Scientific rationale

Recent observational results from space missions, such as the Solar Dynamics Observatory (SDO), Interface Region Imaging Spectrograph (IRIS), Kepler and others that were launched in the past 10 years, as well as from the new large solar and stellar telescopes, such as GREGOR and ALMA, and advanced instrumentation, have convincingly demonstrated that the progress in our understanding of how magnetic fields are generated, emerge from the interior, organize in active regions, and cause powerful eruptions can be achieved only by developing a unified approach and studying relationships between solar and stellar magnetism. Developing a synergy of solar and stellar astronomy is essential in solving grand-challenge problems of the primary mechanisms of stellar magnetic activity and its effects on star-planet relations.

An important key issue is that the same or similar phenomena occur on the Sun and other stars under different conditions (different age, metallicity, rotation rate etc) and studying these similarities and differences will help to uncover the underlying physical mechanisms, their evolution in time and impacts. Specifically, the proposed program includes discussions of new emerging topics that are of interest to both solar and stellar astronomers, such as magnetic field diagnostics of the chromosphere and corona using observations of chromospheric lines and initial data from ALMA (which became available earlier this year), detection of stellar magnetospheres, and detailed mapping of the magnetic fields on the surface of stars. Previously, such mapping was possible only for the Sun. Therefore, it is very interesting and important to compare the solar and stellar results, and also discuss how the surface magnetism structure and evolution are related to the generation of magnetic in the solar and stellar interiors.

In this respect, tremendous progress has been achieved from analysis of helioseismology and asteroseismology data from SDO and Kepler, as well as from observations of the solar and stellar variability. Curiously, one of the IAUS 273 discussions was about a delay of the start of Solar Cycle 24. Now, eight years later, we know that it was not just a delay, but the whole cycle is very weak and unusual in many respects including further decline of helioseismic signatures of the next cycle. This long decline in solar activity undoubtedly will be a topic of future IAU Symposia, but at this point, it is very important to discuss the place of the Sun in the general picture of stellar cycles. This picture that is emerging from analysis of the Kepler and supporting ground-based spectroscopic data reveals new scaling laws and relations that needs to be taken into account in solar magnetism studies. Recent theoretical studies based on advanced supercomputer simulations revealed a key role of magnetism in establishing the solar and stellar differential rotation laws, and now it is important to discuss observational tests of the theoretical predictions based on the currently available and future data.

One of the puzzles of solar and stellar magnetism is related to the origin of extreme flare events. Despite the very weak magnetic cycle the Sun produced, in the cycle declining phase in 2017, some of the strongest flares in the history of observations. This raises the question of how this phenomenon may be related to the magnetism of stars that produce superflares, and what physical mechanism may cause such extreme events. Another important topic for the joint solar-stellar discussions is the influence of magnetism on solar and stellar variability. A recent interest of stellar astronomers in the details of solar variability (e.g. effects of magnetic fields on granulation) is caused by the need to understand and characterize the stellar ‘jitter’ for detection of Earth-type planets.

The interest in understanding the role of stellar magnetism in star-planet relations is driven by the need to determine conditions for habitability. In this aspect, the goal of this Symposium will be to discuss properties of solar and stellar coronae and winds, and their interactions with planetary magnetospheres. Compared to the solar system, in many recently discovered planetary systems the stellar winds are substantially stronger, and planets are much closer to their parent stars. This creates extreme conditions for magnetic interactions and extreme radiation environments, which depend on the state of stellar magnetic activity. The discussion of this new problem which is beyond the traditional study horizon, will undoubtedly raise interest in understanding the broader impacts of magnetic activity and lead to new ideas. We propose to include talks on observational and theoretical studies, as well as on attempts to model star-planet interaction in laboratory experiments.

The Symposium will include an open public session on solar eclipses and planetary transits. The goal of this session is to discuss how the eclipses and transits provide new information about solar and stellar magnetic fields. In particular, the total solar eclipses provide high-resolution measurements of magnetic field in the low corona, which cannot obtained by any other means. In addition, this session will present a broad historical overview of solar eclipses, planetary transits, their role in astronomy, as well as a general talk on habitability of exoplanets. We anticipate that this session will be of great public interest.

The Symposium will be held in Chilean city of Copiapo close to the path of the total solar eclipse. A bus tour to the La Silla Observatory to watch the total eclipse will be organized.