Abstract
Mantle control on planetary dynamos is often studied by imposing heterogeneous core-mantle boundary (CMB) heat flux patterns on the outer boundary of numerical dynamo simulations. These patterns typically enter two main categories: Either they are proportional to seismic tomography models of Earth’s lowermost mantle to simulate realistic conditions, or they are represented by single spherical harmonics for fundamental physical understanding. However, in reality the dynamics in the lower mantle is much more complicated and these CMB heat flux models are most likely oversimplified. Here we term alternative any CMB heat flux pattern imposed on numerical dynamos that does not fall into these two categories, and instead attempts to account for additional complexity in the lower mantle. We review papers that attempted to explain various dynamo-related observations by imposing alternative CMB heat flux patterns on their dynamo models. For present-day Earth, the alternative patterns reflect non-thermal contributions to seismic anomalies or sharp features not resolved by global tomography models. Time-dependent mantle convection is invoked for capturing past conditions on Earth’s CMB. For Mars, alternative patterns account for localized heating by a giant impact or a mantle plume. Recovered geodynamo-related observations include persistent morphological features of present-day core convection and the geomagnetic field as well as the variability in the geomagnetic reversal frequency over the past several hundred Myr. On Mars the models aim at explaining the demise of the paleodynamo or the hemispheric crustal magnetic dichotomy. We report the main results of these studies, discuss their geophysical implications, and speculate on some future prospects.
Highlights
General Various geophysical observations have been recovered by imposing heterogeneous heat flux patterns on the outer boundary of numerical dynamo simulations
For the Earth, the most popular prescribed heat flux patterns are either proportional to seismic velocity anomalies obtained from tomography models of the lowermost mantle to mimic realistic conditions, or spherical harmonic degree and order 2 which is the dominant term in these tomography models
Some mantle convection models constrained by reconstructed time-dependent plate motions suggest that the supercontinent formation and breakup was accompanied by substantial change in mantle convection pattern (Zhang and Zhong 2011; Zhang et al 2010; Zhong et al 2007). According to these dynamical models, the low seismic velocity structures below the Pacific and Africa predate and postdate, respectively, the Pangea breakup (McNamara and Zhong 2005; Zhang et al 2010). These time-dependent mantle convection patterns would inevitably correspond to time-dependent core-mantle boundary (CMB) heat flux with patterns distinctively different from that inferred from present-day tomography models
Summary
General Various geophysical observations have been recovered by imposing heterogeneous heat flux patterns on the outer boundary of numerical dynamo simulations. According to these dynamical models, the low seismic velocity structures below the Pacific and Africa predate and postdate, respectively, the Pangea breakup (McNamara and Zhong 2005; Zhang et al 2010) These time-dependent mantle convection patterns would inevitably correspond to time-dependent CMB heat flux with patterns distinctively different from that inferred from present-day tomography models. It has been proposed that the hemispheric dichotomy in Mars’ crustal magnetic field was caused by heterogeneous CMB heat flux when the Martian paleo dynamo operated (Stanley et al 2008) The magnitude of this dichotomy (Amit et al 2011a) is recovered only with strongly convecting reversing dynamo models (Dietrich and Wicht 2013). In order to record hemispheric magnetization, the Martian crust should have cooled over a shorter period than the typical chron time (Lebrun et al 2013), which poses a constraint on the feasibility of the hemispherical dynamo scenario for explaining the hemispheric dichotomy in Mars’ crustal magnetic field (Dietrich and Wicht 2013; Monteux et al 2015)
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