1. Space and Atmospheric Physics Group, Imperial College London, The Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK.
2. Atmospheric Physics Laboratory, Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK.
3. Centre for Planetary Sciences, University College London, UK.
4. Department of Physics and Astronomy, University of Iowa, 613 Van Allen Hall, Iowa City, IA 52242–1479, USA.
5. Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK.