The Kinetic Structure of Collisionless Slow Shocks and Reconnection Exhausts

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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 various 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 (Pparallel>Pperpendicular) 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-$mu0(Pparallel-Pperpendicular)/B2 tends to plateau at 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.

An explanation for the critical value 0.25 is proposed by examining anisotropic fluid theories, in particular the Anisotropic Derivative Nonlinear-Schrodinger-Burgers equations, 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 transition point in a compound slow shock(SS)/rotational discontinuity(RD) wave. This work implies that it is a pair of compound SS/RD waves that bounds the reconnection outflow, instead of a pair of switch-off slow shocks as in Petschek's model.