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Geomagnetic polar minima do not arise from steady meridional circulation

Cao, Hao and Yadav, Rakesh K. and Aurnou, Jonathan M. (2018) Geomagnetic polar minima do not arise from steady meridional circulation. Proceedings of the National Academy of Sciences of the United States of America, 115 (44). pp. 11186-11191. ISSN 0027-8424. PMCID PMC6217441. doi:10.1073/pnas.1717454115.

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Observations of the Earth’s magnetic field have revealed locally pronounced field minima near each pole at the core–mantle boundary (CMB). The existence of the polar magnetic minima has long been attributed to the supposed large-scale overturning circulation of molten metal in the outer core: Fluid upwells within the inner core tangent cylinder toward the poles and then diverges toward lower latitudes when it reaches the CMB, where Coriolis effects sweep the fluid into anticyclonic vortical flows. The diverging near-surface meridional circulation is believed to advectively draw magnetic flux away from the poles, resulting in the low intensity or even reversed polar magnetic fields. However, the interconnections between polar magnetic minima and meridional circulations have not to date been ascertained quantitatively. Here, we quantify the magnetic effects of steady, axisymmetric meridional circulation via numerically solving the axisymmetric magnetohydrodynamic equations for Earth’s outer core under the magnetostrophic approximation. Extrapolated to core conditions, our results show that the change in polar magnetic field resulting from steady, large-scale meridional circulations in Earth’s outer core is less than 3% of the background field, significantly smaller than the ∼ 100% polar magnetic minima observed at the CMB. This suggests that the geomagnetic polar minima cannot be produced solely by axisymmetric, steady meridional circulations and must depend upon additional tangent cylinder dynamics, likely including nonaxisymmetric, time-varying processes.

Item Type:Article
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URLURL TypeDescription Information CentralArticle
Cao, Hao0000-0002-6917-8363
Yadav, Rakesh K.0000-0002-9569-2438
Additional Information:© 2018 The Author(s). Published under the PNAS license. Edited by Peter L. Olson, Johns Hopkins University, Baltimore, MD, and approved September 18, 2018 (received for review October 4, 2017). PNAS published ahead of print October 16, 2018. We thank the editor and three reviewers for their constructive comments, which greatly improved this manuscript. J.M.A. gratefully acknowledges the support of the National Science Foundation Geophysics Program (Award EAR-1547269). The two higher Ekman number 3D dynamo simulations were carried out at Gesellschaft für wissenschaftliche Datenverarbeitung mbH, Göttingen (GWDG) and at Rechenzentrum Garching der Max-Planck-Gesellschaft (RZG). The lowest Ekman number case was carried out on the ALCF Mira supercomputer (DE-AC02-06CH11357) through the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. Author contributions: H.C. and J.M.A. designed research; H.C. and R.K.Y. performed research; H.C. and R.K.Y. analyzed data; and H.C. and J.M.A. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. Data deposition: The snapshots reported in this paper are available in Movie S1 and on Figshare, This article contains supporting information online at
Funding AgencyGrant Number
Department of Energy (DOE)DE-AC02-06CH11357
Subject Keywords:geodynamo; polar magnetic minima; polar vortex; gyroscopic pumping; flow magnetic field interactions
Issue or Number:44
PubMed Central ID:PMC6217441
Record Number:CaltechAUTHORS:20181016-142315503
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Official Citation:Geomagnetic polar minima do not arise from steady meridional circulation. Hao Cao, Rakesh K. Yadav, Jonathan M. Aurnou. Proceedings of the National Academy of Sciences Oct 2018, 115 (44) 11186-11191; DOI: 10.1073/pnas.1717454115
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:90284
Deposited By: Tony Diaz
Deposited On:16 Oct 2018 21:36
Last Modified:02 Mar 2022 17:04

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