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Quantitative models are developed for representing the global distribution of the average magnetic field produced by the region 1 and 2 Birkeland current systems. The problem is solved in four following steps: (1) constructing a realistic tilt‐dependent model of the Birkeland current sheets, based on the formalism of Euler potentials, (2) numerically computing their field at a large number of points within the modeling region, (3) finding a best‐fit analytical approximation for that field, and (4) adding a current‐free shielding field which confines the Birkeland field within the model magnetopause. At low altitudes, the model field‐aligned currents reach the ionosphere along eccentric ovals, which fit the observed region 1 and 2 zones of Iijima and Potemra, and they continue there as horizontal currents. At larger distances, the nightside region 1 currents map to the plasma sheet boundary layer and are then diverted toward the tail flanks, while currents in the dawn‐dusk and dayside sectors connect directly to the higher‐latitude magnetopause. The region 2 current closes azimuthally near the equator, forming a spread‐out partial ring current system. The described approach allows a great flexibility in the geometry of the Birkeland currents, making it feasible to infer their properties from spacecraft data.
Tsyganenko et al. (Sun,) studied this question.