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Lookup NU author(s): Dr Yue-Kin TsangORCiD
This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
© 2024 The Author(s).The ratio of the magnetic power spectrum and the secular variation spectrum measured at the Earth's surface provides a timescale as a function of spherical harmonic degree l. is often assumed to be representative of timescales related to the dynamo inside the outer core and its scaling with l is debated. To assess the validity of this surmise and to study the time variation of the geomagnetic field inside the outer core, we introduce a magnetic timescale spectrum that is valid for all radius r above the inner core and reduces to the usual at and above the core-mantle boundary (CMB). We study in a numerical geodynamo model. At the CMB, we find that is valid at both the large and small scales, in agreement with previous numerical studies on. Just below the CMB, the scaling undergoes a sharp transition at small l. Consequently, in the interior of the outer core, exhibits different scaling at the large and small scales, specifically, the scaling of becomes shallower than at small l. We find that this transition at the large scales stems from the fact that the horizontal components of the magnetic field evolve faster than the radial component in the interior. In contrast, the magnetic field at the CMB must match onto a potential field, hence the dynamics of the radial and horizontal magnetic fields are tied together. The upshot is becomes unreliable in estimating timescales inside the outer core. Another question concerning is whether an argument based on the frozen-flux hypothesis can be used to explain its scaling. To investigate this, we analyse the induction equation in the spectral space. We find that away from both boundaries, the magnetic diffusion term is negligible in the power spectrum of. However, is controlled by the radial derivative in the induction term, thus invalidating the frozen-flux argument. Near the CMB, magnetic diffusion starts to affect rendering the frozen-flux hypothesis inapplicable. We also examine the effects of different velocity boundary conditions and find that the above results apply for both no-slip and stress-free conditions at the CMB.
Author(s): Tsang Y-K, Jones CA
Publication type: Article
Publication status: Published
Journal: Geophysical Journal International
Year: 2024
Volume: 239
Issue: 1
Pages: 1-16
Print publication date: 01/10/2024
Online publication date: 05/07/2024
Acceptance date: 03/07/2024
Date deposited: 19/08/2024
ISSN (print): 0956-540X
ISSN (electronic): 1365-246X
Publisher: Oxford University Press
URL: https://doi.org/10.1093/gji/ggae234
DOI: 10.1093/gji/ggae234
Data Access Statement: The numerical code used for the geodynamo simulations reported in this paper is at https://github.com/Leeds-Spherical-Dynamo. For access to the Github repository, please contact the authors
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