ABSTRACT:

Most of our current knowledge of the upper atmosphere and ionosphere ofVenus has corne from the various aeronomy experiments on the Pioneer Venus Orbiter (PVO) which was inserted into a highly eccentric orbit around Venus on Decernber 4, 1978. The PVO provided ionospheric data till Oct. 7, 1992 after which it entered the dense atrnosphere and incinerated. We now know that Venus has an extended atmosphere with C02 dominating in the lower thermosphere and O and He at higher altitudes. The atmosphere is not very sensitive to solar EUV variability with the exospheric temperature changing only by about 60 K over a solar cycle. In the upper ionosphere, major ion is 0+ above about 200 km, but lower down 02+ dominates and forms the main ionospheric peak around 150 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. In spite of the long Venus night, a substantial nightside ionosphere exists mainly due to transterminator flow of 0' from the dayside during solar maximurn. During solar minimum, electron precipitation provides an equal or higher contribution to the maintenance.