The structure of the magnetic reconnection exhaust boundary

Abstract

The structure of shocks that form at the exhaust boundaries during collisionless reconnection of anti-parallel fields is studied using particle-in-cell (PIC) simulations and modeling based on the anisotropic magnetohydrodynamic equations. Large-scale PIC simulations of reconnection and companion Riemann simulations of shock development demonstrate that the pressure anisotropy produced by counterstreaming ions within the exhaust prevents the development of classical Petschek switch-off-slow shocks (SSS). The shock structure that does develop is controlled by the firehose stability parameter ɛ=1-μ0(P‖-P⊥)/B2 through its influence on the speed order of the intermediate and slow waves. Here, P‖ and P⊥ are the pressure parallel and perpendicular to the local magnetic field. The exhaust boundary is made up of a series of two shocks and a rotational wave. The first shock takes ɛ from unity upstream to a plateau of 0.25 downstream. The condition ɛ=0.25 is special because at this value, the speeds of nonlinear slow and intermediate waves are degenerate. The second slow shock leaves ɛ=0.25 unchanged but further reduces the amplitude of the reconnecting magnetic field. Finally, in the core of the exhaust, ɛ drops further and the transition is completed by a rotation of the reconnecting field into the out-of-plane direction. The acceleration of the exhaust takes place across the two slow shocks but not during the final rotation. The result is that the outflow speed falls below that expected from the Walén condition based on the asymptotic magnetic field. A simple analytic expression is given for the critical value of ɛ within the exhaust below which SSSs no longer bound the reconnection outflow.