1. Center for Space Research and Department of Physics, Massachusetts Institute of Technology, Cambridge 02139
2. Laboratory for Extraterrestrial Physics, Goddard Space Flight Center, Greenbelt, Maryland 20771
3. Department of Atmospheric Sciences, University of California, Los Angeles
4. Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, West Germany
5. Jet Propulsion Laboratory, California Institute of Technology, Pasadena 91103