近地磁尾方位角流期间的场向电流增强

以往研究表明，地向高速流在近地磁尾可演化为方位角流，电离层内的方位角流和极光膨胀活动发生时，通过磁力线追踪到近地磁尾等离子体片的对应部分通常为地向和方位角高速流.通过对2016-2018年THEMIS卫星计划中THA，THD，THE三颗卫星同时观测到的数据进行分析、甄别后，在筛选出的62个事件中挑选一个典型的方位角流事件，与过去单颗卫星在不同时间段内的观测数据统计平均给出的结果进行对比分析发现，三颗卫星同时观测到的数据与过去单颗卫星在不同时间段内观测数据的统计平均结果存在较大差异.通过比较此事件期间等离子体流在xy平面的流场图发现，场向电流的大小与三颗卫星同时观测到的流场剪切度有较好的相关性.研究结果表明，方位角流期间近地磁尾和电离层通过场向电流耦合时，场向电流并不是在一个较宽的磁地方时内均匀分布，而是在一个局域化区域及较短的时间内产生强场向电流，这可能是由于方位角流在xy平面内的强剪切而造成的. 

Enhancement of Field-aligned Current during the Azimuthal Flow in the Near-Earth Magnetotail

Past researches have shown that the high-speed flow can evolve into azimuth flow at the near-Earth magnetotail. The magnetospheric counterpart of azimuthal flow and auroral expansion activities in the ionosphere commonly showed features of fast azimuthal and earthward flows in the near-Earth plasma sheet. There are 62 azimuthal flow events when three satellites (THA, THD, THE) of THEMIS were located in the magnetotail from 2016 to 2018, one typical case is selected to study in detail after further investigating of these events. Their relative position and simultaneous observation provide us a good opportunity to further calculate the azimuthal pressure gradient and the radial and the field-aligned current. Our results show that the calculation based on the simultaneous observations of the three satellites are quite different from that obtained by the previous observation of a single satellite in different periods:the value of the radial current from joint observation is much smaller and the average value is only about 15% of the previously single satellite at different times; while the value of the field-aligned current from joint observation exceeds that of single satellite for about 1/3 periods of this event, the peak value is even about one order of magnitude larger than that of a single satellite. Thermal pressure does not vary linearly with azimuth angle but changes sharply within a small azimuthal range. By furtherly investigating the flow field in the xy plane during part periods of this azimuthal flow event, it is found that the magnitude of the field-aligned current had a good correlation with the flow shear simultaneously observed by the three satellites. When the near-Earth magnetotail and the ionosphere are coupled by field-aligned current during the azimuthal flow event, our results show that the field-aligned current is not evenly distributed in wide magnetic local time, but is enhanced shortly in a localized region which is possibly due to strong shear of the azimuthal flow in the xy plane.