177 research outputs found
New mechanism of generation of large-scale magnetic field in a sheared turbulent plasma
A review of recent studies on a new mechanism of generation of large-scale
magnetic field in a sheared turbulent plasma is presented. This mechanism is
associated with the shear-current effect which is related to the W x J-term in
the mean electromotive force. This effect causes the generation of the
large-scale magnetic field even in a nonrotating and nonhelical homogeneous
sheared turbulent convection whereby the alpha 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 k^{-2}, where k is
the small-scale wave number. We discuss here also the nonlinear mean-field
dynamo due to the shear-current effect and take into account the transport of
magnetic helicity as a dynamical nonlinearity. The magnetic helicity flux
strongly affects the magnetic field dynamics in the nonlinear stage of the
dynamo action. When the magnetic helicity flux is not small, the saturated
level of the mean magnetic field is of the order of the equipartition field
determined by the turbulent kinetic energy. The obtained results are important
for elucidation of origin of the large-scale magnetic fields in astrophysical
and cosmic sheared turbulent plasma.Comment: 7 pages, Planetory and Space Science, in pres
Large-scale instability in a sheared nonhelical turbulence: formation of vortical structures
We study a large-scale instability in a sheared nonhelical turbulence that
causes generation of large-scale vorticity. Three types of the background
large-scale flows are considered, i.e., the Couette and Poiseuille flows in a
small-scale homogeneous turbulence, and the "log-linear" velocity shear in an
inhomogeneous turbulence. It is known that laminar plane Couette flow and
antisymmetric mode of laminar plane Poiseuille flow are stable with respect to
small perturbations for any Reynolds numbers. We demonstrate that in a
small-scale turbulence under certain conditions the large-scale Couette and
Poiseuille flows are unstable due to the large-scale instability. This
instability causes formation of large-scale vortical structures stretched along
the mean sheared velocity. The growth rate of the large-scale instability for
the "log-linear" velocity shear is much larger than that for the Couette and
Poiseuille background flows. We have found a turbulent analogue of the
Tollmien-Schlichting waves in a small-scale sheared turbulence. A mechanism of
excitation of turbulent Tollmien-Schlichting waves is associated with a
combined effect of the turbulent Reynolds stress-induced generation of
perturbations of the mean vorticity and the background sheared motions. These
waves can be excited even in a plane Couette flow imposed on a small-scale
turbulence when perturbations of mean velocity depend on three spatial
coordinates. The energy of these waves is supplied by the small-scale sheared
turbulence.Comment: 12 pages, 14 figures, Phys. Rev. E, in pres
Competition of rotation and stratification in flux concentrations
In a strongly stratified turbulent layer, a uniform horizontal magnetic field
can become unstable to spontaneously form local flux concentrations due to a
negative contribution of turbulence to the large-scale (mean-field) magnetic
pressure. This mechanism, called the negative effective magnetic pressure
instability (NEMPI), is of interest in connection with dynamo scenarios where
most of the magnetic field resides in the bulk of the convection zone, and not
at the bottom. Recent work using the mean-field hydromagnetic equations has
shown that NEMPI becomes suppressed at rather low rotation rates with Coriolis
numbers as low as 0.1.}{Here we extend these earlier investigations by studying
the effects of rotation both on the development of NEMPI and on the effective
magnetic pressure. We also quantify the kinetic helicity from direct numerical
simulations (DNS) and compare with earlier work.}{To calculate the rotational
effect on the effective magnetic pressure we consider both DNS and analytical
studies using the approach. To study the effects of rotation on the
development of NEMPI we use both DNS and mean-field calculations of the 3D
hydromagnetic equations in a Cartesian domain.}{We find that the growth rates
of NEMPI from earlier mean-field calculations are well reproduced with DNS,
provided the Coriolis number is below about 0.06. In that case, kinetic and
magnetic helicities are found to be weak. For faster rotation, dynamo action
becomes possible. However, there is an intermediate range of rotation rates
where dynamo action on its own is not yet possible, but the rotational
suppression of NEMPI is being alleviated.}{Production of magnetic flux
concentrations through the suppression of turbulent pressure appears to be
possible only in the upper-most layers of the Sun, where the convective
turnover time is less than 2 hours.}Comment: 13 pages, 13 figures submitted to A&
Intense bipolar structures from stratified helical dynamos
We perform direct numerical simulations of the equations of
magnetohydrodynamics with external random forcing and in the presence of
gravity. The domain is divided into two parts: a lower layer where the forcing
is helical and an upper layer where the helicity of the forcing is zero with a
smooth transition in between. At early times, a large-scale helical dynamo
develops in the bottom layer. At later times the dynamo saturates, but the
vertical magnetic field continues to develop and rises to form dynamic bipolar
structures at the top, which later disappear and reappear. Some of the
structures look similar to spots observed in the Sun. This is the
first example of magnetic flux concentrations, owing to strong density
stratification, from self-consistent dynamo simulations that generate bipolar,
super-equipartition strength, magnetic structures whose energy density can
exceeds the turbulent kinetic energy by even a factor of ten.Comment: Published in Monthly Notices of Royal Astronomical Societ
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
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