Axisymmetric Scattering of p Modes by Thin Magnetic Tubes

TL;DR

This paper investigates how acoustic p-mode waves scatter and are absorbed by thin magnetic fibrils in a stratified atmosphere, revealing weak mode mixing but significant phase shifts that depend on mode and frequency.

Contribution

It provides a detailed analysis of wave scattering and absorption by thin magnetic tubes in a stratified environment, highlighting differences from unstratified cases and quantifying phase shifts.

Findings

Mode mixing is weak for thin tubes.

Absorption coefficients are small and frequency-dependent.

Phase shifts can be large, up to 15 degrees, and vary with mode and frequency.

Abstract

We examine the scattering of acoustic p-mode waves from a thin magnetic fibril embedded in a gravitationally stratified atmosphere. The scattering is mediated through the excitation of slow sausage waves on the magnetic tube, and only the scattering of the monopole component of the wavefield is considered. Since such tube waves are not confined by the acoustic cavity and may freely propagate along the field lines removing energy from the acoustic wavefield, the excitation of fibril oscillations is a source of acoustic wave absorption as well as scattering. We compute the mode mixing that is achieved and the absorption coefficients and phase shifts. We find that for thin tubes the mode mixing is weak and the absorption coefficient is small and is a smooth function of frequency over the physically relevant band of observed frequencies. The prominent absorption resonances seen in previous studies of unstratified tubes are absent. Despite the relatively small absorption, the phase shift induced can be surprisingly large, reaching values as high as 15 degree for f modes. Further, the phase shift can be positive or negative depending on the incident mode order and the frequency.