Chemical Convection and Stratification in the Earth's Outer Core

Bouffard, Mathieu; Choblet, G.; Labrosse, S.; Wicht, Johannes

doi:10.3389/feart.2019.00099

Cited by 36 publications

(25 citation statements)

References 100 publications

(159 reference statements)

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“…To circumvent these issues, a "particle-in-cell" (PIC) method has been developed (Bouffard, 2017;Bouffard et al, 2019). It models the compositional field in the limit L 1 as a collection of advected particles, while keeping an Eulerian description for velocity and temperature fields.…”

Section: Methodsmentioning

confidence: 99%

“…The density stratification may have a thermal and/or compositional origin (e.g. Buffett & Seagle, 2010;Davies et al, 2018;Nakagawa, 2018;Bouffard et al, 2019). Indeed, the thermal conductivity has been revised upward by ab-inito calculations (Pozzo et al, 2012;de Koker et al, 2012;Pozzo et al, 2013) and experiments (Gomi et al, 2013;Ohta et al, 2016;Konôpková et al, 2016).…”

Section: Towards Planetary Applications and Beyondmentioning

confidence: 99%

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Rotating double-diffusive convection in stably stratified planetary cores

Monville

Vidal

Cébron

et al. 2019

Geophysical Journal International

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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.

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Section: Methodsmentioning

confidence: 99%

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

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“…The question of whether or not Earth's liquid outer core contains a stratified layer just below its outer boundary has long been debated (Braginsky 1967(Braginsky , 1987Whaler 1980;Gubbins 2007;Hardy & Wong 2019). A stratified layer may result from the pooling of buoyant elements released from the freezing of the solid inner core (Braginsky 2006;Bouffard et al 2019), diffusion from the mantle above (Jeanloz 1990;Buffett & Seagle 2010) or subadiabatic thermal effects (Pozzo et al 2012). Within a strongly stratified layer, the dynamics would be very different to the remainder of the convecting core because spherical radial motion would be suppressed (Braginsky 1999;Davies et al 2015;C o xet al 2019).…”

Section: Stratification Of Earth's Corementioning

confidence: 99%

Enhanced magnetic fields within a stratified layer

Hardy

Livermore

Niesen

2020

Geophysical Journal International

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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.

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“…The large-scale atmospheric circulation is in turbulent regime and its structure has been studied [2]. The earth's outer core convection is large Ra and high Pr RBC with rotation and chemical reaction [3]. In the engineering field, RBC can be observed in single silicon crystal manufacturing process by Czochralski method [4].…”

Section: Introductionmentioning

confidence: 99%

Rayleigh-Bénard Convection of Paramagnetic Liquid under a Magnetic Field from Permanent Magnets

2020

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The convection control is important in terms of the heat transfer enhancement and improvement of the applied devices and resultant products. In this study, the convection control by a magnetic field from block permanent magnets is numerically investigated on the Rayleigh-Bénard convection of paramagnetic fluid. To enhance the magnetic force from the available permanent magnets, pairs of alternating-pole magnets are employed and aligned near the bottom heated wall. The lattice Boltzmann method is employed for the computation of the heat and fluid flow with the consideration of buoyancy and magnetothermal force on the working fluid. It is found that, since the magnetic force at the junction of pair magnets becomes strong remarkably and in the same direction as the gravity, descending convection flow is locally enhanced and the pair of symmetrical roll cells near the magnet junction becomes longitudinal. The local heat transfer corresponds to the affected roll cell pattern; locally enhanced at the magnet junctions and low heat transfer area is shifted aside the magnet outer edge. The averaged Nusselt number on the hot wall also increases proportionally to the magnetic induction but it is saturated at high magnetic induction. This suggests the roll cell pattern is no more largely affected at extremely-high magnetic induction.

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Chemical Convection and Stratification in the Earth's Outer Core

Cited by 36 publications

References 100 publications

Rotating double-diffusive convection in stably stratified planetary cores

Rotating double-diffusive convection in stably stratified planetary cores

Enhanced magnetic fields within a stratified layer

Rayleigh-Bénard Convection of Paramagnetic Liquid under a Magnetic Field from Permanent Magnets

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