The effects of strong temperature anisotropy on the kinetic structure of collisionless slow shocks and reconnection exhausts. Part II: Theory

TL;DR

This paper develops a theoretical model explaining how strong temperature anisotropy influences the structure of collisionless slow shocks and reconnection exhausts, revealing a critical anisotropy value linked to mode degeneracy and wave transitions.

Contribution

It introduces an anisotropic fluid theory-based explanation for the critical anisotropy effect, challenging the traditional Petschek model of reconnection outflows.

Findings

Critical anisotropy epsilon=0.25 is linked to mode degeneracy.

Compound slow shock/rotational discontinuity waves bound reconnection outflows.

Rarity of Petschek-like switch-off slow shocks explained by this model.

Abstract

Simulations of collisionless oblique propagating slow shocks have revealed the existence of a transition associated with a critical temperature anisotropy epsilon=1-mu_0(P_parallel-P_perpendicular)/ B^2 = 0.25 (Liu, Drake and Swisdak (2011)). An explanation for this phenomenon is proposed here based on anisotropic fluid theory, in particular the Anisotropic Derivative Nonlinear-Schrodinger-Burgers equation, with an intuitive model of the energy closure for the downstream counter-streaming ions. The anisotropy value of 0.25 is significant because it is closely related to the degeneracy point of the slow and intermediate modes, and corresponds to the lower bound of the coplanar to non-coplanar transition that occurs inside a compound slow shock (SS)/rotational discontinuity (RD) wave. This work implies that it is a pair of compound SS/RD waves that bound the outflows in magnetic reconnection, instead of a pair of switch-off slow shocks as in Petschek's model. This fact might explain the rareness of in-situ observations of Petschek-reconnection-associated switch-off slow shocks.