The equatorward boundary of the diffuse auroral zone, as viewed via the 6300-¿ (O(1D)) emission line, has been examined by using ground-based photometer data. These observations are supplemented with in situ measurements of ionospheric parameters by satellite-borne instrumentation within the photometer field of view. We find high spatial correlation among 6300-¿ emission, low-energy (E<1 keV) electron precipitation, and the high-latitude recovery of ambient electron density which constitutes the poleward cliff of the main ionospheric trough. Time resolution afforded by this ground-based potical method is especially suited for monitoring the ionospheric signature of this low-energy particle precipitation over broad longitudes. A model is developed of the statistical equatorward boundary of the diffuse aurora as a function of magnetic local time and magnetic activity (as monotored by Kp index). The local time dependence of this boundary exhibits a latitudinal morphology similar to that of the earthward equatorial plasma sheet-that is, asymmetric about local midnight and skewed toward lesser invariant latitude values within the morning sector.