Non-thermal line broadening due to braiding-induced turbulence in solar coronal loops

¹ School of Mathematical and Physical Sciences, University of Newcastle, Callaghan, NSW 2308, Australia
e-mail: david.pontin@newcastle.edu.au
² School of Science and Engineering, University of Dundee, Nethergate, Dundee DD1 4HN, UK
³ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

Received: 26 January 2020
Accepted: 11 May 2020

Abstract

Aims. Emission line profiles from solar coronal loops exhibit properties that are unexplained by current models. We investigate the non-thermal broadening associated with plasma heating in coronal loops that is induced by magnetic field line braiding.

Methods. We describe the coronal loop by a 3D magnetohydrodynamic model of the turbulent decay of an initially-braided magnetic field. From this, we synthesised the Fe XII line at 193 Å that forms around 1.5 MK.

Results. The key features of current observations of extreme ultraviolet (UV) lines from the corona are reproduced in the synthesised spectra: (i) Typical non-thermal widths range from 15 to 20 km s⁻¹. (ii) The widths are approximately independent of the size of the field of view. (iii) There is a correlation between the line intensity and non-thermal broadening. (iv) Spectra are found to be non-Gaussian, with enhanced power in the wings of the order of 10–20%.

Conclusions. Our model provides an explanation that self-consistently connects the heating process to the observed non-thermal line broadening. The non-Gaussian nature of the spectra is a consequence of the non-Gaussian nature of the underlying velocity fluctuations, which is interpreted as a signature of intermittency in the turbulence.