Linkage of field-aligned to horizontal ionospheric current, is a significant aspect in general coupling mechanisms between the magnetosphere and ionosphere. The magnetosphere generates field-aligned current that drives horizontal ionospheric conduction currents. This research provides a detailed, quantitative modelling of the ionospheric and field-aligned currents in the area, or band, covered by the higher latitude set of field-aligned (region-1 Birkeland) current, with the assumption that Birkeland current is uniformly distributed there. Current conservation implies that field-aligned current is balanced by the divergence of ionospheric current. Solution of the divergence equation, .with the area poleward of the band taken to be an insulator, appropriate boundary conditions (inferred from lower latitude modelling), and a local-time dependent conductivity allows horizontal ionospheric current- and electric field, and Birkeland current to be deduced. The new model combines with existing detailed models of the lower latitude region, forming an effectively global computer model. Application of the theoretical model to the 19 September 1976 substorm event yields many interesting features. Among these are a westward electrojet extending from about 19LT past midnight to about 9LT, with a maximum strength of -11x1^5 amp in post-midnight hours. TMs is in reasonable agreement with, observational data for substorm conditions. The calculated eastward electrojet is comparable to (but slightly smaller than) the eastward electrojet in the region just; equatorward of the one treated here, although the lower latitude electrojet; extends to ~23LT. Hear midnight, the westward electrojet modelled here is ~3 times larger than the westward electrojet in the sub-polar region. Theoretical calculations for the 19 September 1976 event using the present model indicate that electric field reversals fall within the studied region in four out of six satellite passes (corresponding to those passes when concentration of Birkeland current is toward the equatorward edge of the band). Although detailed agreement with data is modest, this model agrees with data on a crucial characteristic. In both the present; theory and data, the electric field reversal usually occurs poleward of most of the region-1 current, which means that most region-1 current flows into or out of regions of sunward convection. Since currents flowing in the investigated high-latitude region often have dominant effects on ground magnetic variations (even on magnetograms of low-latitude observations), agreement between theoretical and observed magnetograms for the modelled substorm event is greatly improved by inclusion of the large part of the westward electrojet that flows in the higher latitude region. Contributions of the present work to calculations of Joule heating of the upper atmosphere are significant, with Joule heating poleward of the equatorward edge of region-1 currents comprising ~3% of the total in the modelled event of 19 September 1976. Values for the strength of the electrojet and amount of Joule heating predicted from a simple Cowling conductivity band picture (with no radial current flowing into the band) agree with the present model to within ~2%.