[1] Space Sciences Laboratory, University of California, Berkeley, CA 94720, United States
[2] Department of Space Sciences, University of Michigan, Ann Arbor, MI 48109-2143, United States
[3] NASA Goddard Space Flight Center, Greenbelt, MD 20771, United States
[4] Centre D'Etude Spatiale des Rayonnements, F-31028 Toulouse, France