MODELING THE GLOBAL MAGNETIC-FIELD OF THE LARGE-SCALE BIRKELAND CURRENT SYSTEMS

Quantitative models are developed for representing the global distribu tion of the average magnetic field produced by the region 1 and 2 Birk eland current systems. The problem is solved in four following steps: (1) constructing a realistic hit-dependent model of the Birkeland curr ent sheets, based on the formalism of Euler potentials, (2) numericall y computing their field at a large number of points within the modelin g region, (3) finding a best-fit analytical approximation for that fie ld, and (4) adding a current-free shielding held which confines the Bi rkeland held within the model magnetopause. At low altitudes the model field-aligned currents reach the ionosphere along eccentric ovals, wh ich fit the observed region I and 2 zones of Iijima and Potemra, and t hey continue there as horizontal currents. At larger distances the nig htside region I currents map to the plasma sheet boundary layer and ar e then diverted toward the tail flanks, while currents in the dawn-dus k and dayside sectors connect directly to the higher-latitude magnetop ause. The region 2 current closes azimuthally near the equator, formin g a spread-out partial ring current system. The described approach all ows a great flexibility in the geometry of the Birkeland currents, mak ing it feasible to infer their properties from spacecraft data.