1. College of Physics, Sichuan University, Chengdu 610065, China
2. Southwestern Institute of Physics, P.O. Box 432, Chengdu 610041, China
3. Department of Astrophysical Sciences and Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543, USA
4. Department of Physics and Astronomy, University of California, Irvine, California 92697, USA