ENERGY-BALANCE IN THE SOLAR TRANSITION REGION .3. HELIUM EMISSION IN HYDROSTATIC, CONSTANT-ABUNDANCE MODELS WITH DIFFUSION

In our previous papers we described the mathematical formalism and the computed results for energy-balance hydrostatic models of the solar transition region. This region, characterized by a steep temperature gradient, is the interface between hot coronal material at 10(6) K and much cooler chromospheric material having temperatures at or below 10(4) K, and forms a narrow emission rim observed at the solar limb. In our model calculations we balance the radiative losses from the transition region with the total energy downflow at each depth in the atmosphere. The models include a detailed treatment of particle diffusion (in this case described by the ambipolar diffusion velocity), including the resulting departures from local ionization equilibrium and the transport of ionization energy. Our previous calculations of the hydrogen spectra agree reasonably well with the observed Lyalpha and Lybeta line intensities and profiles and account for the observed spatial variations of these lines. In this paper we discuss in some detail the limitations of the hydrostatic and one-dimensional assumptions used. Then we analyze the determination of helium emission when diffusion is included. We use transport coefficients estimated from kinetic theory to determine the helium departures from local ionization balance. We calculate the helium spectra for each of our models (A, C, F, and P) and evaluate the role of helium in the energy transport. Also, we investigate the effects of coronal illumination on the structure of the transition region and upper chromosphere, and show how coronal illumination affects various EUV lines and the He I 10830 angstrom line. Comparing with both absolute intensities and detailed line profiles, we show that our models are consistent not only with the observed hydrogen spectra but also with the available helium spectra.