1. Institut für Geophysik 1 , Sonneggstrasse 5, ETH Zürich, Zürich CH-8092, Switzerland
2. Department of Magnetohydrodynamics, Institute for Fluid Dynamics, Helmholtz-Zentrum Dresden-Rossendorf 2 , Bautzner Landstrasse 400, D-01328 Dresden, Germany
3. Aix Marseille University, CNRS, Centrale Marseille, IRPHE, 13013 Marseille 3 , France