The geometry of the Venus nightside ionosphere is reexamined on the basis of plasma and magnetic field data obtained with the Pioneer Venus Orbiter (PVO) and the Mariner 5 spacecraft. There is evidence indicating that the shocked solar wind that streams over the magnetic polar regions of the Venus ionosphere experiences a sharp expansion and also a strong deceleration to speeds much smaller than those present in the outer ionosheath. It is argued that the deficiency of solar wind momentum that is inferred from the PVO and Mariner 5 measurements along the flanks of the Venus ionosheath may be responsible for the erosion of the polar upper ionosphere in plasma channels that extend downstream from the magnetic polar regions. The erosion of the Venus ionosphere is mostly produced within those channels and represents the main source of material delivered to the Venus wake. The configuration that is proposed accounts for the PVO observation of plasma clouds and ionospheric holes in the nightside hemisphere and leads to a mass loss smaller than that expected from a global erosion. The formation and evolution of the ionospheric polar channels should also be important in the solar wind interaction and erosion of the Mars ionosphere. ¿ 2001 American Geophysical Union