# Torsional Alfv\'en resonances as an efficient damping mechanism for non-radial oscillations in red giant stars

## TL;DR

This paper proposes a new damping mechanism for non-radial oscillations in red giant stars, involving resonant interactions with Alfvén modes in magnetized cores, explaining observed weak dipole modes.

## Contribution

It introduces a novel damping process via Alfvén resonances that accounts for weak dipole modes in red giants, integrating magnetic effects into stellar oscillation theory.

## Key findings

- Damping rates match observed weak mode amplitudes.
- Resonant Alfvén modes effectively damp oscillations in magnetized stellar cores.
- Magnetic fields can damp oscillations without disrupting mode structure.

## Abstract

Stars are self-gravitating fluids in which pressure, buoyancy, rotation and magnetic fields provide the restoring forces for global modes of oscillation. Pressure and buoyancy energetically dominate, while rotation and magnetism are generally assumed to be weak perturbations and often ignored. However, observations of anomalously weak dipole mode amplitudes in red giant stars suggest that a substantial fraction of these are subject to an additional source of damping localised to their core region, with indirect evidence pointing to the role of a deeply buried magnetic field. It is also known that in many instances the gravity-mode character of affected modes is preserved, but so far no effective damping mechanism has been proposed that accommodates this aspect. Here we present such a mechanism, which damps the oscillations of stars harbouring magnetised cores via resonant interactions with standing Alfv\'en modes of high harmonic index. The damping rates produced by this mechanism are quantitatively on par with those associated with turbulent convection, and in the range required to explain observations, for realistic stellar models and magnetic field strengths. Our results suggest that magnetic fields can provide an efficient means of damping stellar oscillations without needing to disrupt the internal structure of the modes, and lay the groundwork for an extension of the theory of global stellar oscillations that incorporates these effects.