1. Department of Applied Physics and Applied Mathematics, Columbia University, New York, NY 10027;
2. Ames Laboratory, US Department of Energy, Ames, IA 50011;
3. Department of Earth and Environmental Sciences, Columbia University, New York, NY 10027;
4. Lamont–Doherty Earth Observatory, Columbia University, Palisades, NY 10964;
5. Department of Physics, Iowa State University, Ames, IA 50011