A new model of solar EUV irradiance variability 1. Model formulation

We present a new model of solar irradiance variability at extreme ultraviol et; wavelengths (EUV, 50-1200 Angstrom). In this model, quiet Sun, coronal hole, and active region intensities for optically thin emission lines are c omputed from emission measure distributions determined from spectrally and spatially resolved observations, For optically thick emission lines and con tinua, empirical values are used. The contribution of various solar feature s to the spectral irradiance variability is determined from a simple model of limb-brightening and full-disk solar images taken at the Big Bear Solar Observatory and by the Soft X-Ray Telescope on Yohkoh. To extend our irradi ance model beyond the time period covered by the available images, we use c orrelations with proxies for solar activity. Comparisons with the available irradiance data from the Atmospheric Explorer E (AE-E) spacecraft show tha t our model is capable of reproducing the rotational modulation of the EUV irradiance near solar maximum. The AE-E data, however, show systematically more solar cycle variability than our model estimates.