Department of Physics, Division of geophysics and astronomy, P.O. Box 64, University of Helsinki, Finland

Space Sciences Laboratory, University of California, Berkeley, CA, USA

Space Sciences Laboratory, University of California, Berkeley, CA, USA

Department of Astronomy, University of Maryland, College Park, MD, USA

Heliophysics Science Division, NASA Goddard Space Flight Center, MD, USA

Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA, USA