Studying dynamics of energy spectrum of solar diurnal variations in cosmic rays during solar activity cycles 20–25, using method of crossed muon telescopes

The cosmic ray (CR) intensity recorded by ground-based detectors experiences solar diurnal variations (SDVs) associated with the existence of anisotropic angular distribution of CRs in near-Earth space. Long-term observations show that SDVs exhibit a dependence on the solar activity cycle, experiencing periodic 11- and 22-year variations. Such behavior of SDVs is linked to a change in the nature of galactic CR propagation in the heliosphere when it changes during a solar activity cycle. On the other hand, this phenomenon can be partially due to a change in the magnitude of CR drift by the geomagnetic field associated with changes in the SDV energy spectrum. In this work, we determine the dynamics of the SDV energy spectrum in solar activity cycles. The solution to this problem presents certain difficulties associated with peculiarities of ground-based CR recording and with the sensitivity of CR detectors to changes in the state of environment. Therefore, we employ an approach using crossed muon telescopes to estimate it, which allows us to bypass the above difficulties. We analyze data from Yakutsk, Nagoya, Sao Martinho, and Hobart muon telescopes for 1972–2022. It is shown that at solar minima during periods of positive polarity of the Sun's general magnetic field, a significant softening of the spectrum is observed. The results are discussed.