# The EUV spectrum of the Sun: quiet and active Sun irradiances and chemical composition

## TL;DR

This study benchmarks new atomic data against EUV irradiance measurements of the quiet and active Sun, revealing close agreement with observations and insights into solar composition and plasma variations across different activity levels.

## Contribution

It provides a comprehensive validation of atomic data against EUV spectra and demonstrates that the solar corona's composition is nearly photospheric up to 1 MK, with variations at higher temperatures.

## Key findings

- Excellent agreement (within 20%) between predicted and observed irradiances.
- Coronal composition up to 1 MK is nearly photospheric.
- Higher-temperature plasma shows factor of 2 variation in active regions.

## Abstract

We benchmark new atomic data against a selection of irradiances obtained from medium-resolution quiet Sun spectra in the EUV, from 60 to 1040 \AA. We use as a baseline the irradiances measured during solar minimum on 2008 April 14 by the prototype (PEVE) of the Solar Dynamics Observatory Extreme ultraviolet Variability Experiment (EVE). We take into account some inconsistencies in the PEVE data, using flight EVE data and irradiances we obtained from Solar & Heliospheric Observatory (SoHO) Coronal Diagnostics Spectrometer (CDS) data. We perform a differential emission measure and find overall excellent agreement (to within the accuracy of the observations, about 20%) between predicted and measured irradiances in most cases, although we point out several problems with the currently available ion charge state distributions. We used the photospheric chemical abundances of Asplund et al. (2009). The new atomic data are nearly complete in this spectral range, for medium-resolution irradiance spectra. Finally, we use observations of the active Sun in 1969 to show that also in that case the composition of the solar corona up to 1 MK is nearly photospheric. Variations of a factor of 2 are present for higher-temperature plasma, which is emitted within active regions.These results are in excellent agreement with our previous findings.