1. Physical Sciences Division; British Antarctic Survey, Natural Environment Research Council; Cambridge; U.K.
2. Department of Physics; University of Otago; Dunedin; New Zealand
3. Sodankylä Geophysical Observatory; University of Oulu; Sodankylä; Finland
4. Global Seismology and Geomagnetism Group; British Geological Survey, Natural Environment Research Council; Edinburgh; U.K.
5. Space Plasma Environment and Radio Science Group, InfoLab 21, Department of Communication Systems; Lancaster University; Lancaster; U.K.