Triggering mechanism of magnetic reconnection in a collisionless plasma

用三维电磁粒子模拟方法研究了无碰撞等离子体中磁重联的触发机制。主要结果如下：a.在无碰撞等离子体中，存在两种类型的触发机制，它们打破了磁场的冻结状态，导致了磁重联。一种是由于波粒相互作用，这是电流片中异常电阻率的一个原因。另一种是由于粒子动力学效应，这种效应在粒子尺度上变得非常明显，如电子无碰撞皮肤深度和离子拉莫尔半径。B.在没有任何驱动源的情况下，电流层中存在两种等离子体不稳定性，即低混合漂移不稳定性(LHDI)和漂移扭结不稳定性(DKI)。LHDI生长在电流层的外围，但不能穿透到中性层附近的高贝塔区域。由于LHDI对电流片的非线性变形，DKI在中性片附近后期被激发，并可能是电阻率反常的原因，导致中性片上的无碰撞重联。C.当存在外部驱动电场时，对流电场通过粒子动力学效应穿透到电流层，在DKI被激发之前，对流电场触发了无碰撞重联。由于收敛的等离子体流对电流层的压缩，DKI的生长速率比无驱动场情况下更大。D.DKI通过反常离子加热形成各向异性的离子分布。

Triggering mechanism of magnetic reconnection in a collisionless plasma is investigated by means of three-dimensional electromagnetic particle simulation. Main results are summarized as follows.A. There exist two types of triggering mechanisms which break the frozen-in condition of magnetic field and lead to magnetic reconnection in a collisionless plasma. One is due to the wave-particle interaction which is a cause of anomalous resistivity in the current sheet. The other is due to the particle kinetic effect which becomes significant in a particle scale such as an electron collisionless skin depth and an ion Larmor radius.B. When there is not any driving source, two kinds of plasma instabilities grow in the current layer, I.e., the lower hybrid drift instability (LHDI) and the drift kink instability (DKI) instability. The LHDI grows in the periphery of the current layer, but it can not penetrate into a high beta region in the vicinity of the neutral sheet. The DKI is excited near the neutral sheet in a late period as a result of the nonlinear deformation of the current sheet by the LHDI, and can be a cause of anomalous resistivity, leading to collisionless reconnection at the neutral sheet.C. When an external driving field exists, the convective electric field penetrates into the current layer through the particle kinetic effect and collisionless reconnection is triggered by the convective electric field earlier than the DKI is excited. The growth rate of the DKI becomes larger compared with no driving field case, due to the compression of current layer by the convergent plasma flow.D. The anisotropic ion distribution is formed through the anomalous ion heating by the DKI.