Graz University of Technology, Graz, Austria

Leibniz Institute of Atmospheric Physics, Kühlungsborn, Germany

now at: German Aerospace Center, Institute of Atmospheric Physics, Oberpfaffenhofen, Germany

Norwegian Defence Research Establishment, Kjeller, Norway

University of Oslo, Norway

University of Oslo, Norway

on leave from: Norwegian Defence Research Establishment, Kjeller, Norway

Space Research Institute, Austrian Academy of Sciences, Graz, Austria

Department of Physics, University of Colorado, Boulder, USA

Department of Physics, University of Colorado, Boulder, USA

Department of Physics and Astronomy, Hanover, NH 03755, USA