Non-thermal line-broadening in solar prominence

TL;DR

This study demonstrates that non-thermal velocities are the primary cause of line broadening in quiescent solar prominences, supported by simultaneous multi-line observations and analysis of line width ratios.

Contribution

It provides the first detailed analysis of line width ratios across multiple lines in prominences, emphasizing the dominance of non-thermal velocities over thermal effects.

Findings

Non-thermal velocities dominate line broadening in prominences.

Line width ratios support the presence of non-thermal motions.

Observed low kinetic temperatures suggest non-thermal effects are key.

Abstract

We show that the line broadening in quiescent solar prominences is mainly due to non-thermal velocities. We have simultaneously observed a wide range of optically thin lines in quiescent prominences, selected for bright and narrow Mg\,b emission without line satellites from macro-shifts. We find a ratio of reduced widths of H-gamma and H-delta of 1.05 +-0.03 which can hardly be attributed to saturation, since both are optically thin for the prominences observed: tau(gamma)<0.3 ; tau(delta)<0.15. We confirm the ratio of reduced widths of He4772(triplet) and He5015(singlet of 1.1 +-0.05 at higher significance and detect a width ratio of Mgb2 and Mg4571 (both from the triplet system) of 1.3 +-0.1. The discrepant widths of lines from different atoms, and even from the same atom, cannot be represented by a unique pair [T_kin ; V_nth]. Values of T_kin deduced from observed line radiance using models, indicate low temperatures down to T_kin~5000K. Non-thermal velocities, related to different physical states of the respective emitting prominence region, seem to be the most important line broadening mechanism.