Observations of electron density and plasma drift velocity made during noon, June 25, 1990, to noon, June 27, 1990, by the middle and upper atmosphere (MU) radar at Shigaraki, Japan, are investigated by comparing them with values modeled by the Sheffield University plasmasphere-ionosphere model. The model calculations, with inputs for the E¿B drift and neutral wind velocities derived from the observed plasma drift velocities, give diurnal variations for the electron density profile in reasonable accord with the observations and show that at the magnetic latitude of the MU radar (25 ¿N) the effects of the west-east electric field on the electron density profile are comparable to the effects arising from the neutral wind. The observed feature that the F-region electron density is lower at around noon than during the morning and presunset periods results mainly from the effect of the daytime poleward neutral wind. At night the observations show an increase in the F2-layer peak electron density accompanied by a lowering of the F layer. This feature is shown to be caused by the postsunset eastward electric field which gives rise to a plasma flux from the plasmasphere to the ionosphere. It is found that the effects arising from the electric field are different from those arising from its replacement by an equivalent neutral wind derived from the E¿B drift velocity.¿ 1997 American Geophysical Union