1. School of Earth and Space Exploration, Arizona State University , Tempe, AZ 85287, USA
2. Department of Physics, Winona State University , Winona, MN 55987, USA
3. Department of Physics and Astronomy, University of New Mexico , 210 Yale Blvd NE, Albuquerque, NM 87106, USA
4. Commonwealth Scientific and Industrial Research Organisation (CSIRO), Space & Astronomy , P.O. Box 1130, Bentley, WA 6102, Australia