Modulation of the solar daily geomagnetic variation

A least squares spectral analysis is used to investigate cyclic and seasonal changes of the harmonic coefficients of the solar daily and semi-diurnal variation over the 24 year interval from 1 January 1960 to 31 December 1983 at a single magnetic observatory (Dourbes, Belgium). As a preparation for the treatment of other long runs of observations a statistical method is presented to combine the output of the spectral and harmonic analyses of a group of stations. The annual means of the Fourier coefficients are significantly correlated with the solar cycle. The spectra show peaks at periods of 11 years, 1 year, 6 months and 27 days, which entails an important amplitude and phase modulation of the daily and semi-diurnal variation. No simple relationship between the peaks in the broad solar rotation band can be proposed.