[1] Univ Paris Diderot, CNRS, Inst Phys Globe Paris, Equipe Geomagnetisme,UMR7154, F-75238 Paris 05, France
[2] Univ Northampton, Sch Sci & Technol, Northampton NN2 6JD, England
[3] Kingston Univ, Sch Geog Geol & Environm, Kingston Upon Thames KT1 2EE, Surrey, England