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https://stockholmuniversity.zoom.us/j/940229961
The magnetic field in the Sun undergoes a cyclic modulation with a reversal typically every 11 years due to a dynamo operating under the surface. Other solar-like stars with outer convective envelopes show cyclic modulation of their magnetic activity, the level and cycle period being related to their rotation rate. This is suggestive of a common dynamo mechanism.
Here we present results of 3D MHD convective dynamo simulations of slowly and rapidly rotating solar-type stars, where the interplay between convection and rotation self-consistently drives a large-scale magnetic field. With the help of the test-field method, we are able to measure the turbulent transport coefficients in these simulations and therefore gain insight about the dynamo mechanism operating in them. It allows us to explain the weak dependency of the cycle period found in the moderate rotation regime using a Parker dynamo wave operating in our simulations. Furthermore, we find that the alpha effect becomes highly anisotropic at rapid rotation, which can explain the high degree of non-axisymmetry of the magnetic field in observations and models of rapid rotating stars. Taking all dynamo effects into account, we find three distinct regimes. For slow rotation, the α and Rädler effects are dominating in presence of anti-solar differential rotation. For moderate rotation, α and Ω effects are dominant, indicative of αΩ or α2Ω dynamos in operation, producing equatorward-migrating dynamo waves with the qualitatively solar-like rotation profile. For rapid rotation, an α2 mechanism, with an influence from the Rädler effect, appears to be the most probable driver of the dynamo. Our study reveals the presence of a large variety of dynamo effects beyond the classical αΩ mechanism.
Stars spinning faster than the Sun are expected to also produce larger amounts of magnetic helicity at their surfaces. In the Sun, magnetic helicity is essential for the release of energy leading to the eruption of plasma via coronal mass ejection and is thought to play an important role in the heating process of the coronal plasma. Using MHD simulations of solar coronae, we find a power law relation between the surface magnetic helicity and the temperature and activity of these coronae, suggesting an important role of magnetic helicity production in understanding the rotational dependence of stellar activity.
https://ui.adsabs.harvard.edu/abs/2020A%26A...642A..66W/abstract