Geomagnetic Dipole Changes and Upwelling/Downwelling at the Top of the Earth's Core

Huguet, Ludovic; Amit, Hagay; Alboussière, Thierry

doi:10.3389/feart.2018.00170

Cited by 14 publications

(14 citation statements)

References 77 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Nevertheless, in the absence of fluid flow there may still be a local energetic exchange, as demonstrated by the regularized 1840.0-2015.0 diffusion model, for which we find an overall threefold increase of the magnetic energy on the CMB. This result also contrasts with a previous suggestion that time variability in the CMB magnetic energy relates only to fluid flow (Huguet et al 2018). However, it should be noted that such local purely diffusive field growth is inherently transient, which explains the observed overall increase of modelling errors with time (e.g.…”

Section: Discussioncontrasting

confidence: 97%

“…Gubbins & Roberts 1987) and this regularization term will therefore penalize magnetic energy over the entire time period. Nevertheless, this constraint still permits any local exchange of magnetic energy, such as the overall energetic growth at the CMB observed for the 20th century (Huguet et al 2018). The choice of this regularization differs from that adopted elsewhere, for example, the dissipation norm of Gubbins & Bloxham (1985) and is motivated here by the very simple structure (see Appendix D for a derivation):…”

Section: A Penalized Variational Approachmentioning

confidence: 99%

See 1 more Smart Citation

Modelling decadal secular variation with only magnetic diffusion

Metman

Livermore

Mound

et al. 2019

Geophysical Journal International

View full text Add to dashboard Cite

SUMMARY 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 optimized full-sphere initial magnetic field structure whose diffusive evolution best fits, over various time windows, a time-dependent magnetic field model. We present models that are regularized 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. With these regularized models, over time periods of up to 30 yr, it is possible to fit COV-OBS.x1 within one standard deviation at all times. For time windows up to 102 yr 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 per cent 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.

show abstract

Section: Discussioncontrasting

confidence: 97%

Section: A Penalized Variational Approachmentioning

confidence: 99%

Modelling decadal secular variation with only magnetic diffusion

Metman

Livermore

Mound

et al. 2019

Geophysical Journal International

View full text Add to dashboard Cite

show abstract

“…However, from a more fundamental point of view, (1) is affected not only by regional spatio-temporal field variations, but also by global changes, such as the historical geomagneic dipole decrease (e.g. Gubbins, 1987;Olson and Amit, 2006;Finlay, 2008;Huguet et al, 2018). with no pattern change, a fixed critical threshold (such as 32000 nT) for the SAA would suggest that its area increases despite no regional variation.…”

Section: Methodsmentioning

confidence: 99%

Non-monotonous growth and motion of the South Atlantic Anomaly

Amit

Terra-Nova

Lézin

et al. 2020

Preprint

Self Cite

View full text Add to dashboard Cite

The South Atlantic Anomaly (SAA) is a region at Earth’s surface where the intensity of the magnetic field is particularly low. Accurate characterization of the SAA is important for both fundamental understanding of core dynamics and the geodynamo as well as societal issues such as the erosion of instruments at surface observatories and onboard spacecrafts. Previous studies applied crude measures of the SAA. Here we propose new measures to better characterize the SAA area and center, accounting for global dipole changes and shape anisotropy. Applying our characterization to a geomagnetic field model covering the historical era, we find that the mean SAA area increase and westward drift rates are twice slower than previously reported. Our results reveal that the SAA area and center are much more time-dependent, including episodes of area decrease, eastward drift and rapid southward drift. We interpret these special events in terms of the secular variation of relevant large-scale geomagnetic flux patches on the core-mantle boundary. Our characterization may be used as a constraint on Earth-like numerical dynamo models.

show abstract

“…However, some seismic studies (Alexandrakis and Eaton, 2010; Irving et al, 2018) find that a stratified layer is not required. Additionally, concentrated patches of magnetic flux at the core-mantle boundary (CMB) (Amit, 2014) and secular variation of the total geomagnetic energy at the CMB (Huguet et al, 2018) are hard to explain without radial motions near the top of the core that are difficult to reconcile with a thick and strongly stratified global layer. Nevertheless, a variety of origin mechanisms have been proposed that could produce thermal and/or compositional stratification (e.g.…”

Section: Introductionmentioning

confidence: 99%

Scaling Laws for Regional Stratification at the top of Earth’s Core

Mound¹,

Davies²

2019

Preprint

View full text Add to dashboard Cite

Seismic and geomagnetic observations have been used to argue both for and against a global stratified layer at the top of Earth’s outer core. Recently, we used numerical models of turbulent thermal convection to show that imposed lateral variations in core-mantle boundary (CMB) heat flow can give rise to regional lenses of stratified fluid at the top of the core while the bulk of the core remains actively convecting. Here we develop theoretical scaling laws to extrapolate the properties of regional stratified lenses measured in simulations to the conditions of Earth’s core. We estimate that regional stratified lenses in Earth’s core have thicknesses of up to a few hundred kilometres and Brunt-Vaisala frequencies of hours, consistent with independent observational constraints. The location, thickness, and strength of the stratified regions would change over geological time scales in response to the slowly evolving CMB heat flux heterogeneity imposed by mantle convection.

show abstract

Geomagnetic Dipole Changes and Upwelling/Downwelling at the Top of the Earth's Core

Cited by 14 publications

References 77 publications

Modelling decadal secular variation with only magnetic diffusion

Modelling decadal secular variation with only magnetic diffusion

Non-monotonous growth and motion of the South Atlantic Anomaly

Scaling Laws for Regional Stratification at the top of Earth’s Core

Contact Info

Product

Resources

About