Analysis of periodicities observed in neutron monitor counting rates and its relationship with the sunspot number

Résumé

The main goal of this thesis is to improve the understanding of the periodicities detected in neutron monitor counting rates and their relationship with the solar activity measured by Sunspot number. For this purpose we compared and selected neutron monitor stations from around the world based on proposed quality indices, and built a global monitor called Global Neutron Monitor (GNM) by averaging of the selected stations. The behavior of this virtual monitor represents the global network of neutron monitors. To obtain the periodicities in the neutron monitor counting rates, Morlet wavelet analysis have been applied. We are interested in studying the periodicities in GNM from a few days to ∼ 2 years throughout five solar cycles (from 1964 to 2019). The periodicities found in this interval are: ∼ 13.5 days, ∼ 27 days, ∼ 46–64 days, ∼ 79–83 days, Rieger-type period (∼ 134–190 days), ∼ 225–309 days, ∼ 1.06–1.15 years, ∼ 1.31–1.40 years and ∼ 1.6–2.2 years. Some of these periodicities may be related to the solar dynamo and specifically may be due to magnetic Rossby waves of poloidal number m = 1. On the other hand, we find an inverse linear relationship between the average sunspot number in each solar cycle and the duration of the ∼ 1.6–2.2 year period detected in neutron monitor counting rates. Shorter periodicities are obtained for solar cycles with a larger sunspots number and vice versa. This empirical relationship is thoroughly explored throughout this study, wherein estimations are made regarding the duration of the periodicity ranging from 1.6 to 2.2 years in neutron monitor counting rates spanning Solar Cycles 7 to 18, predating the existence of neutron monitors. Furthermore, a prediction is made regarding the duration of this periodicity for the ongoing Solar Cycle 25. The range of ∼ 1.6–2.2 years can be explained by assuming variations of the magnetic field strength in the solar tachocline along with Rossby waves. Based on this assumption, we estimate that a magnetic field of ∼ 7–25 kG in the solar tachocline is responsible for ∼ 1.6–2.2 year period.