A Statistical MMS Survey of Properties and Origin of Electrostatic Solitary Waves in Quasi-perpendicular Earth's Bow Shocks

We performed a statistical analysis of more than 1200 electrostatic bipolar structures measured by Magnetospheric Multiscale spacecrafts in 9 quasi-perpendicular Earth's bow shock crossings, ranging in amplitude from 10 mV/m up to 800 mV/m. The bipolar structures propagate highly oblique to the shock normal with velocities on the order of ion-acoustic velocity and have spatial scales of a few Debye lengths. Through interferometry analysis, we find that the majority of the bipolar structures have negative electrostatic potentials, and about 3% have positive potentials. Our analysis leads to the interpretation that negative potential structures are ion phase space holes produced by ion two-stream instability between incoming and reflected ions near the ramp of the shock, while positive potential structures are either electron holes or ion-acoustic solitons with the latter interpretation being more plausible. We present a summary of the physical natures of bipolar structures and discuss how these results are in agreement with this theory on their generation mechanism.