MODELING SOLAR EXTREME-ULTRAVIOLET IRRADIANCE VARIABILITY USING EMISSION MEASURE DISTRIBUTIONS

We introduce a new model of solar irradiance variability at extreme ul traviolet (EUV) wavelengths. The model combines a spectral emission li ne database, solar emission measure distributions, and estimates from ground-based solar images of the fraction of the Sun covered by the va rious types of activity to synthesize the irradiance. Using Ca II K-li ne images, the model can be used to estimate the irradiance from EUV l ine emission formed in the upper chromosphere and lower transition reg ion. Comparisons of this new model with existing empirical models reve al both similarities and disagreements in the absolute magnitude, the amplitude of the rotational modulation, and the intermediate-term sola r cycle variability of the predicted fluxes.