A New Geomagnetic Field Model for Southern Africa Based on 2005 Ground Survey Data

Geomagnetic field observations over southern Africa, including countries like South Africa, Namibia, and Botswana, were conducted during October and November 2005 as part of a collaborative project, called COMPASS (COmprehensive Magnetic Processes under the African Southern Sub-continent) between the Hermanus Magnetic Observatory (HMO) in South Africa and the GeoForschungsZentrum Potsdam (GFZ) in Germany. For this purpose some 40 repeat stations were identified, separated by distances ranging from 300 till 400 km. These stations form part of a network of 75 repeat stations, established by the HMO during the last 60 years, and visited at regular intervals of 5 years till 2000. Due to the rapid secular variation change over this area, it was decided to conduct annual surveys at a reduced number of repeat stations. Therefore, between 2000 and 2004 surveys were conducted at a total of 12 stations on an annual basis. This did however not allow for good spatial resolution, hence the increased number of stations in 2005. For the field survey of 2005 the stations were also selected to form two independent sets of 20 beacons each, enabling one to visit these at alternative years respectively. During this campaign 2 teams, consisting of both HMO and GFZ observers, using similar DI Flux theodolites and fluxgate variometers, gathered geomagnetic field data over southern Africa. 300 WORDS Results obtained from this field survey, together with information from the 3 continuously recording magnetic observatories in southern Africa at Hermanus, Hartebeesthoek and Tsumeb, have been used to model the main geomagnetic field, employing a polynomial approach.