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

Research areas:
Year:
2009
Authors:
Journal:
EARTH PLANETS AND SPACE
Volume:
61
Number:
11
Pages:
1255-1268
ISSN:
1343-8832
BibTex:
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 oil 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 oil a lower mantle tomography model (Masters et al.. 2000) with a combined thermal and post-Perovskite phase transition interpretation. We impose these patterns as cuter boundary conditions Oil 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-2(0) pattern. Boundary-driven time-average surface flow with some equatorial asymmetry is organized in the shelf 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 sonic places due 10 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.