A numerical model of the dayside aurora

Abstract

A time‐independent model of the dayside auroras is presented within the framework of the pressure‐gradient‐driven mechanism for the generation of the region 1 field‐aligned current (FAC) in the low‐latitude boundary layer (LLBL). Either of two types of the dayside aurora, the bright spots (BSs) or the radially aligned arcs (RAs), arises from spatial modulation of the inner boundary of an LLBL particle population, but the respective source populations have different contents. BSs are produced when the boundary of a majority of the LLBL particle population is modulated. RAs are produced by large‐amplitude modulation occurring on a sharp boundary of a small fraction of the LLBL particle population. The required flux tube content, δN, of the source population of RAs is roughly 10% of that of the ambient population, if the scale length for ∇δN at the inner boundary is comparable to the magnetopause thickness. Using the 1989 Tsyganenko model, the FAC densities from the perturbed inner boundary of the particle population are evaluated and the energy fluxes of electrons precipitating to the ionosphere are then calculated on the basis of the Knight current‐voltage relation. This model can demonstrate the characteristic patterns of the RAs as well as the BSs. It also predicts that (1) both types of aurora with high luminosity as well as inverted‐V structures appear more frequently in the afternoon than in the morning, (2) this morning‐afternoon asymmetry is more pronounced for the case of BSs than RAs, and (3) during periods of northward interplanetary magnetic field, RAs can be distributed over a large area of the polar cap. Now the MLT‐dependent features of the dayside auroras, the appearances of “hot spot” and “warm spot,” and the large‐scale region 1 FAC generation can be understood systematically in view of the distortion of the (average) inner boundary of an LLBL particle population (i.e., nonalignment of the boundary with the average magnetic drift direction).