Large-scale dynamos in rapidly rotating plane layer convection

Abstract

Context: Convectively-driven flows play a crucial role in the dynamo processes that are responsible for producing magnetic activity in stars and planets. It is still not fully understood why many astrophysical magnetic fields have a significant large-scale component. Aims: Our aim is to investigate the dynamo properties of compressible convection in a rapidly rotating Cartesian domain, focusing upon a parameter regime in which the underlying hydrodynamic flow is known to be unstable to a large-scale vortex instability. Methods: The governing equations of three-dimensional nonlinear magnetohydrodynamics (MHD) are solved numerically. Different numerical schemes are compared and we propose a possible benchmark case for other similar codes. Results: In keeping with previous related studies, we find that convection in this parameter regime can drive a large-scale dynamo. The components of the mean horizontal magnetic field oscillate, leading to a continuous overall rotation of the mean field. Whilst the large-scale vortex instability dominates the early evolution of the system, it is suppressed by the magnetic field and makes a negligible contribution to the mean electromotive force that is responsible for driving the large-scale dynamo. The cycle period of the dynamo is comparable to the ohmic decay time, with longer cycles for dynamos in convective systems that are closer to onset. In these particular simulations, large-scale dynamo action is found only when vertical magnetic field boundary conditions are adopted at the upper and lower boundaries. Strongly modulated large-scale dynamos are found at higher Rayleigh numbers, with periods of reduced activity ("grand minima"-like events) occurring during transient phases in which the large-scale vortex temporarily re-establishes itself, before being suppressed again by the magnetic field.

16 pages, 16 figures, to appear in Astronomy & Astrophysics. The material in the Appendix could form the basis for a possible benchmarking exercise; we would encourage anyone who might be interested in participating in such an exercise to contact the authors

References in corpus (10)

• Magnetic field generation in fully convective rotating spheres
• Approaching the Asymptotic Regime of Rapidly Rotating Convection: Boundary Layers vs Interior Dynamics
• Inverse cascade and symmetry breaking in rapidly-rotating Boussinesq convection
• Large-scale dynamos in turbulent convection with shear
• Large-scale vortices in rapidly rotating Rayleigh-Bénard convection
• Large-scale Dynamo Action Driven by Velocity Shear and Rotating Convection
• Generation of magnetic fields by large-scale vortices in rotating convection
• Mean-Field Modeling of ${\alpha }^2$-Dynamo Coupled with Direct Numerical Simulations of Rigidly Rotating Convection
• Magnetic energy dissipation and mean magnetic field generation in planar convection driven dynamos
• Long-term Evolution of Large-scale Magnetic Fields in Rotating Stratified Convection

Cited by in corpus (10)

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• Turbulent processes and mean-field dynamo
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• Strong large scale magnetic fields in rotating convection-driven dynamos: the important role of magnetic diffusion
• Asymptotic behaviour of rotating convection-driven dynamos in the plane layer geometry
• Convection-driven kinematic dynamos with a self-consistent shear flow
• Impacts of small-scale dynamo on rotating columnar convection in stellar convection zones
• Shear-driven magnetic buoyancy in the solar tachocline: The mean electromotive force due to rotation
• Magnetorotational instability in a solar near-surface mean-field dynamo