1. Space Sciences Laboratory, University of California at Berkeley, Berkeley, California 94720, USA
2. New Jersey Institute of Technology, Newark, New Jersey 07102, USA
3. Space Research Institute of Russian Academy of Sciences, Moscow 117997, Russia
4. Institute of Geophysics and Planetary Sciences, University of California, Los Angeles, California 90095, USA
5. Department of Physics, University of California at Berkeley, Berkeley, California 94720, USA