  The asteroseismology of red giant stars has continued to yield surprises
since the onset of high-precision photometry from space-based observations. An
exciting new theoretical result shows that the previously observed suppression
of dipole oscillation modes in red giants can be used to detect strong magnetic
fields in the stellar cores. A fundamental facet of the theory is that nearly
all the mode energy leaking into the core is trapped by the magnetic greenhouse
effect. This results in clear predictions for how the mode visibility changes
as a star evolves up the red giant branch, and how that depends on stellar
mass, spherical degree, and mode lifetime. Here, we investigate the validity of
these predictions with a focus on the visibility of different spherical
degrees. We find that mode suppression weakens for higher degree modes with an
average reduction in the quadrupole mode visibility of up to 49% for the least
evolved stars in our sample, and no detectable suppression of octupole modes,
in agreement with the theoretical predictions. We furthermore find evidence for
the influence of increasing mode lifetimes on the measured visibilities along
the red giant branch, in agreement with previous independent observations.
These results support the theory that strong internal magnetic fields are
responsible for the observed suppression of non-radial modes in red giants. We
also find preliminary evidence that stars with suppressed dipole modes on
average have slightly lower metallicity than normal stars.
