The coupling between the daytime ionosphere and the overlying magnetosphere is investigated theoretically. Steady state plasma densities and fluxes are computed in the height range 150--3000 km from continuity and momentum equations for O+ and H+ ions. It is found that O+ densities near the F2 layer peak are remarkably insensitive to plasma densities in the overlying magnetosphere, in sharp contrast to the nighttime results reported earlier (Park and Banks, 1974). Plasma flow out of the daytime ionosphere increases with decreasing pressure in the magnetosphere, but the upward H+ flux reaches a well-known saturation limit before it can significantly reduce O+ densities in the F2 layer. Conversely, if the magnetospheric pressure is sufficiently high, plasma can flow downward into the sunlit ionosphere. However, in both cases the influence of such flow on ionospheric densities is not large owing to the dominating effect of photo-ionization within the F region. In the top side ionosphere is found that H+ densities and the H+-O+ transition height depend strongly on plasma densities in the overlying magnetosphere. It is concluded that the daytime plasmapause should be associated with the H+ trough in the top side ionosphere but not with the O+ trough within the F2 layer. In contrast, the nighttime plasmapause should produce a trough in both H+ and O+ densities. It is also concluded that plasma outflow cannot be responsbile for depressions in NmF2 observed in sunlit regions during ionospheric storms. However, it appears that changes in neutral gas composition or the O++N2 reaction rate can easily account for such depressions.