1. Department of Astronomy, University of Geneva , Chemin d’Ecogia 16, CH-1290 Versoix , Switzerland
2. Massachusetts Institute of Technology, Kavli Institute for Astrophysics and Space Research , 77 Massachusetts Ave., Cambridge, MA 02139 , USA
3. International Space Science Institute (ISSI) , Hallerstrasse 6, CH-3012 Bern , Switzerland
4. Physikalisches Institut, University of Bern , Sidlerstrasse 5, CH-3012 Bern , Switzerland
5. SRON Netherlands Institute for Space Research , Niels Bohrweg 4, NL-2333 CA Leiden , the Netherlands
6. Anton Pannekoek Institute, University of Amsterdam , Postbus 94249, NL-1090 GE Amsterdam , the Netherlands