Quasi-geostrophic flows responsible for the Secular Variation of the Earth’s magnetic field

A large jet encircling the inner core and carrying a significant part of the core angular momentum and axial vortices of ∼ 700 km diameter mainly clustering around the cylinder tangent to the solid inner core, are inferred from geomagnetic secular variation. These results are presented in the form of maps of axial vorticity in an equatorial section. The study takes advantage of the high precision of recent geomagnetic field models derived from satellite data. The tangent cylinder is considered as impenetrable to core flows and, outside of it, the same eddies account for magnetic field induction at the core surface in both hemispheres. New regularizations are suggested from dynamical considerations. It is found that medium and small scale velocity fields contribute significantly to the large scale secular variation. Accordingly, final models of core flows are calculated after an iterative process, whereby the magnetic field variation produced by small scale stochastic magnetic fields and medium to small scale computed velocity fields are incorporated into the inversion itself as modelling errors. This study represents a significant step in an effort to join geomagnetic observations and the fluid core dynamics on short timescales.