The spectral dependence of facular contrast and solar irradiance variations

We present model calculations of facular and sunspot contrasts as a function of wavelength and limb angle on the Sun. These are the first such calculations; they assume LTE and are based on opacity distribution functions (ODFs). The calculated facular contrasts as a function of limb angle fit into the general picture of contrast measurements, and the behaviour of the contrast with wavelength at a given limb angle is in excellent agreement with the measurements. The calculated intensity spectra are used to construct the solar flux spectrum for different levels of solar activity. It is assumed that the irradiance or flux variations are due to changes in the sunspot and facular filling factors. The model atmosphere used to calculate the facular intensities has been tuned so that the calculated irradiance variations match the observed total and spectral irradiance variations during the last solar cycles. The model calculations have also been used to estimate the relative importance of continuum and spectral-line variations in producing irradiance variations. The results suggest that the continuum variations only contribute negligibly to the total irradiance variations on solar-cycle time scales.