Records of the historical secular variation of the Earth's magnetic field give us a glimpse on the geodynamo inner workings. Interpretation and/or prediction of the secular variation presume 1) Informations on the force balance in the Earth's core on fast timescales 2) Informations on the internal magnetic and velocity fields spatial scales. We have infered a resolved component of the quasi-geostrophic time-varying velocity field for epochs after 1920. We discuss the characteristics of the magnetic field in the core interior, which is required to support this flow.