Observations from the Pioneer Venus orbiter magnetometer reveal the presence of large-scale magnetic fields in the dayside ionosphere during or after periods of high solar wind dynamic pressure. Various hypotheses have been proposed concerning the spatial and temporal evolution of these field structures. Here we examine the hypothesis that the field is produced by a diffusion/convection process rather than by currents driven by electric fields resulting from the solar wind interaction. Dynamic pressure variations occur on various time scales at Venus, producing transient and quasi-steady magnetization features. A one-dimensional diffusion/convection calculation is performed, using typical iono-pause field and pressure values for a variety of altitudes. The Venus ionopause is considered to be the altitutde at which ionospheric thermal pressure is equal to the magnetosheath magnetic pressure, which in turn is well correlated with the normal component of solar wind dynamic pressure. A subsolar model of downward plasma velocity in the 140--290 km range is used. The calculated growth of the ionospheric magnetic field, and the resulting quasi-steady altitude profiles, compare favorably with the observed profiles. The majority of the observed magnetic structures are best explained as the quasi-steady effects of the prevailing solar wind dynamic pressure, which determines the altitude and magnetic field strength of the ionopause boundary.