The early history of 1600 till 1932:

The historical development of geomagnetism in southern Africa covers a period of approximately 400 years till present. The requirements of navigation, rather than any scientific interest in geomagnetism, prompted the recording of data of magnetic field components at the Cape even before 1600. During the era of exploration between 1600 and 1700, visiting seamen from Europe have been responsible for the early magnetic observations in South Africa. In fact the earliest recorded observation of magnetic declination on land was made at Mosselbay in 1595 by Cornelis Houtman [1], commander of a Dutch fleet on its way to India, who obtained a value of 0°. Earlier shipborn measurements by the Portuguese navigator De Castro in 1538 indicate that the magnetic declination at the Cape was then about 6° East of true North.

The first systematic observations in South Africa resulted from the establishment, in 1841, of a worldwide network of observation stations. One of these stations was built in the grounds of the Royal Observatory at the Cape of Good Hope [2], with the Royal Artillery detachment responsible for all measurements. After the magnetic observatory was destroyed by fire in 1852, a period of 80 years elapsed before a magnetic observatory worthy of name was again established in South Africa. During that period observations of magnetic declination had been made by surveyors in different parts of the interior of the country. 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 [1], assisted by Prof. J. T. Morrison of the University of Stellenbosch between 1898 and 1906. Measurements of magnetic declination, inclination and horizontal intensity were made using Kew magnetometers and Dover dip circles.

The Period 1932 till Present:

The Polar Year of 1932-1933 prompted the establishment of a magnetic observatory at the University of Cape Town in 1932 under the leadership of Prof. Alexander Ogg[3]. International organizations provided grants for buildings and the loan of instruments for standard observations. Figure 1 shows the buildings at the University of Cape Town that were constructed to serve as a magnetic observatory. Establishing the Magnetic Observatory on a temporary basis for the duration of the Polar Year, was only the beginning to have it as a permanent facility. In May 1933 the International Commission for the Polar Year passed the following unanimous resolution:

"The Commission considers it highly desirable that the magnetic station of Cape Town, South Africa, should be made a permanent station because of its situation in the Southern Latitude, from which, considering the network of existing stations, magnetic observations are particularly required".

The subsequent electrification of the suburban railway network however created a disturbing influence, to the extent that accurate observations were becoming almost impossible. A new site was identified in Hermanus, because it was sufficiently remote from electric railway disturbances and had been proved by a magnetic survey to be suitable in other respects. The Hermanus Magnetic Observatory officially commenced operation on January 1, 1941. Figure 2 shows the first buildings in 1941 of the Hermanus Magnetic Observatory. A photo taken of the Observatory around 1960 can be seen in Figure 3. Since then the area housing the magnetic observatory in Hermanus has been expanded to 16 ha to eliminate any man-made noise from the surrounding suburban expansions.

Figure 1: The Magnetic Observatory (1932)

Figure 2: The Magnetic Observatory (1941)

Figure 3: The Magnetic Observatory (1960)

One of the major contributions to geomagnetism in southern Africa was the institution of a long-term magnetic secular variation programme in 1938-39. The programme consisted initially of 44 permanent field stations covering, with a fairly uniform distribution, the whole of South Africa, Namibia and Botswana, but has since been expanded to Zimbabwe as well, with a total of 75 stations. To ensure exact reoccupation during subsequent surveys, concrete beacons were erected at each field station to mark the position of the instruments during observations.

A major undertaking during the period 1960-1975 was the establishment of other magnetic observatories in southern Africa. In 1964 an opportunity arose to establish a magnetic observatory on the premises of the ‘Forschungsstation Jonathan Zenneck’, a permanent ionospheric observation station of the Max Planck Institut für Aeronomie outside Tsumeb in Namibia. Two more recording stations were also established in South Africa – one at Hartebeesthoek in 1972, and the other at Grahamstown in 1974. Problems at Grahamstown eventually led to the closure of the station in 1980.

Since then both Hermanus and Hartebeesthoek became INTERMAGNET Observatories, with Tsumeb obtaining the same status in November 2004. Figure 4 shows the positions of the secular variation stations in southern Africa, as well as the 3 INTERMAGNET magnetic observatories at Hermanus, Hartebeesthoek and Tsumeb. The data from Hermanus are used together with those of Honolulu, San Juan, and Kakioka to derive the Dst index, with Hermanus the only Southern Hemisphere magnetic observatory of these 4 observatories.

Since the first magnetic observatory was founded in Cape Town in 1841, geomagnetism has grown in stature where it is now part of a research program at the Hermanus Magnetic Observatory, exploiting ground observations as well as data from low-earth orbit satellites like Ørsted and CHAMP to investigate the peculiar behaviour of the field over southern Africa [4].

References

[1] Beattie, J.C., Report of a magnetic survey of South Africa, Cambridge University Press, 1909

[2] Observations made at the Magnetical and Meteorological Observatory at the Cape of Good Hope: Vol 1 – Magnetical Observations, 1841 to 1846” (published in 1851).

[3] Trigonometrical Survey Office, Results of observations made at the Magnetic Observatory, University of Cape Town, 1939.

[4] Kotzé, P.B., The time-varying geomagnetic field of southern Africa, Earth Planets Space, 55, 111-116, 2003.