sources. The generation of the observed ESWs could be due to ion/electron acoustic instabilities. We assume that the observed ESWs mark the secondary separatrices of the secondary reconnections and provide an energy transfer from suprathermal to cold electrons in collisionless plasma.