1. Department of Physics and Astronomy; University of Leicester; Leicester UK
2. Formerly at Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics; University of California; Los Angeles California USA
3. Institute of Geophysics and Planetary Physics; University of California; Los Angeles California USA
4. Department of Earth, Planetary, and Space Sciences; University of California; Los Angeles California USA
5. Department of Atmospheric, Oceanic, and Space Science; University of Michigan; Ann Arbor Michigan USA
6. National Science Foundation; Arlington Virginia USA
7. Department of Physics and Astronomy; University of California; Los Angeles California USA