Abstract

(with Thomas Gastine) Large-scale interannual to secular changes in Earth’s magnetic field are caused by magnetic induction and, to a lesser extent, by magnetic diffusion.Since 1999,  the almost continuous observation of these changes by dedicated satellites enabled the investigation of their causes with an unprecedented level of accuracy -let us hope that this will continue after the Swarm mission ends! In particular, several wavy patterns have been identified at the top of the core, and tentatively ascribed to certain categories of waves whose existence is predicted by theory. Waves will be discussed at length during the workshop; here I will instead review the properties of the background flow on top of which these waves ride. The background flow, which varies on convective (secular) time scales, consists of a planetary scale circulation comprising a strong westward current at low latitude under the Atlantic ocean and a high-latitude jet close to the tangent cylinder under Siberia and Canada.                           I will discuss the ability of dynamo models to account for these properties, and more generally, what current simulations can and can not achieve, notably in the light of the output of an extreme direct numerical simulation 250 times supercritical performed at an Ekman number equal to 10-7.