A model of the wavelength dependence of solar irradiance variations

The variation of the solar irradiance over the solar cycle has a strong wavelength dependence, being larger at shorter wavelengths. Here we present simple models of the spectral dependence of irradiance variations between solar activity maximum and minimum. We find that the observations (which concentrate on the UV) cannot be reproduced by a change in effective temperature of the Sun (or of parts of its surface) alone. We can, however, reproduce the data with either a 2-component or a 3-component model, of which one component is the quiet Sun, another is a facular component, and the third (in the case of the 3-component model) represents the temperature stratification of sunspots. The facular component is found to be very close to the facular models F or P of Fontenla et al. (1993). The success of these models supports the assumption underlying many studies of total solar irradiance variations that these are caused mainly by magnetic fields at the solar surface. Our investigation also allows an improved estimate of the relative contribution of the various layers in the solar photosphere and of the different wavelength regions to the total irradiance variations.