Periodic Reporting for period 4 - UniSDyn (Building up a Unified Theory of Stellar Dynamos)
Berichtszeitraum: 2023-01-01 bis 2024-04-30
Magnetic fields are ubiquitous in the universe. The special property of cosmic magnetism is that, in the majority of objects hosting magnetic fields, those fields are organized, such that some meaningful averaging can reveal global structure and systematic behaviour. In the Sun, averaging over longitude reveals the equatorward migration of the emergence region of the sunspots, forming the famous butterfly diagram. Further, vigorous turbulence is present in a wide variety of astrophysical systems, and yet they still exhibit organized magnetic fields. These observations prompt the search for a theory to explain how order can arise and sustain itself in such chaos. We claim that the available theories are incomplete, especially in the case of solar-like stars which becomes apparent if we view the Sun as one star among many.
During the UniSDyn project these problems were attacked with novel simulation and data analysis tools. At the heart of these tools are graphics-processing accelerated algorithms that have enabled simulations with unprecedented resolutions, reaching out to more star-like regimes. Data-analysis tools that can interrogate the turbulent flow have also been developed, enabling us to measure and characterise the turbulent transport in stellar convection zones. With the help of these tools we have performed improved convection dynamo simulations to serve as laboratories from which we have measured, investigated, and characterised the turbulent transport coefficients. Finally, global dynamo models incorporating the turbulent effects in full have been constructed based on these results. In conclusion, the project has significantly increased our knowledge of highly turbulent plasma in stellar convection zones, and about the dynamo processes occurring within them.
The toolbox that was developed during the project has direct applications in other fields of astrophysics, such as accretion and galactic disk dynamos, and industry, such as combustion engines and fusion reactors. Implementations to galactic dynamo investigations were already successfully implemented during the project.
During the UniSDyn project these problems were attacked with novel simulation and data analysis tools. At the heart of these tools are graphics-processing accelerated algorithms that have enabled simulations with unprecedented resolutions, reaching out to more star-like regimes. Data-analysis tools that can interrogate the turbulent flow have also been developed, enabling us to measure and characterise the turbulent transport in stellar convection zones. With the help of these tools we have performed improved convection dynamo simulations to serve as laboratories from which we have measured, investigated, and characterised the turbulent transport coefficients. Finally, global dynamo models incorporating the turbulent effects in full have been constructed based on these results. In conclusion, the project has significantly increased our knowledge of highly turbulent plasma in stellar convection zones, and about the dynamo processes occurring within them.
The toolbox that was developed during the project has direct applications in other fields of astrophysics, such as accretion and galactic disk dynamos, and industry, such as combustion engines and fusion reactors. Implementations to galactic dynamo investigations were already successfully implemented during the project.
During the UniSDyn project we developed a new approach to model and understand stellar dynamos with novel simulation and data analysis tools. We also performed extensive work to constrain the models by observations of stellar magnetic fields. Related to the observational work, we, e.g. analysed stellar chromospheric activity data and obtained the strongest confirmation so far of convective turbulence being a required ingredient of stellar dynamos (these results were disseminated in an article in Nature Astronomy by Lehtinen et al., 2020). At the heart of these tools are graphics-processing accelerated algorithms that have enabled simulations with unprecedented resolutions. This work culminated in the release of the full-fledged stencil library Astaroth (disseminated in Pekkilä et al., 2020, in the leading computer science journal Parallel Computing), now used in many other research groups world-wide outside our institution. With the help of these computational tools, we have now been able to reach out to more star-like regimes, such as investigated small magnetic Prandtl number plasmas, closely matching the conditions in the bottom of the solar convection zone. We were able to verify favourable conditions for the excitation of a solar small-scale dynamo in deeper depths of the convection zone, before believed to occur only in the surface regions (disseminated in Nature Astronomy by Warnecke et al., 2023). We also developed data-analysis tools that can interrogate the turbulent flow, enabling us to measure and characterise the turbulent transport in stellar convection zones (disseminated in the Astrophysical Journal, Käpylä et al, 2020, 2022). With the help of these tools we have performed improved convection dynamo simulations to serve as laboratories from which we have measured, investigated, and characterised the turbulent transport coefficients (disseminated in Astronomy and Astrophysics, Warnecke et al., 2023). Finally, global dynamo models incorporating the turbulent effects in full have been constructed based on these results (disseminated e.g. in Astrophysical Journal letters by Warnecke et al., 2021). All the discoveries obtained during the project point to the direction of stellar magnetism being based on two different dynamo instabilities, the large-scale (global) and the small-scale (fluctuating) dynamo mechanism and their interactions. Hence, global-scale magnetism and kinetic and magnetic turbulent fluctuations are intimately coupled. Dynamo models incorporating both these dynamo mechanisms are essential to understand stellar dynamos.
The proof-of-concept chain performed with the test-field method has brought the project science to a realm, where the dynamo solutions realized in the global magnetoconvection simulations can be understood from first physical principles, not having to resort to proxies, approximations, and phenomenological arguments, as has been the case before. This has allowed us to investigate the history and evolution of the dynamo in solar-like stars across their main-sequence life, and to bring novel understanding about the Sun’s history and also its future. The rapid progress made during the project allows us to study the dynamo processes in other types of stars, where the observational information is scarcer and more unreliable than for the Sun. The envisioned applications to other domains of astrophysics (galactic dynamo, magnetism in protostellar disks, where stars are formed, planetary dynamos, industrial application) are now in our reach.