Photon scattering in the solar ultraviolet lines of He i and He ii

Observations made with the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory (SOHO) are used to investigate the behaviour of the intensities of the emission lines of He i, He ii and O iii at the quiet Sun-centre and at θ= 60° towards the equatorial limb. The aim is to examine the possible effects of photon scattering on the spatial variation of the optically thick helium lines. At the quiet Sun-centre, we find that, in agreement with previous work, the ratios of the intensities of the He i 584-Å and He ii 304-Å lines to those of the O iii 600-Å line decrease systematically as the intensity of the O iii line increases. However, we find that the dependence of these ratios on the O iii intensity is not unique, but differs between the individual regions studied. Similar results are found at θ= 60°. We have also used line intensities and intensity ratios to investigate limb-to-disc effects and variations across a sample of supergranulation cell boundaries and adjacent cell interiors at both locations. The results do not exclude photon scattering as the cause of the larger observed ratios in cell interiors. The differences between the apparent widths of boundaries in O iii at Sun-centre and 60° show that the emitting material is extended in height, which will aid the process of scattering into cell interiors. Photon scattering could also account for the lack of oscillations in the He i intensities in a cell interior studied by Pietarila & Judge. Three-dimensional radiative transfer calculations in chosen geometries are now needed to account for the observations in detail.