1. Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20707.
2. Department of Physics and Astronomy, University of Kansas, Lawrence, KS 66044.
3. Max-Planck-Institüt für Aeronomie, 3411 Lindau, Federal Republic of Germany.
4. Department of Astronomy, University of Maryland, College Park, MD 20742.
5. AT&T Bell Laboratories, Murray Hill, NJ 07974.