1. School of Earth and Environment, University of Leeds, Leeds LS29JT, UK
2. Geomagnetism Laboratory, Department of Earth, Ocean and Ecological Sciences, University of Liverpool, Liverpool L697ZE, UK
3. Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris-Diderot, CNRS, 1 rue Jussieu, F-75005 Paris, France