Convection-driven geodynamo models

There has been signi cant progress in the development of numerical geodynamo mod-els over the last ve years. Advances in computer technology have made it possible to perform three-dimensional simulations, with thermal or compositional convection as the driving mechanism. These numerical simulations give reasonable results for the morphology and strength of the eld at the core{mantle boundary, and the mod-els are also capable of giving reversals and excursions which can be compared with palaeomagnetic observations; they also predict di¬erential rotation between the inner core and the mantle. However, there are still a number of fundamental problems associated with the simulations, which are proving hard to overcome. Despite the advances in computing power, the models are still expensive and take a long time to run. This problem may diminish as faster machines become available, and new numerical methods exploit parallelization e¬ectively, but currently there are no practical schemes available which work at low Ekman number. Even with turbulent values of the di¬usivities (and the question of whether iso-tropic di¬usivities are appropriate is still unresolved), the appropriate dynamical regime has not yet been reached. In consequence, modelling assumptions about the nature of the ®ow near the boundaries have to be made, and di¬erent choices can have profound e¬ects on the dynamics. The nature of large-scale magnetoconvection at small E is still not well understood, and until we have more understanding of this issue, it will be di ¯ cult to have a great deal of con dence in the predictions of the numerical models