\section{Main Text}
\label{main}

Despite rapid progress in the discovery and characterization of magnetic fields at the surfaces of stars, very little is known about internal stellar magnetic fields. This has prevented the development of a coherent picture of stellar magnetism and the evolution of magnetic fields within stellar interiors.

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 \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), as illustrated in Fig. \ref{fig:cartoon}. Mixed modes \cite{Beck_2011} have made it possible to distinguish between hydrogen and helium-burning red giants \cite{Bedding_2011,Mosser_2014} and have been used to measure the rotation rate of red giant cores \cite{Beck_2012,Mosser_2012}.

A group of red giants with depressed dipole modes were identified using {\it Kepler} observations \cite{Mosser_2011}, see also Fig. \ref{fig:moneyplot}. These stars show normal radial modes (spherical harmonic degree $\ell=0$), but exhibit dipole ($\ell=1$) modes whose amplitude is much lower than usual.  Until now, the suppression mechanism was unknown \cite{Garcia_2014}. Below, we demonstrate that dipole mode suppression may result from strong magnetic fields within the cores of these red giants.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  