关于磁重联区三维磁零点的卫星观测研究

磁重联是等离子体中磁场能量快速转换为粒子动能和热能的主要途径,是空间物理和等离子体物理中的重要现象.磁重联过程发生在磁场的拓扑分形面上;这种分形面一般是由磁零点附近磁场梯度张量的本征矢量确定的.磁零点的三维特性要求至少空间4点的同时测量.Cluster计划提供了迄今为止唯一的卫星测量手段.基于Poincare指数方法分析Cluster卫星在地球磁尾的探测数据,(1) 找到了磁重联过程中存在4种磁零点的证据;(2) 发现磁零点周围磁场的空间特征尺度大约为离子惯性长度,从而首次揭示霍尔效应可能在三维磁重联中起重要作用;(3) 在重联区找到匹配的磁零点对,并计算出零点连线的长度,确定了所产生的磁场的拓扑分形面;(4) 发现在零点连线附近存在低杂波频率的电磁振荡,这为磁重联过程中可能的电子加速、加热机制提供了观测基础.

Studies of the Three-Dimensional Magnetic Null Based on Sattellite Observations in the Reconnection Region

Magnetic reconnection is considered to be of crucial importance not only to space science but also to magnetically confined fusion,as well as many basic processes in astrophysics and cosmology.In general,except on a closed field line,three-dimensional(3D) magnetic reconnection occurs on the separator connecting magnetic nulls and separatrices,which are determined by the eigenvectors of the magnetic field gradient matrixes of the null pair.Due to the 3D geometric property,the null can only be detected by an at least 4-point measurement so far uniquely provided by the Cluster mission.Here the authors review the recent progresses in applying the Poincare method of magnetic null identification to find out:(1) the in situ evidences of four types of nulls(A,B,As and Bs) and their fans and spines;(2) the spatial scale of the spine structure of the null being of the order of local ion inertial length scale,suggesting that the Hall effect should be important in 3D reconnection dynamics;(3) the in situ evidence for the existence of A-B,B-As null pairs and the separator lines combining the null pairs;and(4) the existence of a lower hybrid oscillation in the immediate vicinity of the A-B separator line,which provides a possible mechanism for electron heating and acceleration in the reconnection process.It is the first in situ evidence for complete 3D reconnection geometry and associated dynamics,a crucial step toward to establish an observational framework of 3D reconnection.