Southern Africa's geomagnetic secular variation

The geomagnetic field is a function of space coordinates, which varies differently at each location with time. The secular change is not regular over the Earth, giving rise to regions where the field changes more rapidly than elsewhere, for instance southern Africa. The Hermanus Magnetic Observatory routinely conducts geomagnetic repeat surveys, covering South Africa, Namibia, Zimbabwe and Botswana. Spherical cap modelling of field survey and observatory secular variation data at 5-year intervals between 1975 and 2000 shows that a geomagnetic jerk occurred between 1980 and 1985 over southern Africa. Results obtained prior to 1980 reveal that the secular variation of declination over the subcontinent was predominantly in a positive direction, except for a small eastern coastal region. Between 1980 and 1985 this pattern changed abruptly, showing a positive tendency in the northern parts of the subcontinent, while the southern region displayed a negative secular variation. This trend continued during 1990-95 as well as 1995-2000. The consequence of this behaviour is that the gradient in declination over southern Africa is increasing with time. Secular variation models are based on data from 70 repeat stations as well as the magnetic observatories at Hermanus and Hartebeesthoek in South Africa, and Tsumeb in Namibia. In addition to the observed secular variation pattern of declination, the total magnetic field at Hermanus has decreased by approximately 20% since 1941. This rapid weakening of the dipole field could also be connected, as has been suggested, with the growth of a patch of reversed flux at the core-mantle boundary below southern Africa. This asymmetric state of the geodynamo could eventually lead to a field reversal similar to the last one about 780 000 years ago.