Plasma density measurements from a polar-orbiting Defense Meteorological Satellite Program satellite at 840 km have shown the existence of significant plasma depletions near the magnetic equator in the postsunrise hours. The phenomenon is observed regularly during magnetically quiet times near the equinoxes. The magnitude of the depletion depends upon longitude and the time elapsed since sunrise in the lower F region. It is significantly less pronounced or absent near the solstices. During magnetically active periods the low-latitude topside ionosphere is modified due to vertical E¿B plasma drifts in the predawn sector. The quiet time equinox observations are interpreted in terms of a diurnal flux tube filling model. It is suggested that the observed equatorial depletions result from a pair of plasma fronts that are moving upward from conjugate ionospheres and that are associated with the early stages of the postsunrise refilling process.