Abstract. A study of the cosmic-ray intensity power spectrum using the Climax Neutron Monitor data in the frequency range from 10^-9 Hz to 10^-7 Hz (which corresponds to periodicities from 11 years to a few months) during the period 1953–1996, was carried out by means of the successive approximations method of analysis and was compared against the power spectrum and the maximum entropy methods. The contributions of the time evolution of several peaks to the global one were obtained. Except for the well-known 11-year and the 1-year variations, peaks at 7.7, 5.5, 2 and 1.7 years are found. Several peaks with periods less than 10 months have appeared in our analysis, while the occurrence of 5.1 months is obtained in all the examined solar cycles with a strong signature in cycle 21. Transitions of these quasi-periodicities are seen in power spectra plots. Some of them can be attributed to the modulation of the cosmic ray intensity by solar activity. Others are sporadic and have been previously attributed to the interplanetary magnetic field. The results obtained support once again the argument regarding the difference in the solar activity between odd and even solar cycles.

Key words. Interplanetary physics (Cosmic rays, Interplanetary magnetic fields)