垂直磁重联平面的驱动流对磁岛链影响的模拟

Effect of out-of-plane driving flow on formation of plasmoids in current sheet system

近20年来,大量的磁岛链现象从空间、天体物理到磁约束实验室等离子体中被观察到,并且有关磁岛链现象的许多物理特性可以直接被计算机模拟结果所证实.磁岛链理论在磁重联理论中的重要进展为快速磁重联的发生机制提供了更加具有说服力的解释.本文采用二维三分量的磁流体力学模型,数值研究了不同宽度和不同强度的垂直平面驱动流对磁重联中磁岛链不稳定性的影响,并分析了导向场和垂直平面的驱动流对磁岛链的共同作用.研究结果表明:垂直平面驱动流的宽度越宽或强度越强,越容易产生磁岛链结构.电流片中的小磁岛个数及重联率随着垂直平面驱动流宽度及强度的增加而增加.另外,导向场会改变重联平面内磁岛链的对称性.相同导向场情况下,驱动流强度越大,小磁岛的增长速度越快.

In the last two decades, a wide variety of plasmoids events have been observed, ranging from space and astrophysical phenomenon to magnetically confined laboratory plasmas, in which there are a lot of evidence of observational plasmoid-like features supported by direct large-scaled computer simulations. A super-Alfvénic instability, named plasmoid instability, occurs in an extended current sheet, when the Lundquist number exceeds a critical value. The large-aspect-ratio current sheet is fragmented by generating, growing, coalescing and ejecting of plasmoids so that this phenomenon has been proposed as a possible mechanism for fast reconnection scenario. This super-Alfvénic plasmoid instability has been usedin the significant new development of reconnection theory, and thus can provide alternative and more convincing mechanism for fast reconnection.In this work, a "driving" kind of shear flow in the out-of-plane direction is imposed on a two-dimensional, threecomponent magnetohydrodynamic model with a current sheet system to study the dynamic process of the plasmoids in a current sheet system. The effect of the width and strength of the driving flow on the reconnection rate of plasmoids are numerically analyzed in detail. It is found that the plasmoids are easily formed in the case of strong and wide out-of-plane driving flow. The reconnection rate and the number of the plasmoids increase with the driving flow width and/or driving flow strength increasing. In the presence of guiding field, it is found that the symmetry of the plasmoids is broken in the reconnection plane. In addition,for the fixed guiding field, the growth rate of plasmoids increases much faster when the strength of driving flow increases.