Modelling stellar irradiances III: a predictive model for solar EUV irradiances

Abstract

ABSTRACT We present a new physically driven model for solar spectral irradiances, developed based on a combination of up-to-date atomic data and empirical correlations of line irradiances derived from SDO/EVE (Solar Dynamics Observatory Extreme Ultraviolet Variability Experiment) observations. We have used the correlation of irradiances with the F30 cm radio flux proxy of activity, as it correlates strongly with coronal EUV emission, compared to the F10.7 cm, and it has high values during Solar cycle 25, consistent with a strong solar maximum (Deliporanidou et al. 2025). We incorporate key plasma parameters, such as the electron density and we employ the latest atomic data from chianti v.11 (Dufresne et al. 2024) to reproduce the observed EUV spectral variability with improved accuracy and physical consistency. Our model achieves agreement with the observed spectra from SDO/EVE within approximately 20 per cent across the EUV range. We also include improved models for H i Lyman $\alpha$ series, as well as the He i series in the EUV. We report and quantify for the first time the variability of the He and H Lyman continua over the solar cycle, in terms of the irradiances. Finally, we measure the average coronal density and use it to refine the modelling of density-sensitive EUV lines, and we discuss abundance variations. We confirm the presence of a small first ionization potential effect in the 1 MK corona, while finding that abundances in the 3 MK plasma remain essentially coronal and constant across two solar cycles.