1. Atmospheric Oceanic and Planetary Physics, Department of Physics, University of Oxford, Clarendon Laboratory, Parks Road, Oxford OX1 3PU, UK;
2. Space Science and Technology Department, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon., OX11 0QX, UK;
3. Department of Earth and Planetary Sciences, 329 Olin Hall, The Johns Hopkins University, Baltimore, MD 21218, USA