Simultaneous observations on April 28, 2001 by Cluster and DMSP-F14 reveal a stable discrete auroral arc and fluxes of field-aligned accelerated electrons and ions coincident with a magnetospheric plasma interface at an altitude of 4.5 RE in the dusk sector. We compare satellite data with a quasi-stationary magnetosphere-ionosphere coupling model based on a Vlasov solution for the magnetospheric generator. The model provides a self-consistent magnetospheric electric potential matching the Cluster observations. The ionospheric potential is derived from the current continuity equation and gives a field-aligned potential drop and a flux of precipitating energy in agreement with the DMSP data. Model results and data analysis suggest a quasi-stationary field-aligned acceleration of auroral electrons and ions with a magnetospheric generator. We associate the generator with the convergent perpendicular electric field at the interface of the plasma sheet boundary layer with the lobe or at the inner edge of the low latitude boundary layer. Copyright 2009 by the American Geophysical Union.