1. a Institute for Meteorology and Climate Research, Forschungszentrum Karlsruhe and University Karlsruhe , Postfach 3640, 76021, Karlsruhe, Germany
2. b Service d'Aéronomie, CNRS , 91370, Verrières-le-Buisson Cedex, France
3. c Swedish Institute of Space Physics , Box 812, 5-98 128, Kiruna, Sweden