1. Department of Physics, Imperial College London, Prince Consort Road, London SW7 2AZ, UK
2. LPC2E, CNRS, Université d’Orléans, F-45100 Orléans, France
3. Swedish Institute of Space Physics, Ångström Laboratory, Lägerhyddsvägen 1, SE-752 37 Uppsala, Sweden
4. Physikalisches Institut, University of Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland
5. Swedish Institute of Space Physics, PO Box 812, SE-981 28 Kiruna, Sweden
6. Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510, USA