A 2-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 horiz ontal plasma transport. The continuity and momentum equations are solv ed self-consistently, using the observed plasma temperatures. It is fo und that the vertical velocity is upward (similar to 10 m/s) at high a ltitudes (z > 240 km) in the dayside ionosphere. Below 240 ion, on the other band, the vertical velocity is downward and is basically the sa me as that obtained by one-dimensional models, The results suggest tha t the nightside ionosphere can be maintained by day-to-night plasma tr ansport, as was suggested by previous studies. It is also shown that t he horizontal pressure gradient forces are sufficient to produce the o bserved acceleration of ionospheric plasma at least under some conditi ons. However, the results do not rule out the: possibility that the so lar wind momentum is transported to the ionosphere, which drives the d ay-to-night flow in the Venus ionosphere.