Institution: | (1) Dept. of Applied Physics and Information Science, University of California, San Diego, La Jolla, Calif, USA;(2) Max-Planck-Institut für Aeronomie, Katlenburg-Lindau, FRG;(3) Present address: Goddard Institute for Space Studies, New York, USA;(4) Present address: Department of Mathematics, University of Khartoum, Sudan |