MODELLING AND ANALYSIS OF SOUTHERN AFRICAN GEOMAGNETIC FIELD OBSERVATIONS: 1840 UNTIL 1903

Abstract

The requirements of navigation, rather than any scientific interest in geomagnetism, prompted the recording of data of magnetic field components at the Cape of Good Hope (CGH), even before 1600. The importance of knowing the deviation of a compass needle from true north was sufficient to warrant the establishment of a recording station at the Cape in 1841. The first magnetic survey of the Union of South Africa, comparable with that of developed countries in the Northern Hemisphere, was carried out by Beattie (1909), assisted by J.T. Morrison of the University of Stellenbosch, who subsequently reduced the data to 1st July 1903. In this paper we compare the measurements in Cape Town between 1840 until 1903, as well as the field survey of 1903.5, to COV-OBS and gufm1 , historical global geomagnetic field models covering the observatory and satellite period of 1840 until 2010 and 1590 until 1990, respectively, while declination observations show clear evidence of a geomagnetic jerk around 1880 to 1883, which correlates with similar observations in Europe a few years earlier (1870). The field survey data of 1903.5 are used to derive a Spherical Cap Harmonic Model for southern Africa using observations of declination (D), inclination (I) and horizontal intensity (H). We also used secular variation measurements for D, I and H obtained during 1900 to 1905 at a few select stations to derive a secular variation model for Southern Africa based on a polynomial approach.