Numerical simulation of global formation of auroral arcs

Abstract

Global simulation of auroral arcs is performed, based on the feedback theory of auroral arcs (Sato, 1978), for a three‐dimensionally coupled ionosphere‐magnetosphere system which includes two pairs of large‐scale Birkeland currents, large‐scale polar cap electric fields, and a day‐night asymmetry of the electron density distribution. Simulation results have shown that auroral arcs are formed in the dark sector of the auroral oval, more preferentially in the evening sector. They usually appear in multiples with a shape elongating in the east‐west direction, each arc being a couple of thousand kilometers in length and 10 to 40 km in width. A pair of small‐scale, upward and downward, Birkeland currents is associated with each arc, the density of which becomes 10 to 200 µA/m² at 110‐km height, and the intensity of the associated electrojet reaches 5 to 20 kA. Each arc is strongly polarized in the direction of the large‐scale north‐south current so that the electric field inside the arc is reduced considerably from its ambient value. What controls the formation of auroral arcs (growing speed, locations, arc width, etc.) is examined in detail. For example, the arc width becomes sharper, as the bounce time of the Alfvén wave decreases. The overall structure of auroras is largely dependent upon the large‐scale Birkeland current, the electric field, and the electron density distribution in the auroral oval.