1. Department of Physics, Imperial College London , London, SW7 2AZ , UK
2. Department of Physics, Umeå University, 901 87 , Umeå , Sweden
3. Laboratory for Atmospheric and Space Physics, University of Colorado , Boulder, CO, 80303 , USA
4. Department of Mathematics, Physics and Electrical Engineering, Northumbria University , Newcastle Upon Tyne,NE1 8ST , UK
5. Lagrange , OCA, CNRS, UCA, Nice, 34229 , France
6. LPC2E , CNRS, Orleans, 45071 , France
7. Swedish Institute of Space Physics, 98128 , Kiruna , Sweden
8. Physikalisches Institut, University of Bern, 3012 , Bern , Switzerland