On the lower altitude limit of the Venusian ionopause

It has been observed from the plasma experiments on the Pioneer Venus Orbiter that the altitude of the upper boundary of the ionosphere (viz ionopause) decreases in response to increasing solar wind dynamic pressure. However, at pressures above about 2.5 × 10−8 dynes cm−2, the further decrease in the ionopause height is rather small. Following the model of Cloutier et al., [1969] we suggest that during high solar wind conditions, when the ionopause is formed at lower altitudes, the solar wind induces vertical and horizontal flows which sweep away the ionospheric plasma that is produced locally by photoionization. As a result a disturbed photo‐dynamical ionosphere is formed which has the scale height of the ionizable neutral constituent. We show that such a photo‐dynamical ionosphere is observed at the subsolar ionopause under these conditions. As a consequence of this interaction, the ionopause altitude is observed to follow the small scale height of the ionizable species, atomic oxygen, showing only small changes with solar wind pressure.