GEOMAGNETIC SECULAR VARIATION AND THE 1969-1970 SECULAR JERK AT THE AFRICAN OBSERVATORIES

All-day annual mean values of the D, H and Z components of the geomagnetic field from ten observatories in the African continent are used to study long period secular variations, effects of external signals for the variation in the recorded field and for the signatures of geomagnetic jerks in the African region. Polynomials of second or third degree are found to be adequate to represent the secular variation in the field for most of the observatories. This shows that no strong local anomalies exist over much of the continent and the source of the secular variation is deep seated. Departures from these trends are observed over the Southern African region. The residuals in the geomagnetic field components are shown to exhibit parallelism with the periods corresponding to the double solar cycle only for some of the stations. A clear latitudinal distribution in the geomagnetic component that exhibits the 1969–70 secular jerk is shown. The jerk appears in the plots of the first differences in H for the southern most observatories of Hermanus, Hartebeesthoek, and Tsumeb, while the Z plots show the jerk for near equatorial and equatorial stations of Antananarivo, Luanda Belas, Bangui and Addis Ababa. There is some indication for this jerk in the first difference plots of D for the northern stations of M\'Bour and Tamanrasset. The plots of D rather strongly suggest the presence of a jerk around 1980 at most of the stations. Key words/phrases : Magnetic observatories, polynomial fitting, residuals, secular variation, secular jerk SINET: Ethiopian Journal of Science Vol. 27 (2) 2004: 105–120

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