1. British Antarctic Survey, High Cross; Madingley Road, Cambridge CB3 0ET UK
2. Lamont-Doherty Earth Observatory; P.O. Box 1000, 61 Route 9W Palisades New York 10964 USA
3. Niels Bohr Institute for Astronomy, Physics and Geophysics, Department of Geophysics; University of Copenhagen; Juliane Maries Vej 30, DK-2100 Copenhagen Denmark