EFFECTS OF MG II AND CA II IONIZATION ON ABINITIO SOLAR CHROMOSPHERE MODELS

We compute acoustically heated solar chromosphere models considering radiation damping by (non-LTE) emission from H- and by Mg II and Ca II emission lines. The radiative transfer equations for the Mg II k and Ca II K emission lines are solved using the core-saturation method with complete redistribution. The Mg II k and Ca II K cooling rates are compared with the VAL model C. We include several substantial improvements over the work of Ulmschneider et al. (1987). First, we remove the dependency of the results from the core-wing separation points by improving the frequency integration method. Second, we treat the ionization equilibria of Mg II-III and Ca II-III in a non-LTE approximation using a two-level atom (instead of LTE). We find that the rapid temperature rises caused by the ionization of Mg II are not formed in the middle chromosphere, but occur at larger atmospheric heights. Our models represent the temperature structure of the "real" solar chromosphere much better. This result is a major precondition for the study of ab-initio models for solar flux tubes based on MHD wave propagation and also for ab-initio models for the solar transition layer. We have checked the dependence of our results on the radiative transfer method by comparing our computations with those using the operator perturbation method of Kalkofen (1984) and those using the method of Carlsson (1986), which combines complete linearization techniques with approximate lambda operators.