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tokens: Acknowledgements Financial support was provided by the German Federal Ministry of Education and Research (BMBF in the framework of the Verbundforschung, D-LOFAR 05A11BAA). The authors would like to thank Mark DeRosa’s help for extending the magnetic field calculations beyond the default distance, NASA’s SDO and ESA’s PROBA2 teams as well as the DAM and ARTEMIS observatories for providing open access to their data, and J. Anderson, A. Warmuth, J. Hessel, and G. Heald for their careful reading of the manuscript. LOFAR, designed and constructed by ASTRON, has facilities in several countries that are owned by various parties (each with their own funding sources) and are collectively operated by the International LOFAR Telescope (ILT) foundation under a joint scientific policy. C. F. acknowledges financial support from the “Agence Nationale de la Recherche” through grant ANR-09-JCJC-0001-01.