Dept. of Physics and Astronomy, University of Iowa, Iowa City, IA, USA

Institute for Physical Science and Technology, University of Maryland, College Park, MD, USA

School of Space Research, Kyung Hee University, Yongin, Gyeonggi 446-701, Korea

Dept. of Physics and Astronomy, University of Iowa, Iowa City, IA, USA

LESIA-CNRS, Observatoire de Paris, Meudon, France

Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, USA