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The emission in the near ultraviolet Ca II H&K lines, often quantified via the S-index, has been serving as a prime proxy of solar and stellar magnetic activity. Despite the broad usage of the S-index, the link between the coverage of a stellar disk by magnetic features and Ca IIH&K emission is not fully understood. In order to fill this gap we developed a physics-based model to calculate the solar S-index. To this end, we made use of the distributions of the solar magnetic features derived from the simulations of magnetic flux emergence and surface transport, together with the Ca IIH&K spectra synthesized using a non-local thermodynamic equilibrium (non-LTE) radiative transfer code. We show that the value of the solar S-index is influenced by the inclination angle between the solar rotation axis and the observer’s line-of-sight, i.e. the solar S-index values obtained by an out-of-ecliptic observer are different from those obtained by an ecliptic-bound observer. This is important for comparing the magnetic activity of the Sun to other stars. We computed time series of the S-index as they would be observed at various inclinations dating back to 1700. We find that depending on the inclination and period of observations, the activity cycle in solar S-index can appear weaker or stronger than in stars with a solar-like level of magnetic activity. We show that there is nothing unusual about the solar chromospheric emission variations in the context of stars with near-solar magnetic activity.

Many solar-type stars with near-solar rotation periods exhibit much stronger variability than the Sun (Reinhold et al. 2020). Some of these stars even show very regular, sine-like light curves. Motivated by solar activity complexes, we developed a numerical model to quantify the effect of active-region (AR) nesting and active longitudes on stellar brightness variations in the rotational time scale. Modelling ARs with facular and spot components, we simulated light curves covering four years and using the Kepler passband. We found that the combined effect of the degree of nesting and the activity level, both being somewhat higher than on the Sun, can explain the whole range of observed light-curve amplitudes of solar-like stars. While nesting at random longitudes can explain variability amplitudes and light-curve morphology in many cases, active-longitude-type nesting reproduces sine-like light curves and the highest amplitude variability.

The emission in the near ultraviolet Ca II H&K lines, often quantified via the S-index, has been serving as a prime proxy of solar and stellar magnetic activity. Despite the broad usage of the S-index, the link between the coverage of a stellar disk by magnetic features and Ca II H&K emission is not fully understood. In order to fill this gap we developed a physics-based model to calculate the solar S-index. To this end, we made use of the distributions of the solar magnetic features derived from the simulations of magnetic flux emergence and surface transport, together with the Ca II H&K spectra synthesized using a non-local thermodynamic equilibrium (non-LTE) radiative transfer code. We show that the value of the solar S-index is influenced by the inclination angle between the solar rotation axis and the observer’s line-of-sight, i.e. the solar S-index values obtained by an out-of-ecliptic observer are different from those obtained by an ecliptic-bound observer. This is important for comparing the magnetic activity of the Sun to other stars. We computed time series of the S-index as they would be observed at various inclinations dating back to 1700. We find that depending on the inclination and period of observations, the activity cycle in solar S-index can appear weaker or stronger than in stars with a solar-like level of magnetic activity. We show that there is nothing unusual about the solar chromospheric emission variations in the context of stars with near-solar magnetic activity.

Recent developments in instrumentation and observational techniques have opened a new window for stellar magnetic field studies. In particular Zeeman Doppler Imaging (ZDI) is now routinely used to recover the large-scale magnetic field topologies of stars different from the Sun, including several planet-hosting stars. These stellar magnetic fields intimately affect the environment around late-type stars by driving the coronal high-energy radiation (EUV/X-rays), transient events (e.g. flares and coronal mass ejections), and the development of stellar winds and astrospheres. These elements can have a strong impact in the evolution of planetary systems via star-planet interactions and erosion of exoplanetary atmospheres. In this context, the results from ZDI data-driven, detailed 3D MHD modeling of the coronal conditions and circumstellar environment around three planet hosting stars are presented. For one of the considered systems (HD 1237), we investigate the interactions of the magnetized stellar wind with the exoplanet, assuming a Jupiter-like magnetosphere around it.

Our understanding of solar magnetic fields is significantly more detailed than we can achieve for other stars. However, over the last few decades, techniques such as Zeeman Doppler Imaging (ZDI), have delivered major advances in the study of stellar magnetic fields, to the point where we can now map the global surface magnetic topology of other stars and use these to model the coronal magnetic fields and even the stellar wind produced by the star. These advances now allow us to compare the Sun with other solar-type stars and make increasingly detailed comparisons of the behaviour and generation of their dynamo magnetic fields. In this review I will give an overview of what ZDI has taught us about the magnetic fields of other stars and how our study of the Sun can inform our understanding of the dynamos operating across a range of solar-type stars.

