1. Institute of Physics, University of Graz, 8010 Graz, Styria, Austria
2. Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Styria, Austria
3. Institut für Geophysik und extraterrestrische Physik, TU Braunschweig, 38106 Braunschweig, Germany
4. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Kanagawa 252-5210, Japan
5. Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom
6. Department of Physics and Astronomy, Dartmouth College, Hanover, New Hampshire 03755, USA