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.