The main trough in the topside ionosphere has been studied using the thermal positive ion and electron densities measured on 15,000 orbits over a 3-year period (1969--1972) by means of spherical electrostatic analyzers aboard the Isis I and Injun 5 satellites in the 560- to 3600-km altitude range. The trough is found to be a persistent feature at night with an occurrence frequency of approximately 95%. The occurrence frequency decreases to approximately 60% near the dawn-dusk meridian and to approximately 48% near local noon. At altitudes below about 1500 km during quiet to moderate conditions (Kp?3o) the trough equatorward boundary is found at L=3.5¿0.5 near midnight and L=12.5¿1.0 near local noon. The equatorward edge of the trough in the nighttime sector lies near the L shell where the plasmapause has been observed. Near local noon the trough occurs at the equatorword edge of the magnetospheric cleft and is at significantly higher L values than those reported for the dayside plasmapause. The seasonal variation of the trough location at a given local time is negligible except near sunrise. With increasing altitude between 1500 and 3600 km the equatorial boundary of the trough moves to continually lower latitudes during the night hours. The equatorward trough wall becomes a dominant feature of the trough, often extending from 15¿ to 20¿ in width during quiet magnetic periods. The poleward edge of the trough becomes less well marked with increasing altitude, often being defined only by a sharp spike in ionization extending over a few degrees within the auroral zone. At altitudes above 1500 km on the dayside, two independent troughs or density gradients are observed. The trough at high latitudes located at the equatorward edge of the cusp gradually decreases in amplitude with increasing altitude. This trough is tentatively identified as the dayside analog of the high-latitude trough observed on the nightside by Ogo 6. Its development is attributed to the depleting effects of enhanced ion chemical reactions in the presence of convective electric fields. The lower latitude dayside trough, observed between L=2 and 6, results from the partial depletion of flux tubes in the outer plasmasphere.