A physical explanation for the magnetic decrease ahead of dipolarization fronts
- 1UCL Mullard Space Science Laboratory, Dorking, RH5 6NT, UK
- 2Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095-1567, USA
- 3School of Earth and Space Sciences, Peking University, Beijing 100871, China
- 4Space Science Institute, School of Astronautics, Beihang University, Beijing 100191, China
- 5Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Physics, Shandong University, Weihai 264209, China
- 6Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
Abstract. Recent studies have shown that the ambient plasma in the near-Earth magnetotail can be compressed by the arrival of a dipolarization front (DF). In this paper we study the variations in the characteristics of currents flowing in this compressed region ahead of the DF, particularly the changes in the cross-tail current, using observations from the THEMIS satellites. Since we do not know whether the changes in the cross-tail current lead to a field-aligned current formation or just form a current loop in the magnetosphere, we thus use redistribution to represent these changes of local current density. We found that (1) the redistribution of the cross-tail current is a common feature preceding DFs; (2) the redistribution of cross-tail current is caused by plasma pressure gradient ahead of the DF and (3) the resultant net current redistributed by a DF is an order of magnitude smaller than the typical total current associated with a moderate substorm current wedge (SCW). Moreover, our results also suggest that the redistributed current ahead of the DF is closed by currents on the DF itself, forming a closed current loop around peaks in plasma pressure, what is traditionally referred to as a banana current.