The spatial distribution of electric fields, conductances, and currents of steadily drifting medium-scale (15--50 km) arcs in the evening sector (20--23 magnetic local time (MLT)) is obtained from European Incoherent Scatter Radar (EISCAT) and optical ground-based measurements. The current systems of stable arcs residing in the northward convection electric field region show a consistent pattern: currents flow downward on the equatorward side of the arcs, then poleward, and upward from the arcs. In one event where the arcs are located in a region of convection reversals, the current pattern is more complicated. Most of the arcs are associated with an enhanced northward-directed electric field region on the equatorward side of the arc, colocated with downward field-aligned currents (FACs) and suppressed E and F region electron densities. The width of the region of the enhanced electric field is one to four times the width of the arc. In some cases, the electron density reduction is so pronounced that the region can be described as an auroral ionospheric density cavity. The electrostatic magnetosphere--ionosphere coupling model of arcs predicts that the width L of an arc is related to the ionospheric Pedersen conductance ΣP and the field-aligned conductance K by $L = sqrt{Sigma_{P}/K}$. This study shows that stable medium-scale arcs in the evening sector obey this equation. A value of K = 2 ¿ 10-8 S m-2 is obtained for 15--35 km wide arcs. It is argued that the large value of the field-aligned conductance cannot be interpreted in terms of the adiabatic theory. Possibly the high value of K results from nonadiabatic processes acting on the current-carrying electrons.