Non‐LTE Radiative Transfer in Model Prominences. I. Integrated Intensities of HeiTriplet Lines

Abstract

In this work we use new results of radiative transfer calculations out of local thermodynamical equilibrium to study the triplet lines emitted by neutral helium in solar quiescent prominences. We compare two types of prominence atmospheres: isothermal and isobaric models versus nonisothermal and nonisobaric ones. We can thus investigate the effect of the presence of a prominence-to-corona transition region (PCTR) on the emergent intensities in detail. It is found that the presence of the PCTR affects the emitted intensities of the triplet lines, even though they are formed in the central parts of the prominence. We show that the inclusion of a transition region reduces the impact of collisional excitation at high temperatures in comparison with the isothermal and isobaric case. A simple study of helium energy level populations shows how statistical equilibrium is changed when a transition region is present. This points to the necessity of including an interface between the prominence body and the corona to predict all emergent intensities, whatever the region of formation of the radiation. We have found a correlation between most of the He I triplet line ratios and the altitude of the model prominence. Comparisons of our predicted intensity ratios with observations yield generally good agreement. Remaining discrepancies may be resolved by extrapolating our predicted results to higher altitudes.