1. Department of Physics; University of Otago; Dunedin New Zealand
2. British Antarctic Survey; Cambridge UK
3. Geomagnetic Laboratory; Natural Resources Canada; Ottawa Ontario Canada
4. Department of Physics and Astronomy; University of Calgary; Calgary Alberta Canada
5. Cooperative Institute for Research in Environmental Sciences; University of Colorado; Boulder Colorado USA
6. Also at National Geophysical Data Center; National Oceanic and Atmospheric Administration; Boulder Colorado USA