Magnetic field lines, electric fields and equipotentials have been mapped throughout the magnetosphere in the vicinity of strong Birkeland currents. It was found that a uniform electric field at either the ionospheric or the equatorial end of a field line can map to a highly structured field at the other end if strong Birkeland currents are located nearby. The initiation of sheet currents of the region 1 - region 2 scale size and intensity resulted in magnetic field line displacements of about 1/2 hour in local time between equatorial and ionospheric end points. As a result, a uniform dawn to dusk electric field at the equator mapped to an ionospheric electric field with strong inward pointing components (a southward component at the northern current sheet and a northward component at the southern current sheet) in the dusk hemisphere. Similar distortions were produced by Birkeland currents associated with narrow east-west-aligned auroral arcs.

A specific model for the auroral current system, based on ionospheric measurements during a large substorm, was used to study effects seen during disturbed periods. An iterative procedure was developed to generate a self-consistent current system even in the presence of highly twisted field lines. The measured ionospheric electric field was projected to the equatorial plane in the presence of the model Birkeland current system. It was concluded that such projections are more reliable if it is possible to start with an approximate equipotential structure in the ionosphere rather than attempting to project individual electric field point measurements. Several physical processes were seen to influence ionospheric and equatorial electric fields, and the associated plasma convection, during a substorm. Electric fields well tailward of the current diversion region near the equator and well poleward of the auroral display in the ionosphere were controlled by processes taking place at high altitude.

Measurements are consistent with the presence of a roughly dawn to dusk equatorial electric field in this region. The diversion of cross tail current to the ionosphere takes place at an apparent westward traveling surge (WTS) and at the eastward end of the substorm current system. Ionospheric polarization appears to control the electric field pattern within this region. Two processes are important in producing spiral patterns near the strongest Birkeland current densities, at the WTS. Parallel electric fields and an inward pointing perpendicular electric field above the WTS produce circular plasma drift in the topside ionosphere. In addition, magnetic field distortion produces spiral magnetic field lines and a characteristic winding/unwinding plasma motion in the same region. ¿ American Geophysical Union 1989