MEAN-FIELD MODELING OF AN α
                    <sup>2</sup>
                    DYNAMO COUPLED WITH DIRECT NUMERICAL SIMULATIONS OF RIGIDLY ROTATING CONVECTION

Masada, Youhei; Sano, Takayoshi

doi:10.1088/2041-8205/794/1/l6

Cited by 15 publications

(14 citation statements)

References 27 publications

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“…(2013) investigated the phase relation between toroidal and poloidal magnetic fields in their oscillatory convectively driven dynamo with equatorward migration and found a phase shift of , which is compatible with what is expected for an oscillatory dynamo. Masada & Sano (2014) confirmed this finding for a dynamo in Cartesian geometry and reinforced the suggestion that the solar dynamo might indeed be of type. Then, Cole et al.…”

Section: The Solar Dynamosupporting

confidence: 53%

Advances in mean-field dynamo theory and applications to astrophysical turbulence

Brandenburg

2018

J. Plasma Phys.

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Recent advances in mean-field theory are reviewed and applications to the Sun, late-type stars, accretion disks, galaxies, and the early Universe are discussed. We focus particularly on aspects of spatio-temporal nonlocality, which provided some of the main new qualitative and quantitative insights that emerged from applying the test-field method to magnetic fields of different length and timescales. We also review the status of nonlinear quenching and the relation to magnetic helicity, which is an important observational diagnostic of modern solar dynamo theory. Both solar and some stellar dynamos seem to operate in an intermediate regime that has not yet been possible to model successfully. This regime is bracketed by antisolar-like differential rotation on one end and stellar activity cycles belonging to the superactive stars on the other. The difficulty in modeling this regime may be related to shortcomings in modelling solar/stellar convection. On galactic and extragalactic length scales, the observational constraints on dynamo theory are still less stringent and more uncertain, but recent advances both in theory and observations suggest that more conclusive comparisons may soon be possible also here. The possibility of inversely cascading magnetic helicity in the early Universe is particularly exciting in explaining the recently observed lower limits of magnetic fields on cosmological length scales. Such magnetic fields may be helical with the same sign of magnetic helicity throughout the entire Universe. This would be a manifestation of parity breaking. † Email address for correspondence: brandenb@nordita.org † In the following, we continue using the lowercase symbols u and b instead of U ′ and B ′ to denote fluctuations of U and B.

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Section: The Solar Dynamosupporting

confidence: 53%

Advances in mean-field dynamo theory and applications to astrophysical turbulence

Brandenburg

2018

J. Plasma Phys.

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“…It is well known that the rotating convection system spontaneously generates a mean kinetic helicity with a north-south antisymmetry, i.e., in the case of the eastward rotation like our PNS model, bulk positive helicity in north and negative in south, because of the Coriolis force acting on the convection flow (e.g., Miesch 2005). From preceding studies on stellar and solar dynamos (e.g., Charbonneau 2014; Brun & Browning 2017, for reviews), we expect that the mean kinetic helicity and its accompanying turbulent electro-motive fore (EMF) would be the origin of the large-scale magnetic component even in our PNS system (e.g., Racine et al 2011;Masada & Sano 2014). Although it is difficult to evaluate quantitatively the role of the turbulent EMF in the complicated PNS dynamo system, we can appraise it at least qualitatively based on the mean- field dynamo (MFD) theory.…”

Section: Discussionmentioning

confidence: 90%

Convection and Dynamo in Newly-born Neutron Stars

Masada,

Takiwaki,

Kotake

2020

Preprint

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We report results from a convection dynamo simulation of proto-neutron star (PNS), with a nuclear equation of state (EOS) and the initial hydrodynamic profile taken from a neutrino radiation-hydrodynamics simulation of a massive stellar core-collapse. A moderately-rotating PNS with the spin period of 170 ms in the leptondriven convection stage is focused. We find that large-scale flow and thermodynamic fields with north-south asymmetry develop in the turbulent flow, as a consequence of the convection in the central part of the PNS, which we call as a "deep core convection". Intriguingly, even with such a moderate rotation, large-scale, 10 15 G, magnetic field with dipole symmetry is spontaneously built up in the PNS. The turbulent electro-motive force arising from rotationally-constrained core convection is shown to play a key role in the large-scale dynamo. The large-scale structures organized in the PNS may impact the explosion dynamics of supernovae and subsequent evolution to the neutron stars.

