The Heating of Solar Coronal Loops by Alfvén Wave Turbulence

• Published in: The Astrophysical journal Vol. 849; no. 1; pp. 46 - 68
• Main Authors: van Ballegooijen, A. A., Asgari-Targhi, M., Voss, A.
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.11.2017; IOP Publishing
• Subjects: ALFVEN WAVES; Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; Braiding; CHROMOSPHERE; COMPARATIVE EVALUATIONS; Computational fluid dynamics; Computer simulation; Constraint modelling; Corona; Coronal loops; DISTRIBUTION; EMISSION; Emission measurements; Fluctuations; Fluid flow; Flux; HEAT; Heating; HEATING RATE; Line spectra; MAGNETIC FIELDS; Magnetohydrodynamic turbulence; MAGNETOHYDRODYNAMICS; magnetohydrodynamics (MHD); PHOTOSPHERE; RANDOMNESS; SIMULATION; SOLAR CORONA; SPACE; SUN; Sun: corona; Sun: magnetic fields; Three dimensional models; THREE-DIMENSIONAL CALCULATIONS; Tubes; TURBULENCE; waves
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aa9118

In this paper we further develop a model for the heating of coronal loops by Alfvén wave turbulence (AWT). The Alfvén 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 magnetohydrodynamic 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 nonthermal widths are predicted to be about 27 km s−1, 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.