Forward Modelling of Standing Kink Modes in Coronal Loops I. Synthetic Views

TL;DR

This paper models the synthetic emission of a coronal loop supporting kink MHD waves, revealing how different wave components affect observable spectral features and providing insights into coronal loop diagnostics.

Contribution

It introduces a novel semi-torus model of a coronal loop with detailed wave decomposition, linking MHD wave theory to observable spectral signatures.

Findings

Spectral line broadening caused by quadrupole term.

Oscillatory intensity and width variations at half the kink period.

Quadrupole term observable as pendular motion in front view.

Abstract

Kink magnetohydrodynamic (MHD) waves are frequently observed in various magnetic structures of the solar atmosphere. They may contribute significantly to coronal heating and could be used as a tool to diagnose the solar plasma. In this study, we synthesise the \ion{Fe}{9} $\lambda171.073$ emission of a coronal loop supporting a standing kink MHD mode. The kink MHD wave solution of a plasma cylinder is mapped into a semi-torus structure to simulate a curved coronal loop. We decompose the solution into a quasi-rigid kink motion and a quadrupole term, which dominate the plasma inside and outside the flux tube, respectively. At the loop edges, the line-of-sight integrates relatively more ambient plasma, and the background emission becomes significant. The plasma motion associated with the quadrupole term causes spectral line broadening and emission suppression. The periodic intensity suppression will modulate the integrated intensity and the effective loop width, which both exhibit oscillatory variations at half of the kink period. The quadrupole term can be directly observed as a pendular motion at front view.