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“…The direct observational knowledge of the change of these quantities during a grand minimum-type event, however, is very limited, and therefore the mean-field modeling approach is problematic. Another possibility is to seek answers from direct numerical simulations of turbulent convection either in local (e.g., Käpylä et al 2013a;Masada & Sano 2014) or global domains (e.g., Ghizaru et al 2010;Käpylä et al 2012;Augustson et al 2015;Fan & Fang 2014;Mabuchi et al 2015;Simitev et al 2015). This is particularly promising in the latter case, where it is possible to directly track the change of all relevant dynamo drivers, provided that a desired type of dynamo solution is found.…”

Section: Introductionmentioning

confidence: 99%

Multiple dynamo modes as a mechanism for long-term solar activity variations

Käpylä

Olspert

et al. 2016

A&A

104

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Context. Solar magnetic activity shows both smooth secular changes, such as the modern Grand Maximum, and quite abrupt drops that are denoted as grand minima, such as the Maunder Minimum. Direct numerical simulations (DNS) of convection-driven dynamos offer one way of examining the mechanisms behind these events. Aims.In this work, we analyze a solution of a solar-like DNS that was evolved for roughly 80 magnetic cycles of 4.9 years and where epochs of irregular behavior are detected. The emphasis of our analysis is to find physical causes for such behavior. Methods. The DNS employed is a semi-global (wedge-shaped) magnetoconvection model. For the data analysis tasks we use Ensemble Empirical Mode Decomposition and phase dispersion methods, as they are well suited for analyzing cyclic (non-periodic) signals. Results. A special property of the DNS is the existence of multiple dynamo modes at different depths and latitudes. The dominant mode is solar-like (equatorward migration at low latitudes and poleward at high latitudes). This mode is accompanied by a higher frequency mode near the surface and at low latitudes, showing poleward migration, and a low-frequency mode at the bottom of the convection zone. The low-frequency mode is almost purely antisymmetric with respect to the equator, while the dominant mode has strongly fluctuating mixed parity. The overall behavior of the dynamo solution is extremely complex, exhibiting variable cycle lengths, epochs of disturbed and even ceased surface activity, and strong short-term hemispherical asymmetries. Surprisingly, the most prominent suppressed surface activity epoch is actually a global magnetic energy maximum; during this epoch the bottom toroidal magnetic field obtains a maximum, demonstrating that the interpretation of grand minima-type events is non-trivial. The hemispherical asymmetries are seen only in the magnetic field, while the velocity field exhibits considerably weaker asymmetry. Conclusions. We interpret the overall irregular behavior as being due to the interplay of the different dynamo modes showing different equatorial symmetries, especially the smoother part of the irregular variations being related to the variations of the mode strengths, evolving with different and variable cycle lengths. The abrupt low-activity epoch in the dominant dynamo mode near the surface is related to a strong maximum of the bottom toroidal field strength, which causes abrupt disturbances especially in the differential rotation profile via the suppression of the Reynolds stresses.

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MEAN-FIELD MODELING OF AN α ² DYNAMO COUPLED WITH DIRECT NUMERICAL SIMULATIONS OF RIGIDLY ROTATING CONVECTION

Cited by 15 publications

References 27 publications

Advances in mean-field dynamo theory and applications to astrophysical turbulence

Advances in mean-field dynamo theory and applications to astrophysical turbulence

Convection and Dynamo in Newly-born Neutron Stars

Multiple dynamo modes as a mechanism for long-term solar activity variations

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MEAN-FIELD MODELING OF AN α 2 DYNAMO COUPLED WITH DIRECT NUMERICAL SIMULATIONS OF RIGIDLY ROTATING CONVECTION

Cited by 15 publications

References 27 publications

Advances in mean-field dynamo theory and applications to astrophysical turbulence

Advances in mean-field dynamo theory and applications to astrophysical turbulence

Convection and Dynamo in Newly-born Neutron Stars

Multiple dynamo modes as a mechanism for long-term solar activity variations

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Product

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MEAN-FIELD MODELING OF AN α ² DYNAMO COUPLED WITH DIRECT NUMERICAL SIMULATIONS OF RIGIDLY ROTATING CONVECTION