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

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 0 2 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.

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

“…More vigorous convection leads to stronger filtering of the CMB order 2 content and the ICB pattern remains dominantly order 1 (Aubert et al 2008), whereas slightly supercritical convection leaves the ICB pattern with nearly non-distorted order 2 ). Amit and Choblet (2009) found that the ratio of order 1 to order 2 in Fig. 4b is significantly larger than that in 4a.…”

“…The heat flux based Ra (9) was used with either generic thermochemical convection (Amit and Choblet 2009;Amit et al in press;Amit and Choblet 2012) or purely thermal convection (Monteux et al 2015). Alternatively, when convection is dominantly chemical, Ra can be defined based on the rate of light elements release into the outer core due to inner core freezing as (Olson 2007a)…”

“…Here we review the papers by Amit and Choblet (2009), Amit and Choblet (2012), and Amit et al (in press). As in previous studies, a reference CMB heat flux anomaly of purely thermal origin δq T was assumed to be linearly proportional to the lowermost mantle seismic shear velocity anomaly δv s :…”

“…However, a shift between the locations of the large heat flux structures and the patches was observed, with the shift value depending on the dynamo internal parameters (Aubert et al 2007;Olson and Christensen 2002;Takahashi et al 2008). Alternatively, some geomagnetic (or paleomagnetic) spectral information, e.g., the number of patches at each hemisphere (corresponding to the dominant order), may be used as an observational constraint (Amit and Choblet 2009).…”

“…2a), with the spreading effect well approximated by a tanh function. Following equation (B8) of Nakagawa and Tackley (2008), Amit and Choblet (2009) attempted to parameterize the thermal-phase interpretation δq Tp as The non-dimensional parameters in (15) are the amplitude A, the tanh spreading factor B, and the offset D. For a given amplitude A, the spreading factor B is constrained by reaching the tanh asymptote while maintaining an inclined step function (rather than Z-like curve). Figure 3b shows a heat flux anomaly map that accounts for post-perovskite according to (15).…”