1. KTH Royal Institute of Technology, Stockholm 100 44, Sweden
2. Physics Department, St. Petersburg State University, St. Petersburg 198504, Russia
3. Laboratory for Atmospheric and Space Physics (LASP), University of Colorado Boulder, Boulder, Colorado 80303, USA
4. Institute for Modeling Plasma, Atmospheres and Cosmic Dust, NASA/SSERVI, Moffet Field, California 94035, USA