1. Swarthmore College , Department of Physics and Astronomy, Swarthmore, PA 19081, USA
2. NASA/Goddard Space Flight Center , Code 662, Greenbelt, MD 20771, USA
3. Department of Geology and Astronomy , West Chester University, West Chester, PA 19383, USA
4. Department of Physics and Astronomy, University of Delaware , Newark, DE 19716, USA
5. Department of Physics and Astronomy, Howard University , Washington, DC 20059, USA
6. Center for Research and Exploration in Space Science and Technology , NASA/GSFC, Greenbelt, MD 20771, USA