Time course of semidiurnal variations in the intensity of cosmic rays according to the data of ground-based detectors for 1971-2021
Galactic cosmic rays (CR) registered by ground-based detectors are subjected to the main influencing factor along their propagation path - the solar wind, which fills the entire heliosphere. Modulation of CR by the heliosphere leads to the formation of their anisotropic spatial-angular distribution in interplanetary space. In the data of ground-based detectors, such anisotropy manifests itself, due to the rotation of the Earth around its axis, in the form of periodic daily fluctuations in the registered intensity. The parameters of such fluctuations are determined by the state of solar activity and the corresponding changes in the large-scale structure. As a rule, the amplitude of such oscillations is small and amounts to only hundredths of the total flux of CR. However, the anisotropy of CR is an important source of information about the properties and structure of the entire heliosphere. The current picture of modulation of CR in the heliosphere suggests the existence of their excess from directions across the lines of force of the interplanetary magnetic field. Experimental data confirm this picture and, on average, the maximum of fluctuations is observed within 3 and 15 hours local time. Considering that the direction of the field lines remains almost unchanged, it is fair to expect that the aforementioned maximum time should also remain unchanged. However, long-term observations have shown that, depending on the level of solar activity, the time of the maximum can significantly shift to an earlier time. This paper is an attempt to explain the observed amplitude-phase oscillations of semidiurnal variations in the intensity of cosmic rays. Based on current ideas about the interaction of CR with the Earth's atmosphere and the geomagnetic field, calculations of the expected semidiurnal variations in the intensity of CR recorded by world networks of muon detectors and neutron monitors have been carried out. Comparison of the calculated and observational data has shown that the observed shift can be explained by the dynamics of the energy spectrum of semidiurnal CR variations. The result obtained confirms and supplements the results obtained earlier by other authors, and it can also be considered as an indication for the further development of the theory of heliospheric CR modulation.