Improving the atomic modelling for solar UV radiative transfer calculations

TL;DR

This paper enhances atomic models in solar UV radiative transfer calculations by incorporating new data and updated processes, leading to improved agreement with observations and reducing the need for ad hoc opacity adjustments.

Contribution

It introduces updated atomic data and modeling for low charge states of C, Si, and S into the Lightweaver code, improving UV continuum predictions.

Findings

Better match with quiet Sun UV observations.

Elimination of the need for 'missing opacity' hypothesis.

Significant impact on UV continuum modeling.

Abstract

Radiative transfer calculations have been produced over the years for many lines and continua in the UV wavelength range of solar and cool stellar atmospheres for a variety of conditions. Despite significant improvements in computing power and availability of atomic data over time, atomic models are often still limited in size and rely on approximations for data. There have also been inconsistencies in the way photo-ionisation and radiative recombination have been treated. Here, we incorporate into the Lightweaver radiative transfer code new data and updated modelling of atomic processes for the low charge states of C, Si and S. Data are taken from the CHIANTI database and other widely-available sources for the relevant elements. We show the significant impact this has on the UV continua in the 1100-1700{\AA} region, especially for Si. The results are in much better agreement with averaged, quiet Sun observations, and remove the need to invoke "missing opacity" to resolve discrepancies. The present treatment has important implications for radiative transfer calculations and the model atmospheres used as inputs.