Effects of a strong magnetic field on internal gravity waves: trapping, phase mixing, reflection and dynamical chaos

TL;DR

This paper investigates how strong magnetic fields in stars affect internal gravity waves, revealing phenomena like trapping, phase mixing, reflection, and chaos, which influence stellar oscillation spectra and may explain observed anomalies.

Contribution

It provides a detailed analytical and numerical analysis of gravity wave interactions with strong magnetic fields, uncovering new wave behaviors and their implications for stellar physics.

Findings

Wave dynamics become chaotic beyond a critical magnetic field strength.

Waves are either reflected or trapped depending on their frequency and orientation.

Trapping regions lead to energy loss via Alfven wave phase mixing.

Abstract

The spectrum of oscillation modes of a star provides information not only about its material properties (e.g. mean density), but also its symmetries. Spherical symmetry can be broken by rotation and/or magnetic fields. It has been postulated that strong magnetic fields in the cores of some red giants are responsible for their anomalously weak dipole mode amplitudes (the "dipole dichotomy" problem), but a detailed understanding of how gravity waves interact with strong fields is thus far lacking. In this work, we attack the problem through a variety of analytical and numerical techniques, applied to a localised region centred on a null line of a confined axisymmetric magnetic field which is approximated as being cylindrically symmetric. We uncover a rich variety of phenomena that manifest when the field strength exceeds a critical value, beyond which the symmetry is drastically broken by the Lorentz force. When this threshold is reached, the spatial structure of the g-modes becomes heavily altered. The dynamics of wave packet propagation transitions from regular to chaotic, which is expected to fundamentally change the organisation of the mode spectrum. In addition, depending on their frequency and the orientation of field lines with respect to the stratification, waves impinging on different parts of the magnetised region are found to undergo either reflection or trapping. Trapping regions provide an avenue for energy loss through Alfven wave phase mixing. Our results may find application in various astrophysical contexts, including the dipole dichotomy problem, the solar interior, and compact star oscillations.