1. Center for Space Research and Department of Physics, Massachusetts Institute of Technology, Cambridge 02139
2. Laboratory for Planetary Atmospheres, Goddard Space Flight Center, Greenbelt, Maryland 20771
3. Laboratory for Extraterrestrial Physics, Goddard Space Flight Center
4. Department of Atmospheric Sciences, University of California, Los Angeles 90024
5. Department of Physics and Astronomy, University of Iowa, Iowa City 52242
6. Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, West Germany