1. Space Science and Engineering Division, Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA.
2. Department of Physics and Astronomy, University of Texas, San Antonio, TX 78249, USA.
3. Department of Applied Mathematics, University of Colorado, Boulder, CO 80309, USA.
4. Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA.
5. Department of Astronomy and Carl Sagan Institute, Cornell University, Ithaca, NY 14853, USA.