Metman, MC, Livermore, PW orcid.org/0000-0001-7591-6716, Mound, JE orcid.org/0000-0002-1243-6915 et al. (1 more author) (2019) Modelling decadal secular variation with only magnetic diffusion. Geophysical Journal International, 2019 (Supplement_1). S58-S82. ISSN 0956-540X
Abstract
Secular variation (SV) of Earth’s internal magnetic field is the sum of two contributions, one resulting from core fluid flow and the other from magnetic diffusion. Based on the millenial diffusive timescale of global-scale structures, magnetic diffusion is widely perceived to be too weak to significantly contribute to decadal SV, and indeed is entirely neglected in the commonly adopted end-member of frozen-flux. Such an argument however lacks consideration of radially fine-scaled magnetic structures in the outermost part of the liquid core, whose diffusive timescale is much shorter. Here we consider the opposite end-member model to frozen flux, that of purely diffusive evolution associated with the total absence of fluid flow. Our work is based on a variational formulation, where we seek an optimised full-sphere initial magnetic field structure whose diffusive evolution best fits, over various time windows, a time-dependent magnetic field model. We present models which are regularised based on their magnetic energy, and consider how well they can fit the COV-OBS.x1 ensemble mean using a global error bound based on the standard deviation of the ensemble. within the core. For With the se regularised models, over time periods of up to 30 years, it is possible to fit COV-OBS.x1 within one standard deviation at all times. For time windows up to 102 years we show that our models can fit COV-OBS.x1 when adopting a time-averaged global uncertainty. Our modelling is sensitive only to magnetic structures in approximately the top 10% of the liquid core, and show an increased surface area of reversed flux at depth. The diffusive models recover fundamental characteristics of field evolution including the historical westward drift, the recent acceleration of the North Magnetic Pole and reversed-flux emergence. Based on a global time-averaged residual, our diffusive models fit the evolution of the geomagnetic field comparably, and sometimes better than, frozen-flux models within short time windows.
Metadata
Item Type: | Article |
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Authors/Creators: |
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Editors: |
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Copyright, Publisher and Additional Information: | © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. |
Keywords: | Dynamo: theories and simulations; Magnetic field variations through time; Electromagnetic theory; Inverse theory; Numerical approximations and analysis |
Dates: |
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Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Environment (Leeds) > School of Earth and Environment (Leeds) > Inst of Geophysics and Tectonics (IGT) (Leeds) |
Depositing User: | Symplectic Publications |
Date Deposited: | 19 Feb 2019 10:23 |
Last Modified: | 25 Jun 2023 21:43 |
Status: | Published |
Publisher: | Oxford University Press |
Identification Number: | 10.1093/gji/ggz089 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:142658 |