On the Nature of Nonthermal Broadening of Spectral Lines Observed by IRIS

TL;DR

This study investigates the physical origins of nonthermal broadening in solar spectral lines, revealing that plasma flows along magnetic fields and velocity gradients significantly contribute to the observed broadening.

Contribution

It demonstrates a correlation between nonthermal broadening, magnetic field alignment, and plasma flows, providing new insights into the processes causing spectral line broadening in the solar transition region.

Findings

Nonthermal broadening correlates with magnetic field alignment.

Downward plasma flows contribute to line broadening.

Velocity gradients within flowing plasma influence spectral line widths.

Abstract

The origin of nonthermal broadening in solar spectra is one of the long-standing questions in solar physics. Various processes have been invoked - including unresolved flows, waves, and turbulent processes - but definitive answers are lacking. To investigate the physical processes responsible for nonthermal broadening, we examine its relation with the angle between the magnetic field and the line of sight in three different closed-field regions above plage regions at different locations on the solar disk. We obtained the nonthermal width of transition-region Si IV 1403 {\AA} spectra observed in active regions by the Interface Region Imaging Spectrograph, after subtraction of the thermal and instrumental line broadening. To investigate the dependence of the measured broadening on the viewing angle between the line of sight and magnetic field direction, we determined the magnetic field direction at transition-region heights using nonlinear force-free extrapolations based on the observed photospheric vector magnetic field taken by the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We found that the nonthermal broadening shows a correlation with downward motion (redshifts)and alignment between the magnetic field and the observer's line-of-sight direction. Based on the observed correlations, we suggest that velocity gradients within plasma flowing down along the magnetic field may lead to a significant portion of the observed nonthermal broadening of transition-region spectral lines in closed fields above plage regions.