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
Received: 2 October 2013
Accepted: 15 July 2015
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, ΔλD/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: τγ ≤ 0.3, τδ ≤ 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 [Tkin; Vnth]. Values of Tkin deduced from observed line radiances using models indicate low temperatures down to Tkin ≈ 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.
Key words: Sun: filaments, prominences
© ESO, 2015