On mode conversion and wave reflection in magnetic Ap stars

TL;DR

This study explores how strong magnetic fields in Ap stars influence the reflection and energy loss of high-frequency acoustic waves, revealing that magnetic field orientation significantly affects wave energy distribution and retention.

Contribution

The paper introduces a combined numerical and analytical approach to analyze wave reflection and energy flux in magnetized stellar atmospheres, focusing on high-frequency oscillations.

Findings

Energy losses are highest where magnetic fields are vertical.

A fraction of wave energy remains trapped in the oscillation cycle.

Inclined magnetic fields help retain wave energy at higher frequencies.

Abstract

We investigate the effect of a strong large scale magnetic field on the reflection of high frequency acoustic modes in rapidly oscillating Ap stars. To that end we consider a toy model composed of an isothermal atmosphere matched onto a polytropic interior and determine the numerical solution to the set of ideal magneto-hydrodynamic equations in a local plane-parallel approximation with constant gravity. Using the numerical solution in combination with approximate analytical solutions that are valid in the limits where the magnetic and acoustic components are decoupled, we calculate the relative fraction of energy flux that is carried away in each oscillation cycle by running acoustic waves in the atmosphere and running magnetic waves in the interior. For oscillation frequencies above the acoustic cutoff we show that most energy losses associated with the presence of running waves occur in regions where the magnetic field is close to vertical. Moreover, by considering the depth dependence of the energy associated with the magnetic component of the wave in the atmosphere we show that a fraction of the wave energy is kept in the oscillation every cycle. For frequencies above the acoustic cutoff frequency such energy is concentrated in regions where the magnetic field is significantly inclined in relation to the local vertical. Even though our calculations were aimed at studying oscillations with frequencies above the acoustic cutoff frequency, based on our results we discuss what results may be expected for oscillations of lower frequency.