Observations of enhanced ac electric field noise about Pioneer Venus periapsis are shown to be related to spacecraft-generated impact ionization of the ambient CO2. The frequency of the electric field noise is found to peak in the vicinity of the CO+2 ion plasma frequency and to closely follow the form of the neutral CO2 density profile. When the electric field noise in all channels is normalized by the square root of the CO2 number density, the ratio is constant. Since the impact electron density ne, measured by the Pioneer Venus Langmuir probe, is observed to scale directly with the neutral CO2, the growth of the electric field amplitude is found to be linear in time with a growth rate proportional to the CO+2 ion plasma frequency. On the basis of these results the impact ionization-driven instability is shown to be the ion acoustic instability. This contrasts with the impact ionization process at earth where the lower beta plasma allows the modified two-stream instability to develop rather than the ion acoustic instability. Implications for the lack of observations by Pioneer Venus or reflected-O+-driven instabilities, as have been proposed for the space shuttle, are discussed.