Measurements of the ion escape rates from Venus for solar minimum

We report the first direct measurements of the Venusian atmospheric erosion rate due to the interaction with the solar wind. The erosion through the ion escape is determined during the period of the minimum solar activity from 24 May 2006 to 12 December 2007. The ion fluxes are measured in the energy range 10 eV to 25 keV by an ion mass spectrometer on board the Venus Express spacecraft and sampled statistically dense in the volume in the Venusian wake. The rates are Q(H+) = 7.1 . 10(24) s(-1), Q(He+) = 7.9 . 10(22) s(-1), and Q(O+) = 2.7 . 10(24) s(-1). The reported escape rates measured for the solar minimum are close to the rates estimated for the solar maximum from the Pioneer Venus Orbiter observations. We may thus propose that the atmospheric loss due to solar wind interaction depends weakly on the solar conditions. The paper also presents in detail how the global escape rates are deduced from the in situ measurements.