Fourth Harmonic Behaviour of the 27-Day Periodicity in Galactic Cosmic Rays During Different Solar Magnetic Polarity Intervals

Galactic cosmic rays as observed by the Hermanus and Jungfraujoch Neutron Monitors between 1960 and 2018 have been analysed using various spectral analysis techniques. Daily mean neutron monitor measurements are used to identify how several harmonics of the roughly 27-day synodic rotation period change during each of these years. Spectral analysis using Lomb-Scargle and Morlet wavelet techniques of Hermanus and Jungfraujoch data revealed in particular that the fourth harmonic (∼7-day) of the solar rotation period is exceptionally strong during the minima of solar cycles when A < 0 (solar dipole pointing south) in comparison to the minima of cycles when A > 0 (solar dipole pointing north). The power spectrum results obtained in this investigation showed that galactic cosmic rays at both Hermanus and Jungfraujoch exhibit peculiar short-term periodicity behaviour as a result of solar polarity dependent magnetic drifts during negative minima.