Dayside ion composition measurements made by the orbiter ion mass spectrometer and the orbiter electron temperature probe on the Pioneer Venus orbiter are used to infer the dominant processes involved in the dynamic response of the Venus ionosphere to the solar wind. The analysis is confined to the topside ionsophere in the vicinity of the subsolar point, where the ionosphere--solar wind interaction is expected to be maximized. Height profiles ofthe ion composition and plasma temperatures in the main body of the topside ionosphere, lying between the ionopause and chemical equilibrium regions, reveal that the ionosphere is in a compressed state. This region of the ionosphere is interpreted in terms of a stationary equilibrium where the compression is derived from the ponderomotive force j¿B. The estimated magnitude of this force is confirmed by the magnetic field measurements made by the orbiter magnetometer.