1. Department of Physics, University of Helsinki , Helsinki, FI 00014, Finland
2. Department of Physics, University of Wisconsin – Madison , 1150 University Avenue, Madison, WI 53706, USA
3. Space Science Institute , Boulder, CO 80301, USA
4. NASA Goddard Space Flight Center , Greenbelt, MD 20771, USA
5. University of Maryland, Baltimore County , Goddard Planetary Heliophysics Institute, Baltimore, MD 21250, USA