1. Osservatorio Astronomico di Bologna, INAF, Via Ranzani 1, 40127 Bologna, Italy
2. University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
3. Max-Planck-Institut für Plasmaphysik and Excellence Cluster Universe, Boltzmannstrasse 2, 85748 Garching, Germany
4. Max-Planck-Institute für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany