1. The Institute of Physical and Chemical Research, Hirosawa, Wako-shi, Saitama 351-01
2. Science and Engineering Research Laboratory, Waseda University, Kikuicho, Shinjuku-ku, Tokyo 162
3. Faculty of General Education, Ehime University, Bunkyo-cho, Matsuyama-shi, Ehime 790
4. Space Sciences Laboratory, University of California, Berkeley, California 94720, USA