Dept. Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK

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

Dept. Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK

Dept. Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK

Blackett Laboratory, Imperial College, London, SW7 2BZ, UK

CESR/CNRS-Université de Toulouse, 9 Avenue du Colonel Roche, B.P. 4346, 31028, Toulouse Cedex 4, France

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

National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA

Dept. Physics and Astronomy, University of Leicester, Leicester, LE1 7RH, UK