The ion mass spectrometer on board the Isis 2 satellite has provided detailed information on absolute concentrations of the principal ion species near the fixed altitude of 1400 km. This analysis describes the latitudinal and longitudinal variations if such concentrations for dip latitudes below 50¿, in the afternoon (14-17.5 hours) and predawn (02-05.5 hours) local times, during undisturbed seasonal periods characteristic of late April/early May. The predawn level ionization at 1400 km is comparable to that observed in the afternoon. Although H+ is the dominant nighttime ion, the principal predawn longitudinal variations relate to the seasonal asymmetry of O+ and to the shape and extent of the low-latitude He+ depressions. Two distinct classes of daytime ionospheric behavior are observed. Certain longitude regions are distinguished by broad summer plateaus of O+, separated by sharp equatorial gradients from concentration depressions at winter latitudes. The major species in the summer and winter hemispheres are O+ and H+, respectively, while He+ exhibits the characteristic equatorial trough. Alternatively, a region of longitude has been observed where the seasonal variation of O+ is less pronounced and where He+ possesses a symmetrical peak at the dip equator. Both direct photoionization processes and neutral winds are considered capable of producing these daytime effects. The data confirm the strong solar-geomagnetic seasonal control over the topside ion distribution. More comprehensive comparisons of ion profiles between different longitudes and seasons are possible when the magnetic dip equator is adopted as reference for the solar direction and hence for the effective ionospheric season.