Study of the electron distribution function stability in the edges of auroral arcs

Garcia, G. and Forme, F.

CETP, Velizy, France

Thanks to satellites such as FAST or FREJA, we know that large field-aligned currents are commonly observed in the ionosphere. We know that the better the spatial resolution, the higher the downward current densities. These large parallel current densities imply the presence of a parallel electric field. So, we can wonder if this parallel electric field could modify the electron distribution functions. These current densities could be the cause of many phenomena such as tall red rays or triggering of unstable ion acoustic waves.

All the studies of this region were made with fluid model supposing that the kinetic perturbations could be neglected. The aim of our work is to study the dynamic of collisionnal plasma under a parallel electric field with a kinetic model. We consider the issue of electrons moving through an ionospheric gas of positive ions and neutrals under the influence of a parallel electric field. We developed a kinetic model of collisions, including electron/electron, electron/ion and electron/neutral collisions. We use a Fokker-Planck approach to describe binary collisions between charged particles with a long range interaction. We present the essential elements of this collision operator: The Langevin equation for electron/electron and electron/ion collisions and the Monte-Carlo and null collision methods for electron/neutral collisions. We calculate a self-consistent electric field.

The Kinetic model shows that for large field-aligned current densities, the electron distributions are non-Maxwellian. The electron distributions have a suprathermal tail due to runaway electrons. Therefore the threshold for e.g. the ion-acoustic instability is modified. We will compare the stability thresholds obtained with our kinetic model and with a fluid model.