1. Department of Physics and Astronomy, University College London , London WC1E 6BT, UK
2. Department of Physics, University of Warwick , Coventry CV4 7AL, UK
3. Institute of Astronomy, University of Cambridge , Madingley Road, Cambridge CB3 0HA, UK
4. Laboratory of Atmospheric and Space Physics, University of Colorado Boulder , Boulder, CO 80309, USA
5. Department of Physical Sciences, Embry-Riddle Aeronautical University, Daytona Beach, FL 32114, USA
6. Department of Astronomy, Boston University , 725 Commonwealth Ave., Boston, MA 02215, USA