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

Abstract

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