Near‐terminator Venus ionosphere: Evidence for a dawn/dusk asymmetry in the thermosphere

Recent models of the near‐terminator ionosphere of Venus constructed using neutral density profiles from the VTS3 model of Hedin et al. (1983) have shown that altitudes of the electron density peaks are in agreement with those measured by Pioneer Venus (PV) Orbiter Radio Occultation (ORO) and other radio occultation profiles in the solar zenith angle (SZA) range 60 to 70°, where they are near 140 km (Fox, 2007). The model peaks in the 75–85° range, however, do not decrease in altitude to near 135 km, as do the PV ORO electron density peaks shown in the study of Cravens et al. (1981). We investigate here possible reasons for this decrease. The PV Orbiter Neutral Mass Spectrometer (ONMS) measured densities of CO 2 , O, CO, N 2 , N, and He for many of the first 600 orbits. We have chosen 10 orbits in the dawn sector and 12 orbits in the dusk sector for which the solar zenith angles at periapsis were in the 75–85° range, and we have examined the ONMS density profiles reported in the PV Unified Abstract Data System. We find that for most of the orbits, the appropriately normalized ONMS measured densities for CO 2 and O are, however, either similar to or larger than those generated from the VTS3 model for the same solar zenith angle and F 10.7 flux, and the use of these densities in our models would therefore produce a higher, rather than a lower, peak. The VTS3 models are, however, not expected to be accurate in the terminator region because of the small number of spherical harmonics used in the models and the large density changes that are expected near the terminators. We have also investigated a possible dawn/dusk asymmetry in the ionosphere. All the low‐altitude PV radio occultation electron density peaks reported in the study of Cravens et al. (1981) in the 70 to 85° range were in the dawn sector at high latitudes. In the VTS3 models, the exospheric temperatures are predicted to be smaller at dawn that at dusk, but the asymmetries are confined to the region above ∼165 km. Thus use of the VTS3 model densities and temperatures in the near‐terminator dawn sector models cannot produce electron density peaks that are lower in altitude than those in the dusk sector. We suggest that there is a high‐latitude asymmetry between the dawn and dusk neutral densities that extends down to within ∼20 km above the expected altitude of the electron density peaks, and that produces a significantly asymmetrical ionosphere.