Plasma density profiles in the equatorial electrojet region reveal that the ionization density at 100 km increases by a factor of 2 to 10 from the time of sunset to midnight. In addition, a valley in plasma density centered around 120 km altitude deepens by a factor of 2 to 5 during the same period. These two aspects are explained on the basis of the dynamical features peculiar to the electrojet region. It is demonstrated that the vertically downward motion of positive ions due to Pedersen currents is very important in the redistribution of ionization. The valley is shown to be formed at 120 km, where the vertical ion velocity maximizes.