Helium Emissions Observed in Ground-Based Spectra of Solar Prominences

Abstract

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 [ 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.

Non-thermal line-broadening in solar prominences

“…Ramelli et al (2012) find a small reduced width of Δλ D /λ 0 (4571) = 0.9 × 10 −5 . We verified this at Locarno in Oct. 2013 and again find Δλ D /λ 0 = 1.0 × 10 −5 .…”

“…Recently, Ramelli et al (2012) and Wiehr et al (2013) resumed such observations and found that "cool" emissions (e.g. of Na D or Mg b) are accompanied by rather broad Table 3, here with error bars).…”

“…These observations were taken consecutively and might thus be influenced by time evolution or drifts of the prominence over the spectrograph slit. Ramelli et al 2012), we selected quiescent prominences with bright but narrow Mg b emissions without marked macro shifts, thus assuring a "spectral photon concentration". The positions of the observed prominences are listed in Table 1.…”

“…For a determination of absolute spectral radiance values, we took spectra of the disk centre with an integration time of 1 s. Ramelli et al (2012) describe the reduction procedure. We integrated the line profiles over 15 pixels in the spatial direction, corresponding to 3.4 arcsec, i.e.…”

Evidence for the Two-fluid Scenario in Solar Prominences

“…Prominence emission lines are superposed by parasitic light originating from imperfect telescope optics (rather than from Earth's atmosphere; see Stellmacher & Wiehr, 1970). We take spectra of the "aureola" in the immediate (emission-free) prominence neighborhood with almost the same slit inclination to the solar limb.…”