465 research outputs found
Differential rotation and meridional flow of Arcturus
The spectroscopic variability of Arcturus hints at cyclic activity cycle and
differential rotation. This could provide a test of current theoretical models
of solar and stellar dynamos. To examine the applicability of current models of
the flux transport dynamo to Arcturus, we compute a mean-field model for its
internal rotation, meridional flow, and convective heat transport in the
convective envelope. We then compare the conditions for dynamo action with
those on the Sun. We find solar-type surface rotation with about 1/10th of the
shear found on the solar surface. The rotation rate increases monotonically
with depth at all latitudes throughout the whole convection zone. In the lower
part of the convection zone the horizontal shear vanishes and there is a strong
radial gradient. The surface meridional flow has maximum speed of 110 m/s and
is directed towards the equator at high and towards the poles at low latitudes.
Turbulent magnetic diffusivity is of the order --. The conditions on Arcturus are not favorable for a
circulation-dominated dynamo
Dissipative and nonaxisymmetric standard-MRI in Kepler disks
Deviations from axial symmetry are necessary to maintain self-sustained
MRI-turbulence. We define the parameters region where nonaxisymmetric MRI is
excited and study dependence of the unstable modes structure and growth rates
on the relevant parameters. We solve numerically the linear eigenvalue problem
for global axisymmetric and nonaxisymmetric modes of standard-MRI in Keplerian
disks with finite diffusion. For small magnetic Prandtl number the microscopic
viscosity completely drops out from the analysis so that the stability maps and
the growth rates expressed in terms of the magnetic Reynolds number Rm and the
Lundquist number S do not depend on the magnetic Prandtl number Pm. The minimum
magnetic field for onset of nonaxisymmetric MRI grows with Rm. For given S all
nonaxisymmetric modes disappear for sufficiently high Rm. This behavior is a
consequence of the radial fine-structure of the nonaxisymmetric modes resulting
from the winding effect of differential rotation. It is this fine-structure
which presents severe resolution problems for the numerical simulation of MRI
at large Rm. For weak supercritical magnetic fields only axisymmetric modes are
unstable. Nonaxisymmetric modes need stronger fields and not too fast rotation.
If Pm is small its real value does not play any role in MRI.Comment: 4 pages, 6 figures, A&A Lette
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
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