Fig. \ref{fig:Bc} shows the value of $B_{c,{\rm min}}$ as stars evolve up the RGB. We have calculated $B_{c,{\rm min}}$ for angular frequencies $\omega \! = \! 2 \pi \nu_{\rm max}$, and evaluated $\nu_{\rm max}$ using the scaling relation proposed by \cite{Brown_1991}. On the lower RGB, where $\nu_{\rm max} \! \gtrsim \! 250\,\mu$Hz, field strengths of order $B_{c,{\rm min}} \! \gtrsim 10^6 \, {\rm G}$ are required for magnetic suppression. As stars evolve up the red giant branch, the value of $B_{c,{\rm min}}$ decreases sharply, primarily because $\nu_{\rm max}$ decreases. By the luminosity bump (near $\nu_{\rm max} \! \sim 40\,\mu$Hz), field strengths of only $B_{c,{\rm min}} \! \sim \! \!10^4 \, {\rm G}$ are sufficient for magnetic suppression. Magnetic suppression during the sub-giant phase (higher $\nu_{\rm max}$) and in higher mass stars ($M \! \gtrsim \! 2 M_\odot$) may be less common due to the larger field strengths required.

For a given field strength, there is a transition frequency $\nu_c$ below which modes will be strongly suppressed and above which modes will appear normal. Stars which show this transition are especially useful because they allow for an inference of $B_r$ at the H-burning shell via Eqn. \ref{eqn:Bc}, evaluated at the transition frequency $\omega \! = \! 2 \pi \nu_c$.  The RGB star KIC8561221 shows this transition \cite{Garcia_2014}. Using the observed value of $\nu_c \! \approx \! 600 \, \mu{\rm Hz}$, we infer that the radial component of the magnetic field within the H-burning shell is $B_r \! \approx \! 1.5 \times 10^{7} \, {\rm G}$, although we cannot rule out the presence of stronger fields away from the H-burning shell. This large field strength may indicate KIC8561221 is the descendant of a magnetic Ap star whose internal field was much stronger than the typical surface fields of $B \! \sim \! 3 \, {\rm kG}$ of Ap stars \cite{Auri_re_2007}.

In principle, it is possible that another symmetry-breaking mechanism within the core could suppress dipole mode amplitudes. The only other plausible candidate is rapid core rotation. In order for rotation to strongly modify the incoming waves such that they will be trapped in the core, the core must rotate at a frequency comparable to $\nu_{\rm max}$, roughly two orders of magnitude faster than the values commonly measured in red giant cores \cite{Beck_2011,Mosser_2012,deheuvels_2014}. The depressed dipole mode star KIC8561221 \cite{Garcia_2014} does not exhibit rapid core rotation and disfavors the rotation scenario.

A magnetic field of amplitude $B \! > \! 10^4 \, {\rm G}$ (see Fig. \ref{fig:Bc}) could be present in the core of a red giant if it was retained from previous phases of stellar formation/evolution (supplementary material). These strong fields may reside within the inner core with little external manifestation apart from the reduced visibility of the dipole modes. However, fields of similar amplitude have been discussed in order to explain the suppression of thermohaline mixing in a small fraction of red giant stars, as inferred from the observations of their surface abundances \cite{Charbonnel_2007}. The inferred core field strength of $B_r \! \gtrsim \! 1.5 \! \times \! 10^7 \, {\rm G}$ in KIC8561221 shows very strong magnetic fields ($B \! \gg \! 10^6 \, {\rm G}$) can exist within the radiative cores of early RGB stars. Since these fields are likely inherited from previous stages of stellar evolution, slightly weaker ($B \! \gg \! 10^5 \,{\rm G})$ fields could exist in the cores of exceptional very highly magnetized main sequence stars.

