Particle acceleration linked to Alfven wave propagation on small scale density gradients

We study how Alfven waves propagate in the presence of sharp density gradie nts in the direction perpendicular to the ambient magnetic field. A fully e lectromagnetic electron guiding centre code is used for the simulation. Dur ing the propagation, initially parallel (k(parallel to) = 0), transverse sc ales of the order of c/w(pe) are quickly reached which contributes to the c reation of a significant parallel component of the electric field in the re gion of density inhomogeneity. The effects of this field on the velocity di stribution functions are then discussed. In particular, we show that they c an present a strong deviation from their initial Gaussian shape (global shi ft in energy) due to the action of the parallel electric field. Evidences a re 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 cavitie s. (C) 2001 Elsevier Science Ltd. All rights reserved.