Regular space-borne measurements since 1978 have revealed significant solar irradiance variations on up to decadal time scales. Variability on time scales of several hours and longer is linked to the magnetic activity of the Sun. Concurrently, ground-based photometric measurements of Sun-like stars uncovered similar variations, although their variability patterns show a much wider variety from which we can perhaps infer other causes of solar irradiance variations.

Despite significant progress, our knowledge and understanding of solar and stellar magnetically driven variability is still incomplete. Some of the critical open issues are:

• the magnitude (e.g., in the UV part of the spectrum) and even the phase (in the visible) of the irradiance variation over the solar cycle;


• the connection between stellar activity and photometric brightness variability;


• is solar variability typical or anomalous with respect to the majority of lower main sequence stars?


• the presence and magnitude of secular changes in solar surface magnetic field and irradiance;


• the origin of the solar and stellar cycles;


• the impact of stellar variability on exo-planets.

Implications of the magnetic variability on the Sun and other stars were jointly discussed at the IAUS264 in 2009, more than 4 years ago. Since then considerable progress has been made both in observations and modelling. New state-of-the-art solar and stellar data, obtained over the last few years, as well as advances in theoretical modelling of the solar and stellar atmospheres, allow us to reach a completely new level in understanding of the solar and stellar magnetic variations and begin tackling many long-standing problems. For example:

• Novel high resolution data obtained by the HINODE mission (launched on September 22, 2006), the Solar Dynamic Observatory (launched on February 11, 2010), and the SUNRISE balloon-borne solar observatory (flights in June 2009 and June 2013) make it possible to put important constraints on the relationship between the magnetic field strength and the spectral contrasts of solar active regions, which are crucial for successful modeling of solar irradiance variability.


• The 3D MHD simulations of solar and stellar atmospheres have reached a new level of realism, thus providing crucial information about the key physical processes driving solar irradiance variability and thus allowing a significant step toward the more physics-based models of solar irradiance changes.


• The unprecedented precision of broadband stellar photometry achieved with the Kepler (launched on March 7, 2009) and Corot (launched on December 27, 2006) space missions initiated a new era in studying stellar photometric variabilities and enable improved solar and stellar data links. The Kepler and Corot data make it possible to constrain models of the irradiance variability against a rich stellar data set and to understand whether the photometric variability of Sun-like stars can be reproduced by the solar paradigm.


• Lengthened ground-based stellar photometric data (mostly obtained with automated telescopes) have significantly increased the number of stars observed over even decade-long stellar cycles. Ground-based transiting exo-planet search programmes are maturing and starting to offer a wealth of data on stellar variability to complement space-based missions. Systematic spectroscopic monitoring of active stars now allows a more reliable assessment of the link between stellar photometric variability and chromospheric activity.

With this pace of discovery, solar and stellar researchers would greatly profit from closer collaboration: solar scientists by broadening their models to reproduce not just the variability of one star, and stellar astronomers by gaining more insights into the physical causes of stellar variability. Such interactions would also help to understand whether the observed patterns of stellar variability are consistent with the solar paradigm, and potentially dismiss certain physical classes of dynamos.

The solar-stellar comparison has a huge potential for improving our understanding of solar variability. For example, it is still unclear whether the surprisingly high solar UV variability and possible negative correlation of the visible irradiance with solar activity over the 11-year cycle, as recently suggested by the SORCE/SIM data, is compatible with stellar observations and whether solar rotational variability is consistent with the Kepler and Corot data. The stellar data can also help constrain the solar irradiance variability on centennial time-scales, which is particularly important for understanding the role of the Sun in natural climate change. The comparison of solar and stellar irradiance data also helps to better understand the general concepts of magnetic variability. For example, the more rapidly rotating stars are expected to have different latitudinal distributions of active regions on their surfaces. The size distribution of active regions may also depend on the stellar activity level, while their contrasts can depend on effective stellar temperature. These factors affect the patterns of stellar magnetic variability, suggesting stellar irradiance data can help constrain the basic properties of stellar activity cycles. The observed range of time scales of solar and stellar irradiance variability has implications for understanding the dynamo processes and poses challenges for dynamo models.

Solar irradiance variations are critical inputs for climate models, and solar-stellar connections can indicate the range of secular variability in irradiances that are not yet definitively observed with the existing data record. Additionally, stellar brightness variations are one of the critical limitations on the detectability of exoplanets and may lead to generalizations on planet habitability.

The recent advances summarized above make bringing together the stellar and solar communities timely. Since IAU General Assemblies are widely attended by both communities, a Focus Meeting during the 2015 IAU GA is an excellent opportunity to understand magnetically driven brightness variations of the Sun and stars and to trigger joint discussions and collaboration between these complementary groups.