1. Departmant of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden
2. Space and Plasma Physics, EES, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden
3. Laboratoire de Physique des Gaz et des Plasmas, UMR 7198 CNRS, Université Paris-Sud, 91405 Orsay Cedex, France