6 Acknowledgements
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.