Ion‐Beam‐Associated Electric‐Field Orientation of Electrostatic Solitary Waves in Magnetotail Earthward Ion Flows

Abstract

We report MMS observations of electrostatic solitary waves (ESWs) in two magnetotail earthward ion‐flow events. In the first event, the ion beam is nearly antiparallel to the background magnetic field, and the associated ESWs exhibit predominantly parallel bipolar electric fields with negligible perpendicular components. In the second event, the ion beam has a significant perpendicular velocity component, and the associated ESWs show bipolar variations in both parallel and perpendicular electric field components. Variance analysis of the three‐component electric field defines the maximum‐variance direction L , which represents the principal electric field orientation of each ESW. The statistical distributions of the L directions show that they are not randomly distributed but are closely aligned with the local ion‐beam direction. These observations reveal a systematic association between the local ion‐beam direction and the electric field orientation of slow electron holes, suggesting that ion beams may play an important role in organizing oblique electrostatic structures in magnetotail earthward ion flows.