We have studied the 27-day variations and their harmonics in Galactic cosmic ray (GCR) intensity, solar wind velocity, and interplanetary magnetic field (IMF) components during the recent prolonged solar minimum 23/24. The time evolution of the quasi-periodicity in these parameters connected with the Sun’s rotation reveals that the synodic period of these variations is ≈ 26 – 27 days and is stable. This means that the changes in the solar wind speed and the IMF are related to the Sun’s near-equatorial regions in considering the differential rotation of the Sun. However, the solar wind parameters observed near the Earth’s orbit provide only the conditions in the limited local vicinity of the equatorial region in the heliosphere (within ± 7^∘ in latitude). We also demonstrate that the observed period of the GCR intensity connected with the Sun’s rotation increased up to ≈ 33 – 36 days in 2009. This means that the process that drives the 27-day GCR intensity variations takes place not only in the limited local surroundings of the equatorial region but in the global 3-D space of the heliosphere, covering also higher latitude regions. A relatively long period (≈ 34 days) found for 2009 in the GCR intensity gives possible evidence of the onset of cycle 24 due to active regions at higher latitudes and rotating slowly because of the Sun’s differential rotation. We also discuss the effect of differential rotation on the theoretical model of the 27-day GCR intensity variations.

Using measurements from the PAMELA and ARINA spectrometers on board the Resurs-DK1 satellite, we have examined the 27 day intensity variations in galactic cosmic ray (GCR) proton fluxes in 2007–2008. The PAMELA and ARINA data allow for the first time a study of time profiles and the rigidity dependence of the 27 day variations observed directly in space in a wide rigidity range from ∼300 MV to several gigavolts. We find that the rigidity dependence of the amplitude of the 27 day GCR variations cannot be described by the same power law at both low and high energies. A flat interval occurs at rigidity GV with a power-law index γ = −0.13 ± 0.44 for PAMELA, whereas for R ≥ 1 GV, the power-law dependence is evident with index γ = −0.51 ± 0.11. We describe the rigidity dependence of the 27 day GCR variations for PAMELA and ARINA data in the framework of the modulation potential concept using the force-field approximation for GCR transport. For a physical interpretation, we have considered the relationship between the 27 day GCR variations and solar wind plasma and other heliospheric parameters. Moreover, we have discussed possible implications of MHD modeling of the solar wind plasma together with a stochastic GCR transport model concerning the effects of corotating interaction regions.

Aims.We studied the 27-day variations of galactic cosmic rays (GCRs) based on neutron monitor (NM), ACE/CRIS, STEREO, and SOHO/EPHIN measurements in the solar minima 23/24 and 24/25, which are characterized by the opposite polarities of solar magnetic cycle. We used the opportunity to reanalyze the polarity dependence of the amplitudes of the recurrent GCR variations in 2007–2009 for the negativeA < 0 solar magnetic polarity and to compared it with the clear periodic variations related to solar rotation in 2017–2019 for positiveA > 0.Methods.We used the Fourier analysis method to study the periodicity in the GCR fluxes. Because the GCR recurrence is a consequence of solar rotation, we analyzed not only GCR fluxes, but also solar and heliospheric parameters to examine the relations of the 27-day GCR variations and heliospheric as well as solar wind parameters.Results.We find that the polarity dependence of the amplitudes of the 27-day variations of the GCR intensity and anisotropy for NMs data is kept for the last two solar minima: 23/24 (2007–2009) and 24/25 (2017–2019), with greater amplitudes in the positiveA > 0 solar magnetic polarity. ACE/CRIS, SOHO/EPHIN, and STEREO measurements are not governed by this principle of greater amplitudes in the positiveA > 0 polarity. The GCR recurrence caused by the solar rotation for low-energy (< 1 GeV) cosmic rays is more sensitive to the enhanced diffusion effects, resulting in the same level in 27-day amplitudes for the positive and negative polarities. In contrast, the high-energy (> 1 GeV) cosmic rays that are registered by NMs are more sensitive to the large-scale drift effect, which leads to the 22-year Hale cycle in the 27-day GCR variation, with the larger amplitudes in theA > 0 polarity than inA < 0.