1. Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA
2. Space Sciences Laboratory, University of California, Berkeley, California 94720, USA
3. Department of Physics and the Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742, USA
4. Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA
5. Department of Astronomy and Astrophysics, University of Chicago, Chicago, Illinois 60673, USA
6. Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona 85719, USA
7. Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA