1. Science Institute, University of Iceland, Dunhaga 3, IS-107 Reykjavik, Iceland
2. School of Science and Engineering, Reykjavik University, Menntavegur 1, IS-102 Reykjavik, Iceland
3. Department of Space and Plasma Physics, School of Electrical Engineering and Computer Science, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden