Data from polar-orbiting satellites have confirmed and existence of field-aligned current flow into and out of the auroral oval. Recently, it has been found that the inward/outward flow exceeds the outward/inward flos in the morning/evening sector. In this paper we present the ground-based signatures of the net field-aligned current flow, and using these signatures, we are able to establish the diurnal variation of the field-aligned currenst penetrating the auroral oval. Information on the diurnal variation of auroral zone electric fields is used to develop a current circuit in which the net upward and downward field-aligned currents are linked by ionospheric currents. In particular, it is believed that net downward current flow across local magnetic noon feeds both the eastward and the westward electrojets, while the region of the Harang discontinuity near local magnetic midnight is the site of poleward-flowing Hall current and westward Pedersen current.