Spectral Analysis of Solar and Geomagnetic Parameters in Relation to Cosmic-ray Intensity for the Time Period 1965 – 2018

Spectral analysis of solar and geomagnetic parameters as well as of cosmic-ray intensity was performed aiming to identify possible new periodicities and confirm the well-known ones. Specifically, short-, mid-, and long-term periodicities of these parameters such as sunspot number, Bz-component of the interplanetary magnetic field, geomagnetic Ap index, and cosmic-ray intensity over the time period 1965 – 2018, covering five solar cycles from Cycles 20 to 24, are presented. For this purpose, two different techniques, fast Fourier transformation and wavelet analysis, have been used in order to ensure accuracy in the frequency values and also their localization in the time series. The periodicities resulting from our comprehensive study, including the well-known 11-year and 27-day periods, the harmonics of the 5.5-year and of the 6-, 9-, and 13.9-day periods, respectively, and the ≈ 1.3-year and 1.7-year periods, were found in all of the above parameters except for the Bz-component of the interplanetary magnetic field. New periodicities such as the ≈ 10-month period for sunspot number and cosmic-ray intensity and the ≈ 3-year period for sunspot number, Ap index, and cosmic-ray intensity, were also determined. Furthermore, the newly introduced splitting of the 27-day periodicity into two adjacent peaks was confirmed in the Fourier spectra of the interplanetary magnetic field and the geomagnetic Ap index. It was concluded that several common periodicities appear in solar activity: the Ap index, and the cosmic-ray intensity. This result, in association with the fact that the spectral behavior of geomagnetic-activity parameters, provides invaluable information about the physical processes involved, and indicates that the Ap index might be used as a suitable index for space-weather forecasting.