1. School of Ocean and Earth Sciences and Technology (SOEST), Hawaii Institute of Geophysics and Planetology (HIGP), Paleomagnetic Laboratory, University of Hawaii, 2525 Correa Rd, Honolulu, HI 96822, USA
2. University of Alaska, Fairbanks, Alaska, USA, and Space and Atmospheric Physics Group, The Blackett Laboratory, Imperial College of Science, Technology and Medicine, London SW7 2BZ, UK