1. Rudolf Peierls Centre for Theoretical Physics, University of Oxford, OX1 3PU Oxford, UK
2. Ludwig-Maximilians-Universität, Fakultät für Physik Schellingstr. 4, D-80799 München, Germany
3. Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK