The midlatitude ionosphere has been known for decades to exhibit correlated (or anticorrelated, depending on the coordinate system) perpendicular and parallel meridional ion velocities. This correlation is indicative of horizontal plasma flow, despite the highly magnetized character of the midlatitude F region. This coupling of the perpendicular and parallel dynamics is a consequence of extremely weak atmosphere-ionosphere coupling and the strong tendency for the neutral atmosphere to flow horizontally. Three physical mechanisms have previously been identified as effecting this coupling: ion drag, the F region dynamo, and enhanced gravity-driven diffusion. We point out some difficulties with one of these processes and present two additional mechanisms, wind-driven ionospheric equilibrium and gravity wave seeding of plasma instabilities, which can also result in horizontal plasma flow. Although these additional mechanisms further complicate the observational distinction between the possible mechanisms, we argue that an appropriately clustered array of instruments can still distinguish which mechanism is responsible for any particular geophysical occurrence.