1. Institute of Geophysics and Planetary Physics; University of California; Los Angeles California USA
2. On leave from Jet Propulsion Laboratory; California Institute of Technology; Pasadena California USA
3. Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Physics; Shandong University; Weihai China
4. Space Sciences Laboratory; University of California; Berkeley California USA