802 research outputs found
Helicity-vorticity turbulent pumping of magnetic fields in solar convection zone
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
\tau-approximation. We estimate the magnitude of the drift velocity and find
that it can be several 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. 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.Comment: 19 pages, 8 figures, submitted to GAF
Advances in mean-field dynamo theories
We give a short introduction to the subject and review advances in
understanding the basic ingredients of the mean-field dynamo theory. The
discussion includes the recent analytic and numerical work in developments for
the mean electromotive force of the turbulent flows and magnetic field, the
nonlinear effects of the magnetic helicity, the non-local generation effects in
the dynamo. We give an example of the mean-field solar dynamo model that
incorporates the fairly complete expressions for the mean-electromotive force,
the subsurface shear layer and the conservation of the total helicity. The
model is used to shed light on the issues in the solar dynamo and on the future
development of this field of research.Comment: Revision2, replaced two figures by color version, 12 pages, IAU 294
Proceeding
Magnetic helicity in non-axisymmetric mean-field solar dynamo
The paper address the effects of magnetic helicity conservation in a
non-linear nonaxisymmetric mean-field solar dynamo model. We study the
evolution of the shallow non-axisymmetric magnetic field perturbation with the
strength about 10G in the solar convection zone. The dynamo evolves from the
pure axisymmetric stage through the short (about 2 years) transient phase when
the non-axisymmetric m=1 dynamo mode is dominant to the final stage where the
axisymmetry of the dynamo is almost restored. It is found that magnetic
helicity is transferred forth and back over the spectral space during the
transient phase. Also our simulations shows that the non-axisymmetric
distributions of magnetic helicity tend to follows the regions of the Hale
polarity rule.Comment: 8 pages, 4 figures, to appear in Magnehydrodynamics, "Russian
Magnetohydrodynamics 2015", Perm', June, 201
The shear-induced alpha-effect and long-term variations in solar dynamo
The consequences of the shear-induced alpha effect to the long-term
modulation of magnetic activity are examined with the help of the axisymmetric
numerical dynamo model that includes the self-consistent description of the
angular momentum balance, heat transport and magnetic field generation in the
spherical shell. We find that the shear contributions to alpha effect can
complicate the long-term behaviour of the large-scale magnetic activity and
differential rotation in nonlinear dynamo. Additionally we consider the impact
secular magnetic activity variations to the secular modulations of the solar
luminosity and radius.Comment: AOGS 2nd Annual Meeting 200
Helicity--vorticity turbulent pumping of magnetic fields in the solar dynamo
The interaction of helical convective motions and differential rotation in
the solar convection zone results in turbulent drift of a large-scale magnetic
field. We discuss the pumping mechanism and its impact on the solar dynamo.Comment: 2 pages, IAU 294 Proceeding
Benchmarking the solar dynamo with Maxima
Recently, Jouve et al(A&A, 2008) published the paper that presents the
numerical benchmark for the solar dynamo models. Here, I would like to show a
way how to get it with help of computer algebra system Maxima. This way was
used in our paper (Pipin & Seehafer, A&A 2008, in print) to test some new ideas
in the large-scale stellar dynamos. In the present paper I complement the
dynamo benchmark with the standard test that address the problem of the
free-decay modes in the sphere which is submerged in vacuum.Comment: 8 pages, 1 figur
Dependence of stellar magnetic activity cycles on rotational period in nonlinear solar-type dynamo
We study turbulent generation of large-scale magnetic fields using nonlinear
dynamo models for solar-type stars in the range of rotational periods from 14
to 30 days. Our models take into account non-linear effects of dynamical
quenching of magnetic helicity, and escape of magnetic field from the dynamo
region due to magnetic buoyancy. The results show that the observed correlation
between the period of rotation and the duration of activity cycles can be
explained in the framework of a distributed dynamo model with a dynamical
magnetic feedback acting on the turbulent generation either from magnetic
buoyancy or magnetic helicity. We discuss implications of our findings for the
understanding of dynamo processes operating in solar-like stars.Comment: 21 pages, 9 figure
On the turbulent sources of the solar dynamo
We revisit the possible turbulent sources of the solar dynamo. Studying
axisymmetric mean-field dynamo models, we find that the large-scale poloidal
magnetic field could be generated not only by the famous alpha effect, but also
by the Omega x J and shear-current effects. The inclusion of these additional
turbulent sources alleviates several of the known problems of solar mean-field
dynamo models.Comment: 7th Int. PAMIR Conf. on Fundamental and Applied MHD (Giens, France,
2008
Does nonaxisymmetric dynamo operate in the Sun?
We explore effects of random non-axisymmetric perturbations of kinetic
helicity (the effect) and diffusive decay of bipolar magnetic regions
on generation and evolution of large-scale non-axisymmetric magnetic fields on
the Sun. Using a reduced 2D nonlinear mean-field dynamo model and assuming that
bipolar regions emerge due to magnetic buoyancy in situ of the large-scale
dynamo action, we show that fluctuations of the effect can maintain
the non-axisymmetric magnetic fields through a solar-type
dynamo process. It is found that diffusive decay of bipolar active regions is
likely to be the primary source of the non-axisymmetric magnetic fields
observed on the Sun. Our results show that the non-axisymmetric dynamo model
with stochastic perturbations of the effect can explain periods of
extremely high activity (`super-cycle' events) as well as periods of deep
decline of magnetic activity. We compare the models with synoptic observations
of solar magnetic fields for the last four activity cycles, and discuss
implications of our results for interpretation of observations of stellar
magnetic activity.Comment: 15 pages, 11 figures, accepted in Ap
The mean-field solar dynamo with double cell meridional circulation pattern
Recent findings of helioseismology as well as advances in direct numerical
simulations of global dynamics of the Sun have indicated that in each solar
hemisphere the meridional circulation forms the two cells along the in the
convection zone. We investigate properties of a mean-field solar dynamo with
such double-cell meridional circulation. The dynamo model also includes the
realistic profile of solar differential rotation (including the tachocline and
subsurface shear layer), and takes into account effects of turbulent pumping,
anisotropic turbulent diffusivity, and conservation of magnetic helicity.
Contrary to previous flux-transport dynamo models, it is found that the dynamo
model can robustly reproduce the basic properties of the solar magnetic cycles
for a wide range of model parameters and the circulation speed. The best
agreement with observations is achieved when the surface speed of meridional
circulation is about 12 m/s. For this circulation speed the simulated sunspot
activity shows good synchronization with the polar magnetic fields. Such
synchronization was indeed observed during the past sunspot cycles 21 and 22.
We compare theoretical and observed phase diagrams of the sunspot number and
the polar field strength and discuss the peculiar properties of Cycle 23.Comment: 19 pages, 7 figures, revise
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