Strong mean field dynamos require supercritical helicity fluxes

2010

A&A

Self Cite

Aims. Twisted magnetic fields are frequently seen to emerge above the visible surface of the Sun. This emergence is usually associated with the rise of buoyant magnetic flux structures. Here we ask how magnetic fields from a turbulent large-scale dynamo appear above the surface if there is no magnetic buoyancy. Methods. The computational domain is split into two parts. In the lower part, which we refer to as the turbulence zone, the flow is driven by an assumed helical forcing function leading to dynamo action. Above this region, which we refer to as the exterior, a nearly force-free magnetic field is computed at each time step using the stress-and-relax method. Results. Twisted arcade-like field structures are found to emerge in the exterior above the turbulence zone. Strong current sheets tend to form above the neutral line, where the vertical field component vanishes. Time series of the magnetic field structure show recurrent plasmoid ejections. The degree to which the exterior field is force free is estimated as the ratio of the dot product of current density and magnetic field strength to their respective rms values. This ratio reaches values of up to 95% in the exterior. A weak outward flow is driven by the residual Lorentz force.

Section: Dynamo Saturationmentioning

confidence: 98%

Surface appearance of dynamo-generated large-scale fields

Warnecke

2010

A&A

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“…In a periodic domain, the dynamo is expected to reach and sustain mean fields of the order of the inverse square root of the scale separation ratio (Blackman & Brandenburg 2002). However, in an open domain the final saturation field strength is always of the order of R −1/2 m (Brandenburg & Dobler 2001;Brandenburg & Subramanian 2005), although the mean field may reach an early equipartition strength peak just at the time when the small-scale field reaches saturation (Fig. 3).…”

Section: Nonlinear Interface Dynamo Without Helicity Fluxesmentioning

confidence: 99%

“…The poleward-propagating mode is now driven by supercritical diffusive helicity fluxes, as opposed to supercritical Vishniac & Cho fluxes (see Brandenburg & Subramanian 2005, for examples of this behaviour). For the same model (R m = 2 × 10 5 ) there exists a critical κ c ∼ 10 −5 such that the secondary dynamo wave does not appear if κ 0 < κ c .…”

Section: Supercritical Diffusive Magnetic Helicity Fluxesmentioning

confidence: 99%

Magnetic helicity fluxes in interface and flux transport dynamos

Chatterjee

Gómez

2010

A&A

Self Cite

Context. Dynamos in the Sun and other bodies tend to produce magnetic fields that possess magnetic helicity of opposite sign at large and small scales, respectively. The build-up of magnetic helicity at small scales provides an important saturation mechanism. Aims. In order to understand the nature of the solar dynamo we need to understand the details of the saturation mechanism in spherical geometry. In particular, we aim to understand the effects of magnetic helicity fluxes from turbulence and meridional circulation. Methods. We consider a model with only radial shear confined to a thin layer (tachocline) at the bottom of the convection zone. The kinetic α owing to helical turbulence is assumed to be localized in a region above the convection zone. The dynamical quenching formalism is used to describe the build-up of mean magnetic helicity in the model, which results in a magnetic α effect that feeds back on the kinetic α effect. In some cases we compare these results with those obtained from a model with a simple algebraic α quenching formula.Results. In agreement with earlier findings, the magnetic α effect has the opposite sign compared with the kinetic α effect and leads to a catastrophic decrease of the saturation field strength proportional to the inverse magnetic Reynolds number. At high latitudes this quenching effect can lead to secondary dynamo waves that propagate poleward because of the opposite sign of α. These secondary dynamo waves are driven by small-scale magnetic helicity instead of the small-scale kinetic helicity. Magnetic helicity fluxes both from turbulent mixing and from meridional circulation alleviate catastrophic quenching. Interestingly, supercritical diffusive helicity fluxes also give rise to secondary dynamo waves and grand minima-like episodes.

“…In open domains there is the possibility that the resulting magnetic field strength can still decrease to catastrophically quenched values unless there is a finite divergence of the magnetic helicity flux (Brandenburg & Subramanian 2005a). Such a flux can be driven efficiently in the presence of shear.…”

Section: Discussionmentioning

confidence: 99%

Advances in Theory and Simulations of Large-Scale Dynamos

2009

Space Sci Rev

Recent analytical and computational advances in the theory of large-scale dynamos are reviewed. The importance of the magnetic helicity constraint is apparent even without invoking mean-field theory. The tau approximation yields expressions that show how the magnetic helicity gets incorporated into mean-field theory. The test-field method allows an accurate numerical determination of turbulent transport coefficients in linear and nonlinear regimes. Finally, some critical views on the solar dynamo are being offered and targets for future research are highlighted.