Modelling of solar and stellar brightness variations

The emergence of  magnetic field on stellar surfaces leads to the formation of magnetic features, such as dark spots and bright faculae. The expected facular and spot signals in stellar data are quite different, as they have distinct temporal and spectral profiles. The plethora of photometric data obtained by past and current space missions, plus the high precision transmission spectroscopy data brought by the James Webb Space Telescope have underlined the needs for a better understanding and modelling of stellar brightness variations on various timescales.

Here we present various modelling efforts that have allowed to understand solar brightness variations in the broader context of stellar variability. To model the magnetic component of the variability we follow the SATIRE approach of calculating brightness variations, which was shown to reproduce the solar variability in great detail. We connect variability of the Total Solar Irradiance (i.e. the spectral integrated solar flux) to that measured in various spectral passbands, routinely used in broad-band photometry (e.g., CoRoT, Kepler, TESS. etc).

Recent simulations with the 3D MHD radiative code MURaM made it also possible to calculate variability of spectral solar irradiance 
brought by the granulation which dominates total variability on timescales below about a day. By combining calculations of spectral solar irradiance variability brought by magnetic features and granulation we obtain a model capable of returning variability on timescales from minutes to decades. Currently, massive efforts are put into extending these calculations to stars with various values of metallicity and effective temperature.