On the effect of photospheric magnetic fields on solar surface brightness -: Results of radiative MHD simulations

Magnetic features in the solar surface layers are assumed to be a main cause for solar irradiance variations on timescales of the solar cycle and shorter. Realistic radiative MHD simulations of photospheric magneto-convection allow us to study the interaction between magnetic features, convective flows and radiation in detail and help to understand the physical mechanisms underlying irradiance variations, whilst at the same time allowing direct comparison with observations. We report on a series of local-box MHD simulations covering magneto-convection in the photosphere from quiet Sun to strong plage conditions. We find that for average magnetic field strengths corresponding to plage or facular regions, the total emergent radiation flux is increased by 2 - 3 % relative to quiet Sun conditions, mainly as a result of increased radiative losses along inclined rays from the hot walls of faculae. For stronger average fields, the radiative energy output drops below quiet Sun levels, partly due to a darker appearance of magnetic features near disk center, partly due to reduced granule brightness. The simulations underline that the modification of convective and radiative energy transport in the photosphere due to surface magnetism is a viable mechanism for solar irradiance variations. The center-to-limb variation of bolometric intensity and facular contrast show good agreement with observations, suggesting that realistic radiative MHD simulations can be useful for refined models of solar irradiance variations.