Study of the 27 Day Variations in GCR Fluxes during 2007–2008 Based on PAMELA and ARINA Observations

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 similar to 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 R = < 0.6-1.0 > GV with a power-law index gamma = -0.13 +/- 0.44 for PAMELA, whereas for R >= 1 GV, the power-law dependence is evident with index gamma = -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.