505 research outputs found
Solar parity issue with flux-transport dynamo
We investigated the dependence of the solar magnetic parity between the
hemispheres on two important parameters, the turbulent diffusivity and the
meridional flow, by means of axisymmetric kinematic dynamo simulations based on
the flux-transport dynamo model. It is known that the coupling of the magnetic
field between hemispheres due to turbulent diffusivity is an important factor
for the solar parity issue, but the detailed criterion for the generation of
the dipole field has not been investigated. Our conclusions are as follows. (1)
The stronger diffusivity near the surface is more likely to cause the magnetic
field to be a dipole. (2) The thinner layer of the strong diffusivity near the
surface is also more apt to generate a dipolar magnetic field. (3) The faster
meridional flow is more prone to cause the magnetic field to be a quadrupole,
i.e., symmetric about the equator. These results show that turbulent
diffusivity and meridional flow are crucial for the configuration of the solar
global magnetic field.Comment: 19 pages, 5 figure
Polar Field Puzzle: Solutions from Flux-Transport Dynamo and Surface Transport Models
Polar fields in solar cycle 23 were about 50% weaker than those in cycle 22.
The only theoretical models which have addressed this puzzle are surface
transport models and flux-transport dynamo models. Comparing polar fields
obtained from numerical simulations using surface flux transport models and
flux-transport dynamo models, we show that both classes of models can explain
the polar field features within the scope of the physics included in the
respective models. In both models, how polar fields change as a result of
changes in meridional circulation depends on the details of meridional
circulation profile used. Using physical reasoning and schematics as well as
numerical solutions from a flux-transport dynamo model, we demonstrate that
polar fields are determined mostly by the strength of surface poloidal source
provided by the decay of tilted, bipolar active regions. Profile of meridional
flow with latitude and its changes with time have much less effect in
flux-transport dynamo models than in surface transport models.Comment: ApJ (accepted
Flux-dominated solar dynamo model with a thin shear layer
Flux-dominated solar dynamo models have demonstrated to reproduce the main
features of the large scale solar magnetic cycle, however the use of a solar
like differential rotation profile implies in the the formation of strong
toroidal magnetic fields at high latitudes where they are not observed. In this
work, we invoke the hypothesis of a thin-width tachocline in order to confine
the high-latitude toroidal magnetic fields to a small area below the overshoot
layer, thus avoiding its influence on a Babcock-Leighton type dynamo process.
Our results favor a dynamo operating inside the convection zone with a
tachocline that essentially works as a storage region when it coincides with
the overshoot layer.Comment: 4 pages, 2 figures, accepted for publication in Astronomische
Nachrichte
The subsurface-shear shaped solar dynamo
We propose a solar dynamo model distributed in the bulk of the convection
zone with the toroidal magnetic field the flux concentrated in the near-surface
layer. We show that if the boundary conditions at the top of the dynamo region
allow the large-scale toroidal magnetic fields to penetrate closer to the
surface, then the pattern of the modeled butterfly diagram for the toroidal
magnetic fields in the upper part of the convection zone is formed by the
surface rotational shear layer. The model is in agreement with observed
properties of the magnetic solar cycle.Comment: Accepted for ApJ
Large-scale solar cycle features of solar photospheric magnetic field
It is well accepted that the solar cycle originates from a
magnetohydrodynamics dynamo deep inside the Sun. Many dynamo models have long
been proposed based on a lot of observational constraints. In this paper, using
342 NSO/Kitt Peak solar synoptic charts we study the solar cycle phases in
different solar latitudinal zones to set further constraints. Our results can
be summarized as follows. (1) The variability of solar polar regions' area has
a correlation with total unsigned magnetic flux in advance of 5 years. (2) The
high-latitude region mainly appears unipolar in the whole solar cycle and its
flux peak time lags sunspot cycle for 3 years. (3) For the activity belt, it is
not surprised that its phase be the same as sunspot's. (4) The flux peak time
of the low-latitude region shifts forward with an average gradient of 32.2
. These typical characteristics may provide some hints for
constructing an actual solar dynamo.Comment: 6 pages, 4 figures; Accepted by AdSR
Sustained magneto-shear instabilities in the solar tachocline
We present nonlinear three-dimensional simulations of the stably-stratified
portion of the solar tachocline in which the rotational shear is maintained by
mechanical forcing. When a broad toroidal field profile is specified as an
initial condition, a clam-shell instability ensues which is similar to the
freely-evolving cases studied previously. After the initial nonlinear
saturation, the residual mean fields are apparently too weak to sustain the
instability indefinitely. However, when a mean poloidal field is imposed in
addition to the rotational shear, a statistically-steady state is achieved in
which the clam-shell instability is operating continually. This state is
characterized by a quasi-periodic exchange of energy between the mean toroidal
field and the instability mode with a longitudinal wavenumber m=1. This
quasi-periodic behavior has a timescale of several years and may have
implications for tachocline dynamics and field emergence patterns throughout
the solar activity cycle.Comment: 5 pages, 3 figures (eps format). Fig. 3 also in jpg format. Submitted
to Astrophysical Journal Letter
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