Seasonal trends of the cosmic ray intensity observed by 16 neutron monitors for 1964–2020

Abstract

The intensity of neutrons as secondary cosmic rays has typical modulations, such as diurnal and solar cyclic variation or transient events. Jeong and Oh (2020) examined the seasonal variation using data of the Oulu neutron monitor, but the seasonal variations and their origins were difficult to determine. In this study, the daily data of 16 neutron monitors from 1964 to 2020 were used to examine the seasonal variation and its origin. The solar cyclic variations were eliminated by normalizing to the yearly mean or median and selecting the representative value of each day from the mean or median for 57 years. Sixteen neutron monitors showed a clear seasonal trend for four ways of data transformation based on the Cox and Stuart trend test. Furthermore, the seasonal trend remained steadfast, excluding the outliers of each year to remove the variations by sporadic effects. The seasonal trend showed the maximum in January and the minimum in June. Out of 16 neutron monitors, an attempt was made to find the groups with a homogeneous seasonal trend. The statistical clustering method using the K-means idea provides two types of groups: Thule-type and Newark-type. The difference between the two groups showed the different recovery period from the minimum value and the combination of semiannual wave. This difference may result from a combination of terrestrial and extraterrestrial effects depending on the neutron monitor’s latitude and altitude. We considered the various origins of the seasonal trends including terrestrial and extraterrestrial factors. The seasonal trends may originate from the combination of various sources, including terrestrial and extraterrestrial factors rather than from one independent factor.