2021ApJ...908..205A

Solar extreme-ultraviolet (EUV: 100-1100 Å) radiation rises in the 10⁴ K upper chromosphere and hotter layers extending into the million-degree corona. Much of the EUV normally is not visible in other stars owing to interstellar H I absorption. The present study derived power-law scaling relations for EUV species based on irradiance spectra from the Extreme Ultraviolet Variability Experiment on board the Solar Dynamics Observatory, over the rise and fall of recent sunspot Cycle 24. These relations not only can test solar models, but also allow "invisible" stellar EUV emissions to be reconstructed from more accessible ultraviolet (UV: 1100-3000 Å) proxies, with implications for heating, ionization, and erosion of exoplanet atmospheres by Sun-like hosts. Hot EUV coronal tracers like Si XII 520 Å (8 MK) show strongly curved power laws relative to chromospheric H I 1025 Å Lyβ, as was seen in Paper I for 0.2-2 keV soft X-rays versus Mg II 2800 Å. The brightest EUV feature, He II 303 Å, exhibits a nearly 1:1 correlation with Lyβ (and H I 1215 Lyα, but slightly steeper than 1:1 relative to Mg II). The second brightest, C III 977 Å, has a shallower power-law slope of ∼0.5. Correlations for low first ionization potential species, such as Mg, Si, and Fe, might be affected by cycle-dependent chemical fractionation. Parameterized flux-flux relations are presented for most of the strong EUV (and selected UV) features, together with fluxes of individual lines and broad EUV wavelength bands at the highs and lows of Cycle 24.