the short story
Red giants are constantly ringing due to gas motions in their outer layers, and, because of this, they constantly vibrate at a large number of notes. However, for many giants, a large number of these notes are muffled. Curiously, the muffled notes seem to be exactly the ones that we know extend all the way to the star’s center, meaning that the muffling probably happens deep inside the star. Many experts think that strong magnetic fields are responsible.
If the magnetic field is strong enough, the kinds of waves inside the cores of red giants will gradually develop finer and finer detail until they can be erased by some other fluid or magnetic process.
the longer story
About 20% of red giants have suppressed dipole and quadrupole oscillations, but normal-amplitude radial modes. This suggests that some core mechanism is responsible for their damping, with strong core magnetism being a very promising candidate. However, the manner in which this suppression occurs is poorly understood, because the propagation of gravity waves under strong magnetic fields is not well-understood, especially in realistic field geometries.
We demonstrate using a combination of analytical and numerical calculations that all magnetogravity modes will be refracted upwards at some strong-enough field onto a “slow magnetic” branch, after which they approach infinite wavenumber. At this time, they are easily damped out by either viscosity or Ohmic diffusion. The tesseral and sectoral (m≠0) modes also develop sharp features in their fluid displacements, in the vicinity of critical latitudes corresponding to resonances with Alfvén waves.
This work was presented at the 2023 meeting of the TESS/Kepler Asteroseismic Scientific Consortium (TASC7/KASC14) in Honolulu, HI. The slides can be found here.
the longest story
This work is described in our paper in the Monthly Notices of the Royal Astronomical Society:
Nicholas Rui 