1. Applied Physics Laboratory, The Johns Hopkins University, Laurel, MD 20707-6099
2. University of Kansas, Lawrence, KS 66044
3. Max-Planck-Institut für Aeronomic, Postfach 60, 3411 Lindau, Federal Republic of Germany
4. University of Maryland, College Park, MD 20742
5. AT&T Bell Laboratories, Murray Hill, NJ 07974
6. University of Iowa, Iowa City IA 52242