Kelvin-Helmholtz vortices in the magneotail-flank

Kelvin-Helmholtz涡是磁尾翼边界大尺度等离子体输运的候选因子。为了理解MHD尺度KH涡旋在非线性区域的性质，我们进行了包括有限电子惯性的二维数值模拟。假设存在非零的平面内磁场，当稳定磁场张力较弱时，形成很好的卷起的涡旋，并且磁力线被拉伸以在涡旋内形成反平行的几何形状。然后触发重新连接。这一过程已经在电阻MHD方法中得到了研究，但是，本文首次从跨尺度耦合的角度证明了这一过程。另一方面，当张力较强时，磁场分量的符号是否在剪切层上发生变化，其结果会有很大的不同。(1)当磁分量在剪切层上不改变符号时，不稳定性的影响只是使剪切层加宽。(2)当磁分量在剪切层上改变符号时，已经处于不稳定性增长的早期阶段，部分电流片被流动箍缩，发生重联。重联产生的流动有助于涡的增长，在这种情况下得到的涡的尺寸比没有重联时大得多。111更有趣的是，我们发现强磁张力和反平行几何的组合是产生大尺度等离子体混合的最佳条件。

The Kelvin-Helmholtz (KH) vortices are a candidate agent for large-scale plasma transport across the magnetotail flank boundary. To understand the nature of MHD-scale KH vortices in non-llinear regimes, we have performed two-dimensional numerical simulations including finite electron inertia. Non-zero in-plan magnetic field is assumed to be present.When the stabilizing magnetic tension is weak, nicely rolled-up vortices are formed and field lines are so stretched to form an anti-parallel geometry within the vortices. Then reconnection is triggered. Such a process has been studied in resistive MHD approaches, however, this study is the first to prove the process from the cross-scale coupling point of view.On the other hand, when the tension is strong, the results are drastically dofferent depending on whether the sign of the magnetic field component changes across the shear layer or not.(1) When the magnetic component does not change sign across the shear layer, the effects of the instability is only to broaden the shear layer.(2) When the magnetic component changes sign across the shear layer, already in the early phase of the instability growth, a part of the current sheet is pinched by the flow and reconnection takes place. Then the flow produced by reconnection assists the vortex to grow and the size of the vortex obtained in this situation I smuch larger than the one without reconnection. What is more, the large vortex is filled with mixed plasma, that is, large scale plasma mixing across the boundary is attained in this situationWhat is even more interesting, we have found that the combination of strong magnetic tension and anti-parallel geometry is the best for creating large scale plasma mixing across the velocity shear layer.