The fratricide ofαΩ dynamos by theirα2siblings

Hubbard, Alexander; Rheinhardt, M.; Brandenburg, Axel

doi:10.1051/0004-6361/201116705

Cited by 13 publications

(11 citation statements)

References 28 publications

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“…In that gauge, magnetic helicity evolves merely like a passive scalar. Numerical experiments confirm that, within error bars, there is no magnetic helicity flux in situations where the formula for the VC flux would have predicted one (Hubbard & Brandenburg 2010b).…”

Section: Resultsmentioning

confidence: 53%

“…It should also be noted that the theoretical foundations of the VC flux have been called into question (Hubbard & Brandenburg 2010b). The criticism is connected with the fact that a shear produces spurious divergences of magnetic helicity flux that can only be removed in a gauge where the scalar potential is equal to the dot product of velocity and vector potential.…”

Section: Resultsmentioning

confidence: 99%

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Shear-driven and diffusive helicity fluxes in αΩ dynamos

Gómez

Chatterjee

Brandenburg

2010

Monthly Notices of the Royal Astronomical Society

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We present non‐linear mean‐field αΩ dynamo simulations in spherical geometry with simplified profiles of kinetic α effect and shear. We take magnetic helicity evolution into account by solving a dynamical equation for the magnetic α effect. This gives a consistent description of the quenching mechanism in mean‐field dynamo models. The main goal of this work is to explore the effects of this quenching mechanism in solar‐like geometry, and in particular to investigate the role of magnetic helicity fluxes, specifically diffusive and Vishniac–Cho (VC) fluxes, at large magnetic Reynolds numbers (Rm). For models with negative radial shear or positive latitudinal shear, the magnetic α effect has predominantly negative (positive) sign in the Northern (Southern) hemisphere. In the absence of fluxes, we find that the magnetic energy follows an R−1m dependence, as found in previous works. This catastrophic quenching is alleviated in models with diffusive magnetic helicity fluxes resulting in magnetic fields comparable to the equipartition value even for Rm= 107. On the other hand, models with a shear‐driven Vishniac–Cho flux show an increase in the amplitude of the magnetic field with respect to models without fluxes, but only for Rm < 104. This is partly a consequence of assuming a vacuum outside the Sun which cannot support a significant VC flux across the boundary. However, in contrast to the diffusive flux, the VC flux modifies the distribution of the magnetic field. In addition, if an ill‐determined scaling factor in the expression for the VC flux is large enough, subcritical dynamo action is possible that is driven by the action of shear and the divergence of magnetic helicity flux.

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Section: Resultsmentioning

confidence: 53%

Section: Resultsmentioning

confidence: 99%

Shear-driven and diffusive helicity fluxes in αΩ dynamos

Gómez

Chatterjee

Brandenburg

2010

Monthly Notices of the Royal Astronomical Society

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“…On this occasion we mention that, using the same setup as here, Hubbard et al (2011) have demonstrated that, after some initial saturation phase, the aW dynamo mode is "fratricided" by a separate 2 a mode and eventually the initial aW model µ -dependence, whereas blue dotted line represents Equation (8). The data points for initial growth phase (t 0.6 diff t < ) are not used for this plot.…”

Section: Large R M and Large D: Regime Ivmentioning

confidence: 76%

Is the Small-Scale Magnetic Field Correlated With the Dynamo Cycle?

Karak

Brandenburg

2015

ApJ

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The small-scale magnetic field is ubiquitous at the solar surface-even at high latitudes. From observations we know that this field is uncorrelated (or perhaps even weakly anticorrelated) with the global sunspot cycle. Our aim is to explore the origin, and particularly the cycle dependence, of such a phenomenon using three-dimensional dynamo simulations. We adopt a simple model of a turbulent dynamo in a shearing box driven by helically forced turbulence. Depending on the dynamo parameters, large-scale (global) and small-scale (local) dynamos can be excited independently in this model. Based on simulations in different parameter regimes, we find that, when only the large-scale dynamo is operating in the system, the small-scale magnetic field generated through shredding and tangling of the large-scale magnetic field is positively correlated with the global magnetic cycle. However, when both dynamos are operating, the small-scale field is produced from both the small-scale dynamo and the tangling of the large-scale field. In this situation, when the large-scale field is weaker than the equipartition value of the turbulence, the small-scale field is almost uncorrelated with the large-scale magnetic cycle. On the other hand, when the large-scale field is stronger than the equipartition value, we observe an anticorrelation between the smallscale field and the large-scale magnetic cycle. This anticorrelation can be interpreted as a suppression of the smallscale dynamo. Based on our studies we conclude that the observed small-scale magnetic field in the Sun is generated by the combined mechanisms of a small-scale dynamo and tangling of the large-scale field.

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“…Recently, Mitra et al (2010) reported an intriguing numerical finding in their forced helical turbulence that α 2 dynamo can yield solar-like equatorward migration of magnetic activity belts (see also Schrinner et al 2011). The superiority of the α 2 mode over αΩ mode at the nonlinear stage was found by Hubbard et al (2011). Furthermore, a connection between α 2 dynamo mode and solar magnetism was suggested in some recent results of global MHD dynamos (e.g., Simard et al 2013;Käpylä et al 2013b).…”

Section: Summary and Discussionmentioning

confidence: 96%

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

Masada¹,

Sano²

2014

ApJ

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The mechanism of large-scale dynamos in rigidly rotating stratified convection is explored by direct numerical simulations (DNS) in Cartesian geometry. A mean-field dynamo model is also constructed using turbulent velocity profiles consistently extracted from the corresponding DNS results. By quantitative comparison between the DNS and our mean-field model, it is demonstrated that the oscillatory α 2 dynamo wave, excited and sustained in the convection zone, is responsible for large-scale magnetic activities such as cyclic polarity reversal and spatiotemporal migration. The results provide strong evidence that a nonuniformity of the α-effect, which is a natural outcome of rotating stratified convection, can be an important prerequisite for large-scale stellar dynamos, even without the Ω-effect.

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The fratricide ofαΩ dynamos by theirα²siblings

Cited by 13 publications

References 28 publications

Shear-driven and diffusive helicity fluxes in αΩ dynamos

Shear-driven and diffusive helicity fluxes in αΩ dynamos

Is the Small-Scale Magnetic Field Correlated With the Dynamo Cycle?

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

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The fratricide ofαΩ dynamos by theirα2siblings

Cited by 13 publications

References 28 publications

Shear-driven and diffusive helicity fluxes in αΩ dynamos

Shear-driven and diffusive helicity fluxes in αΩ dynamos

Is the Small-Scale Magnetic Field Correlated With the Dynamo Cycle?

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

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The fratricide ofαΩ dynamos by theirα²siblings

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