On the Formation of Super-Alfv\'enic Flows Downstream of Collisionless Shocks

TL;DR

This paper demonstrates that firehose-unstable fluctuations and compressive heating drive super-Alfvénic flows downstream of collisionless shocks, with in situ MMS data supporting the kinetic process's role in jet formation.

Contribution

It provides a theoretical and observational analysis showing how firehose instability accelerates plasma flows downstream of shocks, highlighting kinetic effects in magnetosheath jet formation.

Findings

Super-Alfvénic jets are driven by firehose instability and compressive heating.

Approximately 11% of plasma measurements in jets show firehose-unstable fluctuations.

Downstream flow can be accelerated by a factor of 2 to 4 due to these processes.

Abstract

Super-Alfv\'enic jets, with kinetic energy densities significantly exceeding that of the solar wind, are commonly generated downstream of Earth's bow shock under both high and low beta plasma conditions. In this study, we present theoretical evidence that these enhanced kinetic energy flows are driven by firehose-unstable fluctuations and compressive heating within collisionless plasma environments. Using a fluid formalism that incorporates pressure anisotropy, we estimate that the downstream flow of a collisionless plasma shock can be accelerated by a factor of 2 to 4 following the compression and saturation of firehose instability. By analyzing quasi-parallel magnetosheath jets observed in situ by the Magnetospheric Multiscale (MMS) mission, we find that approximately 11\% of plasma measurements within these jets exhibit firehose-unstable fluctuations. Our findings offer an explanation for the distinctive generation of fast downstream flows in both low ($\beta<1$) and high ($\beta>1$) beta plasmas, and provide new evidence that kinetic processes are crucial for accurately describing the formation and evolution of magnetosheath jets.