Secular Variations of the Planetary‐Scale Geomagnetic Anomalies

Abstract

AbstractThe characteristics of a planetary‐scale geomagnetic anomaly are described by using “unsigned magnetic flux” through the anomaly region. The eighth generation of the international geomagnetic reference filed (IGRF) is analyzed to study the secular variations of 5 planetary‐scale geomagnetic anomalies during the 1900‐2000 period. The results show that the magnetic fluxes through the southern Atlantic anomaly (SAT), Australian anomaly (AUS), and African anomaly (AF) have increased by more than 200 MWb, the increase of the flux through the Eurasian anomaly (EA) is smaller (157MWb), while the flux through the North America anomaly (NAM) has decreased by 50MWb. On the other hand, the variations of the anomaly areas are within 10%. The westward drift at the core‐mantle boundary (CMB) is less than 0.1°/a, being much slower than that at the Earth's surface (0.2°/a). This contrast may be attributed to the different westward drift velocities of the various harmonics in the geomagnetic field: the westward drift at the Earth's surface is controlled by the dominant low‐degree harmonics, while the westward drift at the CMB largely depends on high‐degree harmonics. Consequently, it should be careful to take the westward drift velocity at the Earth's surface as the characteristic velocity of the fluid flow in the outer core.