From the Viking and, more recently, the FAST spacecraft observations, it is known that the auroral acceleration regions correspond to relatively small-scale density cavities (a few kilometers to a few tens of kilometers in a direction perpendicular to the magnetic field). In order to study how Alfv¿n waves can contribute to the auroral acceleration, we analyze their propagation in the presence of the sharp density gradients that characterize the edges of these cavities. It is shown that an Alfv¿n wave packet entering into the cavity quickly develops short perpendicular length scales in the region of density inhomogeneity. Transverse scales of the order of c/&ohgr;pe are reached after a propagation of a few tenths of second, which corresponds to ~2000 km in the cavity. This contributes to the creation of significant parallel electric field in the region of density gradient. Its amplitude is enhanced by the formation, on the gradients, of strong space charges due to the ion polarization drift. As the auroral cavities are known to be strong current regions, the density gradients would thus be the sites of particularly powerful wave/particle energy transfer and consequently, if the incident flux of Alfv¿n waves coming from the magnetosphere is high enough, of the strongest particle acceleration. ¿ 1999 American Geophysical Union