Persistent anomalies in the geomagnetic field arise from top-down control on the geodynamo

Abstract The most prominent feature of the current geomagnetic field is arguably the South Atlantic Anomaly (SAA), a region where the geomagnetic field is exceptionally weak. The origin and evolution of the SAA are still enigmatic. Recent geomagnetic field models put its origin under the Indian Ocean, but this contradicts a suggested link between the SAA and the large low-shear-velocity province (LLSVP) underneath the African continent. This contradiction arises from a sparse paleomagnetic data kernel especially lacking Southern Hemisphere data. Here we present a global geomagnetic field model that includes new paleomagnetic data from Réunion Island located in the Indian Ocean – a key region to unravel the history of the SAA. Our model reveals that not only the SAA but also other anomalous features of the geomagnetic field arise from two distinct latitudinal bands just East of both LLSVPs. This suggests that the LLSVPs disturb the generally westward flow in the top of the outer core, either through topography on the core-mantle-boundary, by thermal gradients, or a combination of both. Such local disturbances in the geodynamo may then evolve chaotically while moving slowly westwards and dwindling in strength.