  We present a simple (stationary) mechanism capable of generating the auroral
downward field-aligned electric field {that is} needed for {accelerating the
ionospheric electron component up into the magnetosphere and confining the
ionospheric ions at low latitudes (as is required by observation of an
ionospheric cavity in the downward auroral current region). The lifted
ionospheric electrons carry the downward auroral current. Our model is based on
the assumption of collisionless reconnection in the tail current sheet. It
makes use of the dynamical difference between electrons and ions in the ion
inertial region surrounding the reconnection {\sf X}-line which causes Hall
currents to flow. We show that the spatial confinement of the Hall magnetic
field and flux to the ion inertial region centred on the {\sf X}-point
generates a spatially variable electromotive force which is positive near the
outer inflow boundaries of the ion inertial region and negative in the central
inflow region. Looked {at} from the ionosphere it functions like a localised
meso-scale electric potential.} The positive electromotive force gives rise to
upward electron flow from the ionosphere {during substorms (causing `black
aurorae')}. A similar positive potential is identified on the earthward side of
the fast reconnection outflow region which has the same effect, explaining the
observation that auroral upward currents are flanked from both sides by narrow
downward currents. keywords{Field-aligned auroral currents, parallel fields,
Hall field in reconnection, substorms}
