Resonant absorption in complicated plasma configurations: applications to multi-stranded coronal loop oscillations

TL;DR

This study investigates how transverse oscillations in a complex, multi-stranded coronal loop are excited and damped through resonant absorption, revealing energy transfer mechanisms in realistic solar plasma structures.

Contribution

It introduces a more realistic multi-stranded loop model and demonstrates resonant absorption and mode conversion despite complex geometries.

Findings

Resonant absorption causes damping of transverse oscillations.

Energy conversion occurs both at the loop edge and inside the structure.

Global modes couple with local Alfvén waves regardless of geometry.

Abstract

We study the excitation and damping of transverse oscillations in a multi-stranded model of a straight line-tied coronal loop. The transverse geometry of our equilibrium configuration is quite irregular and more realistic than the usual cylindrical loop model. By numerically solving the time-dependent ideal magnetohydrodynamic equations in two dimensions we show how the global motion of the whole bundle of strands, excited by an external disturbance, is converted into localized Alfv\'enic motions due to the process of resonant absorption. This process produces the attenuation of the transverse oscillations. At any location in the structure two dominant frequencies are found, the frequency of the global mode, or quasi-mode, and the local Alfv\'en frequency. We find that the mechanism of mode conversion, due to the coupling between fast and Alfv\'en waves, is not compromised by the complicated geometry of the model. We also show that it is possible to have energy conversion not only at the external edge of the composite loop but also inside the structure. The implications of these results and their relationship with the observations are discussed.