Regional stratification at the top of Earth's core due to core–mantle boundary heat flux variations

Mound, J. E.; Davies, C. J.; Rost, Sebastian; Aurnou, J.

doi:10.1038/s41561-019-0381-z

Cited by 67 publications

(65 citation statements)

References 61 publications

(44 reference statements)

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This may favour an outer sub-adiabatic thermal stratification, but large thermodynamical uncertainties remain (Williams, 2018). Moreover, Mound et al (2019) pointed out that this outermost stratification may be regional (rather than global), being generated by the lateral variations in heat flux at the coremantle boundary . Stratification may be also sustained by the accumulation of light elements (e.g.…”

Section: Towards Planetary Applications and Beyondmentioning

confidence: 99%

Rotating double-diffusive convection in stably stratified planetary cores

Monville

Vidal

Cébron

et al. 2019

Geophysical Journal International

View full text Add to dashboard Cite

In planetary fluid cores, the density depends on temperature and chemical composition, which diffuse at very different rates. This leads to various instabilities, bearing the name of double-diffusive convection. We investigate rotating double-diffusive convection (RDDC) in fluid spheres. We use the Boussinesq approximation with homogeneous internal thermal and compositional source terms. We focus on the finger regime, in which the thermal gradient is stabilising whereas the compositional one is destabilising. First, we perform a global linear stability analysis in spheres. The critical Rayleigh numbers drastically drop for stably stratified fluids, yielding large-scale convective motions where local analyses predict stability. We evidence the inviscid nature of this large-scale double-diffusive instability, enabling the determination of the marginal stability curve at realistic planetary regimes. In particular, we show that in stably stratified spheres, the Rayleigh numbers Ra at the onset evolve like Ra ∼ Ek −1 , where Ek is the Ekman number. This differs from rotating convection in unstably stratified spheres, for which Ra ∼ Ek −4/3 . The domain of existence of inviscid convection thus increases as Ek −1/3 . Second, we perform nonlinear simulations. We find a transition between two regimes of RDDC, controlled by the strength of the stratification. Furthermore, far from the RDDC onset, we find a dominating equatorially anti-symmetric, large-scale zonal flow slightly above the associated linear onset. Unexpectedly, a purely linear mechanism can explain this phenomenon, even far from the instability onset, yielding a symmetry breaking of the nonlinear flow at saturation. For even stronger stable stratification, the flow becomes mainly equatorially-symmetric and intense zonal jets develop. Finally, we apply our results to the early Earth core. Double diffusion can reduce the critical Rayleigh number by four decades for realistic core conditions. We suggest that the early Earth core was prone to turbulent RDDC, with large-scale zonal flows.

show abstract

Section: Towards Planetary Applications and Beyondmentioning

confidence: 99%

Rotating double-diffusive convection in stably stratified planetary cores

Monville

Vidal

Cébron

et al. 2019

Geophysical Journal International

View full text Add to dashboard Cite

show abstract

“…Although our simulations approach the limit of what is computationally feasible, they remain far from the parameter regime for the Earth's core. In particular, estimates of the relevant parameters suggest that Ra may be far larger and E far smaller in the Earth than in our simulations (Mound et al, 2019). The value of q is uncertain in the Earth as it requires knowledge of both the temperature structure and thermal conductivity of the lowermost mantle and the total superadiabatic CMB heat flow; nevertheless, its value in the Earth may be an order of magnitude larger than in our simulations (Mound et al, 2019).…”

Section: Introductionmentioning

confidence: 52%

“…An alternative view of core stratification has recently been suggested from numerical modelling in which stratification is caused, rather than opposed, by lateral CMB heat flow variations; furthermore, the resultant stratification is found to be confined into regional lenses, rather than a global layer (Mound et al, 2019). In some cases, 1D averaging over strong and laterally extensive regional inversion lenses can produce an apparent global stratification despite there being radial motion throughout the core including its outermost regions.…”

Section: Introductionmentioning

confidence: 99%

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

Mound¹,

Davies²

2019

Preprint

Self Cite

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

“…Finally, we note that the appropriate description of a stratified layer may in fact need to be more complex than a single uniform layer that we assume. Numerical simulations of core flow with heterogeneous CMB heat flux by Mound et al (2019) find that localized subadiabatic regions that are stratified are possible amid the remaining actively convecting liquid. If indeed local rather than global stratification is the more appropriate model for the Earth's outermost core then the condition of requiring an exact Malkus state would not apply, and the constraints on the magnetic field would be weakened by the existence of regions of non-zero radial flow.…”

Section: Further Extensionsmentioning

confidence: 98%

“…It has been found that the inclusion of a thin stable layer in numerical models can act to destabilize the dynamo, through generating a thermal wind which creates a different differential rotation pattern in the core (Stanley & Mohammadi 2008). Additionally many distinctive features of the geomagnetic field are not reproduced, as strong stratification leads to the disappearance of reverse flux patches and suppression of all non-axisymmetric magnetic field components (Christensen & Wicht 2008;Mound et al 2019).…”

Section: Stratification Of Earth's Corementioning

confidence: 99%

Enhanced magnetic fields within a stratified layer

Hardy

Livermore

Niesen

2020

Geophysical Journal International

View full text Add to dashboard Cite

SUMMARY 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.

show abstract

Regional stratification at the top of Earth's core due to core–mantle boundary heat flux variations

Abstract: This is a repository copy of Regional stratification at the top of Earth's core due to core-mantle boundary heat flux variations.

Cited by 67 publications

References 61 publications

Rotating double-diffusive convection in stably stratified planetary cores

Rotating double-diffusive convection in stably stratified planetary cores

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

Enhanced magnetic fields within a stratified layer

Contact Info

Product

Resources

About