A study of the propagation of Alfven waves in the auroral density cavities

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 Alfve n waves can contribute to the auroral acceleration, we analyze their propag ation in the presence of the sharp density gradients that characterize the edges of these cavities. It is shown that an Alfven wave packet entering in to the cavity quickly develops short perpendicular length scales in the reg ion of density inhomogeneity. Transverse scales of the order of c/omega(pe) are reached after a propagation of a few tenths of second, which correspon ds to similar 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 know n to be strong current regions, the density gradients would thus be the sit es of particularly powerful wave/particle energy transfer and consequently, if the incident flux of Alfven waves coming from the magnetosphere is high enough, of the strongest particle acceleration.