Liu Yi-Hsin, Hesse M, Li T C, Kuznetsova M, Le A
Department of Physics and Astronomy, Dartmouth College, Hanover, NH, USA.
Department of Physics and Technology, University of Bergen, Bergen, Norway.
J Geophys Res Space Phys. 2018 Jun;123(6):4908-4920. doi: 10.1029/2018JA025410. Epub 2018 May 31.
The orientation and stability of the reconnection x line in asymmetric geometry is studied using three-dimensional (3-D) particle-in-cell simulations. We initiate reconnection at the center of a large simulation domain to minimize the boundary effect. The resulting x line has sufficient freedom to develop along an optimal orientation, and it remains laminar. Companion 2-D simulations indicate that this x line orientation maximizes the reconnection rate. The divergence of the nongyrotropic pressure tensor breaks the frozen-in condition, consistent with its 2-D counterpart. We then design 3-D simulations with one dimension being short to fix the x line orientation but long enough to allow the growth of the fastest growing oblique tearing modes. This numerical experiment suggests that reconnection tends to radiate secondary oblique tearing modes if it is externally (globally) forced to proceed along an orientation not favored by the local physics. The development of oblique structure easily leads to turbulence inside small periodic systems.
利用三维粒子模拟研究了不对称几何结构中重联x线的取向和稳定性。我们在一个大模拟域的中心引发重联，以尽量减少边界效应。由此产生的x线有足够的自由度沿最优取向发展，并且保持层流状态。伴随的二维模拟表明，这种x线取向使重联率最大化。非旋转变压力张量的散度打破了冻结条件，这与其二维对应情况一致。然后，我们设计一维较短的三维模拟来固定x线取向，但长度要足够长，以允许最快增长的斜向撕裂模增长。这个数值实验表明，如果重联在外部（全局）被迫沿着局部物理不支持的取向进行，那么它倾向于辐射出二次斜向撕裂模。在小周期系统中，斜向结构的发展很容易导致湍流。
