Conversion of Internal Gravity Waves into Magnetic Waves

TL;DR

This paper investigates how internal gravity waves in stars convert into magnetic waves in the presence of strong magnetic fields, explaining observed suppression of certain oscillation modes in red giants.

Contribution

It provides analytic solutions and numerical validation for wave conversion mechanisms in stellar interiors with magnetic fields, advancing understanding of asteroseismic phenomena.

Findings

Perfect conversion of IGWs into magnetic waves beyond a critical magnetic field strength

Downward IGWs convert into upward Alfvénic waves that dissipate

Strong magnetic fields can suppress dipole oscillation modes in red giants

Abstract

Asteroseismology probes the interiors of stars by studying oscillation modes at a star's surface. Although pulsation spectra are well understood for solar-like oscillators, a substantial fraction of red giant stars observed by Kepler exhibit abnormally low-amplitude dipole oscillation modes. Fuller et al. (2015) suggests this effect is produced by strong core magnetic fields that scatter dipole internal gravity waves (IGWs) into higher multipole IGWs or magnetic waves. In this paper, we study the interaction of IGWs with a magnetic field to test this mechanism. We consider two background stellar structures: one with a uniform magnetic field, and another with a magnetic field that varies both horizontally and vertically. We derive analytic solutions to the wave propagation problem and validate them with numerical simulations. In both cases, we find perfect conversion from IGWs into magnetic waves when the IGWs propagate into a region exceeding a critical magnetic field strength. Downward propagating IGWs cannot reflect into upward propagating IGWs because their vertical wavenumber never approaches zero. Instead, they are converted into upward propagating slow (Alfvenic) waves, and we show they will likely dissipate as they propagate back into weakly magnetized regions. Therefore, strong internal magnetic fields can produce dipole mode suppression in red giants, and gravity modes will likely be totally absent from the pulsation spectra of sufficiently magnetized stars.