Multiwavelength spectroscopic observations of a quiescent prominence

¹ Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210023, China
² Institut d’Astrophysique Spatiale, CNRS/Université Paris-Sud, Université Paris-Saclay, 91405 Orsay, France
³ Astronomical Institute of the CAS, Fričova 298, 25165 Ondřejov, Czech Republic
⁴ Max Planck Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany

^⋆⋆ Corresponding authors: lfeng@pmo.ac.cn, jean-claude.vial@universite-paris-saclay.fr

Received: 4 December 2023
Accepted: 14 September 2025

Abstract

Aims. In this paper we focus on the analysis of the multiwavelength spectroscopic observations of a quiescent prominence. The spectral and geometrical parameters in the prominence were derived and used to constrain the nonlocal thermodynamic equilibrium (NLTE) radiative transfer models of the prominence. Applying this method with multiwavelength observations provides a good opportunity to reduce the large range of thermodynamic parameters in solar prominences.

Methods. As far as velocities are concerned, we used time-slice and optical flow methods in order to derive the plane-of-sky (POS) velocities in the prominence, and used gravity center and peak position methods on Mg II h&k and H I Lyα profiles to compute the line-of-sight (LOS) velocities. As far as densities and temperatures are concerned, we used the integrated intensities and full width at half maximum (FWHM) values of the Hα and the Ca II H together with Mg II h&k lines to compare with values derived from the NLTE radiative transfer computations. Ionization degree and thickness of the prominence plasma could be further derived.

Results. Opposite flows are observed along two strands between prominence barbs. The POS velocity can reach 20 km s⁻¹ and the largest LOS velocity is > 90 km s⁻¹. The derived electron densities range from 6.5 × 10⁹ to 2.7 × 10¹⁰ cm⁻³ and the derived total hydrogen densities range from 7.4 × 10⁹ to 6.6 × 10¹⁰ cm⁻³ in different regions of the studied prominence. The temperature ranges from 7000 to 14 000 K. The ionization degree of hydrogen is in the range of 0.40 to 0.91. The comparison between averaged and modeled profiles of Mg II and Lyα lines shows that macro-velocities of 15 and 20 km s⁻¹ are required, respectively.

Conclusions. The bulk motions among prominence barbs indicate that the prominence plasma is not confined within magnetic dips but exhibits a large-scale behavior. The presence of high-speed cool plasma flows, along with a wide range of plasma densities and temperatures, suggests that the prominence plasma is far from thermodynamic equilibrium and is inherently dynamic in nature.