The ionosphere and upper atmosphere of Venus: A review

Abstract

Most of our current knowledge of the upper atmosphere and ionosphere of Venus has come from the various aeronomy experiments on the Pioneer Venus Orbiter (PVO) which was inserted into a highly eccentric orbit around Venus on December 4, 1978. The PVO provided ionospheric data till Oct. 7, 1992 after which it entered the dense atmosphere and incinerated. We now know that Venus has an extended atmosphere with CO₂ dominating in the lower thermosphere and O and He at higher altitudes. The atmosphere is not very sensitive to solar EUV variability with the daytime exospheric temperature changing only by about 60 K over a solar cycle. This weak: response has been explained due to very strong 15 μm cooling from CO₂, collisionally excited by oxygen atoms. This cooling approximately balances the EUV and UV heating. And in the upper ionosphere, major ion is O⁺ above about 200 km, but lower down O₂⁺ dominates and forms the main ionospheric peak around 140 km. The ionosphere responds very strongly to changes in solar activity and solar zenith angle. The planet has no intrinsic magnetic field and therefore the solar wind interacts directly with its ionosphere resulting in a sharp density gradient (called the ionopause), above the top of the ionosphere. Inspite of the long Venus night, a substantial nightside ionosphere exists mainly due to transterminator flow of O⁺ from the dayside during solar maximum. During solar minimum, electron precipitation provides an equal or higher contribution to the maintenance.