1. School of Nuclear Science and Technology and Department of Modern Physics, University of Science and Technology of China, Hefei, Anhui 230026, China
2. Key Laboratory of Geospace Environment, CAS, Hefei, Anhui 230026, China
3. Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 0854, USA
4. LSEC, Academy of Mathematics and Systems Science, Chinese Academy of Sciences, P.O. Box 2719, Beijing 100190, China