Conversion and Damping of Nonaxisymmetric Internal Gravity Waves in Magnetized Stellar Cores

Abstract 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 asteroseismic observations of global oscillations that result from the coupling of acoustic waves in the convective zone to internal gravity waves (IGWs) in the radiative interior. Recent modeling efforts have implicated 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 work shows 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. Here, we extend these results to the nonaxisymmetric case, for which the IGWs and SM waves are coupled to a continuous spectrum of Alfvén waves (AWs). We consider a Cartesian model of the radiative interior with uniform stratification and a spatially varying, current-free magnetic field. Using a Wentzel–Kramers–Brillouin approximation to solve for the vertical mode structure, corroborated with numerical simulations, we show that IGWs convert to up-going SM waves, which resonate with the Alfvén spectrum and produce mixed SM–AW modes. We find 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, resulting in rapid damping. Our results suggest that energy in both axisymmetric and nonaxisymmetric IGWs is lost via interactions with a strong magnetic field.