The longitudinal variations of the topside ionosphere at low latitudes observed by the Hinotori satellite during equinoctial periods at high solar activity are studied using the Sheffield University plasmasphere-ionosphere model (SUPIM). The model values show that both the neutral wind and E¿B drift velocities make important contributions to the observed longitudinal variations in the topside ionosphere. The displacement of the geographic and magnetic equators and the magnetic declination angle, which give rise to conjugate-hemisphere differences in the neutral wind in the magnetic meridian, are the principal causes of the observed north-south asymmetries in the electron density about the magnetic equator. A comparison of the modeled and observed electron densities shows that the modeled longitudinal variations are, in general, in qualitative agreement with the observations when the neutral winds are taken from the HWM90 thermospheric wind model. Improved agreement in the magnitudes is achieved if HWM90 is modified so that at low latitudes (1) the eastward component of the zonal wind is increased during the day and decreased at night and (2) the diurnal variations of the meridional wind in the northern hemisphere at eastern longitudes and the equatorward wind at around midnight at western longitudes are reduced. The model reproduces the observed longitudinal variations in the development of the equatorial peak electron density during the day and in the equatorial trough and associated crests during the afternoon and postsunset periods. The trough and crests are most prominent at around 2000 LT, where the crest-to-trough ratio varies from about 1.15 at eastern longitudes to about 2.0 at western longitudes. Model calculations show that the longitudinal differences of these features can arise from a longitudinal variation in the vertical E¿B drift velocity. ¿ American Geophysical Union 1996