Near‐terminator Venus ionosphere: How Chapman‐esque?

We have modeled the near‐terminator ionosphere of Venus for solar zenith angles χ between 60 and 85° in 5° increments, and from 86 to 90° in 1° increments. The most important neutral densities of the background thermospheres have been adopted from the VTS3 model of Hedin et al. (1983), which is based on densities from the Pioneer Venus (PV) Orbiter Neutral Mass Spectrometer (e.g., Niemann et al., 1980) that are normalized to the PV Orbiter Atmospheric Drag data (e.g., Keating et al., 1980). We compare the ion density profiles to those of a Chapman layer and to those obtained from radio occultation data. We determine the best fit values of n max,0 i and k in the Chapman expression n max, χ i = n max,0 i cos( χ ) k for each solar zenith angle interval; using a linear least squares regression for log n max, χ i as a function of log cos( χ ) we also derive the best fit values for the group of ten models for solar zenith angles from 60 to 89°. For a theoretical Chapman layer with plane parallel geometry, k = 0.5, and n max,0 i is the peak electron density at the subsolar point. In the near‐terminator region, the production rates and densities must be computed using spherical geometry, and this alone will cause the derived value of k to be reduced below the theoretical factor of 0.5. For the solar zenith angle models in the range 60–85°, we find that the magnitudes of the both the main ( F 1 ) and lower ( E ) peaks decrease as the solar zenith angle increases in a somewhat Chapman‐like manner, but the altitudes of the F 1 and E peaks increase only slowly from 140 to 144 km and from 125 to 129.5 km, respectively. The nearly constant altitude of the F 1 peaks has been attributed to the collapse of the thermosphere as it merges into the nighttime cryosphere (Cravens et al., 1981). This behavior is quite different from that of Mars, where the electron density peak altitudes have been observed to rise monotonically as the solar zenith angle increases. The predicted behavior for the Venus ionosphere is in general agreement with radio occultation data, but some differences also are observed. We discuss the differences between the modeled and observed peak altitudes and we attribute the differences partly to deficiencies in the VTS3 models and partly to uncertainties in the electron temperatures. There also may be some errors introduced into the radio occultation profiles in the very near‐terminator region due to deviations from spherical symmetry. In addition, differences exist between the measured and model peak magnitudes at solar zenith angles near 60° that we ascribe partly to the use of the S2K v2.22 solar flux model of Tobiska (2004), for which the EUV and soft X‐ray photon fluxes are significantly smaller than those of the S2K v1.24 or the Hinteregger solar flux models.