1. Department of Physics and Astronomy, University of Western Ontario, London, ON N6A 3K7, Canada
2. Institute for Earth and Space Exploration, Western University, London, ON N6A 5B7, Canada
3. Institute of Applied Physics & Oeschger Center for Climate Change Research, Microwave Physics, University of Bern, CH-3012 Bern, Switzerland
4. Institute for Meteorology, Universität Leipzig, D-04103 Leipzig, Germany
5. Swedish Institute of Space Physics (IRF), SE-98192 Kiruna, Sweden
6. Sodankyla Geophysical Observatory, University of Oulu, FI-99600 Oulu, Finland
7. Department of Physics and Astronomy, LE1 7RH, University of Leicester, Leicester, UK