1. Department of Physics and Astronomy, Swarthmore College, Swarthmore, PA 19081, USA
2. Department of Physics and Astronomy & Bartol Research Institute, University of Delaware,Newark, DE 19716, USA
3. NASA/Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
4. Bryn Mawr College, Bryn Mawr, PA 19010, USA