[1] Penn State Univ, Dept Geosci, University Pk, PA 16802 USA
[2] Penn State Univ, Earth & Environm Syst Inst, University Pk, PA 16802 USA
[3] Univ New Hampshire, Inst Study Earth Oceans & Space, Durham, NH 03824 USA
[4] Univ Washington, Appl Phys Lab, Polar Sci Ctr, Seattle, WA 98105 USA