Jim Fuller edited Introduction.tex  almost 9 years ago

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After exhausting hydrogen in their cores, most main sequence stars evolve up the red giant branch (RGB). During this phase, the stellar structure is characterized by an expanding convective envelope and a contracting radiative core. Acoustic waves (p modes) in the envelope can couple to gravity waves (g modes) in the core {\bf \cite{Bedding_2014}}. Consequently, non-radial stellar oscillation modes become mixed modes that probe both the envelope (the p mode cavity) and the core (the g mode cavity), {\bf as illustrated in Figure \ref{fig:cartoon}.} Space-based asteroseismology has opened a window into the interiors of red giants. Mixed modes \cite{Beck_2011} have made it possible to distinguish between hydrogen and helium-burning red giants \cite{Bedding_2011,Mosser_2014}, {\bf and to measure the rotation rate of red giant cores (\cite{Beck_2012,Mosser_2012}).}  The amplitudes of the stellar oscillations depend on the interplay between driving and damping of the modes {\bf \cite{Dupret_2009,Benomar_2014}.} {\bf A group of red giants with depressed dipole modes were identified using {\it Kepler} observations (\cite{Mosser_2011}, see also Figure \ref{fig:moneyplot}).} {\bf These stars show normal radial modes} (spherical harmonic degree $\ell=0$), {\bf but exhibit dipole ($\ell=1$) modes whose amplitude is much lower than usual.} Until now, the depression mechanism was unknown \cite{Garcia_2014}. Below, we demonstrate that dipole mode depression {\bf may result} from strong magnetic fields within the cores of these red giants.