1. Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
2. School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
3. LESIA-Observatoire de Paris, UPMC Univ. Paris 06, Univ. Paris-Diderot, France
4. Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK