Rationale:
Understanding the
physical mechanisms of magnetic fields inside the Sun and
other stars is a long-standing problem of astrophysics. It
is believed that the magnetic fields are generated by
turbulent dynamo processes, which involve a complex
interaction of turbulent convection with differential
rotation, global meridional circulation, and large-scale
flows. The dynamo-generated magnetic fields emerged on the
surface of our Sun and other stars forming sunspots, and
starspots, which are the origin of a chain of space weather
events substantially affecting planetary environments.
However, despite substantial observational and theoretical
efforts, we are still far from understanding the basic
mechanisms of solar and stellar magnetism.
The unexpected sharp decrease in the sunspot number in the past two solar cycles and the unusual development of the current solar cycle raised questions about the origin of the centennial variations of solar activity and new interest in the analysis of historical observations of sunspots and solar activity. This has led to the development of innovative machine-learning techniques for the analysis of historical records and merging them with modern data.
Recent studies of the global magnetic field of the Sun reveal a connection between the polar magnetic strength during the solar minima and the subsequent maxima. Semi-empirical flux-transport and mean-field models can reproduce the sunspot butterfly diagram. Yet, whether they can provide a robust solar-cycle prediction is still under debate. Observations of distant stars have progressed from detection to characterization of global magnetic fields, starspots, and activity cycles. They also found a relationship between the cyclic activity, rotation, and convective properties, which initially showed `active’ and ‘non-active’ classes of magnetic activity, curiously placing the Sun between these branches. However, most recent data show that such classification may not be correct and that these relationships may be significantly more complex.
In addition, observations of magnetic fields and activity on A- and B-type stars raised questions about how stars that are supposed to lack significant convective sub-surface regions, nevertheless, exhibit magnetic fields and whether the dynamo process is the only process that can generate surface magnetism. Recent advances in supercomputing provide an opportunity to model the global solar and stellar dynamics directly from the first physics principles. Initial attempts are underway to link models of dynamo action in stellar interiors with angular momentum loss in magnetized stellar winds. While the sophisticated 3D MHD model can qualitatively reproduce some observed phenomena, they are still far from a quantitative description of solar and stellar magnetism. High-resolution `realistic’ radiative MHD simulations of small patches of solar and stellar surface and subsurface dynamics demonstrated the existence of local dynamo processes which contribute to `magnetic carpets’ covering the stellar and solar surfaces. However, how the local dynamo can affect the global processes is currently unclear.
New significant constraints on dynamo models and theories come from helio- and asteroseismology results which establish the differential rotation laws and meridional circulation patterns. Recent helioseismology results determined the extended solar-cycle pattern of the Sun’s zonal flows (torsional oscillations) migrating with the radius and latitude during the dynamo cycles. These flows are connected to the dynamo processes, but this relationship has been established. In addition, helioseismology discovered cyclic variations of the meridional circulation, which can significantly affect magnetic flux transport, thus challenging the popular flux-transport dynamo models. Essential constraints on dynamo models also come from asteroseismology and magnetic field observations by advanced Zeeman-Doppler imaging techniques that allow the mapping of magnetic fields and their helicity density across stellar surfaces.
The unexpected sharp decrease in the sunspot number in the past two solar cycles and the unusual development of the current solar cycle raised questions about the origin of the centennial variations of solar activity and new interest in the analysis of historical observations of sunspots and solar activity. This has led to the development of innovative machine-learning techniques for the analysis of historical records and merging them with modern data.
Recent studies of the global magnetic field of the Sun reveal a connection between the polar magnetic strength during the solar minima and the subsequent maxima. Semi-empirical flux-transport and mean-field models can reproduce the sunspot butterfly diagram. Yet, whether they can provide a robust solar-cycle prediction is still under debate. Observations of distant stars have progressed from detection to characterization of global magnetic fields, starspots, and activity cycles. They also found a relationship between the cyclic activity, rotation, and convective properties, which initially showed `active’ and ‘non-active’ classes of magnetic activity, curiously placing the Sun between these branches. However, most recent data show that such classification may not be correct and that these relationships may be significantly more complex.
In addition, observations of magnetic fields and activity on A- and B-type stars raised questions about how stars that are supposed to lack significant convective sub-surface regions, nevertheless, exhibit magnetic fields and whether the dynamo process is the only process that can generate surface magnetism. Recent advances in supercomputing provide an opportunity to model the global solar and stellar dynamics directly from the first physics principles. Initial attempts are underway to link models of dynamo action in stellar interiors with angular momentum loss in magnetized stellar winds. While the sophisticated 3D MHD model can qualitatively reproduce some observed phenomena, they are still far from a quantitative description of solar and stellar magnetism. High-resolution `realistic’ radiative MHD simulations of small patches of solar and stellar surface and subsurface dynamics demonstrated the existence of local dynamo processes which contribute to `magnetic carpets’ covering the stellar and solar surfaces. However, how the local dynamo can affect the global processes is currently unclear.
New significant constraints on dynamo models and theories come from helio- and asteroseismology results which establish the differential rotation laws and meridional circulation patterns. Recent helioseismology results determined the extended solar-cycle pattern of the Sun’s zonal flows (torsional oscillations) migrating with the radius and latitude during the dynamo cycles. These flows are connected to the dynamo processes, but this relationship has been established. In addition, helioseismology discovered cyclic variations of the meridional circulation, which can significantly affect magnetic flux transport, thus challenging the popular flux-transport dynamo models. Essential constraints on dynamo models also come from asteroseismology and magnetic field observations by advanced Zeeman-Doppler imaging techniques that allow the mapping of magnetic fields and their helicity density across stellar surfaces.