Non-thermal line-broadening in solar prominences

¹ Institute d’Astrophysique (IAP), 98bis bd d’Arago, 75014 Paris, France
² Institut für Astrophysik der Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
e-mail: ewiehr@astrophysik.uni-goettingen.de

Received: 2 October 2013
Accepted: 15 July 2015

Abstract

Aims. We show that the line broadening in quiescent solar prominences is mainly due to non-thermal velocities.

Methods. 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.

Results. We find a ratio of reduced widths, Δλ_D/λ₀, of Hγ and Hδ of 1.05 ± 0.03, which can hardly be attributed to saturation, since both are optically thin for the prominences observed: τ_γ ≤ 0.3, τ_δ ≤ 0.15. We confirm the ratio of reduced widths of He 4772 (triplet) and He 5015 (singlet) of 1.1 ± 0.05 at higher significance and detect a width ratio of Mg b₂ and Mg 4571 (both from the triplet system) of 1.3 ± 0.1.

Conclusions. 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 radiances using models indicate low temperatures down to T_kin ≈ 5000 K. Non-thermal velocities, related to different physical states of the respective emitting prominence region, seem to be the most important line broadening mechanism.