  We develop a three dimensional (3-D) model of the 27-day variation of
galactic cosmic ray (GCR) intensity with a spatial variation of the solar wind
velocity. A consistent, divergence-free interplanetary magnetic field is
derived by solving the corresponding Maxwell equations with a variable solar
wind speed, which reproduces in situ observed experimental data for the time
interval to be analyzed (24 August 2007-28 February 2008). We perform model
calculations for the GCR intensity using the variable solar wind and the
corresponding magnetic field. Results are compatible with experimental data;
the correlation coefficient between our model predictions and observed 27-day
GCR variation is 0.80 0.05.
