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1.
V. V. Pipin 《Astronomy Letters》2001,27(3):171-175
We investigate the influence of turbulence anisotropy and rotation on the diffusion of a low-concentration passive scalar in a turbulent medium. Using the renormalization the diffusion tensor over the spectrum of turbulent fluctuations, we show that enhanced horizontal mixing reduces the vertical diffusion transport of a passive scalar. Allowance for rotation results in two effects, which have not been noted previously: (1) under the influence of Coriolis forces, horizontal turbulence also produces a vertical diffusion flux, with the horizontal and vertical diffusions being of the same order of magnitude for rapidly rotating stars and (2) in the case of rapid rotation, all diffusion fluxes of a passive scalar decrease in inverse proportion to the square root of the Coriolis number. 相似文献
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V. V. Pipin 《Astronomy Reports》2007,51(5):411-420
We consider a solar dynamo mechanism that generates large-scale magnetic fields due to the combined action of cyclonic flows (the α effect), differential rotation (the Θ effect), and the non-uniformity of large-scale magnetic fields (the Θ × J effect). Our results are based on numerical model which takes into account currently available data on the differential rotation of the convection zone and the intensity of convective flows in the solar interior. A reasonable choice of parameters characterizing the intensity of magnetic-field generation by the α and Θ × J mechanisms can account for an oscillatory dynamo regime with properties similar to the 22-year magnetic-activity cycle of the Sun. We analyze the nonlinear saturation of the generation effects in the large-scale magnetic field, due to either magnetic stresses or the conservation of magnetic helicity. Allowance for the helicity of the small-scale magnetic fields is of crucial importance in limiting the energy of the generated large-scale magnetic field. 相似文献
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Abstractthe effect of the large-scale magnetic fields generated by the solar dynamo on the radiation flux issuing from the convection zone is studied. A governing equation describing convective heat transfer is obtained in the framework of mean-field magnetohydrodynamics, with account for the influence of magnetic fields and differential rotation on the energy budget of the convection zone. The principal effects are illustrated using a one-dimensional numerical model. Calculations indicate that the influence of large-scale magnetic fields can modulate the solar irradiance with a relative amplitude of ～0.07%. 相似文献
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Some consequences of a nonlinear coupling between magnetic field and rotation are studied within a solar type 2D dynamo model for a spherical convective shell. The magnetic feedback on the rotation law produces two main effects. First, the torsional oscillations are excited. Second, a long-term amplitude modulation of the dynamo cycles is produced. The latter may be identified with the grand cycle of solar activity. The dynamo model seems to be in accord with the phase relations between the torsional and magnetic activity oscillations observed in the 11-year cycle as well as in the 55-year grand cycle. It, however, fails to reproduce the observationally suggested global decreasing trend in the equatorial rotation rate. 相似文献
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Abstract A theory of the non-diffusive anisotropic kinetic alpha-effect (“Γ-effect”) for densitystratified rotating turbulent fluids is developed. No limitations on the rotation rate are imposed and the fully nonlinear dependence of the Γ-effect on the angular velocity is studied. When the Coriolis number, ω? = 2τ ω, is small the dimensionless “dynamo number”, Cτ, characterising the power of the Γ-effect, grows with ω?. The dependence, however, reaches a maximum for ω? ～ 2. For still higher rotation rates CΛ decreases as 1/ω?. In opposition, the corresponding number, Cx, of the hydromagnetic α2 -dynamo problems remains finite for very large ω?. Hence, for fast rotation the hydrodynamic Γ-effect is small while the hydromagnetic α-effect remains large. In consequence, the large-scale magnetic and velocity structures are expected to be generated with roughly equal power in slowly rotating objects. In the rapid rotators, however, generation of the large-scale flows is problematic. 相似文献
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V.V. Pipin 《地球物理与天体物理流体动力学》2013,107(1-2):185-206
We study the effect of turbulent drift of a large-scale magnetic field that results from the interaction of helical convective motions and differential rotation in the solar convection zone. The principal direction of the drift corresponds to the direction of the large-scale vorticity vector. Thus, the effect produces a latitudinal transport of the large-scale magnetic field in the convective zone wherever the angular velocity has a strong radial gradient. The direction of the drift depends on the sign of helicity and it is defined by the Parker–Yoshimura rule. The analytic calculations are done within the framework of mean-field magnetohydrodynamics using the minimal τ-approximation. We estimate the magnitude of the drift velocity and find that it can be a few m/s near the base of the solar convection zone. The implications of this effect for the solar dynamo are illustrated on the basis of an axisymmetric mean-field dynamo model with a subsurface shear layer. The model shows that near the bottom of the convection zone the helicity–vorticity pumping results mostly from the kinetic helicity contributions. We find that the magnetic helicity contributions to the pumping effect are dominant at the subsurface shear layer. There the magnitude of the drift velocity is found to be a few cm/s. We find that the helicity–vorticity pumping effect can have an influence on the features of the sunspot time–latitude diagram, producing a fast drift of the sunspot activity maximum at the rise phase of the cycle and a slow drift at the decay phase of the cycle. 相似文献
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We present a straightforward comparison of model calculations for the α-effect, helicities, and magnetic field line twist
in the solar convection zone with magnetic field observations at atmospheric levels. The model calculations are carried out
in a mixing-length approximation for the turbulence with a profile of the solar internal rotation rate obtained from helioseismic
inversions. The magnetic field data consist of photospheric vector magnetograms of 422 active regions for which spatially-averaged
values of the force-free twist parameter and of the current helicity density are calculated, which are then used to determine
latitudinal profiles of these quantities. The comparison of the model calculations with the observations suggests that the
observed twist and helicity are generated in the bulk of the convection zone, rather than in a layer close to the bottom.
This supports two-layer dynamo models where the large-scale toroidal field is generated by differential rotation in a thin
layer at the bottom while the α-effect is operating in the bulk of the convection zone. Our previous observational finding
was that the moduli of the twist factor and of the current helicity density increase rather steeply from zero at the equator
towards higher latitudes and attain a certain saturation at about 12 – 15∘. In our dynamo model with algebraic nonlinearity, the increase continues, however, to higher latitudes and is more gradual.
This could be due to the neglect of the coupling between small-scale and large-scale current and magnetic helicities and of
the latitudinal drift of the activity belts in the model. 相似文献
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Solar Physics - For a given field of magnetic fluctuations the dynamo-α, as well as the kinetic and current helicities, have been computed, assuming that turbulence is subject to magnetic... 相似文献