The structure and variability of ion concentrations, velocities, and electric and magnetic fields in the ionosphere of Venus result from a combination of physical and chemical processes driven by the interaction of the solar wind with the atmosphere of Venus. In this paper we describe these processes and formulate a model describing their interrelationships in a self-consistent manner. We show that the consequences of this model are in agreement with Pioneer Venus observations in the Venus ionosphere. It is shown that the magnetic field configuration of the Venus ionosphere is a steady state global structure which results from a combination of a large-scale ''open'' current system connecting through the ionopause to the postshock solar wind and two small-scale ''closed'' current systems entirely confined within the ionosphere itself and generated by ionospheric flows and shears. A previously detected ionospheric layer in O2 + near 170 km altitude is shown to be a new type of ionospheric layer resulting from transition of solar-wind-driven vertical flow to velocities approaching Mach 1 (sonic flow). Accordingly, this layer has been named the ''sonic flow layer'' or ''S layer.'' It is suggested that this model should apply to other nommagnetic bodies in magnetized plasma flows, such as comets and Jovian satellites. ¿ American Geophysical Union 1987