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

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.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 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 westward (Hermanus and Hartebeesthoek).This results in the spatial gradient over the subcontinent to increase with time.During 2005 a joint field survey campaign was conducted by the Hermanus Magnetic Observatory (HMO) and the GeoForschungsZentrum (GFZ) in southern Africa, including countries like South Africa, Namibia and Botswana in order to characterize the time variation of various 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 as well as the data from the 3 continuous recording magnetic observatories in southern Africa at Hermanus, Hartebeesthoek and Tsumeb, have been used to model the geomagnetic field time variation for 2004-2005, employing a polynomial approach.