1. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA.
2. Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.
3. Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218–2686, USA.
4. University of Colorado, Laboratory for Atmospheric and Space Physics, 1234 Innovation Drive, Boulder, CO 80303, USA.