The Heating of Solar Coronal Loops by Alfven Wave Turbulence

TL;DR

This study models Alfven wave turbulence as a mechanism for heating solar coronal loops, finding it can produce some heating but not enough to match observed temperatures and spectral widths, suggesting the need for alternative explanations.

Contribution

The paper advances the understanding of coronal heating by developing a 3D MHD model of Alfven wave turbulence and comparing its predictions with observational constraints.

Findings

AWT can heat loops to about 2.5 MK, below observed 3-4 MK.

Simulated heating events are short-lived (<1 min) and do not match emission measure distributions.

Predicted spectral line widths are higher than observed, challenging AWT as the sole heating mechanism.

Abstract

In this paper we further develop a model for the heating of coronal loops by Alfven wave turbulence (AWT). The Alfven waves are assumed to be launched from a collection of kilogauss flux tubes in the photosphere at the two ends of the loop. Using a three-dimensional magneto-hydrodynamic (MHD) model for an active-region loop, we investigate how the waves from neighboring flux tubes interact in the chromosphere and corona. For a particular combination of model parameters we find that AWT can produce enough heat to maintain a peak temperature of about 2.5 MK, somewhat lower than the temperatures of 3-4 MK observed in the cores of active regions. The heating rates vary strongly in space and time, but the simulated heating events have durations less than 1 minute and are unlikely to reproduce the observed broad Differential Emission Measure distributions of active regions. The simulated spectral line non-thermal widths are predicted to be about 27 km/s, which is high compared to the observed values. Therefore, the present AWT model does not satisfy the observational constraints. An alternative "magnetic braiding" model is considered in which the coronal field lines are subject to slow random footpoint motions, but we find that such long period motions produce much less heating than the shorter period waves launched within the flux tubes. We discuss several possibilities for resolving the problem of producing sufficiently hot loops in active regions.