Hardy, CM orcid.org/0000-0002-4550-4822, Livermore, PW orcid.org/0000-0001-7591-6716 and Niesen, J orcid.org/0000-0002-6693-3810 (2020) Enhanced magnetic fields within a stratified layer. Geophysical Journal International, 222 (3). pp. 1686-1703. ISSN 0956-540X
Abstract
Mounting evidence from both seismology and numerical experiments on core composition suggests the existence of a layer of stably stratified fluid at the top of Earth’s outer core. In such a layer, a magnetostrophic force balance and suppressed radial motion lead to stringent constraints on the magnetic field, named Malkus constraints, which are a much more restrictive extension of the well known Taylor constraints. Here, we explore the consequences of such constraints for the structure of the core’s internal magnetic field. We provide a new simple derivation of these Malkus constraints, and show solutions exist which can be matched to any external potential field with arbitrary depth of stratified layer. From considerations of these magnetostatic Malkus constraints alone, it is therefore not possible to uniquely infer the depth of the stratified layer from external geomagnetic observations. We examine two models of the geomagnetic field defined within a spherical core, which obey the Taylor constraints in an inner convective region and the Malkus constraints in an outer stratified layer. When matched to a single-epoch geomagnetic potential field model, both models show that the toroidal magnetic field within the outer layer is about 100 times stronger compared to that in the inner region, taking a maximum value of 8 mT at a depth of 70 km. The dynamic regime of such a layer, modulated by suppressed radial motion but also a locally enhanced magnetic field, may therefore be quite distinct from that of any interior dynamo.
Metadata
Item Type: | Article |
---|---|
Authors/Creators: |
|
Copyright, Publisher and Additional Information: | © The Author(s) 2020. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article has been accepted for publication in Geophysical Journal International © The Author(s) 2020. Published by Oxford University Press on behalf of The Royal Astronomical Society. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved. |
Keywords: | Composition and structure of the core; Core; Dynamo: theories and simulations |
Dates: |
|
Institution: | The University of Leeds |
Academic Units: | The University of Leeds > Faculty of Engineering & Physical Sciences (Leeds) > School of Computing (Leeds) The University of Leeds > Faculty of Engineering & Physical Sciences (Leeds) > School of Mathematics (Leeds) > Applied Mathematics (Leeds) 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: | 29 Jun 2020 12:44 |
Last Modified: | 25 Jun 2023 22:19 |
Status: | Published |
Publisher: | Oxford University Press (OUP) |
Identification Number: | 10.1093/gji/ggaa260 |
Open Archives Initiative ID (OAI ID): | oai:eprints.whiterose.ac.uk:162453 |