[1
]Max-Planck Institut für Extraterrestrische Physik, Garching, D-85748 Germany
[2
]Institute of Space and Astronautical Science, Yoshinodai, Sagamihara, Kanagawa 229,
Japan
[3
]Department of Physics, Rikkyo University, Nishi-Ikebukuro, Toshima-ku, Tokyo 171,
Japan