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

TL;DR

This study models large-scale velocity distributions in flaring twisted coronal loops to interpret non-thermal line broadening in EUV spectral lines, linking observed spectral features to physical processes in the solar atmosphere.

Contribution

The paper introduces a resistive MHD model of twisted magnetic fluxtubes that reproduces observed spectral line broadening and shifts in flaring coronal loops, connecting spectral features to plasma heating and turbulence.

Findings

Velocity dispersions increase with temperature from 20-30 km/s at 1 MK to 200-400 km/s at 10-20 MK.

Model results show qualitative and quantitative agreement with observed spectral line broadening.

Velocity dispersions are lower across magnetic fields near foot-points, indicating anisotropic turbulence.

Abstract

Observation of coronal EUV spectral lines sensitive to different temperatures offers an opportunity to evaluate the thermal structure and flows in flaring atmospheres. This can be used to estimate the partitioning between the thermal and kinetic energies. Our aim is to model large-scale (50-10000km) velocity distributions in order to interpret non-thermal broadening of different spectral EUV lines. The developed models allow us to understand the origin of the observed spectral line shifts and broadening, and link these features to particular physical effects in flaring atmospheres. We use ideal MHD to derive twisted magnetic fluxtube configurations in a stratified atmosphere. The evolution of these fluxtubes is followed using resistive MHD, with anomalous resistivity depending on the local density and temperature. The model also takes into account the thermal conduction and radiative losses. 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. Our models show qualitative and quantitative agreement with observations. The line-of-sight (LOS) velocity dispersions demonstrate substantial correlation with the temperature, increasing from about 20-30 km/s around 1 MK to about 200-400 km/s near 10-20 MK. The average LOS velocities also correlate with velocity dispersions, although they demonstrate a very strong scattering, compared to observations. We also note that near foot-points 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.