Multi-point Analysis On Magnetotail Dynamics And Storm Time Ring Current Distribution

The analysis methods based on single point measurement cannot distinguish the variation between temporal and spatial. Recently, the successful launch of multi-point satellite missions, such as Cluster, THEMIS, SWARM and future MMS, provide a good opportunity to solve this problem. In this study, with Cluster and THEMIS 3 to 4 point analysis methods, we strive to figure out the magnetotail flux evolution during substorm and storm, the fine structure of magnetotail flux, storm time ring current distribution and its effect on the geometry of magnetic field lines. The main methods and conclusions are as follows: 1. With the newly developed magnetopause model, we have given the magnetotailstructure parameter and then calculated the magnetotail flux when four Clustersatellites position in the tail lobe region. The magnetotail flux variation duringsubstorm and storm has been investigated. The study shows that, during the growth phase, the magnetotail flux keeps increasing to the maximum value and then starts to decrease, which correspond to the expansion or recovery phase. The substorm and storm activities greatly depend on the value of magnetotail flux.Generally, there is a larger activity when the magnetotail flux is higher, indicating more energy entrance to the magnetosphere. Especially, the magnetotail flux becomes much higher during strong storm events. This work has an important role on quantitative analysis and monitoring of magnetotail energy storage, expansion and release process during substorm and storm. 2. During the year of 2001 to 2004, 13 magnetotail flux ropes(MFRs) crossing events are selected from Cluster to investigate their force-free features(the kind of magnetic field structure satisfying J′ B=0). It is showed that the magnetic field gradient is rather weak near the axis center, where the curvature radius and current density is large. The stronger current density or the lower plasma beta values within a MFR favor the formation of force-free field(in agreement with the theoretic results). The quasi force-free region is generally found to be embedded in the central portion of the MFRs, where the current is approximately field-aligned and proportional to the strength of core field. It is shown that, ~60% of surveyed MFRs can be globally approximated as force-free. The force-free factor is found to be non-constantly varied through the quasi force-free MFR, suggesting that the force-free structure is non-linear. 3. We select 48 storm time Cluster crossing events to investigate the variation of the current density distribution and magnetic configuration of the ring current around4RE. For the first time, it is quantitatively confirmed the distortion of magneticfield lines(MFLs) during magnetic storms. The radius of curvature of the MFLs is found to be increasingly reduced at all local times with the increasing intensity of the storm activities. Especially, the curvature radius of MFLs in the RC region during the severe storm periods can be as small as only a third of that at the quiet time. It is noted that, during storm activities, the radius of curvature of the local MFLs, is smallest on the nightside to duskside, medium on the dawnside, and largest on the dayside. This change in geometry may have significant influence on the particle dynamic process in the plasmasphere, ring current, and radiation belts. 4. Taking advantage of the orbit and small separations of the three THEMIS satellites, for the first time, the equatorial current distribution for the region of4-12 ER in the inner magnetosphere(except dawnside) during magnetic storms is directly obtained from the observation. Statistical study reveals the following global radial distribution features of the equatorial current: within r<4.8 RE(wherer is the geocentric radial distance), the current is mainly eastward; at r=5-7.5 RE,the current is predominantly westward, decreases with r and shows some magnetic local time(MLT) dependence, and beyond r=7.5 RE, the westward current is nearly constant. The results also suggest that the radius that demarcates the shift from eastward to westward current moves towards the Earth with increasing storm activity. During weak storms, the eastward current can extend to beyond r=5 RE.This study is a fundamental for future analysis of storm time ring current distribution and mechanism.