1. Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland 20707
2. Department of Physics, University of Kansas, Lawrence 66044
3. Max-Planck Institute for Aeronomy, D-3411 Katlenburg-Lindau 3, West Germany
4. Applied Physics Laboratory, John Hopkins University
5. Department of Physics and Astronomy, University of Maryland, College Park
6. Applied Physics Laboratory, Johns Hopkins University
7. Bell Laboratories, Murray Hill, New Jersey 07974
8. Department of Physics and Astronomy, University of Maryland