The diffusion process on Si(001) in presence of a 5 keV Ar+ ion-beam has been investigated by monitoring initiation of ripple-pattern formation. The morphology of the surface obtained by scanning tunneling microscopy measurements in ultra-high vacuum were characterized using the height-difference correlation function. These measurements clearly show formation of nanostructured ripple patterns having wavelength ~60 nm and height ~0.32 nm at 200oC. The results demonstrate that ion-beam induced and thermal diffusions cannot be treated as additive processes and the observed enhancement of surface diffusion requires lowering of activation energy that arises due to creation of ion-beam induced vacant regions.