1. CAS Key Laboratory of Geospace Environment, Department of Geophysics and Planetary Science, University of Science and Technology of China, Hefei 230026, China
2. Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543, USA
3. Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA
4. Physics Department, Auburn University, Auburn, Alabama 36849, USA