1. Institue of Applied Physics and Computational Mathematics, Beijing 100088, China
2. CAS Key Lab of Geospace Environment, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China
3. Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
4. Department of Physics, Auburn University, Auburn, Alabama 36849, USA