[1] NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA
[2] Univ Michigan, Dept Atmospher Ocean & Space Sci, Ann Arbor, MI 48109 USA
[3] Univ New Hampshire, Ctr Space Sci, Durham, NH 03824 USA
[4] Univ Calif Los Angeles, Inst Geophys & Planetary Phys, Los Angeles, CA 90065 USA