The computer simulation scheme for calculating the electric fields and currents in the global ionosphere in relation to field-aligned currents for quiet periods that we previously developed is extended to disturbed conditions. In a series of numerical simulations representing various substorms we take into account our current knowledge of characteristics of auroral enhancement in the ionospheric electric conductivities as well as changes in the intensity and location of field-aligned currents. The possible effects of these variable parameters in producing the observed complexity of the electric field and current patterns are studied, with the following main results: (1) The conductivity inhomogeneity in the auroral belt can reproduce the observed local time dependence of the electric fields and currents. (2) The amount of the 'return' currents from the auroral electrojets flowing within the ionosphere is small, an indication that the major part of the electrojets is connected directly to the field-aligned currents. Therefore the ground magnetic perturbations in the polar cap and also in the middle and low latitudes during substorms can be ascribed mainly to the magnetic effects of the field-aligned currents. (3) Even when the intensities of the field-aligned currents and the associated auroral electrojets do increase considerably during substorms over their values of quiet times, the total potential difference across the polar cap does not increase very much, because of a simultaneous increase in the ionospheric conductivities along the nightside auroral oval. (4) No special field-aligned current system in the Harang discontinuity region is required to reproduce this feature. (5) The distribution pattern of the equivalent ionospheric current differs significantly from that of the real ionospheric current vectors. The maximum electrojet intensity seen in the equivalent current system is weaker than that of the real auroral electrojets.