58 research outputs found
Small-scale dynamos in simulations of stratified turbulent convection
Small-scale dynamo action is often held responsible for the generation of
quiet-Sun magnetic fields. We aim to determine the excitation conditions and
saturation level of small-scale dynamos in non-rotating turbulent convection at
low magnetic Prandtl numbers. We use high resolution direct numerical
simulations of weakly stratified turbulent convection. We find that the
critical magnetic Reynolds number for dynamo excitation increases as the
magnetic Prandtl number is decreased, which might suggest that small-scale
dynamo action is not automatically evident in bodies with small magnetic
Prandtl numbers as the Sun. As a function of the magnetic Reynolds number
(), the growth rate of the dynamo is consistent with an scaling. No evidence for a logarithmic increase of the growth rate
with is found.Comment: 6 pages, 5 figures, submitted to Astron. Nach
Long-term variations of turbulent transport coefficients in a solar-like convective dynamo simulation
The Sun, aside from its eleven year sunspot cycle is additionally subject to
long term variation in its activity. In this work we analyse a solar-like
convective dynamo simulation, containing approximately 60 magnetic cycles,
exhibiting equatorward propagation of the magnetic field, multiple frequencies,
and irregular variability, including a missed cycle and complex parity
transitions between dipolar and quadrupolar modes. We compute the turbulent
transport coefficients, describing the effects of the turbulent velocity field
on the mean magnetic field, using the test-field method. The test-field
analysis provides a plausible explanation of the missing cycle in terms of the
reduction of in advance of the reduced surface activity,
and enhanced downward turbulent pumping during the event to confine some of the
magnetic field at the bottom of the convection zone, where local maximum of
magnetic energy is observed during the event. At the same time, however, a
quenching of the turbulent magnetic diffusivities is observed, albeit
differently distributed in depth compared to the other transport coefficients.
Therefore, dedicated mean-field modelling is required for verification.Comment: 11 pages, 12 figures, accepted by AN for 14th Potsdam Thinksho
Magnetorotational instability driven dynamos at low magnetic Prandtl numbers
7 pages, 7 figures, submitted to MNRAS. Version with higher resolution figures is available at http://www.helsinki.fi/~kapyla/publ.htmlNumerical simulations of the magnetorotational instability (MRI) with zero initial net flux in a non-stratified isothermal cubic domain are used to demonstrate the importance of magnetic boundary conditions. In fully periodic systems the level of turbulence generated by the MRI strongly decreases as the magnetic Prandtl number (Pm), which is the ratio of kinematic viscosity and magnetic diffusion, is decreased. No MRI or dynamo action below Pm=1 is found, agreeing with earlier investigations. Using vertical field conditions, which allow magnetic helicity fluxes out of the system, the MRI is found to be excited in the range 0.1Peer reviewe
Effects of a subadiabatic layer on convection and dynamos in spherical wedge simulations
We consider the effect of a subadiabatic layer at the base of the convection
zone on convection itself and the associated large-scale dynamos in spherical
wedge geometry. We use a heat conduction prescription based on the Kramers
opacity law which allows the depth of the convection zone to dynamically adapt
to changes in the physical characteristics such as rotation rate and magnetic
fields. We find that the convective heat transport is strongly concentrated
toward the equatorial and polar regions in the cases without a substantial
radiative layer below the convection zone. The presence of a stable layer below
the convection zone significantly reduces the anisotropy of radial enthalpy
transport. Furthermore, the dynamo solutions are sensitive to subtle changes in
the convection zone structure. We find that the kinetic helicity changes sign
in the deeper parts of the convection zone at high latitudes in all runs. This
region expands progressively toward the equator in runs with a thicker stably
stratified layer.Comment: 32 pages, 18 figures, accepted to GAFD Special issue on 'Recent
Developments in Natural Dynamos
From convective to stellar dynamos
Volume: 6 Host publication title: Astrophysical Dynamics Host publication sub-title: From Stars to GalaxiesNon peer reviewe
Open and closed boundaries in large-scale convective dynamos
9 pages, 7 figures, submitted to Astron. AstrophysEarlier work has suggested that large-scale dynamos can reach and maintain equipartition field strengths on a dynamical time scale only if magnetic helicity of the fluctuating field can be shed from the domain through open boundaries. To test this scenario in convection-driven dynamos by comparing results for open and closed boundary conditions. Three-dimensional numerical simulations of turbulent compressible convection with shear and rotation are used to study the effects of boundary conditions on the excitation and saturation level of large-scale dynamos. Open (vertical field) and closed (perfect conductor) boundary conditions are used for the magnetic field. The contours of shear are vertical, crossing the outer surface, and are thus ideally suited for driving a shear-induced magnetic helicity flux. We find that for given shear and rotation rate, the growth rate of the magnetic field is larger if open boundary conditions are used. The growth rate first increases for small magnetic Reynolds number, Rm, but then levels off at an approximately constant value for intermediate values of Rm. For large enough Rm, a small-scale dynamo is excited and the growth rate in this regime increases proportional to Rm^(1/2). In the nonlinear regime, the saturation level of the energy of the mean magnetic field is independent of Rm when open boundaries are used. In the case of perfect conductor boundaries, the saturation level first increases as a function of Rm, but then decreases proportional to Rm^(-1) for Rm > 30, indicative of catastrophic quenching. These results suggest that the shear-induced magnetic helicity flux is efficient in alleviating catastrophic quenching when open boundaries are used. The horizontally averaged mean field is still weakly decreasing as a function of Rm even for open boundaries.Peer reviewe
Coronal ejections from convective spherical shell dynamos
Volume: 286 Host publication title: Comparative Magnetic Minima: Characterizing quiet times in the Sun and StarsNon peer reviewe
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