1. Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015, USA.
2. Physics Department, The Catholic University of America, Washington, DC 20064, USA.
3. Smithsonian Institution, Washington, DC 20013, USA.
4. Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA.
5. Department of Earth and Planetary Science, American Museum of Natural History, New York, NY 10024, USA.
6. The Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
7. Computer Sciences Corporation, Lanham-Seabrook, MD 20706, USA.