Short- and Mid-term Periodicities Observed in Neutron Monitor Counting Rates throughout Solar Cycles 20–24

Abstract

This paper examines the short- and mid-term periodicities (≲2 yr) in the cosmic-ray flux along 55 yr, from 1964 to 2019. The cosmic-ray flux has been computed by averaging the counting rates, in typified units, of a set of selected neutron monitors. This builds a representative virtual neutron monitor, named the global neutron monitor. The relevant discovered periodicities are ∼13.5, ∼27, ∼46–64, ∼79–83 day; Rieger-type (∼134–190 days); ∼225–309 day; and ∼1.06–1.15, ∼1.31–1.40, and ∼1.75–2.20 yr periods. The same analyses have been applied to the sunspot number (SSN) with the aim to compare the discovered periodicities and look for possible origins of these periodicities. Two main results have been achieved: the periodicities of 77–83 days, 134–190 days (Rieger type), 225–309 days, ∼1.3 yr, and ∼1.7 yr could be related to the solar dynamo, and an inversely linear relationship has been found between the average of the SSN versus the duration time for each solar cycle of the ∼1.75–2.20 yr period.