We use a new nonlinear guiding center particle in cell code to investigate the role of Alfv¿n waves in the auroral particle acceleration. The propagation of an Alfv¿n wave in an inhomogeneous auroral plasma, described by a density cavity, is considered. Parallel electric fields are generated on the edges of the cavity, leading to an efficient electron acceleration and to the formation of electron beams. A beam-plasma instability takes place. It evolves nonlinearly, and small-scale electrostatic structures are created and propagate at a velocity close to the Alfv¿n velocity. A first category of these structures could be assimilated to weak double layers, whereas a second may be related to electron phase space holes. This study demonstrates the potential importance of Alfv¿n waves in the auroral acceleration and the coupling between large-scale electromagnetic and small-scale electrostatic phenomena. ¿ 2001 American Geophysical Union