Distribution of the Projection Position of the Magnetotail FAC in the Northern and Southern Hemisphere Based on Cluster Observations
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摘要: 利用Cluster四颗卫星的磁场探测数据计算磁尾场向电流并投影到极区电离层，研究其投影位置在南北半球的分布规律，统计过程中去除了强磁暴（磁暴主相Dst<–100 nT）期间的场向电流事件。结果显示：磁尾场向电流事件在极区投影位置的纬度分布具有明显的南北半球不对称性，北半球为单峰结构，南半球为双峰结构。在北半球投影到较低纬度（<64°）的场向电流事件数目明显多于南半球，并且所能达到的最低纬度更低；在南半球投影到较高纬度（>74°）的场向电流事件数目明显多于北半球，并且所能达到的最高纬度更高。地磁平静条件下（|AL|<100 nT），磁尾场向电流密度随磁地方时（MLT）呈递增趋势，这一结果与低高度卫星在极区对I区场向电流的探测结果符合很好。研究结果表明，磁尾场向电流投影位置的纬度分布呈现出明显的南北不对称性，这与南北半球磁尾场向电流的空间分布以及磁层中磁场结构具有密切关系。Abstract: Using the magnetic field data of Cluster 4 satellites, the density of Field-Aligned Current (FAC) was calculated and projected to the polar ionosphere, and the distribution of its projected position in the north and south hemispheres was studied. The FAC events during strong magnetic storms are removed from the statistical process (during the main phase of storm Dst<–100 nT). The results showed that: The projection locations of magnetotail FAC events in the polar region are mainly distributed in the Invariant Latitude (ILAT) between 64° and 76°. The distribution in the northern and southern hemispheres is obviously asymmetric, with a single peak structure in the northern hemisphere and a bimodal structure in the southern hemisphere. The number of FAC events that can project to lower latitudes (< 64°) in the northern hemisphere is significantly more than in the southern hemisphere, and can achieve the lowest latitude is lower; The number of FAC events that can project to higher latitudes (>74°) in the southern hemisphere is significantly more than in the northern hemisphere, and can achieve the highest latitude is higher. Distinguish the polarity of FAC, the distribution of earthward FAC and the tailward FAC in the north and south hemisphere is similar. Under quiet geomagnetic conditions (|AL|<100 nT), the current density of FAC in the magnetotail increased with the Magnetic Local Time (MLT) (from 20:00 MLT before midnight to 04:00 MLT after midnight), which was consistent with the previous detection results of Region 1 FAC by low-altitude satellites. The above results show that the latitude distribution of the projected position of the magnetotail FAC presents an obvious north-south asymmetry, which is closely related to the spatial distribution of the magnetotail FAC in the northern and southern hemispheres and the structure of the magnetic field in the magnetosphere. The distribution of the FAC density in the magnetotail with MLT is consistent with the distribution of Region 1 FAC in the polar ionosphere, which provides indirect evidence for the connection between the magnetotail FAC and Region 1 FAC, indicating that the FAC is a large-scale structure from the magnetotail to the ionosphere.
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图 1 Cluster卫星探测到的1839个磁尾场向电流事件在GSM坐标系下xy、xz和yz平面的分布及其在极区的投影（绿色线表示该轨道有可用探测数据，黄色线表示无可用的探测数据）
Figure 1. Distribution of 1839 magnetotail FAC cases detected by the Cluster in GSM coordinates xy, xz and yz planes and their projections in the polar region (Green line indicates that the data is available on the orbit, and the yellow line indicates that no data is available)
图 2 磁尾场向电流事件（在极区投影）数目随不变纬度的变化
Figure 2. Number of FAC in the magnetotail (projection in the polar region) changes with ILAT
图 3 区分南北半球后磁尾场向电流事件（在极区投影）数目随不变纬度的变化
Figure 3. Distinguish the southern hemisphere from the northern hemisphere, the number of FAC in the magnetotail (projection in the polar region) changes with ILAT
图 4 南北半球磁尾地向和尾向场向电流事件（在极区投影）数目随不变纬度的变化
Figure 4. Distinguish the southern hemisphere from the northern hemisphere, the number of earthward (tailward) FAC in the magnetotail (projection in the polar region) changes with ILAT
图 5 |AL|<100 nT条件下磁尾场向电流密度随磁地方时的分布与以往研究中I区（R1）场向电流密度的对比（圆圈附近的数字表示磁尾场向电流事件数目）
Figure 5. Distribution of the FAC density in the magnetotail with MLT is compared with R1 density distribution under the condition |AL|<100 nT （The numbers beside the circles are the numbers of FAC cases in the magnetotail）
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