Surface flux concentrations and spherical alpha-square dynamo

Abstract

In the presence of strong density stratification, turbulence can lead to a large-scale instability of a horizontal magnetic field if its strength is in a suitable range (within a few percent of the turbulent equipartition value). This instability is related to a suppression of the turbulent pressure so that the turbulence contribution to the mean magnetic pressure becomes negative. This results in the excitation of a negative effective magnetic pressure instability (NEMPI). This instability has so far only been studied for an imposed magnetic field. We want to know how NEMPI works when the mean magnetic field is generated self-consistently by an ${\alpha }^2$ dynamo, whether it is affected by global spherical geometry, and whether it can influence the properties of the dynamo itself. We adopt the mean-field approach which has previously been shown to provide a realistic description of NEMPI in direct numerical simulations. We assume axisymmetry and solve the mean-field equations with the Pencil-Code for an adiabatic stratification at a total density contrast in the radial direction of approximately 4 orders of magnitude. NEMPI is found to work when the dynamo-generated field is about 4% of the equipartition value, which is achieved through strong $\alpha$ quenching. This instability is excited in the top 5% of the outer radius provided the density contrast across this top layer is at least 10. NEMPI is found to occur at lower latitudes when the mean magnetic field is stronger. For weaker fields, NEMPI can make the dynamo oscillatory with poleward migration. NEMPI is a viable mechanism for producing magnetic flux concentrations in a strongly stratified spherical shell in which a magnetic field is generated by a strongly quenched $\alpha$ effect dynamo.

published in A&A

References in corpus (6)

• Flux-transport dynamos with Lorentz force feedback on differential rotation and meridional flow: Saturation mechanism and torsional oscillations
• Kinematic alpha effect in isotropic turbulence simulations
• Magnetic fluctuations and formation of large-scale inhomogeneous magnetic structures in a turbulent convection
• New scaling for the alpha effect in slowly rotating turbulence
• Surface appearance of dynamo-generated large-scale fields
• Competition of rotation and stratification in flux concentrations

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• Bipolar region formation in stratified two-layer turbulence
• Bipolar magnetic spots from dynamos in stratified spherical shell turbulence
• Magnetic flux concentrations in a polytropic atmosphere
• Negative Magnetic Diffusivity beta replacing alpha effect in Helical Dynamo