Fanning out of the $f$-mode in presence of nonuniform magnetic fields

TL;DR

This study demonstrates that nonuniform magnetic fields cause the f-mode in stratified layers to fan out in the $k ext{-} u$ diagram, revealing potential for subsurface magnetic field diagnostics.

Contribution

It extends previous work by analyzing the effects of nonuniform magnetic fields and turbulence on the f-mode, showing new features like mode fan-out and low-frequency stripes.

Findings

f-mode fans out in the $k ext{-} u$ diagram due to nonuniform magnetic fields

Mode fan-out is stronger for fields below the surface layer

Low frequency vertical stripes appear at multiples of twice the background field's wavenumber

Abstract

We show that in the presence of a harmonically varying magnetic field the fundamental or $f$-mode in a stratified layer is altered in such a way that it fans out in the diagnostic $k\omega$ diagram, but with mode power also within the fan. In our simulations, the surface is defined by a temperature and density jump in a piecewise isothermal layer. Unlike our previous work (Singh et al. 2014) where a uniform magnetic field was considered, we employ here a nonuniform magnetic field together with hydromagnetic turbulence at length scales much smaller than those of the magnetic fields. The expansion of the $f$-mode is stronger for fields confined to the layer below the surface. In some of those cases, the $k\omega$ diagram also reveals a new class of low frequency vertical stripes at multiples of twice the horizontal wavenumber of the background magnetic field. We argue that the study of the $f$-mode expansion might be a new and sensitive tool to determining subsurface magnetic fields with longitudinal periodicity.