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The rapid temporal evolution of the observed magnetic field and the associated processes in the Earth's liquid outer core


Recent models of the Earth's magnetic field, derived from observatory and satellite data, present rapid temporal evolutions, on sub-decadal time-scales, that have not been seen before. These rapid evolutions are particularly apparent in the secular acceleration, i.e. in the time derivative of the core field secular variation. What is the origin of this signal? How much is it distorted by the conductive mantle? What processes in the core can give rise to these fast evolutions? To address these questions, an international team  has been organized covering three specialties, namely: Data analysis,Core flow modelling and Magnetohydrodynamics.

Regarding the data analysis, the repeatability of the observed signal and its evolution in time has to be investigated. This can be done using the most recent available satellite data, as well as older observatory data. The impact of the conductive mantle on the observed signal needs also to be accounted for. In the derivation of core flow models, little attention is usually given to the temporal evolution of these models. As the magnetic field acceleration is now better resolved, this aspect of the core flow modelling can be also considered. The derivation of these flow models commonly rely on the frozen-flux approximation and quasi-geostrophy assumptions that need to be tested. This can be done by characterizing, under these assumptions, hydromagnetic normal modes in the core. Such modes could be responsible for the observed fast changes in both the field and the flow.

Two short workshops are planned by the team to organize collaboration, build a common research strategy and exchange information. Hopefully, significant progresses will be made in understanding these new aspects of the core field evolution.

Vincent Lesur 2009-07-03