Ion dynamics in the Venus ionosphere

Dynamics play an important role in defining the characteristics of the Venus ionosphere. The absence of a significant internal magnetic field at Venus allows the ionization to respond freely to gradients in the plasma pressure. The primary response to a gradient in plasma pressure is the nightward flow of the ionization away from a photoionization source on the dayside. The flow is approximately symmetric about the Sun-Venus axis and provides the source of O⁺ that maintains the nightside ionosphere during solar maximum. Modelling efforts have generally been successful in describing the average nightward ion velocity. Asymmetric and temporally-variable flow is measured, but is not well described by the models. Departures from axially-symmetric flow described in this paper include ionospheric superrotation at low altitudes and an enhanced flow at high altitude at the dawn terminator. Variability that is the result of changes in the ionopause height induced by changes in solar wind dynamic pressure is especially strong on the nightside. Ion flow to the nightside is also reduced during solar minimum because of a depressed ionopause.