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Large-scale dynamos in rapidly rotating plane layer convection

Lookup NU author(s): Dr Paul Bushby, Professor Axel Brandenburg, Dr Benjamin Favier, Dr Celine Guervilly

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This is the final published version of an article that has been published in its final definitive form by EDP Sciences, 2018.

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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 non-linear 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, the large-scale vortex 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.


Publication metadata

Author(s): Bushby PJ, Käpylä PJ, Masada Y, Brandenburg A, Favier B, Guervilly C, Käpylä MJ

Publication type: Article

Publication status: Published

Journal: Astronomy and Astrophysics

Year: 2018

Volume: 612

Print publication date: 01/04/2018

Online publication date: 24/01/2018

Acceptance date: 15/01/2018

Date deposited: 16/01/2018

ISSN (print): 0004-6361

ISSN (electronic): 1432-0746

Publisher: EDP Sciences

URL: https://doi.org/10.1051/0004-6361/201732066

DOI: 10.1051/0004-6361/201732066


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