The history of the South Atlantic Anomaly as viewed from southern Africa: Potential influences of the African LLVSP on the geodynamo

English Abstract:

A broad low intensity area in Earth’s recent magnetic field spans the southern Atlantic Ocean, Africa and South America. This is commonly called the South Atlantic Anomaly (SAA). The SAA allows a relatively close approach of Earth’s radiation belts, affecting spacecraft operations. The low magnetic intensity decreases the efficiency of magnetic shielding in the region, which can influence atmospheric ozone. Many believe the SAA is linked to the dramatic decay of the dipole geomagnetic field intensity during the last 160 years, and the growth of a reversed core flux patch beneath South Africa. Some have even speculated Earth is heading toward a geomagnetic field reversal. But understanding these phenomena within the context of longer-term geomagnetic history has been limited by a lack of Southern Hemisphere archeomagnetic data. We have recently presented the first archeomagnetic data from Iron Age sites of southern Africa (∼1000-1650 AD) (Tarduno et al., 2014). Magnetic data show a sharp intensity drop at ∼1300 AD, at a rate comparable to modern field changes in the SAA (but to lower values). These changes motivated our conceptual model whereby the recurrence of low field values reflects magnetic flux expulsion from the core, promoted by the unusual core-mantle boundary composition and structure beneath southern Africa defined by seismology (specifically the African Large Low Velocity Seismic Province, or LLVSP). Because the African LLVSP is a longstanding structure, we expect this region to be a steady site of flux expulsion, and perhaps the triggering site for reversals, on time scales of millions of years. Here we discuss our ongoing efforts to extend the archeomagnetic record from southern Africa back in time, and further develop the flux expulsion- African LLVSP hypothesis.