SPECTRUM OF GEOMAGNETIC-ACTIVITY IN THE PERIOD RANGE 5-60 DAYS - POSSIBLE LUNAR INFLUENCES

The series of daily Ap-indices has been subdivided into pentades (1932 -1936 etc.) and spectra with fine-frequency resolution have been calcu lated for the indices in each of these intervals. Daily sunspot number s have been processed in the same way. The average spectrum from all s pectra in the pentades, as well as the spectrum from the whole interva l have been calculated, and significant peaks have been determined. Th ere is a significant difference between the spectra in the pentades co ntaining the solar activity minimum (1932-1936, 1942-1946 etc.) and th ose containing the solar activity maximum (1937-1941, 1947-1951 etc.). Most peaks can be interpreted as a response to solar rotation and to the structure of solar wind speed (two high-speed streams per solar ro tation), both modulated by the Ii-year, annual and semi-annual waves. No significant peak corresponding to the period of the synodic month, or its half has been found. This result suggests that the influence of lunar cycles on some natural phenomena (if any) is not mediated by ge omagnetic activity.