Photoelectron fluxes in the Venus dayside ionosphere

Suprathermal electron fluxes measured in the Venus dayside ionosphere in the energy range from 20 to 50 eV are presented for different altitudes, solar zenith angle (SZA), and solar F-10.7 flux. The average flux is nearly constant for SZAs between 0 degrees and 70 degrees and decreases with increasing altitude. The flux increases with the F-10.7 index more rapidly than the square of the plasma peak density does. Close to the terminator, where the production rate of the photoelectrons is small, strong depletions of nearly 1 order of magnitude occur, often at altitudes above 300 km. The photoelectron fluxes were calculated using the two-stream approximation. The calculations reproduce the shape of the observed photoelectron energy spectrum, the SZA, and the solar F-10.7 dependence reasonably well. The decrease of the flux with altitude needs the introduction of either a loss term or a vertical electric field. The best agreement with the data was obtained for an ionosphere with a reflecting ionopause and either a height and energy integrated flow of 1.10(10) cm(-2) s(-1) toward the terminator or an electric field of 12 mV/km.