Secular variations of the geomagnetic field at the African plate
Geomagnetic secular variations at the African plate consist of shortperiod and longperiod components. Shortperiod SV depends on solar activity. To separate longperiod SV for the Athens magnetic observatories with confidence is impossible so, as rows of mean year values at most observatories are short. We used the data of fifteen observatories, but only MLT, TAN and PLS made observations from the twentieth century beginning.
On the data of these observatories and fragment data another ones, the 80-year period of SV may be discovered. These variations have quasisinusoidal form and their extremums coinside to the solar activity maximuma.
SV are generated by internal and external sources. The internal sources manifest itself at vertical component and external ones in horizontal component of the geomagnetic field. SV (observed variation) may be devided on the quasiconstant (SV1) and variable quantities. We have chosen as SV1 variations at 1979 year. At this year the sign of SV changes at all observatories. There was supposed that SV1 reflects the sources connected to the changes of region tectonic. The differences between SV and SV1 reflects the external sources component, and it is confirmed by its connection to solar activity change.On the coast and islands observatories the values of shortperiod and longperiod variations were increased.
On the tectonic most active east part of African plate SV were more active. Distinctive peculiarity of the SV on the African plate is decreasing of the SV absolute values. It begins at the end of 70-th year of twentieth century. It means the sources of the geomagnetic field relaxe.
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