What we talk about when we talk about geomagnetic surface intensity minima

Today, the most striking feature of surface geomagnetic intensity field is a vast region of abnormally weak field values, reaching ​​as low as 22 µT, making Earth more susceptible e.g. to the arrival of hazardous particles from the solar wind at this region. It has crossed the South Atlantic Ocean, from Africa to South America, in the era of observatories, hence its name: the South Atlantic Anomaly (SAA). Today, its center is located in South America, but not exclusively. Over the last decade, the South Atlantic Anomaly has become more complex with the emergence of a secondary minimum near Africa, as the zero curves of the spatial derivatives of the surface intensity field converge. This dual location of surface intensity minima has been recorded by archaeomagnetism on both continents. Reference curves, based on a new archaeomagnetic database dedicated to South America, accurately record the emergence or appearance of these surface intensity minima over past millennia. The recurrence of surface intensity minima may be ancient in the South Atlantic, as paleomagnetism has also shown the recurrence of a non-dipolar field in this region. Reproducing the position of weak surface field anomalies in dynamo simulations, which account for lateral mantle heterogeneities based on lower mantle seismic tomography, is successful in recovering longitudes but fails to produce surface minima far from the geographic equator. Based on single harmonics, only non-dipolar reversing simulations consistently produce surface minima at mid-latitudes. Studies using high-end simulations also fail to systematically record minima away from the equator. Consequently, numerical dynamos fail to reproduce the latitude of the South Atlantic Anomaly (SAA), or the current morphology of the surface field is rare. Multiple minima scenarios are, in fact, common in Earth's past history. Over the West Pacific surface intensity minima anomaly has been registered by archaeomagnetism, but not only, but also by independent aurora borealis data, called the West Pacific Anomaly (WPA). Recently, ultra low field anomalies (ULFAs) (~ 10 µT) have been observed in both West Pacific and South America raising the possibility that multiple ULFAs have existed over the past few millennia.