On the generation of magnetic dips ahead of advancing dipolarization fronts Dong-Xiao Pan1, Xu-Zhi Zhou1(xzzhou@pku.edu.cn), Quan-Qi Shi2, Jiang Liu3, Vassilis Angelopoulos3, Andrei Runov3, Qiu-Gang Zong1, and Sui-Yan Fu1
1School of Earth and Space Sciences, Peking University, Beijing, China 2School of Space Sciences and Physics, Shandong University at Weihai, Weihai, China 3Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, California, USA
Dipolarizing flux bundles transport magnetic flux to the inner and dayside magnetosphere, heat the plasma sheet, and provide a seed population to the radiation belt. The magnetic perturbation ahead of them, often referred to as a dipolarization front (DF), is asymmetric with a small Bz dip followed by a sharp Bz enhancement. The Bz dip is thought to be generated from dawnward currents carried by DF-reflected ions; after reflection, these earthward moving ions gyrate clockwise and contribute to dawnward diamagnetic currents ahead of the front. Using observations of hundreds of DFs, we investigate this hypothesis. We find that the depth of the Bz dip as a function of the front azimuth depends on DF propagation speed and ambient plasma density. These statistical signatures support the hypothesis that the Bz dip is caused by ion reflection and suggest that secondary currents carried by these reflected ions can reshape the front significantly.
We are grateful to Judy Hohl for editorial support. This research was supported by NSFC grants 41474140 and 41421003. It was also made possible by the THEMIS mission operated under NASA contract NAS5-02099. We thank C. W. Carlson and J. P. McFadden for use of ESA data; D. Larson and R. P. Lin for use of SST data; and K.-H. Glassmeier, U. Auster, and W. Baumjohann for use of FGM data. All the observational data used in this paper are available online at http://themis.ssl.berkeley.edu/index.shtml. The Fortran code for the test particle simulation is available online at https://github.com/ScSwP/DFcode.
Pan, D.-X., X.-Z. Zhou, Q.-Q. Shi, J. Liu, V. Angelopoulos, A. Runov, Q.-G. Zong, and S.-Y. Fu (2015), On the generation of magnetic dips ahead of advancing dipolarization fronts. Geophys. Res. Lett., 42,4256–4262. doi: 10.1002/2015GL064369.