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Self-similarity of the dipole-multipole transition in rapidly rotating dynamos

Majumder, D and Sreenivasan, B and Maurya, G (2024) Self-similarity of the dipole-multipole transition in rapidly rotating dynamos. In: Journal of Fluid Mechanics, 980 .

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Official URL: https://doi.org/10.1017/jfm.2024.1


The dipole-multipole transition in rapidly rotating dynamos is investigated through the analysis of forced magnetohydrodynamic waves in an unstably stratified fluid. The focus of this study is on the inertia-free limit applicable to planetary cores, where the Rossby number is small not only on the core depth but also on the length scale of columnar convection. By progressively increasing the buoyant forcing in a linear magnetoconvection model, the slow magnetic-Archimedean-Coriolis (MAC) waves are significantly attenuated so that their kinetic helicity decreases to zero; the fast MAC wave helicity, on the other hand, is practically unaffected. In turn, polarity reversals in low-inertia spherical dynamos are shown to occur when the slow MAC waves disappear under strong forcing. Two dynamically similar regimes are identified - the suppression of slow waves in a strongly forced dynamo and the excitation of slow waves in a moderately forced dynamo starting from a small seed field. While the former regime results in polarity reversals, the latter regime produces the axial dipole from a chaotic multipolar state. For either polarity transition, a local Rayleigh number based on the mean wavenumber of the energy-containing scales bears the same linear relationship with the square of the peak magnetic field measured at the transition. The self-similarity of the dipole-multipole transition can place a constraint on the Rayleigh number for polarity reversals in the Earth. © The Author(s), 2024. Published by Cambridge University Press.

Item Type: Journal Article
Publication: Journal of Fluid Mechanics
Publisher: Cambridge University Press
Additional Information: The copyright for this article belongs to the Authors.
Keywords: Magnetic polarity; Magnetohydrodynamics, Coriolis; Dynamo theories; Forcings; Geodynamo; Multipoles; Polarity reversal; Rayleigh number; Self-similarities; Slow wave; Waves in rotating fluids, Interactive devices, geodynamo; magnetic field; magnetic reversal; magnetohydrodynamics; Rayleigh number; Rossby number; rotating fluid; stratified fluid
Department/Centre: Division of Mechanical Sciences > Centre for Earth Sciences
Date Deposited: 08 Apr 2024 09:16
Last Modified: 08 Apr 2024 09:16
URI: https://eprints.iisc.ac.in/id/eprint/84694

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