The solar XUV He I and He II emission lines. II - Intensity ratios and distribution functions

From high-resolution solar images we show that the He ii A256 line intensity is very nearly uncorrelated with He ii A3O4 at the same location on the Sun and that the A256 line is formed mainly by the photoionization-recombination process. We also derive center-to-limb variations of He ii A3O4 and A256 and He I A584 and A537 for network and cell regions separately and find that (1), in both network and cells, A3O4 and A584 each limb-brighten in the quiet Sun and limbdarken in coronal holes and (2), for both A3O4 and A584, network and cell regions are each brighter in the quiet Sun than in coronal holes. We conclude that the appearance of dark coronal holes in the helium lines is not a geometrical effect involving the chromospheric network, but is rather an intrinsic property of the atmosphere in both network and cell regions. We suggest that the network and cells can be treated as isolated atmospheres in the solution of the transfer equation in the helium lines. Subject headings: Sun: chromosphere - Sun: corona - Sun: spectra - ultraviolet: spectra