Investigating Global Convective Dynamos with Mean-field Models: Full Spectrum of Turbulent Effects Required

Warnecke, Jörn; Rheinhardt, M.; Viviani, M.; Gent, Frederick A.; Tuomisto, Simo; Käpylä, Maarit J.

doi:10.3847/2041-8213/ac1db5

Cited by 16 publications

(15 citation statements)

References 30 publications

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“…The product of the radial gradient of W and    = + determines the propagation direction of the dynamo wave in αΩ dynamos (Parker 1955;Yoshimura 1975). Figure 3(d) indicates both poleward ( ¶ W < 0 r ) and equatorward ( ¶ W > 0 r ) regions outside the tangent cylinder and predominantly poleward propagation in the lower part of the CZ inside the tangent cylinder.…”

Section: Flow States and Dynamo Considerationsmentioning

confidence: 99%

“…Neither of these possibilities can be ruled out immediately with the data at hand, although the shear near the surface is not very prominent in the current simulations. However, explanations based on such simple models should be considered with caution in light of a recent study by Warnecke et al (2021), who found that explaining the cause and evolution of large-scale magnetism in a spherical shell simulation required a mean-field model with 18 turbulent transport coefficients derived using the test-field method (e.g., Schrinner et al 2007).…”

Section: Flow States and Dynamo Considerationsmentioning

confidence: 99%

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Solar-like Dynamos and Rotational Scaling of Cycles from Star-in-a-box Simulations

Käpylä

2022

ApJL

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Magnetohydrodynamic star-in-a-box simulations of convection and dynamos in a solar-like star with different rotation rates are presented. These simulations produce solar-like differential rotation with a fast equator and slow poles and magnetic activity that resembles that of the Sun with equatorward migrating activity at the surface. Furthermore, the ratio of rotation to cycle period is almost constant, as the rotation period decreases in the limited sample considered here. This is reminiscent of the suggested inactive branch of stars from observations and differs from most earlier simulation results from spherical shell models. While the exact excitation mechanism of the dynamos in the current simulations is not yet clear, it is shown that it is plausible that the greater freedom that the magnetic field has due to the inclusion of the radiative core and regions exterior to the star are important in shaping the dynamo.

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Section: Flow States and Dynamo Considerationsmentioning

confidence: 99%

Section: Flow States and Dynamo Considerationsmentioning

confidence: 99%

Solar-like Dynamos and Rotational Scaling of Cycles from Star-in-a-box Simulations

Käpylä

2022

ApJL

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“…Non-linear feedback also occurs at small scale in dynamo action, and this aspect has been modeled as a simple alpha quenching in this work. Nevertheless, recent studies underline that mean-field coefficients can have non-trivial dependencies, for instance, in high Rossby regime where anti-solar DR are expected (see Warnecke et al 2021, and reference therein). Such complex feedbacks are beyond the scope of the present study and shall be considered in future works (see also Pipin 2021).…”

Section: Model Contextmentioning

confidence: 99%

Impact of anti-solar differential rotation in mean-field solar-type dynamos

Noraz

Brun

Strugarek

et al. 2022

A&A

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Context. Over the course of their lifetimes, the rotation of solar-type stars goes through different phases. Once they reach the zero-age main sequence, their global rotation rate decreases during the main sequence until at least the solar age, approximately following the empirical Skumanich’s law and enabling gyrochronology. Older solar-type stars might then reach a point of transition when they stop braking, according to recent results of asteroseismology. Additionally, recent 3D numerical simulations of solar-type stars show that different regimes of differential rotation can be characterized with the Rossby number. In particular, anti-solar differential rotation (fast poles, slow equator) may exist for high Rossby number (slow rotators). If this regime occurs during the main sequence and, in general, for slow rotators, we may consider how magnetic generation through the dynamo process might be impacted. In particular, we consider whether slowly rotating stars are indeed subject to magnetic cycles. Aims. We aim to understand the magnetic field generation of solar-type stars possessing an anti-solar differential rotation and we focus on the possible existence of magnetic cycles in such stars. Methods. We modeled mean-field kinematic dynamos in solar (fast equator, slow poles) and anti-solar (slow equator, fast poles) differential rotation, using the STELEM code. We consider two types of mean field dynamo mechanisms along with the Ω-effect: the standard α-effect distributed at various locations in the convective envelope and the Babcock-Leighton effect. Results. We find that kinematic αΩ dynamos allow for the presence of magnetic cycles and global polarity reversals for both rotation regimes, but only if the α-effect is saddled on the tachocline. If it is distributed in the convection zone, solar-type cases still possess a cycle and anti-solar cases do not. Conversely, we have not found any possibility for sustaining a magnetic cycle with the traditional Babcock-Leighton flux-transport dynamos in the anti-solar differential rotation regime due to flux addition. Graphic interpretations are proposed in order to illustrate these cases. However, we find that hybrid models containing both prescriptions can still sustain local polarity reversals at some latitudes. Conclusions. We conclude that stars in the anti-solar differential rotation regime can sustain magnetic cycles only for very specific dynamo processes. The detection of a magnetic cycle for such a star would therefore be a particularly interesting constraint in working to decipher what type of dynamo is actually at work in solar-type stars.

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“…Neither of these possibilities can be ruled out immediately with the data at hand, although the shear near the surface is not very prominent in the current simulations. However, explanations based on such simple models should be considered with caution in light of a recent study by Warnecke et al (2021), who found that to explanation the cause and evolution of large-scale magnetism in a spherical shell simulation required a mean-field model with 24 turbulent transport coefficients derived using the test-field method (e.g. Schrinner et al 2007).…”

Section: Flow States and Dynamo Considerationsmentioning

confidence: 99%

Solar-like dynamos and rotational scaling of cycles from star-in-a-box simulations

Käpylä

2022

Preprint

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Magnetohydrodynamic star-in-a-box simulations of convection and dynamos in a solar-like star with different rotation rates are presented. These simulations produce solar-like differential rotation with a fast equator and slow poles, and magnetic activity that resembles that of the Sun with equatorward migrating activity at the surface. Furthermore, the ratio of rotation to cycle period is almost constant as the rotation period decreases in the limited sample considered here. This is reminiscent of the suggested inactive branch of stars from observations and differs from most earlier simulation results from spherical shell models. While the exact excitation mechanism of the dynamos in the current simulations is not yet clear, it is plausible that the greater freedom that the magnetic field has due to the inclusion of the radiative core and regions exterior to the star are important in shaping the dynamo.

show abstract

Investigating Global Convective Dynamos with Mean-field Models: Full Spectrum of Turbulent Effects Required

Cited by 16 publications

References 30 publications

Solar-like Dynamos and Rotational Scaling of Cycles from Star-in-a-box Simulations

Solar-like Dynamos and Rotational Scaling of Cycles from Star-in-a-box Simulations

Impact of anti-solar differential rotation in mean-field solar-type dynamos

Solar-like dynamos and rotational scaling of cycles from star-in-a-box simulations

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