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

Abstract

We present a new model of solar irradiance variability at extreme ultraviolet wavelengths (EUV, 50–1200 Å). In this model, quiet Sun, coronal hole, and active region intensities for optically thin emission lines are computed from emission measure distributions determined from spectrally and spatially resolved observations. For optically thick emission lines and continua, empirical values are used. The contribution of various solar features 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 irradiance model beyond the time period covered by the available images, we use correlations with proxies for solar activity. Comparisons with the available irradiance data from the Atmospheric Explorer E (AE‐E) spacecraft show that 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.