Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK

Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK

Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK

Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland

Space Sciences Lab, University of California, Berkeley, CA 94720-7450, USA

Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland

Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, Graz, 8042, Austria

Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK

CESR, 9 avenue du colonel Roche, Toulouse, 31028, France

Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, Graz, 8042, Austria

now at: Solar-Terrestrial Physics Group, Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Yoshinodai 3-1-1, Sagamihara 229-8510, Japan