Various ionospheric electrodynamic parameters for the period July 23--24, 1983, are calculated by using ground magnetic records from a total of 88 stations in the northern hemisphere. For this purpose, an ''instantaneous'' conductance distribution deduced from the DMSP-F6 bremsstrahlung X ray image data is utilized. Since the conductance distribution is, for the first time, completely independent of ground magnetic data, it is a unique opportunity to examine some of the inherent ambiguity in the magnetogram-inversion technique based on a statistically derived conductance model. Several important conclusions of this study are as follows: (1) The poleward portion of the westward electrojet in the morning sector is dominated by the electric field, while its equatorward portion is dominated by the ionospheric conductance. Although less definite, a reverse trend seems to pervade the eastward electrojet region in the dusk sector. (2) During a quiet or moderately disturbed period, the major electric potential pattern is roughly circumscribed by the auroral zone conductance belt with the subaural zone being a substantially lower electric field region. (3) The global pattern of the equivalent current system resembles the electrical potential distribution during summer conditions.

It may thus be possible to use the equivalent current system as a reasonable approximation of the electric potential distribution in studying the global pattern of magnetospheric convection for prevailing sunlit conditions. (4) The electric potential distribution consists generally of a smooth and well-defined two-cell convection pattern without any significantly localized structure. (5) A sunward convection flow is clearly identified over the polar cap region during strongly northward IMF periods. The multicell nature of the convection pattern is still unclear. (6) During strongly northward IMF periods, significant currents and Joule dissipation are observed in the polar cap region, indicating that the magnetosphere is far from its ground state. (7) The regions of intense Joule heating are generally confined to relatively narrow belts along the auroral electrojets, with the major heating region in the westward electrojet region shifted poleward and the one in the eastward electrojet region shifted equatorward. The Joule dissipation rate is relatively low in the local midnight sector. (8) The presently available statistical conductance models can be used, as a first approximation, to study global-scale polar ionospheric electrodynamics. However, the fact that the statistical models cannot simulate an instantaneous situation severely restricts their usefulness for studying the spatial and temporal variations of individual substorms. ¿ American Geophysical Union 1989