Convection-driven spherical shell dynamos at varying Prandtl numbers

Abstract

(abidged) Context: Stellar convection zones are characterized by vigorous high-Reynolds number turbulence at low Prandtl numbers. Aims: We study the dynamo and differential rotation regimes at varying levels of viscous, thermal, and magnetic diffusion. Methods: We perform three-dimensional simulations of stratified fully compressible magnetohydrodynamic convection in rotating spherical wedges at various thermal and magnetic Prandtl numbers (from 0.25 to 2 and 5, respectively). Results: We find that the rotation profiles for high thermal diffusivity show a monotonically increasing angular velocity from the bottom of the convection zone to the top and from the poles toward the equator. For sufficiently rapid rotation, a region of negative radial shear develops at mid-latitudes as the thermal diffusivity is decreased. This coincides with a change in the dynamo mode from poleward propagating activity belts to equatorward propagating ones. Furthermore, the cyclic solutions disappear at the highest magnetic Reynolds numbers. The total magnetic energy increases with the magnetic Reynolds number in the range studied here ($5-151$), but the energies of the mean magnetic fields level off at high magnetic Reynolds numbers. The differential rotation is strongly affected by the magnetic fields and almost vanishes at the highest magnetic Reynolds numbers. In some of our most turbulent cases we find that two regimes are possible where either differential rotation is strong and mean magnetic fields relatively weak or vice versa. Conclusions: Our simulations indicate a strong non-linear feedback of magnetic fields on differential rotation, leading to qualitative changes in the behaviors of large-scale dynamos at high magnetic Reynolds numbers. Furthermore, we do not find indications of the simulations approaching an asymptotic regime where the results would be independent of diffusion coefficients.

18 pages, 10 figures, submitted to Astron. Astrophys

References in corpus (22)

• Global-Scale Turbulent Convection and Magnetic Dynamo Action in the Solar Envelope
• Persistent Magnetic Wreaths in a Rapidly Rotating Sun
• Magnetic Cycles in a Convective Dynamo Simulation of a Young Solar-type Star
• Galactic dynamo and helicity losses through fountain flow
• Galactic dynamos supported by magnetic helicity fluxes
• Prandtl number dependence of convection driven dynamos in rotating spherical fluid shells
• Magnetically controlled stellar differential rotation near the transition from solar to anti-solar profiles
• Reynolds stress and heat flux in spherical shell convection
• Parameter dependences of convection driven dynamos in rotating spherical fluid shells
• Small-scale magnetic helicity losses from a mean-field dynamo
• On the cause of solar-like equatorward migration in global convective dynamo simulations
• Formation of starspots in self-consistent global dynamo models: Polar spots on cool stars
• Catastrophic alpha quenching alleviated by helicity flux and shear
• Turbulent dynamo with advective magnetic helicity flux
• The Origin of Solar Activity in the Tachocline
• Helicity inversion in spherical convection as a means for equatorward dynamo wave propagation
• Modelling turbulent stellar convection zones: sub-grid scales effects
• Magnetic flux concentrations from turbulent stratified convection
• Effect of the radiative background flux in convection
• Magnetic twist: a source and property of space weather
• Lambda-effect from forced turbulence simulations
• Robustness of oscillatory ${\alpha }^2$ dynamos in spherical wedges

Cited by in corpus (35)

• Advances in mean-field dynamo theory and applications to astrophysical turbulence
• Transition from axi- to nonaxisymmetric dynamo modes in spherical convection models of solar-like stars
• Models for the long-term variations of solar activity
• Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars
• Evolution of solar and stellar dynamo theory
• Dynamo cycles in global convection simulations of solar-like stars
• Rossby and Magnetic Prandtl Number Scaling of Stellar Dynamos
• Consequences of high effective Prandtl number on solar differential rotation and convective velocity
• Convection with Misaligned Gravity and Rotation: Simulations and Rotating Mixing Length Theory
• Small-scale dynamos: From idealized models to solar and stellar applications
• Star-in-a-box simulations of fully convective stars
• Effects of a subadiabatic layer on convection and dynamos in spherical wedge simulations
• Sensitivity to luminosity, centrifugal force, and boundary conditions in spherical shell convection
• Convective velocity suppression via the enhancement of subadiabatic layer: Role of the effective Prandtl number
• Stellar dynamos in the transition regime: multiple dynamo modes and anti-solar differential rotation
• Numerical evidence for a small-scale dynamo approaching solar magnetic Prandtl numbers
• Transition from anti-solar to solar-like differential rotation: Dependence on Prandtl number
• A Babcock-Leighton-type Solar Dynamo Operating in the Bulk of the Convection Zone
• Rotational dependence of turbulent transport coefficients in global convective dynamo simulations of solar-like stars
• Magnetic and rotational quenching of the $\Lambda$ effect
• A Knee-Point in the Rotation-Activity Scaling of Late-type Stars with a Connection to Dynamo Transitions
• Small-scale dynamos in simulations of stratified turbulent convection
• Breaking Taylor-Proudman balance by magnetic field in stellar convection zone
• Magnetohydrodynamic simulations of A-type stars: Long-term evolution of core dynamo cycles
• Long-term variations of turbulent transport coefficients in a solar-like convective dynamo simulation
• Toward fully compressible numerical simulations of stellar magneto-convection with the RAMSES code
• Effects of small-scale dynamo and compressibility on the $\Lambda$ effect
• Acoustic and inertial modes in planetary-like rotating ellipsoids
• Mean-field analysis on large-scale magnetic fields at high Reynolds numbers
• Minimal Roles of Solar Subsurface Meridional Flow in the distributed-shear Babcock-Leighton Dynamo
• Small-scale and large-scale dynamos in global convection simulations of solar-like stars
• Impacts of small-scale dynamo on rotating columnar convection in stellar convection zones
• Shaping core dynamos in A-type stars: The role of dipolar fossil fields
• Flows and dynamos in a model of stellar radiative zones
• Angular momentum transport by magnetoconvection and the magnetic modulation of the solar differential rotation