Affiliation: | Geomagnetism Laboratory, Department of Earth and Ocean Sciences, University of Liverpool, Oliver Lodge Laboratories, Oxford Street, Liverpool L69 7ZE,;United Kingdom. E-mail: Geological Survey of Japan, AIST, Tsukuba 305-8567,;Japan Institute of Earth Sciences, University of Potsdam, Karl-Liebknechtstrasse 24,;Haus 27, 14476 Golm, Germany |