Modelling the southern African geomagnetic field secular variation using ground survey data for 2005

Secular change of the earth's magnetic field is a comparatively regional phenomenon and that does not proceed in a regular manner across the earth. This gives rise to regions where the magnetic field changes more rapidly than elsewhere, like for instance southern Africa. As part of a cooperative project between Germany and South Africa, called Inkaba ye Africa, the COMPASS (COmprehensive Magnetic Processes under the African Southern Sub-continent) program aims to study the regional geomagnetic field and in particular its evolutionary behaviour. In addition to a rapid decrease of the geomagnetic field in this region, as evidenced by the 20% decrease observed at Hermanus, the orientation of the geomagnetic field in southern Africa is also changing rapidly. In the north-western part of southern Africa the declination of the magnetic field is propagating eastward (Tsumeb) and in the south-eastern part it propagates westward (Hermanus and Hartebeesthoek). This results in an overall increase of the spatial gradient over the subcontinent with time. During 2005, a joint field survey campaign was conducted by the Hermanus Magnetic Observatory (HMO) and the GeoForschungsZentrum (GFZ) in South Africa, Namibia and Botswana in order to characterize the time variation of different components of the geomagnetic field. Results obtained from this field survey, together with information obtained from the previous field survey during 2004 at 8 field stations, in addition to such data from the three continuous recording magnetic observatories in Southern Africa at Hermanus, Hartebeesthoek and Tsumeb, are used to model the geomagnetic field time variation for 2004 to 2005, using a polynomial approach.