Dynamics of the energy spectrum of solar-diural variations in the intensity of cosmic rays in 22–25 cycles of solar activity

Periodic  24-hour  variations  in  the  intensity  of  galactic  cosmic  rays,  continuously  observed by  ground-based  detectors,  are  called  solar  diurnal  variations.  The  nature  of  these  variations  lies  in the existence in the interplanetary medium of an anisotropic spatial distribution of galactic cosmic rays, which arises during the interaction of these particles with the heliosphere. It is believed that the physical factors responsible for the observed anisotropy of galactic cosmic rays are the processes of convection, diffusion and their drift. The combination of these factors determines the main parameters of solar diurnal variations, such as amplitude, phase and energy spectrum. To study the energy spectrum of solar-diurnal variations, we used measurement data from muon telescopes of the Yakutsk Cosmic Ray Spectrograph after A.I. Kuzmin and Nagoya stations (Japan). The research approach is based on the idea of the crossed  telescope method, originally designed to take into account the temperature effect. Due to the difference in the receiving characteristics of the crossed northern and southern directions of the above muon telescopes, the intensity variations recorded by them are sensitive to changes in the energy spectrum of solar-diurnal variations. This property was used to assess the dynamics of the energy spectrum of solar-diurnal variations for 22–25 cycles of solar activity. Analysis of the data obtained showed that in the last minimum of solar activity  in  2018-2021.  An  anomalously  early  phase  of  solar-diurnal  variations  was  observed.  To  study this phenomenon, we simulated the ratio and phase difference of a pair of northern and southern crossed  directions  for  both  muon  telescopes  for  different  types  and  values  of  the  energy  spectrum  of  solar-diurnal variations. A comparison of model calculations and observational data has made it possible to establish that during periods of minimum solar activity in the positive polarity of the general magnetic  field of the Sun, a significant softening of the energy spectrum of solar-diurnal variations is observed.  The reasons for the discovered phenomenon are discussed.

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