Dynamo tests for stratification below the core-mantle boundary

Olson, Peter; Landeau, Maylis; Reynolds, Evan

doi:10.1016/j.pepi.2017.07.003

Cited by 46 publications

(74 citation statements)

References 66 publications

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“…This stratified layer may affect the geodynamo (e.g. Olson et al, 2017;Christensen, 2018), e.g. by filtering small-scale internal convective motions (Vidal & Schaeffer, 2015) or trapping waves (Knezek & Buffett, 2018).…”

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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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: 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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“…For the inner boundary fixed codensity is imposed (e.g. Aubert et al 2008;Olson et al 2017). Additional simulations were run with a homogeneous heat flux as references cases.…”

Section: Numerical Dynamosmentioning

confidence: 99%

Preferred locations of weak surface field in numerical dynamos with heterogeneous core–mantle boundary heat flux: consequences for the South Atlantic Anomaly

Terra-Nova

Amit

Choblet

2019

Geophysical Journal International

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S U M M A R YThe present-day geomagnetic field is characterized by a region of weak surface intensity, the socalled South Atlantic Anomaly (SAA). We identify the locations of surface intensity minima in modern, historical and archeomagnetic field models. We then investigate whether lower mantle thermal heterogeneity may explain the location of the SAA. We run numerical dynamos with heterogeneous core-mantle boundary (CMB) heat flux inferred from a lowermost mantle tomography model, varying dynamo internal control parameters as well as the amplitude of the CMB heat flux heterogeneity. Histograms of the longitude and latitude of surface intensity minima show the persistence of different locations. We find two preferred longitudes of surface intensity minima, one close to the present SAA minimum longitude. In contrast, in the dynamo models and in the archeomagnetic field models the surface intensity minima are often close to the equator, whereas the present-day SAA is at mid-latitudes. We demonstrate that the determining ingredients in dynamo models to reproduce the SAA latitude are related to north-south asymmetries of reversed and normal geomagnetic flux on the CMB. The imposed heterogeneous heat flux leads to more convective and magnetic activities in the Northern Hemisphere. Large time-average upwelling structure below the South Atlantic in the dynamo models correlates well with the present-day SAA region. Scaling laws analysis indicates that the persistence of surface minima longitudes is favored by slow rotation, strong convection and large heat flux heterogeneity. Furthermore, increasing mantle control yields two preferred longitudes and southern surface minima, the latter indicating that the present-day southern location of the SAA is mantle controlled. However, the rareness of mid-latitude minima in dynamo models and archeomagnetic field models leads us to speculate that the SAA midlatitude value at present is possibly unusual.

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“…The model setup includes a uniform heat sink in the temperature equation to represent the influence of conduction down the adiabatic gradient (e.g. Olson et al 2017). Stable stratification develops at the top of the core when the volumeintegrated heat sink exceeds the heat flow through the lower (innercore) boundary (at r = R i ).…”

Section: Numerical Dynamo Modelmentioning

confidence: 99%