The dayside Venus ionosphere. II - Combined numerical model of ion and neutral composition above 120 km

The median vertical profile of ion densities for the dayside Venusian ionosphere obtained by the orbiter retarding potential analyzer (ORPA) is simulated by one-dimensional model calculations. The model includes both neutral and ionic chemistry, eddy and molecular diffusion for neutral constituents, and ion-plasma diffusion for ionic constituents. The electron and ion temperatures measured by the ORPA are used to calculate the plasma diffusion coefficients and scale heights for ions. The predicted O2(+) densities below about 200 km agree particularly well with observations by the ORPA, but the model values are significantly less than those measured by the orbiter ion mass spectrometer. The observed ion composition is interpreted in terms of densities of the neutral atmosphere and its composition.