Conversion and Damping of Non-axisymmetric Internal Gravity Waves in Magnetized Stellar Cores

Magnetism is thought to play an important role in the evolution and dynamics of stars, though little is known about magnetic fields deep within stellar interiors. A promising avenue for probing these fields uses asteroseismological observations of global oscillations that result from the coupling of acoustic waves in the stellar convective zone to internal gravity waves (IGWs) in the radiative interior. Recent modeling efforts implicate deep magnetic fields in the suppression of dipole mixed modes observed in 20% of red giants and a number of high-mass main sequence stars. Previous numerical and theoretical efforts show that core magnetic fields could suppress axisymmetric global modes by refracting down-going IGWs into slow-magnetosonic (SM) waves that damp at magnetic cutoff heights. In this work, we extend these results to the non-axisymmetric case, for which the IGWs and SM waves are coupled to a continuous spectrum of azimuthally-directed Alfven waves (AWs). We consider a simple Cartesian model of the radiative interior with uniform stratification and vertically-decaying magnetic field. Using a Wentzel–Kramers–Brillouin (WKB) approximation to solve for the vertical mode structure, corroborated with numerical simulations of the linear magneto-Boussinesq equations, we show that IGWs convert to up-going SM waves, which resonate with the Alfven spectrum and produce mixed SM-AWs modes. We find SM cutoff heights (as in the axisymmetric case), above which the SM/SM-AWs convert to AWs. Latitudinal variations of the background magnetic field lead to phase mixing of the AWs and result in rapid damping. Our results suggest that energy in both axisymmetric and non-axisymmetric IGWs is lost via interactions with a strong magnetic field.