Distribution of the Projection Position of the Magnetotail FAC in the Northern and Southern Hemisphere Based on Cluster Observations

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.