Mr. Yi-Hsin Liu
University of Maryland
The kinetic structure of collisionless slow shocks
A 2-D Riemann problem in Particle-In-Cell simulations is designed to study the development and dynamics of the slow shocks that are thought to form at the boundaries of reconnection exhausts. Unanswered questions remain concerning the kinetic structure of such shocks in a collisionless plasma and the associated mechanisms leading to particle heating (such as suggested in the context of solar flares and the solar wind). 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|| > P⊥) 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 ε = 1 - μ0(P|| - P⊥) / B2 tends to lock into 0.25. This balance arises from the competition between counterstreaming ions, which drives ε 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-Schrödinger-Burgers equations, with an intuitive model of the energy closure for the downstream counterstreaming ions. The anisotropy value of 0.25 is significant because it is closely related to the location where the slow and intermediate waves propagate at the same speed, and corresponds to the lower bound of the coplanar to non-coplanr 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 reconnection outflow, instead of a pair of switch-off slow shocks as in Petschek's model. This fact might explain the rareness of the in-situ observation of Petschek-reconnection-associated slow shocks.