Short and long-term irradiance variability on the Sun and other main sequence stars manifest in both spectroscopic and bolometric brightness variations. These can be particularly evident at shorter wavelengths, for instance in chromospheric Ca II and coronal X-ray emission, but span the entire observed spectrum. These variations are predominantly caused by magnetic-field emergence and evolution due to the complex interaction of gas dynamics and magnetic flux. Concentrations of magnetic fields on solar and stellar surfaces form various observable features, with dark spots and faculae being the most obvious such manifestations. These two features are the main drivers of solar and stellar radiative variability on timescales of about a day and longer.
The last few years have brought significant progress both in modelling and in measuring the variability of solar and stellar radiative outputs. Modeling improvements include new understandings of radiative transfer, magneto-hydrodynamic modeling, flux transport, and the solar dynamo.
Topical Collection solicits papers aimed at observing and modelling irradiance variations of the Sun and Sun-like stars and, in particular, at understanding the links between magnetic fields and the resulting solar and stellar variability. We especially welcome papers based on synergies between solar and stellar research.

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