Transverse, Propagating Velocity Perturbations in Solar Coronal Loops

TL;DR

This paper reviews observations and simulations of transverse velocity perturbations in solar coronal loops, explaining their damping via mode coupling and suggesting possible turbulence development.

Contribution

It combines 3D numerical simulations and analytical models to explain the damping of transverse waves and explores high-frequency power in observations indicating turbulence onset.

Findings

Mode coupling causes damping of transverse waves in coronal loops.

Wave profiles transition from Gaussian to exponential decay with height.

High-frequency power excess suggests potential wave turbulence.

Abstract

This short review paper gives an overview of recently observed transverse, propagating velocity perturbations in coronal loops. These ubiquitous perturbations are observed to undergo strong damping as they propagate. Using 3D numerical simulations of footpoint-driven transverse waves propagating in a coronal plasma with a cylindrical density structure, in combination with analytical modelling, it is demonstrated that the observed velocity perturbations can be understood in terms of coupling of different wave modes in the inhomogeneous boundaries of the loops. Mode coupling in the inhomogeneous boundary layers of the loops leads to the coupling of the transversal (kink) mode to the azimuthal (Alfven) mode, observed as the decay of the transverse kink oscillations. Both the numerical and analytical results show the spatial profile of the damped wave has a Gaussian shape to begin with, before switching to exponential decay at large heights. In addition, recent analysis of CoMP (Coronal Multi-channel Polarimeter) Doppler shift observations of large, off- limb, trans-equatorial loops shows that Fourier power at the apex appears to be higher in the high-frequency part of the spectrum than expected from theoretical models. This excess high-frequency FFT power could be tentative evidence for the onset of a cascade of the low-to-mid frequency waves into (Alfvenic) turbulence.