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    Solid earth sciences

    201509201509

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

    Amit H, Choblet G, Olson P, Monteux J, Deschamps F, Langlais B, Tobie G

    Magnetic field, Dynamo, Core-mantle boundary, Heat flux

    Top: Geomagnetic reversal frequency (yellow and barcode below) based on the Geomagnetic Polarity Time Scale (GPTS) database vs. reversal frequency in dynamo models with time-dependent CMB heat flux (black pluses) and fixed present-day tomographic pattern (red crosses). The Cretaceous Normal Superchron (CNS), Kiaman Reversed Superchron (KRS) and Moyero Reversed Superchron (MRS) are grey shaded.

    Bottom: Polarity bias, i.e. the difference between time spent in normal and reversed polarities normalized by their sum.

    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.