[1] Hermanus Magnet Observ, ZA-7200 Hermanus, South Africa
[2] Eotvos Lorand Univ, Space Res Grp, H-1518 Budapest, Hungary
[3] Univ KwaZulu Natal, Sch Phys, ZA-4000 Durban, South Africa
[4] Eotvos Lorand Univ, Res Grp Geol Geophys & Space Sci, H-1518 Budapest, Hungary
[5] Royal Inst Technol, Sch Elect Engn, Stockholm, Sweden
[6] Uppsala Univ, Dept Space Phys & Astron, Uppsala, Sweden
[7] Univ Otago, Dept Phys, Dunedin 9054, New Zealand