The time-varying geomagnetic field of Southern Africa

The geomagnetic field at any given epoch is a function of space coordinates, varying differently at each location with time. It has been known that secular change is a comparatively local phenomenon and that it does not proceed in a regular way all over the Earth, giving rise to regions where the field changes more rapidly than elsewhere, like for instance southern Africa. The Hermanus Magnetic Observatory routinely executes geomagnetic repeat surveys, which includes 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. The secular variation models are based on 70 repeat station data central differences as well as the 3 magnetic observatories at Hermanus, Hartebeesthoek and Tsumeb (Namibia) and include terms up to spatial degree 3 and temporal degree 2. Although each model allows for 48 coefficients, only 42 were found to be statistically significant.