Radiative transfer effects on hydrogen (and helium) in the solar atmosphere

In this work we present Non-Local Thermodynamic Equilibrium (non-LTE) computations for hydrogen for a VAL-C model of the Sun's atmosphere. The solar atmosphere is represented by a one-dimensional plane-parallel horizontal slab. The purpose of this study is to investigate the effects of the transfer of radiation in the chromosphere and the transition region. In particular, we aim at understanding how the radiative losses in the energy balance for electrons are affected by the non-LTE radiative transfer, which has to be considered in the regions where the temperature is less than 25 000 K. The numerical code used here allows us to study the properties of, and the spectrum emitted by, the hydrogen particles. The non-LTE radiative transfer equations (RT) are solved for all optically thick resonance lines. The solutions of the RT in the optically thick lines affect all population densities of atoms and ions through the statistical equilibrium equations (SE). For the VAL-C atmosphere model there is a peak around 6 10³ K in the net radiative cooling rates due to several lines and continua from hydrogen. To our knowledge this peak has never been considered when evaluating the radiative losses in the chromosphere in the frame of solar wind modelling. We mention some consequences for solar wind models in the description of the chromosphere and the transition region which is often made under the assumptions of full ionization and optically thin plasma.