利用GS流场重构方法研究磁尾等离子体片涡流

2000年9月30日Geotail卫星分别于17∶54∶36~18∶09∶00UT和18∶59∶00~19∶30∶00UT在磁尾晨侧等离子体片内(n≈0.4 cm^-3,T≈6 keV)观测到等离子体涡流事件.本文采用Grad-Shafranov (GS)流场重构技术再现了这些涡流的二维速度场、离子数密度和离子温度的分布图像.结果显示:从地心太阳磁层坐标系(GSM)赤道面上面看, 涡流的尺度约为5000 km×1400 km , 朝地球的运动速度约为15~25 km/s.所有5个涡流的旋转方向都为顺时针方向,旋转周期约为6~11 min.相邻涡流的相互作用导致它们之间的磁场强度增强.考察观测数据发现,涡流内不仅包含等离子体片热等离子体成分，也包含较大通量的类似源自磁鞘的冷等离子体成分(T<1 keV).这与观测到涡流等离子体的平均温度(T≈4 keV)较磁尾等离子体片等离子体的典型温度(T≈6 keV)明显偏低的事实是一致的.不仅如此,离子数密度和温度在结构内的分布也不均匀,数密度在涡流内部偏离中心的位置比较低而在每个涡流的边缘位置比较高,温度的分布大体上与密度相反.分析认为观测到的磁尾等离子体涡流事件可能由发生在低纬边界层的Kelvin-Helmholtz不稳定性引起,涡流结构内的冷等离子体可能来自磁层顶外部的磁鞘.

Study of magnetotail plasma sheet vortices with GS velocity field reconstruction method

Geotail observed series of plasma vortices in the dawnside magnetotail plasma-sheet at intervals 17∶54∶36~18∶09∶00UT and 18∶59∶00~19∶30∶00UT on 30 September 2000. The vortices contain hot plasma-sheet plasma as well as cold (T<1 keV) sheath-like component. From Geotail data, we reconstructed the two-dimensional(2-D) maps of velocity field, ion number density and ion temperature for these vortices using Grad-Shafranov (GS) technique. Combined with analysis of vortex plasma features, our results revealed that: the sense of rotation of the plasma flow is approximately clockwise on the GSM equatorial plane. It has a period of about 6~11 min. The vortices have a scale of about 5000 km×1400 km, and move earthward at the speed of 15~25 km/s.Interaction between vortices will cause enhancement of magnetic field between them. But, this is different from the intense disturbances of magnetic field and velocity caused by Kelvin-Helmholtz (K-H) instability at LLBL. Plasma in vortex has lower mean temperature (T≈4 keV) than that of the plasma sheet plasma (T≈6 keV).However, the distribution of both ion number density and temperature is not uniform in the vortex. Ion density is lower at the inner vortex but higher at the edge of each vortex. Temperature has almost an opposite distribution characteristics with that of density. We believe that the observed vortex events were due to the K-H instability occurred at LLBL and the cold plasma in the vortex may come from the magnetosheath.