1. Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA
2. Pushkov Institute of Terrestrial Magnetism of the Russian Academy of Sciences, St Petersburg Filial. 1 Mendeleevskaya Liniya, St Petersburg 199034, Russia
3. Saint Petersburg State University, Institute of Earth Sciences Universitetskaya nab., 7-9, St. Petersburg 199034, Russia
4. Ecole et Institut de Physique du Globe, CNRS-UMR 7516 5 rue Descartes Strasbourg cedex, France