Fig. 17

Core–mantle boundary origin of the South Atlantic Anomaly and its recent changes analysed using Green’s functions for the Laplace equation under Neumann boundary conditions. Top row: black areas show regions of lowest intensity (under 24,000 nT) selected for analysis, the main anomaly (left column) and the new secondary minimum (right column). 2nd row: combined sensitivities \(G^{\Sigma }_F\) (see Eq. (24)) showing the sensitivity of the field intensity, relative to the dipole, to the core–mantle boundary radial field, for each defined region. 3rd row: taking these sensitivities as weights and multiplying by the radial field at the core–mantle boundary. This shows the parts of the radial field at the core–mantle boundary that in combination are responsible for the main South Atlantic anomaly (left) and the new secondary minimum (right), respectively, in 2020. Bottom row: change in the sensitivity-weighted radial field at the core–mantle boundary between 2014 and 2020, showing the origin of field intensity changes for the selected regions at Earth’s surface