1. The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
2. University of British Columbia, Vancouver, BC V6T 1Z4, Canada.
3. Planetary Science Institute, Tucson, AZ 85719, USA.
4. Goddard Space Flight Center, Greenbelt, MD 20771, USA.
5. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA.
6. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.