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 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.