Plasma motions and non-thermal line broadening in flaring twisted coronal loops

¹ Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, UK
e-mail: mykola.gordovskyy@manchester.ac.uk
² Astronomy & Astrophysics Group, University of Glasgow, Glasgow G15 8QQ, UK

Received: 25 August 2015
Accepted: 27 January 2016

Abstract

Context. Observation of coronal extreme ultra-violet (EUV) spectral lines sensitive to different temperatures offers an opportunity to evaluate the thermal structure and flows in flaring atmospheres. This, in turn, can be used to estimate the partitioning between the thermal and kinetic energies released in flares.

Aims. Our aim is to forward-model large-scale (50–10 000 km) velocity distributions to interpret non-thermal broadening of different spectral EUV lines observed in flares. The developed models allow us to understand the origin of the observed spectral line shifts and broadening, and link these features to particular physical phenomena in flaring atmospheres.

Methods. We use ideal magnetohydrodynamics (MHD) to derive unstable twisted magnetic fluxtube configurations in a gravitationally stratified atmosphere. The evolution of these twisted fluxtubes is followed using resistive MHD with anomalous resistivity depending on the local density and temperature. The model also takes thermal conduction and radiative losses in the continuum into account. The model allows us to evaluate average velocities and velocity dispersions, which would be interpreted as non-thermal velocities in observations, at different temperatures for different parts of the models.

Results. Our models show qualitative and quantitative agreement with observations. Thus, the line-of-sight (LOS) velocity dispersions demonstrate substantial correlation with the temperature, increasing from about 20–30 km s^-1 around 1 MK to about 200–400 km s^-1 near 10–20 MK. The average LOS velocities also correlate with velocity dispersions, although they demonstrate a very strong scattering compared to the observations. We also note that near footpoints the velocity dispersions across the magnetic field are systematically lower than those along the field. We conclude that the correlation between the flow velocities, velocity dispersions, and temperatures are likely to indicate that the same heating mechanism is responsible for heating the plasma, its turbulisation, and expansion/evaporation.