Impulsively generated wave trains in coronal structures: II. Effects of transverse structuring on sausage waves in pressureless slabs

TL;DR

This paper investigates how transverse density structuring affects impulsively generated sausage wave trains in coronal slabs, revealing that spectral features depend on density profiles and can inform on sub-resolution structuring with high-cadence observations.

Contribution

It provides a comprehensive analytical and numerical analysis of trapped sausage modes in coronal slabs, highlighting differences from cylindrical geometries and implications for interpreting observations.

Findings

Coronal slabs trap sausage waves with longer wavelengths than cylinders.

Cutoff wavenumbers depend on the density profile steepness.

Morlet spectra are shaped by group speed curves, revealing sub-structure details.

Abstract

Impulsively generated sausage wave trains in coronal structures are important for interpreting a substantial number of observations of quasi-periodic signals with quasi-periods of order seconds. We have previously shown that the Morlet spectra of these wave trains in coronal tubes depend crucially on the dispersive properties of trapped sausage waves, the existence of cutoff axial wavenumbers and the monotonicity of the dependence of the axial group speed on the axial wavenumber in particular. This study examines the difference a slab geometry may introduce, for which purpose we conduct a comprehensive eigenmode analysis, both analytically and numerically, on trapped sausage modes in coronal slabs with a considerable number of density profiles. For the profile descriptions examined, coronal slabs can trap sausage waves with longer axial wavelengths, and the group speed approaches the internal Alfv\'en speed more rapidly at large wavenumbers in the cylindrical case. However, common to both geometries, cutoff wavenumbers exist only when the density profile falls sufficiently rapidly at distances far from coronal structures. Likewise, the monotonicity of the group speed curves depends critically on the profile steepness right at the structure axis. Furthermore, the Morlet spectra of the wave trains are shaped by the group speed curves for coronal slabs and tubes alike. Consequently, we conclude that these spectra have the potential for telling the sub-resolution density structuring inside coronal structures, although their detection requires an instrumental cadence of better than $\sim 1$ second.