The Acceleration Region of Stable Auroral Arcs

The acceleration region above stable, discrete auroral arcs is reviewed. Substantial observational evidence shows that the acceleration of auroral electrons associated with these arcs is achieved by an electric potential structure above the aurora at altitudes of around 0.5-2 R-E. The morphology, internal structure, and lifetime of the predominantly U-shaped potential structure are discussed, based on observations by a number of spacecrafts. The parallel (to the geomagnetic field) electric field component of the potential structure, which accelerates the auroral electrons, is discussed in terms of relatively recent direct observations. The most important theories for how the parallel electric field is sustained are also described. The altitude distribution predicted from the various theories is compared to observations, and briefly discussed, as well as their relation to each other.