We study how Alfvén waves propagate in the presence of sharp density gradients in the direction perpendicular to the ambient magnetic field. A fully electromagnetic electron guiding centre code is used for the simulation. During the propagation, initially parallel ( k// = 0 ), transverse scales of the order of c / w_pe are quickly reached which contributes to the creation of a significant parallel component of the electric field in the region of density inhomogeneity. The effects of this field on the velocity distribution functions are then discussed. In particular, we show that they can present a strong deviation from their initial Gaussian shape (global shift in energy) due to the action of the parallel electric field. Evidences are then given for a net energy gain of the electrons, to the expense of the wave, during this process. This energy transfer mechanism may be relevant in order to explain the particle acceleration in the auroral plasma cavities.