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We study the relationship of the 27-day variation of the galactic cosmic rayintensity with similar changes of the solar wind velocity and theinterplanetary magnetic field based on the experimental data for the Bartelsrotation period 2379 of 23 November 2007-19 December 2007. We develop a threedimensional (3-D) model of the 27-day variation of galactic cosmic rayintensity based on the heliolongitudinally dependent solar wind velocity. Aconsistent, divergence-free interplanetary magnetic field is derived by solvingMaxwells equations with a heliolongitudinally dependent 27-day variation of thesolar wind velocity reproducing in situ observations. We consider two types of3-D models of the 27-day variation of galactic cosmic ray intensity - (1) witha plane heliospheric neutral sheet, and (2)- with the sector structure of theinterplanetary magnetic field. The theoretical calculation shows that thesector structure does not influence significantly on the 27-day variation ofgalactic cosmic ray intensity as it was shown before based on the experimentaldata. Also a good agreement is found between the time profiles of thetheoretically expected and experimentally obtained first harmonic waves of the27-day variation of the galactic cosmic ray intensity (correlation coefficientequals 0.98 0.02). The expected 27-day variation of the galactic cosmic rayintensity is inversely correlated with the modulation parameter z (correlationcoefficient equals -0.91 0.05) which is proportional to the product of thesolar wind velocity V and the strength of the interplanetary magnetic field B(z VB). The high anticorrelation between these quantities indicates that thepredictable 27-day variation of the galactic cosmic ray intensity mainly iscaused by this basic modulation effect.

On the relationship of the 27-day variations of the solar wind velocity and galactic cosmic ray intensity in minimum epoch of solar activity