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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - UniSDyn (Building up a Unified Theory of Stellar Dynamos)

Teaser

Summary

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.

We propose a coherent plan of advancement in which each theory shall be tested by requiring it also to explain the cyclic dynamo action seen in more active rapid rotators. UNISDYN project attacks these very problems with novel simulations and data analysis tools. Our path to resolve them is to enhance the state-of-the-art stellar dynamo models with the relevant descriptions of the turbulent processes. This goal is reached in three steps. (i) We will produce improved convection dynamo simulations to serve as laboratories from which (ii) the turbulent transport coefficients are directly measured with a novel test methods suite. (iii) Finally, global dynamo models incorporating the turbulent effects in full are constructed based on (i) and (ii) results. These results will allow us to unify stellar dynamo theory for solar-like inactive and rapidly rotating active stars. The developed toolbox has direct applications in other fields of astrophysics, such as accretion and galactic disk dynamos, and industry, such as combustion engines and fusion reactors.

Work performed

During the first eight months of the project, we have performed the first ever proof-of-concept of the turbulent transport coefficients measured from turbulent magnetoconvection, using the test-field method, reproducing the dynamo solution realized in the DNS model, when the coefficients are put back into a mean-field induction equation. This provides a vital validation of the basic methodology that was proposed to be utilized in the project, and gives us a strong urge to go further with our approach. Furthermore, the first steps to extend the test methods suite into conditions more closely resembling the astrophysical objects has been made in the form of the nonlinear test field method. First applications of this method have been performed to strong shear flows, that can be found, e.g., in the solar tachocline. The improvement of the global DNS models has been started, and, thanks to the versatile personnel that we have been able to recruit, the work is now extending beyond just utilizing the Pencil Code to other fully spherical, albeit anelastic, solvers. These opened possibilities to utilize a large suite of MHD codes have pushed us forward to try a completely novel approach of coupling the test methods suite of the Pencil Code to other DNS solvers, using a novel MPI interface.

Final results

The proof-of-concept chain performed with the test-field method has already 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 will allow us to investigate the history and evolution of the dynamo in solar-like stars across their main-sequence life, and bring novel understanding about the Sunâ€™s history and also its future. The rapid progress we are making with the test method extensions will allow 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 soon in our reach.