1. Physics Department, Lancaster University , Lancaster, LA1 4YW , UK
2. Department of Astronomy and Physics, University of Leicester , Leicester, LE1 7RH , UK
3. Department of Physics and Astronomy, University of Southampton , Southampton, SO17 1BJ , UK
4. British Antarctic Survey , Cambridge, CB3 0ET , UK
5. Space Science Institute , Boulder, CO 80301 , USA
6. Department of Physics and Technology, University of Bergen , 5007 Bergen , Norway
7. Department of Arctic Geophysics, University Centre in Svalbard , P.O. Box 156 N-9171 Longyearbyen , Norway
8. Department of Physics, Imperial College London , London, SW7 2AZ , UK