227 research outputs found
Shear-current effect in a turbulent convection with a large-scale shear
The shear-current effect in a nonrotating homogeneous turbulent convection
with a large-scale constant shear is studied. The large-scale velocity shear
causes anisotropy of turbulent convection, which produces the mean
electromotive force \bec{\cal E}^{(W)} \propto {\bf W} {\bf \times} {\bf J}
and the mean electric current along the original mean magnetic field, where
is the background mean vorticity due to the shear and is
the mean electric current. This results in a large-scale dynamo even in a
nonrotating and nonhelical homogeneous sheared turbulent convection, whereby
the effect vanishes. It is found that turbulent convection promotes
the shear-current dynamo instability, i.e., the heat flux causes positive
contribution to the shear-current effect. However, there is no dynamo action
due to the shear-current effect for small hydrodynamic and magnetic Reynolds
numbers even in a turbulent convection, if the spatial scaling for the
turbulent correlation time is , where is the
small-scale wave number.Comment: 8 pages, Physical Review E, in pres
The negative magnetic pressure effect in stratified turbulence
While the rising flux tube paradigm is an elegant theory, its basic
assumptions, thin flux tubes at the bottom of the convection zone with field
strengths two orders of magnitude above equipartition, remain numerically
unverified at best. As such, in recent years the idea of a formation of
sunspots near the top of the convection zone has generated some interest. The
presence of turbulence can strongly enhance diffusive transport mechanisms,
leading to an effective transport coefficient formalism in the mean-field
formulation. The question is what happens to these coefficients when the
turbulence becomes anisotropic due to a strong large-scale mean magnetic field.
It has been noted in the past that this anisotropy can also lead to highly
non-diffusive behaviour. In the present work we investigate the formation of
large-scale magnetic structures as a result of a negative contribution of
turbulence to the large-scale effective magnetic pressure in the presence of
stratification. In direct numerical simulations of forced turbulence in a
stratified box, we verify the existence of this effect. This phenomenon can
cause formation of large-scale magnetic structures even from initially uniform
large-scale magnetic field.Comment: 5 pages, 2 figures, submitted conference proceedings IAU symposium
273 "Physics of Sun and Star Spots
The dynamics of Wolf numbers based on nonlinear dynamo with magnetic helicity: comparisons with observations
We investigate the dynamics of solar activity using a nonlinear
one-dimensional dynamo model and a phenomenological equation for the evolution
of Wolf numbers. This system of equations is solved numerically. We take into
account the algebraic and dynamic nonlinearities of the alpha effect. The
dynamic nonlinearity is related to the evolution of a small-scale magnetic
helicity, and it leads to a complicated behavior of solar activity. The
evolution equation for the Wolf number is based on a mechanism of formation of
magnetic spots as a result of the negative effective magnetic pressure
instability (NEMPI). This phenomenon was predicted 25 years ago and has been
investigated intensively in recent years through direct numerical simulations
and mean-field simulations. The evolution equation for the Wolf number includes
the production and decay of sunspots. Comparison between the results of
numerical simulations and observational data of Wolf numbers shows a 70 %
correlation over all intervals of observation (about 270 years). We determine
the dependence of the maximum value of the Wolf number versus the period of the
cycle and the asymmetry of the solar cycles versus the amplitude of the cycle.
These dependencies are in good agreement with observations.Comment: 9 pages, 13 figures, final revised paper for MNRA
Nonlinear Turbulent Magnetic Diffusion and Mean-Field Dynamo
The nonlinear coefficients defining the mean electromotive force (i.e., the
nonlinear turbulent magnetic diffusion, the nonlinear effective velocity, the
nonlinear kappa-tensor, etc.) are calculated for an anisotropic turbulence. A
particular case of an anisotropic background turbulence (i.e., the turbulence
with zero mean magnetic field) with one preferential direction is considered.
It is shown that the toroidal and poloidal magnetic fields have different
nonlinear turbulent magnetic diffusion coefficients. It is demonstrated that
even for a homogeneous turbulence there is a nonlinear effective velocity which
exhibits diamagnetic or paramagnetic properties depending on anisotropy of
turbulence and level of magnetic fluctuations in the background turbulence.
Analysis shows that an anisotropy of turbulence strongly affects the nonlinear
mean electromotive force. Two types of nonlinearities (algebraic and dynamic)
are also discussed. The algebraic nonlinearity implies a nonlinear dependence
of the mean electromotive force on the mean magnetic field. The dynamic
nonlinearity is determined by a differential equation for the magnetic part of
the alpha-effect. It is shown that for the alpha-Omega axisymmetric dynamo the
algebraic nonlinearity alone cannot saturate the dynamo generated mean magnetic
field while the combined effect of the algebraic and dynamic nonlinearities
limits the mean magnetic field growth. Astrophysical applications of the
obtained results are discussed.Comment: 15 pages, REVTEX
Nonlinear turbulent magnetic diffusion and effective drift velocity of large-scale magnetic field in a two-dimensional magnetohydrodynamic turbulence
We study a nonlinear quenching of turbulent magnetic diffusion and effective
drift velocity of large-scale magnetic field in a developed two-dimensional MHD
turbulence at large magnetic Reynolds numbers. We show that transport of the
mean-square magnetic potential strongly changes quenching of turbulent magnetic
diffusion. In particularly, the catastrophic quenching of turbulent magnetic
diffusion does not occur for the large-scale magnetic fields when a divergence of the flux of the mean-square magnetic
potential is not zero, where is the equipartition mean magnetic
field determined by the turbulent kinetic energy and Rm is the magnetic
Reynolds number. In this case the quenching of turbulent magnetic diffusion is
independent of magnetic Reynolds number. The situation is similar to
three-dimensional MHD turbulence at large magnetic Reynolds numbers whereby the
catastrophic quenching of the alpha effect does not occur when a divergence of
the flux of the small-scale magnetic helicity is not zero.Comment: 8 pages, Physical Review E, in pres
Nonlinear theory of a "shear-current" effect and mean-field magnetic dynamos
The nonlinear theory of a "shear-current" effect in a nonrotating and
nonhelical homogeneous turbulence with an imposed mean velocity shear is
developed. The ''shear-current" effect is associated with the -term in the mean electromotive force and causes the
generation of the mean magnetic field even in a nonrotating and nonhelical
homogeneous turbulence (where is the mean vorticity and is the mean electric current). It is found that there is no quenching of
the nonlinear "shear-current" effect contrary to the quenching of the nonlinear
-effect, the nonlinear turbulent magnetic diffusion, etc. During the
nonlinear growth of the mean magnetic field, the ''shear-current" effect only
changes its sign at some value of the mean magnetic field.
The magnitude determines the level of the saturated mean
magnetic field which is less than the equipartition field. It is shown that the
background magnetic fluctuations due to the small-scale dynamo enhance the
"shear-current" effect, and reduce the magnitude . When the
level of the background magnetic fluctuations is larger than 1/3 of the kinetic
energy of the turbulence, the mean magnetic field can be generated due to the
"shear-current" effect for an arbitrary exponent of the energy spectrum of the
velocity fluctuations.Comment: 16 pages, 4 figures, REVTEX4, revised version, Phys. Rev. E, v. 70,
in press (2004
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