Electron spectrograms from 351 passes of the Isis 1 and 2 satellites were utilized to study statistically the effects of the interplanetary magnetic field (IMF), substorm activity, and the earth's dipole tilt angle on the latitude of the equatorward boundary of the nightside (2000--0400 magnetic local time) 'instantaneous' auroral oval. The boundary location (in invariant latitude) of the instantaneous oval at hourly local time intervals was identified in terms of the equatorward boundary of the diffuse >100-eV electron precipitation. The following characteristics were noted: (1) The north-south component (Bz) of the IMF plays the dominant role in controlling the motion of this boundary. The invariant latitude of the boundary is shown to shift by approximately ¿4¿ depending on the direction of the IMF (northward and southward, respectively) relative to its position corresponding to Bz=0. This indicates an inward motion of the associated boundary in the magnetotail by about 5 earth radii when the IMF changes its direction from northward to southward with large magnitude. There is a significant difference in the amount of the shift between the evening and morning sectors: i.e., for the same decrease in Bz value the boundary moves more equatorward in the morning sector than in the evening sector. When the obtained oval particle boundary was projected onto the equatorial plane of the magnetotail along magnetic field lines, good agreement was found between the projected boundary and the drift boundary (the Alfv¿n layer) of low-energy electrons in the presence of the dawn-dusk electric field. Thus this agreement gives new evidence showing that the diffuse electron precipitation that produces the diffuse aurora originates near and at the inner boundary of the plasma sheet. (2) Substorm activity seems to have a separate role in determining the latitude of the equatorward boundary of the nightside auroral precipitation region. The boundary during substorms periods is statistically found to be 2¿--3¿ lower in invariant latitude than that during quiet times. Even a simple classification into quiet and disturbed conditions improves the accuracy with which the auroral oval location can be inferred. By combing the IMF effect and the substorm effect it is indicated that the boundary is located at the lowest latitudes when a substorm takes place during a southward IMF with large magnitude, whereas the boundary is located in the highest latitudes when the IMF has a northward component during quiet times. (3) The equatorward boundary of the nightside auroral oval is located at higher latitudes in the winter hemisphere than in the summer hemisphere, although this effect of the earth's dipole tilt is usually smaller than the effects of the IMF and substorm activity.