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C. C. Acknowledgements, N.G. was partially supported by the French Centre National dEtudes Spatiales (CNES) for the study of Earth's core dynamics in the context of the Swarm mission of ESA, and ISTerre is part of Labex OSUG@2020 (ANR10 LABX56) Numerical computations were performed at S-CAPAD, IPGP, France, and using HPC resources from GENCI-IDRIS, France (grant 2015-042122) and also at the Froggy platform of the CIMENT infrastructure (https:// ciment.ujf-grenoble.fr) supported by the Rhône-Alpes region (GRANT CPER07_13 CIRA), the OSUG@2020 labex (reference ANR10 LABX56) and the Equip@Meso project (referenceANR-10-EQPX-29-01) We thank the International Space Science Institute for providing support for the workshops of international team no, acknowledges support by the Danish Council For Independent Research?Natural Sciences We thank R. Holme and two anonymous referees for helpful comments, 2011.