High frequency spectral features of galactic cosmic rays at different rigidities during the ascending and maximum phases of the solar cycle 24

Galactic cosmic rays (GCR) intensity during the current solar cycle (SC) 24 exhibit unusual modulation characteristics. In this study, the temporal evolution of the daily GCRs recorded over a wide range of particle rigidities by seven neutron monitor stations located at Thule, Nain, Sanae, Newark, Alma-Ata, Mexico and Athens is investigated. The datasets span the time interval from Jan 2009 to Dec 2015 which cover the ascending and the maximum phases of the present SC 24. The Morlet wavelet analysis is employed to examine the periodicities in the range 8–512 day. At high frequency range, the prominent period of band 20–40 day related to the synodic rotation of the Sun along with its harmonics in the band 8–16 day were detected. In the mid-term range, the well-known ‘Rieger-type periodicity’ of 120–170 day was identified specially during the maximum epoch, i.e. 2012. Another period in the range 250–460 day is significantly appears during nearly the entire epoch, and at almost all stations. It may be considered as annual and near-annual period where it is supposed to be related—during these phases—to the solar magnetic field characteristics, and not to the earth’s heliolatitude variations. Possible explanation of the observed periods are discussed in the light of existing models and earlier findings.