We investigate the azimuthal expansion/motion of auroral bulges in 27 large substorms using global auroral images acquired from the ultraviolet imager on board the Polar spacecraft. The region of each auroral bulge is first isolated using a region-growing image processing technique and the location of the auroral bulge is calculated and represented by its luminosity centroid position. It is found that eastward moving auroral bulges are as common as westward moving ones during the substorm expansion phase for the events studied. To exploit the possible cause of the east-west auroral bulge motion, we further correlate the azimuthal velocity of the auroral bulges with the dawn-dusk component (y-component in geocentric solar magnetic coordinates) of propagated interplanetary magnetic field (IMF) data from the Wind spacecraft. It is found that the centroid of the bulges during the expansion phase tends to move in the same direction as the y-component of IMF. We suggest that the asymmetry in the global magnetospheric/ionospheric convection associated with a nonzero value of IMF By may be the cause of the azimuthal motion of auroral bulge.