1. Department of Physics, University of Helsinki, PO Box 64, FI-00014 Helsinki, Finland
2. Institute of Physics and Technology, Ural Federal University, Mira str 19, 620002 Ekaterinburg, Russia
3. Space Science Institute, 4750 Walnut St, Suite 205 Boulder, CO 80301, USA
4. Finnish Geospatial Research Institute FGI, PO Box 84, FI-00521 Helsinki, Finland