[1] Johns Hopkins Univ, Appl Phys Lab, Laurel, MD 20723 USA
[2] Univ Colorado, Atmospher & Space Phys Lab, Boulder, CO 80303 USA
[3] NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA
[4] Univ Arizona, Lunar & Planetary Lab, Tucson, AZ 85721 USA
[5] Univ Cent Florida, Dept Phys, Orlando, FL 32816 USA
[6] Univ Maryland, Dept Astron, College Pk, MD 20742 USA
[7] Carnegie Inst Washington, Dept Terr Magnetism, Washington, DC 20015 USA