1. Geophysical Institute; University of Alaska; Fairbanks Alaska USA
2. Institute of Space Physics and Applied Technology School of Earth and Space Sciences; Peking University; Beijing China
3. University of Toulouse, UPS-OMP, IRAP; Toulouse France
4. CNRS, IRAP, BP 44346; Toulouse Cedex 4 France
5. Institute for Geophysics and Extraterrestrial Physics; Braunschweig Germany