NLTE model calculations for the solar atmosphere with an iterative treatment of opacity distribution functions

Context. Modeling the variability of the solar spectral irradiance is a key factor in understanding the Sun's influence on the climate of the Earth. Aims. As a first step toward calculating the solar spectral irradiance variations, we reproduce the solar spectrum for the quiet Sun over a broad wavelength range with an emphasis on the UV. Methods. We introduce the radiative transfer code COSI, which calculates solar synthetic spectra under conditions of non-local thermodynamic equilibrium (NLTE). A self-consistent simultaneous solution of the radiative transfer and the statistical equation for the level populations guarantees that the correct physics is considered for wavelength regions where the assumption of local thermodynamic equilibrium (LTE) breaks down. The new concept of iterated opacity distribution functions (NLTE-ODFs) is presented, through which all line opacities are included in the NLTE radiative transfer calculation. Results. We show that it is essential to include the line opacities in the radiative transfer to reproduce the solar spectrum in the UV. Conclusions. Through the implemented scheme of NLTE-ODFs, the COSI code is successful in reproducing the spectral energy distribution of the quiet Sun.