TC-1在近磁尾观测到地向流的偏转

使用TC-1卫星在2004年到2007年磁尾探测数据,将以往高速流的研究拓宽到较低的速度,统计分析其从-13.4RE到-5RE地心距离内的空间演化.研究发现:(1)在向着地球运动的过程中,地向流发生率在日地连线附近减小,但在晨昏两翼的发生率增加,且在黄昏侧的发生率最高;这表明地向流在运动到近地时向着晨昏两翼偏转.(2)越靠近地球,流速V和Vx越小,Vy和Vz的变化幅度较小并且具有明显的晨昏不对称性;所以地向流在近地运动过程中,不仅在晨昏方向上偏转,而且在南北方向上偏转.(3)地向流期间,等离子体密度整体偏小;但是随地心距离的减小,密度整体上逐步增加.(4)平行和垂直于磁场的流速具有明显的晨昏不对称性.在黎明侧的平行流速比黄昏侧大,在黄昏侧的垂直流速比黎明侧大.鉴于较大的垂直流速易触发与电流中断关系密切的不稳定性,我们推测电流中断更容易出现在黄昏侧.(5)除个别位置处的热压和磁压相当外,磁压在总压中一直占据主导地位.日地连线附近的总压较大,晨昏两翼处的总压相对较小;从而在晨昏向上产生较大的压力梯度,导致地向流在晨昏两翼偏转和发生率增大.在晨昏两翼,距离地球较近的位置处观测到了较小的压力;而在日地连线附近,距离地球较远的位置才可以观测到较小的压力;压力分布的这个统计特征说明过去事例研究中电流中断出现在不同的位置可能是由近地磁尾的压力分布造成的.

The deflection of the earthward flow observed by TC-1 in the near magnetotail

Using the data observed by the TC-1 satellite in the magnetotail season from 2004 to 2007, we extended the earthward high-speed flow to the lower speed and statistically investigated its spatial evolution in the geocentric distances between -13.4R_E and -5R_E. Our findings are as follows: (1) When the earthward flow are moving close to the earth, the occurrence rate of the earthward flow decreases mainly around the sun-earth line, the occurrence rate in the dawn and dusk flank does not fall but rise, the highest occurrence rate locates in the dusk flank, which may indicate that the earthward flow is deflecting to the dawn and dusk flank under its process penetrating to the earth. (2) Closer to the earth, the amplitude of V and V_x is smaller; there are slight variations of V_y and V_z and their distributions are dawn-dusk asymmetry; which indicates that the process penetrating to the earth is accompanied by the dawn-dusk and north-south deflection. (3) During the earthward flow, the plasma density is relatively small on the whole; it gradually increases with the decrease of the geocentric distances. (4) The distribution of the parallel and perpendicular flow speed is dawn-dusk asymmetry. The parallel speed in the dawn is larger than that in the dusk. The perpendicular speed in the dusk is larger than that in the dawn. Considering that the large perpendicular flow speed is easy to trigger the instability closely related to the current disruption, we infer that the current disruption occurs easily in the dusk. (5) The magnetic pressure is mostly dominant with few thermal pressures comparable to the magnetic pressure. The total pressure is large near the sun-earth line and small in the dawn and dusk, so large pressure gradient forms both in the dawn and dusk direction; which cause the earthward flow deflect and the occurrence rate increase in the dawn and dusk flank. Lower pressure than the ambient can be observed in the dawn and dusk flank close to the earth, and that can be observed farther away from the earth around the sun-earth line; which suggest that the current disruption occurred at different locations in the past cases may be caused by the pressure distribution close to the earth.