Abstract
Spectral analysis of the Dourbes H component hourly data from the period 1960–1978 revealed the existence of a number of minor terms, in addition to the main solar and lunar peaks. The relative amplitudes of oscillations in the geomagnetic spectrum are unrelated with those predicted through lunar tide theory. The minor terms agree more closely with the 27-day amplitude modulation mechanism. A high frequency resolution power spectrum clearly shows the splitting of the solar diurnal and semi-diurnal line, and even of the lunar semi-diurnal line by the annual variation and its harmonics. The correlation between the amplitude of the M 2 wave and the mean sunspot number is of no significance.
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