On the Nature and Origin of Bipolar Electrostatic Structures in the Earth's Bow Shock

Ivan Y Vasko,Rachel Wang,Stuart D Bale,Anton V Artemyev,Forrest S Mozer

doi:10.3389/fphy.2020.00156

Abstract

We present a statistical analysis of large-amplitude bipolar electrostatic structures measured by the Magnetospheric Multiscale spacecraft in the Earth's bow shock. The analysis is based on 371 large-amplitude bipolar structures collected in nine supercritical quasi-perpendicular Earth's bow shock crossings. We have found that 361 of the bipolar structures have negative electrostatic potentials, and only 10 structures (> 1, a parameter range typical of collisionless shocks in the heliosphere and various astrophysical environments. This analysis indicates that the original mechanism of electron surfing acceleration involving electron phase space holes is not likely to be efficient in realistic collisionless shocks.

Highlights

The Earth’s bow shock is a natural laboratory for probing plasma processes in supercritical collisionless shocks, because the Alfvén Mach number of the solar wind flow typically exceeds the second critical value [1]
Numerical simulations have demonstrated that electrostatic fluctuations and coherent electrostatic structures produced by various instabilities in a shock transition region can result in efficient
The rarity of electron phase space holes among large-amplitude bipolar structures is most likely due to the electron hole transverse instability, the criterion for which is strongly dependent on ωpe/ωce [36, 37]

Summary

INTRODUCTION

The Earth’s bow shock is a natural laboratory for probing plasma processes in supercritical collisionless shocks, because the Alfvén Mach number of the solar wind flow typically exceeds the second critical value [1]. The analysis of spacecraft measurements in the Earth’s bow shock characterized by ωpe/ωce ∼ 100 should advance our understanding of the origin and consequences of electrostatic fluctuations in collisionless shocks under realistic background plasma parameters. Detailed analyses of bipolar structures measured in several crossings of the Earth’s bow shock by Cluster spacecraft [25] and Magnetospheric Multiscale spacecraft (MMS) [26, 27] showed that the bipolar structures had negative electrostatic potentials, which is inconsistent with the interpretation in terms of electron phase space holes. The analysis provides valuable information on instabilities operating in the Earth’s bow shock and reveals the presence, albeit in negligible numbers, of bipolar structures with positive potentials, which could be electron phase space holes. We discuss the implications of our results for the surfing acceleration mechanism, which suggested that electron phase space holes might be involved in efficient electron acceleration in high-Mach-number collisionless shocks [10, 11]

MMS OBSERVATIONS

THEORETICAL INTERPRETATION

DISCUSSION AND CONCLUSION

Findings

DATA AVAILABILITY STATEMENT