Yi-Hsin Liu, J. F. Drake, M. Swisdak
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 epsilon=1-mu_0(P_parallel-P_perpendicular)/ B^2 through its influence
on the speed order of the intermediate and slow waves. Here P_parallel and
P_perpendicular 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 epsilon from unity upstream to a plateau
of 0.25 downstream. The condition epsilon =0.25 is special because at this
value the speeds of nonlinear slow and intermediate waves are degenerate. The
second slow shock leaves epsilon=0.25 unchanged but further reduces the
amplitude of the reconnecting magnetic field. Finally, in the core of the
exhaust epsilon 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 Walen condition based on the asymptotic magnetic field. A simple analytic
expression is given for the critical value of epsilon within the exhaust below
which SSSs no longer bound the reconnection outflow.
View original:
http://arxiv.org/abs/1111.7039
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