1. Birkeland Centre for Space Science, Department of Physics and Technology, University of Bergen, 5020 Bergen, Norway
2. Southwest Research Institute, San Antonio, Texas 78228, USA
3. Dartmouth College, Hanover, New Hampshire 03755, USA
4. NASA, Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
5. Space Research Institute, Austrian Academy of Sciences, 8042 Graz, Austria
6. University of California, Berkeley, California 94720, USA