Mechanisms of auroral particle acceleration

The paper deals with the conditions enabling auroral acceleration, the coupling of the various processes involved, and the feedback mechanisms on the source of energy and mass. Particular emphasis is given to the energy flow, for which four different paths are identified, whose efficiencies and time-scales are being compared. Furthermore, the necessary ingredients of auroral arc formation are discussed, among which current filamentation and dynamical feedback on the source plasma are still largely unknown elements. The paper closes with a conjecture about the origin of auroral folds and curls, which is attributed to the evacuation process of the topside ionosphere, the ensuing modification of the electric potential distribution, generation of small-scale shear flows and their instability.