FORMATION OF Ca II LINES IN SOLAR PROMINENCES

We compute the profiles of the resonance lines and infrared triplet of ionized calcium emitted by some representative models of prominences. These models consist of plane-parallel slabs of different temperatures, pressures and thicknesses, standing vertically above the solar surface. These slabs are assumed to be observed at the limb, and to be perpendicular to the line of sight. They are irradiated by the Sun on both sides, and the incoming intensities, at every relevant wavelength, are taken from observations. The model atom includes 3 stages of ionization (Cai, Caii and Caiii) with 5 discrete levels for Caii. We study the relations between emitted intensities and physical properties of the slabs, and compare the intensities emitted in Caii and hydrogen lines for the same set of models. As a result of ionization, the intensity decreases more rapidly with temperature in Caii lines than in Hi lines, so that the ratio of Caii to Hi line intensities may be used as a temperature indicator. The intensity ratio between resonance and infrared lines of Caii depends principally on the optical thickness of the structure. At high pressure and low temperature, the ratio Caii 8542Å/H is found to increase with pressure. This behaviour, which is due to the saturation of H, is opposite to that found by Heasley and Milkey (1978) for low pressures (optically thin structures).