Morphology of the dayside ionosphere of Venus: Implications for ion outflows

The nightside ionosphere of Venus is formed mostly by day-to-night transport of ions below the ionopause, with a small contribution from precipitation of energetic electrons from the wake. This nightward flux of ions should result in dayside ionospheres that are characterized by smaller electron density scale heights at high altitudes than those that are characteristic of diffusive equilibrium. In order to determine both the maximum possible values of the upward fluxes and the upward fluxes implied by comparison of our computed ion and electron density profiles to measurements, we have constructed more than 60 models of the high solar activity Venus ionosphere for upward velocity boundary conditions ranging from 0 to 3 x 10(5) cm s(-1). We show that as the upward velocity at the top of the model is increased, the high-altitude densities of the atomic ions decrease, as do the total ion and electron densities. The maximum upward O+ flux that we derive is similar to 5.3 x 10(8) cm(-2) s(-1). By comparing our model electron or total ion density profiles to those from Pioneer Venus data, we have derived upward fluxes of O+ that range from 1.8 x 10(8) to 2.5 x 10(8) cm(-2) s(-1) with an average of 2.1 x 10(8) cm(-2) s(-1). This value is roughly equal to estimates of the transterminator ion flux and to estimates of the downward fluxes of ions over the nightside. From this, we infer that the escape flux due to ion outflow is small. We conclude that the upward fluxes of ions that we have inferred from the morphology of the ionosphere contribute mostly to the day-to-night flow and to the formation of the nightside ionosphere and are much less than their source-limited values.