The spectral dependence of facular contrast and solar irradiance variations

We present model calculations of facular and sunspot contrasts as a functio n of wavelength and limb angle on the Sun. These are the first such calcula tions; they assume LTE and are based on opacity distribution functions (ODF s). The calculated facular contrasts as a function of limb angle fit into t he general picture of contrast measurements, and the behaviour of the contr ast with wavelength at a given limb angle is in excellent agreement with th e measurements. The calculated intensity spectra are used to construct the solar flux spect rum for different levels of solar activity. It is assumed that the irradian ce or flux variations are due to changes in the sunspot and facular filling factors. The model atmosphere used to calculate the facular intensities ha s been tuned so that the calculated irradiance variations match the observe d total and spectral irradiance variations during the last solar cycles. The model calculations have also been used to estimate the relative importa nce of continuum and spectral-line variations in producing irradiance varia tions. The results suggest that the continuum variations only contribute ne gligibly to the total irradiance variations on solar-cycle time scales.