Suprathermal electron fluxes measured in the Venus dayside ionosphere
in the energy range from 20 to 50 eV are presented for different altit
udes, solar zenith angle (SZA), and solar F-10.7 flux. The average flu
x is nearly constant for SZAs between 0 degrees and 70 degrees and dec
reases with increasing altitude. The flux increases with the F-10.7 in
dex more rapidly than the square of the plasma peak density does. Clos
e to the terminator, where the production rate of the photoelectrons i
s small, strong depletions of nearly 1 order of magnitude occur, often
at altitudes above 300 km. The photoelectron fluxes were calculated u
sing the two-stream approximation. The calculations reproduce the shap
e of the observed photoelectron energy spectrum, the SZA, and the sola
r F-10.7 dependence reasonably well. The decrease of the flux with alt
itude needs the introduction of either a loss term or a vertical elect
ric field. The best agreement with the data was obtained for an ionosp
here with a reflecting ionopause and either a height and energy integr
ated flow of 1.10(10) cm(-2) s(-1) toward the terminator or an electri
c field of 12 mV/km.