Periodic variation of anomalous cosmic ray oxygen during solar cycles 23 and 24

We investigate the evolution of the 152-156-day Rieger periodicity, the 27-day synodic rotation period and its 13.5 and 9-day harmonic periodicities in anomalous cosmic ray (ACR) oxygen (O) fluxes at the energy range between 8 - 25 MeV/n observed by the Advanced Composition Explorer (ACE) satellite during solar cycles 23 and 24. The ACR oxygen flux data is analysed using the Lomb-Scargle periodogram and Morlet wavelet spectral analysis techniques. Daily mean oxygen fluxes during solar quiet times are used to identify how the ACR oxygens at different energies vary at the Rieger periodicity and at the solar rotation periodicities in each year. This is the first investigation of the periodicity evolution of ACR oxygen ions. Previous investigations have mostly concentrated on the spectral behaviour of GCR particles, in particular their 27-day and 13.5-day periodicities, during solar cycles of opposite polarities. Our analysis revealed a significant temporal and energy dependence in the spectral behaviour of ACR oxygens during the two cycles. An important finding of this investigation, not reported earlier to our knowledge, is the significant increase in the power of the different ACR oxygen periodicities during the minimum between cycles 24/25 (of positive solar polarity) in comparison to the minimum between cycles 23/34 (of negative solar polarity).