We thank D.C. Braun, D.-Y. Chou, S. Couvidat, T.L. Duvall, D. Haber, S. Hughes, R. Komm, A.G. Kosovichev, C. Lindsey, P.H. Scherrer, and J. Zhao for their encouragements, figures, and comments. We are grateful to M.J. Thompson and an anonymous referee for suggesting improvements. SOHO is an international collaboration between the European Space Agency and the National Aeronautics and Space Administration. The GONG project is managed by the National Solar Observatory and is supported by AURA and the National Science Foundation. This work was supported in part by NASA grants NAG5-13261 and NAG5-12452.
© Max Planck Society and the author(s)