Finnish Meteorological Institute, Arctic Research unit, P.O. Box 503, 00101 Helsinki, Finland

Atmospheric Physics Laboratory, UCL, Gower Street, London WC1E 6BT, UK

Swedish Institute of Space Physics, 75121 Uppsala, Sweden

Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan

Applied Physics Laboratory, Johns Hopkins University, 11100 Johns Hopkins Road, Laurel, MD 20723-6099, USA

Solar-Terrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8601, Japan

Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, CO, 80309, USA

GeoForschungsZentrum (GFZ) Potsdam, Telegrafenberg, 14473 Potsdam, Germany

Finnish Meteorological Institute, Arctic Research unit, P.O. Box 503, 00101 Helsinki, Finland

Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University 33, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812, Japan