1. Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, 2455 Hayward St, Ann Arbor, Michigan 48109, USA
2. Georgia Institute of Technology, 225 North Ave NW, Atlanta, Georgia 30332, USA
3. International Space Science Institute, Hallerstrasse 6, CH-3012 Bern, Switzerland and Physikalisches Institut, Universität Bern, Sidlerstrasse 5, CH-3012 Bern, Switzerland