  Using measurements from the PAMELA and ARINA spectrometers onboard the RESURS
DK-1 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 GV. 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.
