tokens: We thank the referee for the helpful comments, and R. Schulz and P.G. Edwards for the useful discussions that improved the manuscript. C.M. acknowledges the support of the Bundesministerium für Wirtschaft und Technologie (BMWi) through Deutsches Zentrum für Luft- und Raumfahrt (DLR) grant 50 OR 1404 and of the Studienstiftung des Deutschen Volkes. This research was funded in part by the National Aeronautics and Space Administration (NASA) through Fermi Guest Investigator grants NNH09ZDA001N, NNH10ZDA001N, and NNH12ZDA001N, by BMWi through DLR grant 50 OR 1311, by Deutsche Forschungsgemeinschaft grant WI1860/10-1, and by an appointment to the NASA Postdoctoral Program at the Goddard Space Flight Center, administered by Oak Ridge Associated Universities through a contract with NASA. We thank J. E. Davis for the development of the slxfig module that has been used to prepare the figure in this work. This research has made use of ISIS functions provided by ECAP/Dr. Karl Remeis-Observatory (Bamberg, Germany) and MIT ( http://www.sternwarte.uni-erlangen.de/isis/ ).
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