The effects of strong temperature anisotropy on the kinetic structure of collisionless slow shocks and reconnection exhausts. Part I: PIC simulations

TL;DR

This paper uses PIC simulations to explore how strong temperature anisotropy influences the structure of collisionless slow shocks and reconnection exhausts, revealing transitions in shock modes and downstream turbulence.

Contribution

It introduces a detailed simulation study of the impact of temperature anisotropy on shock structures and downstream turbulence in collisionless plasmas.

Findings

Large firehose-sense temperature anisotropy downstream

Transition from coplanar slow shock to rotational mode

Downstream epsilon parameter tends to 0.25

Abstract

A 2-D Riemann problem is designed to study the development and dynamics of the slow shocks that are thought to form at the boundaries of reconnection exhausts. Simulations are carried out for varying ratios of normal magnetic field to the transverse upstream magnetic field (i.e., propagation angle with respect to the upstream magnetic field). When the angle is sufficiently oblique, the simulations reveal a large firehose-sense (P_parallel>P_perpendicular) temperature anisotropy in the downstream region, accompanied by a transition from a coplanar slow shock to a non-coplanar rotational mode. In the downstream region the firehose stability parameter epsilon=1-mu_0(P_parallel-P_perpendicular)/ B^2 tends to lock in to 0.25. This balance arises from the competition between counterstreaming ions, which drives epsilon down, and the scattering due to ion inertial scale waves, which are driven unstable by the downstream rotational wave. At very oblique propagating angles, 2-D turbulence also develops in the downstream region.