Volume 500, Number 2, June III 2009
|Page(s)||633 - 646|
|Published online||29 April 2009|
Alpha effect and turbulent diffusion from convection
Observatory, Tähtitorninmäki, PO Box 14, 00014 University of Helsinki, Finland e-mail: email@example.com
2 NORDITA, AlbaNova University Center, Roslagstullsbacken 23, 10691 Stockholm, Sweden
Accepted: 18 March 2009
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 latitude profile for moderate rotation. Turbulent diffusivity, , is proportional to the square of the turbulent vertical velocity in all cases. Whereas 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 -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.
Key words: magnetohydrodynamics (MHD) / convection / turbulence / Sun: magnetic fields / stars: magnetic fields
© ESO, 2009
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.