A two-dimensional model of the Venus ionosphere .1. Unmagnetized ionosphere

A two-dimensional model of the ionosphere of Venus under unmagnetized conditions has been developed in order to study the processes of horizontal plasma transport. The continuity and momentum equations are solved self-consistently, using the observed plasma temperatures. It is found that the vertical velocity is upward (similar to 10 m/s) at high altitudes (z > 240 km) in the dayside ionosphere. Below 240 ion, on the other band, the vertical velocity is downward and is basically the same as that obtained by one-dimensional models, The results suggest that the nightside ionosphere can be maintained by day-to-night plasma transport, as was suggested by previous studies. It is also shown that the horizontal pressure gradient forces are sufficient to produce the observed acceleration of ionospheric plasma at least under some conditions. However, the results do not rule out the: possibility that the solar wind momentum is transported to the ionosphere, which drives the day-to-night flow in the Venus ionosphere.