Modelling short-term spectral irradiance variations

On time-scales of the solar rotation most of the solar irradiance variation s are caused by the changing distribution of solar surface magnetic feature s. We model these short-term irradiance variations using calculations of su nspot and facular contrasts as a function of wavelength and limb angle on t he Sun. The position of active regions on the solar disc is derived from th e MDI magnetograms. The reconstructed irradiance variations are compared wi th total and spectral irradiance measurements obtained by the VIRGO experim ent on SOHO.