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Large-scale and small-scale dynamos generating magnetic flux are fundamental processes in astrophysics, for which solar and stellar dynamos provide crucial paradigms. During the last two decades, a wealth of new observational results, together with a new generation of large-scale numerical simulations, gave important insights in the complex interactions of turbulent convection, rotation, and magnetic fields. They permit tests and calibrations of mean-field models for the generation of large-scale magnetic flux, differential rotation, and meridional circulation in stellar convection zones. Understanding of the underlying processes and the possibiliy to compare with a broad base of observational data puts the necessary parametrizations in simplified dynamo models (e.g., “classical” \(\alpha \)-effect models, flux-transport dynamos, or Babcock-Leighton models) on a firmer basis. At the same time, space-based observations of stars have dramatically increased the amount of information on rotation, age, magnetic activity, and cycles for a wide range of stellar parameters, thus opening the possibility to study dynamo action for a wide range of stellar parameters and evolutionary stages.
These developments led to substantial progress in our understanding of the generation of magnetic fields by self-excited dynamo action in the Sun and other stars. Dynamo models addressing the variation of the magnetic field in the solar interior and at its surface in the course of the 11-yr activity cycle have become more detailed and realistic. The important processes of formation, rise, and emergence of magnetic flux loops in the course of the solar dynamo process are now studied in simulations covering the whole range from the convection zone into the corona. The operation of small-scale dynamo processes is revealed by high-resolution solar observations, which can be directly compared with numerical simulations of radiative magneto-hydrodynamics. The ubiquitous magnetic fields generated by small-scale dynamo action are important for many astrophysical systems. They affect crucial aspects of convective turbulence, such as the transport of energy and angular momentum, as well as the driving of large-scale flows and differential rotation. 3D-MHD simulations show dynamo action with large-scale organization of the generated magnetic field and cyclic polarity reversals.
At this stage of affairs, it was the right time to review and put into perspective the wealth of results from observations, simulations, and simplified models. The one-week international workshop “Solar and Stellar Dynamos: a New Era” hosted by ISSI in Bern brought together more than 50 established as well as young researchers from altogether 13 countries working in the relevant areas: solar and stellar observations from space and ground, numerical simulations, turbulence theory, and dynamo models. This gave the unique opportunity to broadly review the state of the art, outline the open questions, and discuss approaches to make further progress.
This collection summarizes the outcome of the workshop in the form of 15 comprehensive review papers covering the whole range of topics discussed during the meeting. Charbonneau & Sokoloff set the stage for the subsequent papers with a historical and technical introduction, specifically focussing on a number of unresolved “tension points” and questions. The reviews of Norton et al. and Biswas et al., respectively, address the basic observations of the solar cycle and its long-term development. Likewise, observations of magnetic fields and the activity cycles of cool stars are reviewed by Isik et al. and Jeffers et al., respectively. Weber et al. summarize the present understanding of the formation of magnetic structures in the convection zone and their emergence as active regions at the surfaces of the Sun and stars. The subsequent evolution of the emerged magnetic flux at the surface is the topic of the review by Yeates et al. Small-scale dynamo action and its reflection in observations are addressed by Rempel et al. Important ingredients relevant for models of large-scale dynamos are the structure and dynamics of the solar tachocline, the radial shear layer at the bottom of the convection zone (reviewed by Strugarek et al.), and the large-scale flows in the solar convective envelope (reviewed by Hotta et al.). The subsequent reviews cover a wide range of topics relevant for dynamo models: mean-field dynamos and turbulent processes (Brandenburg et al.), flux-transport dynamos and the generation of meridional flows (Hazra et al.), as well observationally guided models based upon the Babcock-Leighton scenario and the role of the surface field (Cameron & Schüssler). How such physical models could be used for the the prediction of solar and stellar cycles is the topic of the review by Bhowmik et al. Finally, Käpylä et al. review the enormous progress that has been made by comprehensive 3D-MHD simulations of solar and stellar dynamos.
Acknowledgements
The editors would like to thank all authors and coordinators for their dedicated efforts to prepare the papers that are assembled in this collection. The friendly and always helpful assistance by the staff of ISSI before, during, and after the workshop was an invaluable support for the participants and the conveners. We also thank the editorial staff of Space Science Reviews for the smooth organization of the publishing process and efficient help whenever technical issues came up. Last not least, we are very grateful to ISSI for generous financial support in organizing the workshop and for the publication of the book.
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Solar and Stellar Dynamos: A New Era
Edited by Manfred Schüssler, Robert H. Cameron, Paul Charbonneau, Mausumi Dikpati, Hideyuki Hotta and Leonid Kitchatinov
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Schüssler, M., Cameron, R., Charbonneau, P. et al. Editorial to the Topical Collection: Solar and Stellar Dynamos: a New Era. Space Sci Rev 220, 2 (2024). https://doi.org/10.1007/s11214-023-01037-y
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DOI: https://doi.org/10.1007/s11214-023-01037-y