Mass motions in optically thin solar transition zone lines

The widths of optically thin EUV spectral lines formed at temperatures between $$\simeq 3\times 10^4$$ and 2 × 10⁵ K in the solar transition zone are larger than expected assuming ionization equilibrium. This result has been interpreted as due to non-thermal mass motions, characterized by a parameter $$\xi\enspace(\text {km s}^{-1})$$. In this paper we discuss observations of spectral line profiles recorded mostly at +2 arcsec outside the limb of the north and south polar coronal holes, and compare the derived values of ξ to values deduced from spectra of quiet Sun regions. These spectra were recorded by the NRL spectrograph on Skylab (1100–2000 Å). Systematic differences in line widths between coronal holes and quiet Sun region spectra are probably no larger than 0.01 Å, for the optically thin lines used in the analysis. Line-of-sight path lengths of the emitting plasma at 2 arcsec above the limb are calculated from the intensity ratios of optically thick lines of C IV, N V, Al III, and Si IV. These ratios should be 2:1 in the optically thin limit, but they are around 1.5 at 2 arcsec above the solar limb. The derived path lengths range from ≃ 1000 to 5000 km.