Fig. 2

Standard deviation associated with the radial component at the Earth’s surface of the sum of the core field and the lithospheric field expanded up to spherical harmonics degree \(\ell =20\). Each panel corresponds to a different epoch which is displayed on their bottom left. Isocontours show the mean \(B_r=0\) solution. White triangles represent the locations of ground-based observatories available at the presented epochs. On the bottom right of each map is indicated the r.m.s standard deviation \(\bar{\sigma }\) in nT