[1] Univ Delaware, Dept Phys & Astron, Newark, DE 19716 USA
[2] Univ Calif Berkeley, Dept Phys, Berkeley, CA USA
[3] Harvard Univ, Ctr Astrophys, Cambridge, MA USA
[4] Univ Arizona, Lunar & Planetary Lab, Tucson, AZ USA
[5] Johns Hopkins Univ, Appl Phys Lab, Laurel, MD USA