1. Space Astrophysics Laboratory, Institute for Space and Terrestrial Science, and Department of Physics and Astronomy, York University, North York, Ontario, Canada M3J 1P3
2. Space Astrophysics Laboratory, Institute for Space and Terrestrial Science, North York, Ontario, Canada M3J 3K1
3. Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125