宁静期间双峰电流片的观测研究

2001年8月19日2011～2030UT，AE指数相对较小(40～130 nT)，Cluster 卫星穿越磁尾电流片.利用Cluster 观测资料分析，发现宁静期间有双峰电流片存在，这期间没有明显的高速流，没有明显的电流片振荡. 同时，进一步分析双峰电流片中的离子特性发现:质子数密度Np在中性线（<Bx> = 0）附近相对较大，呈非对称性分布；质子温度分布比较均匀；在中性线两侧，质子y方向上的流速Vy方向相反. 最后讨论LHDI（低混杂漂移不稳定性）的非线性演化可能是形成宁静期间双峰电流片的主要机制. 这些对进一步完善电流片形成机制可能有很重要的意义.

Cluster observation and analysis of bifurcated current sheet structure in a quiet time

We analyse the vertical structure of the magnetotail current sheet within 2011 - 2030 UT on 19 August 2001, during which the Cluster spacecrafts crossed the current sheet in a quiet time. In the intervals, the current sheet moved slowly, and the value of the AE index was relatively small, about 40 - 130 nT. In the quiet time, without fast plasma flow and without significant flapping motion, we also directly observed the bifurcated current sheet. Ion density, velocity and temperature, measured by Cluster spacecrafts in the current sheet during 2011 -2030 UT 19 August 2001 are discussed. It is shown that ion density was comparatively bigger near the neutral line, presented non-symmetric distribution, but the distribution of temperature was uniform. And it is observed that in the y direction, the proton bulk velocity is opposite on the both sides of the neutral line. Finally we discuss a possible mechanism for the bifurcated current sheet in a quiet time, which is nonlinear evolution of the lower-hybrid drift instability in the current sheet. These may be important for perfecting the formation mechanism of current sheet bifurcation.