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Title:
Alpha effect and turbulent diffusion from convection
Authors:
Käpylä, P. J.; Korpi, M. J.; Brandenburg, A.
Affiliation:
AA(Observatory, Tähtitorninmäki, PO Box 14, 00014 University of Helsinki, Finland ), AB(Observatory, Tähtitorninmäki, PO Box 14, 00014 University of Helsinki, Finland), AC(NORDITA, AlbaNova University Center, Roslagstullsbacken 23, 10691 Stockholm, Sweden)
Publication:
Astronomy and Astrophysics, Volume 500, Issue 2, 2009, pp.633-646 (A&A Homepage)
Publication Date:
06/2009
Origin:
EDP Sciences
Astronomy Keywords:
magnetohydrodynamics (MHD), convection, turbulence, Sun: magnetic fields, stars: magnetic fields
DOI:
10.1051/0004-6361/200811498
Bibliographic Code:
2009A&A...500..633K

Abstract

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 ηt 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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