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Mantle-driven geodynamo features—effects of post-Perovskite phase transition

Earth, Planets and Space volume 61, pages 1255–1268 (2009)Cite this article

Abstract

Exploring the impact of the heterogeneous lower mantle on the geodynamo requires knowledge of the heat flux anomaly across the core-mantle boundary. Most studies so far used a purely thermal interpretation of seismic shear wave anomalies to assign heterogeneous heat flux boundary conditions on numerical dynamo models, ignoring phase transition or compositional origins. A recent study of mantle convection (Nakagawa and Tackley, 2008) provides guidelines to include such non-thermal effects. Here we construct maps of heat flux across the core-mantle boundary based on a lower mantle tomography model (Masters et al, 2000) with a combined thermal and post-Perovskite phase transition interpretation. We impose these patterns as outer boundary conditions on numerical dynamo simulations and study the impact of accounting for post-Perovskite effects on the long-term time-average properties of the dynamo. We then compare our results with geophysical observations. We find in all cases that surface downwellings associated with cyclones concentrate intense non-axisymmetric magnetic flux at high-latitudes, the surface flow contains a large anticlockwise vortex at mid-latitudes of the southern hemisphere, and the inner boundary buoyancy flux is dominated by a Y 02 pattern. Boundary-driven time-average surface flow with some equatorial asymmetry is organized in the shell by quasi-axial convective rolls that extract more buoyancy from low-latitudes of the inner-boundary. These positive inner boundary buoyancy flux structures are found at low-latitudes of the northern hemisphere, in some places due to cyclonic flow at mid-latitudes of the southern hemisphere connecting with higher latitude cyclonic flow in the northern hemisphere. Accounting for post-Perovskite effects improves the recovery of several geodynamo observations, including the Atlantic/Pacific hemispherical dichotomy in core flow activity, the single intense paleomagnetic field structure in the southern hemisphere, and possibly the m = 1 dominant mode of inner-core seismic heterogeneity.

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  1. Laboratoire de Planétologie et de Géodynamique, CNRS, Université de Nantes, Nantes Atlantique Universités, UMR CNRS 6112, 2 rue de la Houssinière, F-44000, Nantes, France

    Hagay Amit & Gaël Choblet

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  1. Hagay Amit
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Correspondence to Hagay Amit.

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Amit, H., Choblet, G. Mantle-driven geodynamo features—effects of post-Perovskite phase transition. Earth Planet Sp 61, 1255–1268 (2009). https://doi.org/10.1186/BF03352978

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Key words

  • Geodynamo
  • mantle tomography
  • post-Perovskite
  • geomagnetic field
  • core flow
  • inner core