Model predictions of the diurnal variations of plasma convection velocities and electron densities in the high-latitude ionosphere were compared with Millstone Hill incoherent scatter radar observations for an equinox day on which there was moderate magnetic activity. On the observation day, three major morphological features were evident at 500 km, includding a dayside high density region, a nightside mid-latitude trough, and a region of slightly enhanced densities in the auroral zone. Although the dayside high density region was due to solar EUV radiation, it was not symmetrical about local noon (1000-1900 LT sector) owing to the effect of horizontal transport. The nightside mid-latitude trough was the deepest, the widest, and reached its most equatorward latitude in the morning sector. The model was able to reproduce these two features quite accurately. In the dusk sector, the trough ws filled in, and its latitudinal extent was restricted by a discrete auroral arc, a feature not included in the ionospheric model. Except for this arc region, the enhanced electron densities in the auroral zone were adequately described by the average precipitation fluxes used in the model. The observed plasma drift velocities were consistent with a two-cell, asymmetric convection pattern with enhanced flow in the dusk sector. Outside the polar cap, the fall-off of the magnetospheric potential with latitude was proportional to the inverse of the sine of colatitude to the fourth power. The convection pattern employed in the model included these features and had a 60 kV cross-polar-cap potential. Efforts to reproduce the observed behavior by using a larger cross-polar-cap potential (90 kV) or a symmetric pattern are also presented. These were generally less successful and demonstrate the sensitivity of the morphology of the F region at high latitudes to the convection process.