Prediction of line intensity ratios in solar prominences

Solar prominences are made of relatively cool and dense plasma embedded in the solar corona, supported and structured by the magnetic field. Since this plasma is definitely out of LTE, the diagnosis of physical conditions in prominences needs the use of specific radiative transfer (RT) codes to predict the spectrum emitted by models and compare it to observations. For optically thin lines, the solution of RT equations in the transition itself is not required, but the emitted intensities depend, via the statistical equilibrium equations, on RT in other transitions which are optically thick. We use two different sets of models. The first one contains monolithic models defined by 5 parameters: temperature, pressure, thickness, microturbulent velocity and altitude above the solar surface. For each parameter, we assume a range of variation. For each model, the values of the 5 parameters are randomly chosen within the corresponding range of variation. The second set contains composite models made of multiple layers, in order to simulate the penetration of radiation into inhomogeneous prominences. We use NLTE radiative transfer codes to compute the intensities of the lines of hydrogen, helium and calcium emitted by each model. So, for any couple of lines, we may obtained their intensity ratio as a function of the 5 parameters. We discuss the behaviour of some of these intensity ratios as a function of the principal parameters and construct distribution diagrams, which are compared to different published observations.