1. SRON Netherlands Institute for Space Research, Sorbonnelaan 2, NL-3584 CA Utrecht, the Netherlands
2. Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, NL-6500 GL Nijmegen, the Netherlands
3. Instituto de Astrofísica de Canarias, E-38200 La Laguna, Tenerife, Spain
4. Departamento de Astrofísica, Universidad de La Laguna, Astrofísico Francisco Sánchez s/n, E-38206 La Laguna, Tenerife, Spain
5. Space Radiation Laboratory, California Institute of Technology, Pasadena, CA 91125, USA
6. Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, D-85748 Garching, Germany
7. Department of Physics, University of Warwick, Coventry CV4 7AL, UK