The low-latitude asymmetric disturbance field, which is so prominent during the developing state of a magnetic storm and which traditionally has been attributed to a dusk-centered partial ring current, recently has been interpreted by Harel et al. (1981) to be the result of incomplete cancellation of their computer-simulated region 1 and 2 Birkeland currents over local time, with net current flowing into the ionosphere at noon and out at midnight. The new interpretation has the advantage of being based on an observed current system rather than on a hypothetical one. We explore the Birkeland current explanation quantitively in terms of a simple analytical model, the details of which are given separately in an appendix. The residual current arises as a result of the divergence of the Hall current at the conductivity discontinuity at the poleward boundary of the auroral oval. This effect rotates the pattern of region 1 Birkeland currents relative to region 2 currents and produces the net current at noon and midnight. The model gives a simple expresion for the amplitude G of the resulting distrubance at the equator in terms of the polar cap potenital &PHgr; and the high-intergated Hall conductivity &Sgr;A H in the auroral oval: G(nT)≈0.017&PHgr;(kV)&Sgr;A H (mhos). The equivalent current pattern for the model is presented and compared to the equivalent current pattern for a similar model developed by Vasyliunas for the substorm. Combining the two patterns in a time-dependent model can account for the observed pattern of phase shifting of the low-latitude asymmetry during substorms.