LIGHT-ION FLOW IN THE NIGHTSIDE IONOSPHERE OF VENUS

The flow characteristics of the light ions H+ and He+ have been studie d in the midnight region of the ionosphere of Venus. Measurements of i on composition, electron and ion temperatures and magnetic fields by i nstruments onboard the Pioneer Venus Orbiter have been used in the ele ctron and ion equations of conservation of mass and momentum to derive the vertical flow velocities of H+ and He+. When average height profi les of the measured quantities were used, H+ was found to flow upward, accelerating to speeds of almost 1 km s-1 at the ion-exobase. In a si milar fashion, He+ was found to flow downward into the neutral atmosph ere where it is readily quenched by charge transfer reactions. The pol arization electric field played an important role in forcing H+ upward , but did not contribute enough to the He+ force balance to produce up ward flow. At the ion-exobase, the outward electric polarization force on H+ was shown to be five times the gravitational force. Using an an alogy with the terrestrial ion-exosphere, H+ was inferred to flow upwa rd into the ionotail of Venus and accelerate to escape speeds. A plane t averaged escape flux of 1.4 x 10(7) cm-2 s-1 was calculated, which i s comparable to hydrogen loss rates estimated by other investigators.