Scouts, Guides, and the Fashioning of Empire, 1919–39

Proctor, Tammy M.

doi:10.2752/9781847888723/fashbodpol0010

Cited by 12 publications

(10 citation statements)

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“…In addition to the α-effect, there are other possible alternatives, in particular the incoherent alpha-shear dynamo (Vishniac & Brandenburg 1997;Proctor 2007), which could produce a large-scale magnetic field even without stratification and hence no regular α-effect. This interpretation has been favoured in some recent work where the shear-current effect was found not to be excited ).…”

Section: Discussionmentioning

confidence: 99%

“…In the turbulent mean-field dynamo picture an α-effect can arise due to helical motions in a stratified fluid in the presence of either rotation or shear, or both Rüdiger & Kitchatinov 2006;Rädler & Stepanov 2006). Other candidates for regenerating large-scale magnetic fields in shearing turbulence include the incoherent αΩ-effect (Vishniac & Brandenburg 1997;Proctor 2007) and the shear-current effect .…”

Section: Introductionmentioning

confidence: 99%

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Large-scale dynamos in turbulent convection with shear

Käpylä¹,

Korpi²,

Brandenburg

2008

A&A

111

129

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Aims. To study the existence of large-scale convective dynamos under the influence of shear and rotation. Methods. Three-dimensional numerical simulations of penetrative compressible convection with uniform horizontal shear are used to study dynamo action and the generation of large-scale magnetic fields. We consider cases where the magnetic Reynolds number is either marginal or moderately supercritical with respect to small-scale dynamo action in the absence of shear and rotation. Our magnetic Reynolds number is based on the wavenumber of the depth of the convectively unstable layer. The effects of magnetic helicity fluxes are studied by comparing results for the magnetic field with open and closed boundaries. Results. Without shear no large-scale dynamos are found even if the ingredients necessary for the α-effect (rotation and stratification) are present in the system. When uniform horizontal shear is added, a large-scale magnetic field develops, provided the boundaries are open. In this case the mean magnetic field contains a significant fraction of the total field. For those runs where the magnetic Reynolds number is between 60 and 250, an additional small-scale dynamo is expected to be excited, but the field distribution is found to be similar to cases with smaller magnetic Reynolds number where the small-scale dynamo is not excited. In the case of closed (perfectly conducting) boundaries, magnetic helicity fluxes are suppressed and no large-scale fields are found. Similarly, poor large-scale field development is seen when vertical shear is used in combination with periodic boundary conditions in the horizontal directions. If, however, open (normal-field) boundary conditions are used in the x-direction, a large-scale field develops. These results support the notion that shear not only helps to generate the field, but it also plays a crucial role in driving magnetic helicity fluxes out of the system along the isocontours of shear, thereby allowing efficient dynamo action.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Large-scale dynamos in turbulent convection with shear

Käpylä¹,

Korpi²,

Brandenburg

2008

A&A

111

129

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“…In this scenario, a planar shear flow is added to a velocity field that may not in itself act as a dynamo. The effect of shear can also be studied in the mean field context (Vishniac & Brandenburg 1997;Proctor 2007;Richardson & Proctor 2010;Mitra & Brandenburg 2012). In all cases, it is observed that dynamo action is made possible (when there is no dynamo in the absence of shear) or enhanced.…”

Section: Introductionmentioning

confidence: 99%

Fluctuating αO dynamos by iterated matrices

Richardson¹,

Proctor²

2012

Monthly Notices of the Royal Astronomical Society: Letters

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Recent publications have suggested various scaling laws for dynamo growth rates when dynamo action is made possible or enhanced by shear. In order to understand the relation between the different scalings, we study a simple model mean field dynamo, in which the α‐effect is piecewise constant in time, allowing a simple derivation of the growth rates in terms of iterated matrices. It is found that all the different scaling laws that appear in earlier work can be seen in different parameter regimes.

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“…Even if the mean value of α is zero, strong enough fluctuations about zero in combination with shear can drive an incoherent α-shear dynamo (e.g. Vishniac & Brandenburg 1997;Proctor 2007). Finally, the shear-current Kleeorin & Rogachevskii 2008) and Ω × J (Rädler 1969;Rädler et al 2003;Pipin 2008) effects may operate even in nonhelical turbulence.…”

Section: Introductionmentioning

confidence: 99%

Alpha effect and turbulent diffusion from convection

Käpylä¹,

Korpi²,

Brandenburg

2009

A&A

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Aims. We study turbulent transport coefficients that describe the evolution of large-scale magnetic fields in turbulent convection. Methods. We use the test field method, together with three-dimensional numerical simulations of turbulent convection with shear and rotation, to compute turbulent transport coefficients describing the evolution of large-scale magnetic fields in mean-field theory in the kinematic regime. We employ one-dimensional mean-field models with the derived turbulent transport coefficients to examine whether they give results that are compatible with direct simulations. Results. The results for the α-effect as a function of rotation rate are consistent with earlier numerical studies, i.e. increasing magnitude as rotation increases and approximately cos θ latitude profile for moderate rotation. Turbulent diffusivity, η t , is proportional to the square of the turbulent vertical velocity in all cases. Whereas η t decreases approximately inversely proportional to the wavenumber of the field, the α-effect and turbulent pumping show a more complex behaviour with partial or full sign changes and the magnitude staying roughly constant. In the presence of shear and no rotation, a weak α-effect is induced which does not seem to show any consistent trend as a function of shear rate. Provided that the shear is large enough, this small α-effect is able to excite a dynamo in the mean-field model. The coefficient responsible for driving the shear-current effect shows several sign changes as a function of depth but is also able to contribute to dynamo action in the mean-field model. The growth rates in these cases are, however, well below those in direct simulations, suggesting that an incoherent α-shear dynamo may also act in the simulations. If both rotation and shear are present, the α-effect is more pronounced. At the same time, the combination of the shear-current and Ω × J-effects is also stronger than in the case of shear alone, but subdominant to the α-shear dynamo. The results of direct simulations are consistent with mean-field models where all of these effects are taken into account without the need to invoke incoherent effects.

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Scouts, Guides, and the Fashioning of Empire, 1919–39

Cited by 12 publications

References 0 publications

Large-scale dynamos in turbulent convection with shear

Large-scale dynamos in turbulent convection with shear

Fluctuating αO dynamos by iterated matrices

Alpha effect and turbulent diffusion from convection

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