Swedish Institute of Space Physics, Box 537, Uppsala, 751 21 Sweden; Centre d'Étude Spatiale des Rayonnements, 9, avenue du Colonel Roche, Toulouse, 31400 France
John Hopkins University, Applied Physics Laboratory, 11100 John Hopkins Rd, Laurel, MD 20723-6099, USA
Swedish Institute of Space Physics, Box 537, Uppsala, 751 21 Sweden