[1] Univ Durham, Sci Labs, Dept Phys, Durham DH1 3LE, England
[2] Anglo Australian Observ, Epping, NSW 2121, Australia
[3] Drexel Univ, Dept Phys, Philadelphia, PA 19104 USA
[4] UCL, Mullard Space Sci Lab, Dorking RH5 6NT, Surrey, England
[5] Univ Oxford, Dept Phys, Oxford OX1 3RH, England
[6] Liverpool John Moores Univ, Birkenhead CH41 1LD, Merseyside, England