A study of the unresolved fine-structure model for the solar transition region

The unresolved fine-structure (UFS) model for the lower transition region was proposed by Feldman as an explanation for a number of puzzling observational results: specifically, the small filling factor of this region, the inability of the observations to resolve the temperature structure, and the observation of persistent redshifted UV emission lines even near the solar limb. It was hypothesized that opacity effects may be able to explain the redshift observations. We consider a simple model for the UFS consisting of a plasma sphere undergoing expansion and contraction due to a time-varying heating. We calculate in detail the line profile of the well-observed C IV 1548 Angstrom line. Our calculations include the effects of both nonequilibrium ionization and radiative transfer. We find that although the model can reproduce some of the features of the observations, such as the line widths, the effect of finite optical depth is to produce a blueshifted peak for the emission line, contrary to observations. The physical origins of this blueshift are discussed. We conclude that unless the pressures of the UFS are significantly higher than the typical pressures assumed for the lower transition region, opacity effects are unlikely to explain the observations.