1. Department of Space and Plasma Physics, School of Electrical Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden
2. Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland
3. Laboratoire de Physique des Gaz et Plasmas - LPGP, UMR 8578 CNRS, Université Paris-Sud, 91405 Orsay Cedex, France
4. CentraleSupélec, Grande Voie des Vignes, 92295 Chatenay-Malabry Cedex, France
5. CNRS, UPR 288 Laboratoire EM2C, Grande Voie des Vignes, 92295 Chatenay-Malabry Cedex, France
6. Plasma and Coatings Physics Division, IFM-Materials Physics, Linköping University, SE-581 83 Linköping, Sweden