In this paper a brief summary will be given of the historical development of geomagnetism as a science in southern Africa and particularly the role played by Hermanus Magnetic Observatory in this regard. From a very modest beginning in 1841 as a recording station at the Cape of Good Hope Hermanus Magnetic Observatory is today part of the South African National Space Agency (SANSA), where its geomagnetic field data are extensively used in international research projects with a wide range.

We use wavelets and Lomb–Scargle spectral analysis techniques to investigate the changing pattern of the different harmonics of the 27-day solar rotation period of the AE index during various phases of different solar cycles between 1960 and 2014. Results show that, particularly during the solar minimum of 23–24 in 2008, the 27-day period is no longer detectable above the 95 % CI, while auroral geomagnetic activity is now dominated by the second (13.5-day) and third (9.0-day) harmonics.