1. Department of Physics, University of Helsinki, P.O. Box 68, 00014 Uusimaa, Finland
2. Center for Astronomy, Institute for Theoretical Astrophysics, Heidelberg University, 69120 Heidelberg, Germany
3. Swedish Institute of Space Physics, P.O. Box 537, SE-751 21 Uppsala, Sweden
4. Space and Earth Observation Centre, Finnish Meteorological Institute, P.O. Box 503, FI-00101 Helsinki, Finland