[1] St Petersburg State Univ, Inst Phys, St Petersburg 198504, Russia
[2] Univ Calif Los Angeles, Inst Geophys & Planetary Phys, Los Angeles, CA 90095 USA
[3] Univ Calif Berkeley, Space Sci Lab, Berkeley, CA 94720 USA
[4] Atmospher & Space Phys Lab, Boulder, CO 80303 USA
[5] Austrian Acad Sci, Space Res Inst, A-8042 Graz, Austria
[6] Univ Calif Los Angeles, Dept Earth & Space Sci, Los Angeles, CA 90095 USA