Helium Emissions Observed in Ground-Based Spectra of Solar Prominences

The only prominent line of singly ionized helium in the visible spectral range, He ii 4686 Å, is observed together with the He i 5015 Å singlet and the He i 4471 Å triplet line in solar prominences. The Na D 2 emission is used as a tracer for He ii emissions which are sufficiently bright to exceed the noise level near 10 −6 of the disk-center intensity. The prominences thus selected are characterized by small non-thermal line broadening and almost absent velocity shifts, yielding narrow line profiles without wiggles. The reduced widths [Δ λ D / λ ] of He ii 4686 Å are 1.5 times broader than those of the He i 4471 Å triplet and 1.65 times broader than those of the He i 5015 Å singlet. This indicates that the He lines originate in a prominence–corona transition region with outwards increasing temperature.