Annual fluctuations in the ionospheric absorption of long and middle waves

The statistical processing and analysis of long-period series based on daily values of the day-time ionospheric absorption in the lower (h less than 75) and upper (h greater than 75) regions are carried out for a period of 18 years (1970-1987). The investigation of the normalized power spectra of the two time series shows that the spectral amplitude of the annual fluctuations in the absorption at 164 kHz predominates over that for semiannual fluctuations. For 1412 kHz, the spectral amplitudes of the annual and semiannual fluctuations are almost the same.