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arXiv 1111.7039physics.plasm-phastro-ph.EPastro-ph.SR
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磁重联排气边界的结构

The structure of the magnetic reconnection exhaust boundary

Yi-Hsin Liu, J. F. Drake, M. Swisdak

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13 pages, 5 figures
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摘要：

采用基于各向异性磁流体动力学方程的粒子模拟和建模方法，研究了反平行场无碰撞重联过程中排气边界处形成的激波结构。大规模的PIC模拟的重联和同伴黎曼模拟的冲击发展表明，排气中的逆流离子产生的压力各向异性，防止经典的Petschek关闭慢冲击（SSS）的发展。激波结构的发展是由水龙带稳定性参数λ =1-μ_0（P_parallel-P_vertical）/ B^2通过它对中间波和慢波速度顺序的影响来控制的。这里P_parallel和P_vertical是平行于和垂直于局部磁场的压力。排气边界是由一系列的两个冲击和旋转波。第一个激波从上游的1 °到下游的0.25 °的平台。条件λ =0.25是特殊的，因为在这个值处，非线性慢波和中间波的速度是退化的。第二次慢激波使θ =0.25不变，但进一步减小了重连磁场的幅度。最后，在排气的核心处，电子密度进一步下降，并且通过重连场旋转到平面外的方向来完成过渡。排气的加速发生在两个慢冲击之间，但不是在最后的旋转期间。结果是流出速度低于基于渐近磁场的Walen条件的预期。一个简单的解析表达式给出的临界值内的排气SSSs不再约束的重联流出。

英文摘要：

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.

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