Department of Earth System Science

Charge exchange between H and H⁺ and momentum transfer between fast O and H provide comparable sources for suprathermal H atoms in Venus' exosphere. The fast O atoms are produced by dissociative recombination of O₂ ⁺. The spatial distribution of suprathermal H was calculated using an approximate numerical solution of the time-independent Boltzmann equation. Sources of suprathermal H atoms were specified on the basis of measurements by Pioneer Venus. Reactions involving H₂ were neglected in the absence of direct experimental information on the concentration of H₂ and on the grounds of indirect arguments suggesting its mixing ratio should be less than 0.5 ppmv. Computed densities of suprathermal H are in satisfactory agreement with profiles derived earlier from analysis of Lyman-α airglow by Mariners 5 and 10, and Veneras 11 and 12. The dayside emission at radial distances larger than 18,000 km is attributed to scattering of solar photons by fast H atoms produced primarily on the nightside near midnight. The nightside ionosphere, and consequently the source of suprathermal H, are expected to vary in response to changes in the solar wind. Observations of the variability of Lyman-α emission on the dayside could provide a useful test of the model, in particular its description of conditions in the nightside ionosphere and its neglect of fast H produced by reactions involving H₂. Charge transfer of H with nightside H⁺ accounts for approximately 70% of the hydrogen escaping from Venus. The total escape rate is estimated to be between 0.4 and 1 × 10⁷atoms cm⁻²s⁻¹.