Multiband modelling of the Sun as a variable star from VIRGO/SoHO data

The time series of total solar irradiance (TSI) and optical spectral irradiance at 402, 500 and 862 nm (SSIs) obtained by the VIRGO experiment on board the satellite SoHO are analysed in order to model their variability in the framework of a purely stellar-like approach. The different time scales of variability are estimated by means of the pooled variance method revealing the growth and decay of sunspots and faculae in active regions, as well as their rotationally modulated visibility. The determination of the rotation period of the Sun from the time modulation of the TSI and SSIs is made difficult by the short lifetimes of photospheric brightness inhomogencities in comparison to the rotation period. Only during the phases with the lowest level of activity of solar cycle 23, when the variability is dominated by long-lived faculae, is it possible to determine the truly solar synodic period. The simultaneous modelling of the rotational modulation of the TSI and SSIs can be performed by means of a simple stellar-like approach which extends the model we previously applied to the TSI modulation alone (Lanza et al. 2003). Our model yields residuals about 20-30 times smaller than the amplitudes of the TSI and SSI variations in all the phases of the 11-yr activity cycle. The determination of the model parameters, including the temperature of the surface brightness inhomogeneities and the trade-off among them are discussed and compared with the results obtained with different modelling approaches. The advantages and the drawbacks of applying the model to other stars are also considered.