Although many features can be seen moving across it at various wavelengths, the Sun is an almost constant star with regard to its radiative output. Its radiative variability has generally only been discovered after the beginning of the space age. For Sun-like stars it is easier to measure their small levels of variability from the ground, leading to some unexpected discoveries that seemed to suggest a behaviour rather different than that displayed by the Sun. However, revisions of the measurements as well as advances in modelling have in recent years shown that some of the main differences between the Sun and stars can be reconciled. A much richer data set of short and medium-term variability has been provided by the COROT and Kepler missions. Although a number of exciting results have already been obtained from these data, many more are expected in the coming years as the data are more thoroughly analysed and models of solar variability are extended to describe stars which, while similar to the Sun, are not exact solar analogs.

The surface magnetic field has substantial influence on various stellar properties that can be probed through various techniques. With the advent of new space-borne facilities such as CoRoT and Kepler, uninterrupted long high-precision photometry is available for hundred of thousand of stars. This number will substantially grow through the forthcoming TESS and PLATO missions. The unique Kepler observations –covering up to 4 years with a 30-min cadence– allows studying stellar variability with different origins such as pulsations, convection, surface rotation, or magnetism at several time scales from hours to years. We study the photospheric magnetic activity of solar-like stars by means of the variability induced in the observed signal by starspots crossing the visible disk. We constructed a solar photometric magnetic activity proxy, Sph from SPM/VIRGO/SoHO, as if the Sun was a distant star and we compare it with several solar well-known magnetic proxies. The results validate this approach. Thus, we compute the Sph proxy for a set of CoRoT and Kepler solar-like stars for which pulsations were already detected. After characterizing the rotation and the magnetic properties of 300 solar-like stars, we use their seismic properties to characterize 18 solar analogs for which we study their magnetism. This allows us to put the Sun into context of its siblings.

Some aspects of stellar magnetic activity such as X-ray emission are relatively easy to observe, while others, such as the geometry of the magnetic field are rather more difficult. Indeed, typically only the large-scale (or low-order) components of the field can be mapped. Most elusive of all is the hot, tenuous stellar wind, yet the wind speed is a crucial quantity governing the angular momentum loss of the star. We demonstrate, however, that wind speeds can be predicted reliably even from fairly low-resolution magnetograms. We use an empirically-derived model of the solar wind that predicts the distribution of wind speeds based on surface magnetograms. Using solar magnetic field measurements spanning several magnetic cycles, we demonstrate how changes in the surface magnetic field on various length-scales affect the coronal structure and hence the wind speed. We find that only low-order field components are needed to characterise the wind speed. We compare the variations in wind speed with variations in the X-ray emission over several cycles and show that while the small-scale field has a significant effect on Lx, it has little effect on the wind speed. This suggests that the large number of stellar magnetograms that are becoming available can be used to predict the stellar wind speeds. Publisher PDF

The rich array of solar magnetic field-related phenomena we see occurs not only on stellar counterparts of our Sun but in stars that represent significant departures in their fundamental parameters from those of the Sun. Though these phenomena appear energetically negligible when compared to the total luminosity of stars, they nevertheless govern the angular momentum evolution and modulate the radiative and particle output of the Sun and late-type stars. The term “The Solar-Stellar Connection” has been coined to describe the solar-stellar synergisms in the investigation of the generation, emergence and coupling of magnetic fields with the outer solar-stellar atmosphere to produce what we broadly refer to as magnetic activity. With the discovery of literally thousands of planets beyond our solar system, the Solar-Stellar-Planet Connection is quickly emerging as a new area of investigation of the impacts of magnetic activity on exoplanet atmospheres. In parallel with this rapid evolution in our perspectives is the advent of transformative facilities for the study of the Sun and the dynamic Universe. The primary focus of this invited talk will be on photometric variations in solar-type stars and the Sun. These brightness variations are associated with thermal homogeneities typically defined by magnetic structures that are also spatially coincident with key radiative proxies. Photometric variability in solar-type stars and the Sun includes transient brightening, rotational modulation by cool spots and cycle-related variability, each with a characteristic signature in time and wavelength. The emphasis of this presentation will be on the relationship between broadband photometric variations and magnetic field-related activity in solar-type stars and the Sun. Facets of this topic will be discussed both retrospectively and prospectively as we enter a revolutionary, new era for astronomy.