Analysis of periodicities of cosmic ray time series located at different geomagnetic locations

We studied the neutron monitor data bases of Mexico City, Oulu, Finlad and Moscow, Russia from 1990 to 2017 to find periodicities in the intensity variations of the cosmic ray flux. We used the wavelet transform to identify mid-term variations present in the records. The corresponding confidence levels are given to the periodicities, as well as the contribution to the total power spectrum of such variations. Results are consistent with previous analysis done for other cosmic ray detectors, showing the relevance of mid-term variations, probably related to phenomena occuring below the solar atmosphere. As a reference, we compare these results with those of classical Fourier analysis based on the discrete Fourier transform, and a fractal analysis, giving consistent results. To the best of our knowledge, this is the first time that a comparative analysis of this kind is done for these three neutron monitor series representing low, medium and high geomagnetic latitudes.