  We present for the first time hourly variations of the spatial density
gradient of 50 GeV cosmic rays within a sample solar rotation period in 2006.
By inversely solving the transport equation, including diffusion, we deduce the
gradient from the anisotropy that is derived from the observation made by the
Global Muon Detector Network (GMDN). The anisotropy obtained by applying a new
analysis method to the GMDN data is precise and free from atmospheric
temperature effects on the muon count rate recorded by ground based detectors.
We find the derived north-south gradient perpendicular to the ecliptic plane is
oriented toward the Helioshperic Current Sheet (HCS) (i.e. southward in the
toward sector of the Interplanetary Magnetic Field (IMF) and northward in the
away sector). The orientation of the gradient component parallel to the
ecliptic plane remains similar in both sectors with an enhancement of its
magnitude seen after the Earth crosses the HCS. These temporal features are
interpreted in terms of a local maximum of the cosmic ray density at the HCS.
This is consistent with the prediction of the drift model for the $A<0$ epoch.
By comparing the observed gradient with the numerical prediction of a simple
drift model, we conclude that particle drifts in the large-scale magnetic field
play an important role in organizing the density gradient, at least in the
present $A<0$ epoch. We also found that corotating interaction regions did not
have such a notable effect. Observations with the GMDN provide us with a new
tool for investigating cosmic ray transport in the IMF.
