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.