Isis 2 electron density profiles of the topside F region obtained during December 1971 have led to a detailed examination of the ionosphere's ‘main trough’ region under an optimum set of geophysical conditions. A total of 30 individual satellite passes during a 21-day period were used to determine the trough's characteristic features under the conditions of midwinter in the northern hemisphere, midnight local time, and very low magnetic activity. The morphological description obtained from this rigidly consistent data base is termed the ‘baselevel trough.’ The latitudinal profile of the trough at hmax over the 40°–80° corrected geomagnetic latitude range shows that while the equatorward edge and the trough minimum are wide features (7° and 10°, respectively), the poleward wall is sharp (∼1½°). The equatorward edge and the trough minimum tend to lose their distinct identities toward higher altitudes, while the magnitude of the poleward wall enhancement remains nearly constant at 2.15 from 450 to 950 km. The trough's poleward wall is found 1°–2° lower in latitude than the statistical position of the equatorward edge of the diffuse aurora. Ratios of topside densities to the peak density reveal that the shape of the Ne(h) profiles is constant from mid-latitudes to the trough minimum. Within the minimum a relatively rapid change in topside shape occurs which marks the light ion depletion region characterized by high H+/O+ transition heights. The northern terminus of the transition height enhancement agrees with the statistical position of the magnetospheric plasmapause. Estimates of plasma temperature (Te + Ti) show only modest enhancements within the trough minimum.