Several quite dissimilar models have been proposed for the direct interaction of the solar wind with planetary ionospheres, based, respectively, on the assumption of (1) restricted flow into the ionosphere, (2) an interface between the solar wind and the ionospheric plasma, or (3) a magnetic field barrier (‘induced magnetopause’). All three require the existence of a standing bow shock in the solar wind. We review these models, discuss their individual limitations, and consider what observational data would distinguish among t