1. Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland 20810
2. Department of Physics and Astronomy, University of Kansas, Lawrence 66044
3. Max-Planck Institute for Aeronomy, D-3411 Katlenburg-Lindau 3, West Germany
4. Applied Physics Laboratory, Johns Hopkins University
5. Department of Physics, University of Arizona, Tucson 85721
6. Department of Physics and Astronomy, University of Maryland, College Park 20742
7. Bell Telephone Laboratories, Murray Hill, New Jersey 07974
8. Department of Physics and Astronomy, University of Maryland