The post-terminator ionosphere of Venus

We have modeled the near and post-terminator thermosphere/ionosphere of Venus with a view toward understanding the relative importance of EUV solar fluxes and downward fluxes of atomic ions transported from the dayside in producing the mean ionosphere. We have constructed one-dimensional thermosphere/ionosphere models for high solar activity for seven solar zenith angles (SZAs) in the dusk sector: 90 degrees, 95 degrees, 100 degrees, 105 degrees, 110 degrees, 115 degrees and 125 degrees. For the first 4 SZAs, we determine the optical depths for solar fluxes from 3 A to 1900 A by integrating the neutral densities numerically along the slant path through the atmosphere. For SZAs of 90 degrees, 95 degrees, and 100 degrees, we first model the ionospheres produced by absorption of the solar fluxes alone; for 95 degrees, 100 degrees, and 105 degrees SZAs, we then model the ion density profiles that result from both the solar source and from imposing downward fluxes of atomic ions, including O+, Ar+, C+, N+, H+, and He+, at the top of the ionospheric model in the ratios determined for the upward fluxes in a previous study of the morphology of the dayside (60 degrees SZA) Venus ionosphere. For SZAs of 110 degrees, 115 degrees and 125 degrees, which are characterized by shadow heights above about 300 km, the models include only downward fluxes of ions. The magnitudes of the downward ion fluxes are constrained by the requirement that the model O+ peak density be equal to the average O+ peak density for each SZA bin as measured by the Pioneer Venus Orbiter Ion Mass Spectrometer. We find that the 90 and 95 SZA model ionospheres are robust for the solar source alone, but the O+ peak density ill the "solar-only" 95 SZA model is somewhat smaller than the average value indicated by the data. A small downward flux of ions is therefore required to reproduce the measured average peak density of O+. We find that, on the nightside, the major ion density peaks do not occur at the altitudes of peak production, and diffusion plays a substantial role in determining the ion density profiles. The average downward atomic ion flux for the SZA range of 90-125 degrees is determined to be about 1.2 x 10(8) cm(-2) s(-1). (C) 2011 Elsevier Inc. All rights reserved.