1. Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
2. Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique (LESIA), Observatoire de Paris, CNRS, UPMC Univ. Paris 06, Univ. Paris-Diderot, 5 place Jules Janssen, Meudon Cedex 92195, France
3. Atmospheric, Oceanic and Planetary Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK