Swedish Institute of Space Physics, P.O. Box 537, 751 21 Uppsala, Sweden

Department of Physics and Astronomy, Space and Plasma Physics, P.O. Box 515, 751 20 Uppsala, Sweden

Swedish Institute of Space Physics, P.O. Box 537, 751 21 Uppsala, Sweden

Swedish Institute of Space Physics, P.O. Box 537, 751 21 Uppsala, Sweden

Swedish Institute of Space Physics, P.O. Box 537, 751 21 Uppsala, Sweden

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

Centre d'Etude Spatiale des Rayonnements, CNRS, Toulouse, France

Department of Physics and Technology, University of Bergen, Bergen, Norway

Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany

Mullard Space Science Laboratory, University College London, Dorking, UK