Modelling Stellar Irradiances – II. Correlations of solar irradiances with proxies of activity along a cycle

ABSTRACT There is a pressing need to model X-ray Ultra-Violet (XUV: 1–30 nm) stellar irradiances, given the scarcity of current measurements. One of the measurable effects of a stellar cycle is the significant (more than one order of magnitude) variation in XUV irradiance. As a first step in modelling stellar irradiances, we present Extreme Ultraviolet (EUV) irradiances in a sample of strong spectral lines formed in different layers and regions of the solar atmosphere, obtained from Solar Dynamics Observatory Extreme Ultraviolet Variability Experiment (SDO EVE). These irradiances span half a solar cycle. We present correlations with several proxies of solar activity, such as the Mg ii index, sunspot numbers, and cm radio fluxes. Among these, the sunspot number proves to be the poorest proxy, whereas the Mg ii index is a very good proxy for coronal lines (hotter temperature lines). We find a relatively strong linear relationship, which enables us to build a model essential for various applications. Additionally, we compare our results with the previous EUV standard solar irradiances reported by Del Zanna and Andretta 2014, derived from Solar and Heliospheric Observatory Coronal Diagnostic Spectrometer (SOHO CDS) data, as well as historical records from the literature. We have also run a DEM analysis on Quiet Sun (QS) and Active Regions (AR) and list the blends and formation temperatures for the strong lines. Finally, we provide a simple routine for deriving the irradiances of the strong lines based on proxy values.