A model of solar total and spectral irradiance variations

We model solar spectral irradiance variations under the assumption that they are produced by sunspots and faculae alone. The model is based on three components, i.e. quiet Sun, assumed to be temporally invariant, sunspots and faculae whose temporal variations are mimicked using time series of sunspot areas and Mg II core-to-wing ratios. The detailed flux spectrum for each component is employed. The results are compared to spectral irradiance measurements at 402 nm, 500 nm and 862 nm obtained between February 22 and December 31, 1996 by the three-channel sunphotometers (SPM) which are part of the VIRGO package onboard SOHO. Our model shows a good correlation with the measured variations of the three colour channels and of the total irradiance. Since it also successfully reproduces changes in the UV spectral irradiance, irradiance variations and observed variations of the faculae-to-spots filling factor ratio since 1978, our model supports earlier suggestions that a large part of the solar irradiance variations is caused by magnetic fields at the solar surface. Finally, we use the model to test whether the difference between the magnitude of solar and stellar brightness variations reported by Lockwood et al. (1992) is due to the particular filters they use for their stellar observations. Our results suggest that this effect can explain only a small part of the discrepancy.