1. Astronomy Department and Space Sciences Laboratory, University of California, Berkeley, CA 94720.
2. Space Sciences Laboratory, University of California, Berkeley, CA 94720.
3. Physics Department and Space Sciences Laboratory, University of California, Berkeley, CA 94720.
4. Max-Planck-Institut für Aeronomie, Lindau, Federal Republic of Germany.
5. Centre d'Etude Spatiale des Rayonnements, CNRS-Toulouse, France.
6. EECS, University of California, San Diego, CA 92093.