Fig. 2

Core-field estimate. a The field lines of the axisymmetric component of the initial field model at epoch 2014.3. They are represented down to the core-mantle boundary, under the usual assumption that the mantle is an electrical insulator. The estimate of the magnetic structure in- and outside the core, obtained by Kalman filtering, is shown to the right. b The color map represents the axisymmetric toroidal (azimuthal) field, which is trapped in the core (scale ±2 mT). c The field lines are those of the axisymmetric poloidal field