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The role of magnetic waves in tangent cylinder convection

Majumder, D and Sreenivasan, B (2023) The role of magnetic waves in tangent cylinder convection. In: Physics of the Earth and Planetary Interiors, 344 .

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Official URL: https://doi.org/10.1016/j.pepi.2023.107105


The secular variation of the geomagnetic field suggests that there are anticyclonic polar vortices in the Earth's core. Under the influence of a magnetic field, the polar azimuthal flow is thought to be produced by one or more coherent upwellings within the tangent cylinder, offset from the rotation axis. In this study, convection within the tangent cylinder in rapidly rotating dynamos is investigated through the analysis of forced magnetic waves. The first part of the study investigates the evolution of an isolated buoyancy disturbance in an unstably stratified rotating fluid subjected to an axial magnetic field. It is shown that the axial flow intensity of the slow Magnetic-Archimedean-Coriolis (MAC) waves becomes comparable to that of the fast MAC waves when |�M/�C|�0.1, where �M and �C are the Alfvén wave and inertial wave frequencies respectively. In spherical shell dynamo simulations, the isolated upwellings within the tangent cylinder are shown to originate from the localized excitation of slow MAC waves in the dipole-dominated regime. Axial flow measurements in turn reveal the approximate parity between the slow and fast wave intensities in this regime, which corresponds to the existence of strong polar vortices in the Earth's core. To obtain the observed peak azimuthal motions of 0.6�0.9� yr�1, the Rayleigh number in the low-inertia geodynamo must be �103 times the Rayleigh number for the onset of nonmagnetic convection. However, if the forcing is so strong as to cause polarity reversals, the field within the tangent cylinder decays away, and the convection takes the form of an ensemble of plumes supported entirely by the fast waves of frequency ���C. The resulting weak polar circulation is comparable to that obtained in nonmagnetic convection. © 2023 Elsevier B.V.

Item Type: Journal Article
Publication: Physics of the Earth and Planetary Interiors
Publisher: Elsevier B.V.
Additional Information: The copyright for this article belongs to author
Keywords: Cylinders (shapes); Geomagnetism; Magnetic fields; Vortex flow, Coriolis; Earths cores; Fast waves; Geodynamo; Magnetic waves; Magnetic-field; Outer core; Polar vortex; Slow magnetostrophic wave; Tangent cylinder, Axial flow, convection; cylinder; geodynamo; geomagnetic field; outer core; polar vortex
Department/Centre: Division of Mechanical Sciences > Centre for Earth Sciences
Date Deposited: 28 Feb 2024 13:11
Last Modified: 28 Feb 2024 13:11
URI: https://eprints.iisc.ac.in/id/eprint/83674

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