1. Astrophysical and Planetary Science Department, University of Colorado, Boulder, CO 80309, USA.
2. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80309, USA.
3. Program in Atmospheric and Oceanic Science, University of Colorado, Boulder, CO 80309, USA.
4. Department of Applied Mathematics, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada.