Towards more realistic core-mantle boundary heat flux patterns: a source of diversity in planetary dynamos

Amit, Hagay; Choblet, G.; Olson, Peter; Monteux, J.; Deschamps, Frédéric; Langlais, B.; Tobie, G.

doi:10.1186/s40645-015-0056-3

Cited by 22 publications

(21 citation statements)

References 197 publications

(386 reference statements)

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“…thermal anomaly from the seismic anomaly is still highly debated (Amit et al 2015a). Finally, the model of Masters et al (2000) is in agreement with the large-scale features of most lower mantle tomography models (Lekic et al 2012).…”

Section: Discussion a N D C O N C L U S I O N Ssupporting

confidence: 73%

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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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Section: Discussion a N D C O N C L U S I O N Ssupporting

confidence: 73%

“…19). Alternatively, the CMB heat flux might be wrong because it relies on an oversimplified lower mantle seismic-thermal relation (Amit et al 2015a). Here we chose to use the tomography model of Masters et al (2000) for several reasons.…”

Section: Discussion a N D C O N C L U S I O N Smentioning

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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“…Such a motion may be confined to a limited longitudinal range if strong lower mantle thermal heterogeneity controls the geodynamo (e.g. Amit et al 2015).…”

Section: O N C L U D I N G R E M a R K Smentioning

confidence: 99%

Polar caps during geomagnetic polarity reversals

Zossi

Fagre

Amit

et al. 2018

Geophysical Journal International

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Changes in the Earth's magnetic field can deeply modify the polar caps and auroral zones, which are the regions of most frequent precipitation of energetic particles. The present field is characterized by a dominant dipole plus weaker multipolar components. The field varies greatly in time, with the most drastic changes being polarity reversals that take place on average every ∼200 000 yr. During a polarity transition the field magnitude may diminish to about 10 per cent of its value prior to the reversal due to a decreasing dipolar component and by becoming mostly multipolar in nature. Polar caps depend on the geomagnetic field configuration so changes in their morphology are expected as a consequence of the variation and reversal of this field. We model polar caps' location by considering a superposition of the internal geomagnetic field and a uniform external field and then following the open field lines to the Earth's surface. Polar caps' location and shape for different magnetic field reversal scenarios are analysed in this work. Two polar caps near the present dipole axis intersection with the Earth's surface prevail for a dipole decrease to a certain extent, below which the southern hemisphere polar cap moves to mid-latitudes. An axial dipole collapse gives a pair of polar caps both at mid-latitudes of the southern hemisphere, while in a dipole rotation scenario the polar caps reside at the equator. If reversals occur due to an energy cascade from the dipole to higher degrees, more than two polar caps may appear. In our energy cascade scenario, four polar caps at various latitudes of both hemispheres prevail. These results indicate that during reversals auroral zones may reach mid-and low-latitude regions, and the atmosphere may become more vulnerable to the direct effect of energetic particle precipitation. This vulnerability is particularly striking at the southern hemisphere where reversed flux patches appear on the core-mantle boundary and weak intensity characterizes the present field at the Earth's surface.

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“…Mantle convection simulations produce lateral temperature anomalies of thousands of Kelvin and lateral CMB heat flow variations greater than the mean CMB heat flow (e.g. Nakagawa & Tackley 2008;Olson et al 2015). These lateral variations will inevitably drive baroclinic flows in the underlying core through the thermal wind, but it is unclear the extent to which they will drive penetrative flow within a strongly stratified region.…”

Section: Introductionmentioning

confidence: 99%

Penetration of boundary-driven flows into a rotating spherical thermally stratified fluid

et al. 2019

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Motivated by the dynamics within terrestrial bodies, we consider a rotating, strongly thermally stratified fluid within a spherical shell subject to a prescribed laterally inhomogeneous heat-flux condition at the outer boundary. Using a numerical model, we explore a broad range of three key dimensionless numbers: a thermal stratification parameter (the relative size of boundary temperature gradients to imposed vertical temperature gradients), 10 −3 S 10 4 , a buoyancy parameter (the strength of applied boundary heat flux anomalies), 10 −3 B 10 6 , and the Ekman number (ratio of viscous to Coriolis forces), 10 −6 E 10 −4 . We find both steady and time-dependent solutions and delineate the temporal regime boundaries. We focus on steady-state solutions, for which a clear transition is found between a low S regime, in which buoyancy dominates dynamics, and a high S regime, in which stratification dominates. For the latter case, the radial and horizontal velocities scale respectively as u r ∼ S −1 , u h ∼ S − 3 4 B 1 4 and are confined to boundary-induced flow within a thin layer of depth (S B) − 1 4 at the outer edge of the domain. For the Earth, if lower-mantle heterogeneous structure is due principally to chemical anomalies, we estimate that the core is in the high-S regime and steady flows † Email address for correspondence: gracecox@cp.dias.ie arXiv:1807.00310v2 [physics.flu-dyn]

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Towards more realistic core-mantle boundary heat flux patterns: a source of diversity in planetary dynamos

Cited by 22 publications

References 197 publications

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

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

Polar caps during geomagnetic polarity reversals

Penetration of boundary-driven flows into a rotating spherical thermally stratified fluid

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