To achieve the objectives of the action, we used state-of-the-art solar observations, simulations of the evolution of magnetic field on the solar surface, SATIRE model for solar irradiance variability, and RT codes RH and NESSY. We synthesised spectra of the quiet-Sun and magnetic features using the RT code NESSY in an optimal non-LTE setup. We tested this setup to reproduce the observations from Atacama Large Millimeter/submillimeter Array and James Clerk Maxwell Telescope. This led to the development of a new model for the sunspot umbra (dark central area of a sunspot) with a better constrained chromosphere for the first time. We also modelled the solar chromospheric emission variations in the near-UV Ca II H&K spectral lines. For a long time, Ca II emission has served as the best proxy for solar and stellar magnetic activity. However, many aspects of this emission remain unexplored. We used the observed surface distribution of solar magnetic features in combination with their non-LTE spectra, in SATIRE model for solar irradiance variability and reproduced the observed variability of the near-UV emission on timescales of the solar activity cycle and the solar rotation. Using this model along with simulations of magnetic field evolution on the surface, we showed that the Sun’s seemingly strong Ca II variability is a bias introduced by the Sun being observed from its near-equatorial plane and during a period of relatively high magnetic activity (Fig. 1). Consequently, there is nothing unusual in the variability of solar Ca II emission. The announcement of the third early release of the data from Gaia space mission and the anticipation of the full data release led to a strong interest in the role of stellar magnetic activity in detecting exoplanets using astrometry technique, which measures the stellar wobbles created by the star-planet interaction. We used the tools developed during the action to calculate the jitter due to magnetic activity (i.e. the displacement of the star’s brightness centre caused by magnetic activity) and showed that the magnitude of the jitter for the Sun observed from different vantage points is comparable to the astrometric signal generated by the Earth (Fig. 2). These results further our understanding of the solar near-UV variability, solar-stellar connection and the role of stellar magnetic activity in the search for exoplanets. The action and the newly fostered collaborations through it has led to two peer-reviewed publications (available on arXiv with free access) and five more are in preparation. In addition, the action results were widely disseminated to the scientific community through seminars, oral and poster presentations at international conferences.
