1. Centre for Astrophysics Research, University of Hertfordshire , College Lane, Hatfield AL10 9AB, UK
2. Institute of Astronomy, University of Cambridge , Madingley Road, Cambridge CB3 0HA, UK
3. Department of Astronomy, University of Michigan , 1085 S. University, Ann Arbor, MI 48109, USA
4. Jet Propulsion Laboratory, California Institute of Technology , Pasadena, CA 91109, USA
5. Space Radiation Laboratory, California Institute of Technology , Pasadena, CA 91125, USA
6. Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL) , Observatoire de Sauverny, CH-1290 Versoix, Switzerland