Fast evolving spatial structure of auroral parallel electric fields

We use a new nonlinear guiding center particle in cell code to investigate the role of Alfven waves in the auroral particle acceleration. The propagation of an Alfven 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 Alfven 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 Alfven waves in the auroral acceleration and the coupling between large-scale electromagnetic and small-scale electrostatic phenomena.