Helium Emissions Observed in Ground-Based Spectra of Solar Prominences

TL;DR

This study reports ground-based observations of helium emission lines in solar prominences, revealing that helium lines originate in a transition region with increasing temperature outward, based on spectral line analysis.

Contribution

It provides new observational data on helium emissions in prominences and interprets the origin of these lines as coming from a transition region with temperature gradients.

Findings

Helium-II 4686 A line is brighter than noise level near 10^-6 of disk-center intensity.

Helium lines are narrower with minimal velocity shifts, indicating low non-thermal broadening.

Helium-II lines are broader than helium-I lines, suggesting formation in a transition region with increasing temperature.

Abstract

The only prominent line of singly ionized helium in the visible spectral range, helium-II 4686 A, is observed together with the helium-I 5015 A singlet and the helium-I 4471 A triplet line in solar prominences. The sodium emission, NaD2, is used as a tracer for helium-II emissions which are sufficiently bright to exceed the noise level near 10^-6 of the disk-center intensity. The so selected prominences are characterized by small non-thermal line broadening and almost absent velocity shifts, yielding narrow line profiles without wiggles. The reduced widths [Delta(lambda_D) / lambda] of helium-II 4686 A are 1.5 times broader than those of helium-I 4471 A triplet and 1.65 times broader than those of helium-I 5015 A singlet. This indicates that the helium lines originate in a prominence--corona transition region with outwards increasing temperature.