Interstellar oxygen in the heliospheric interface: influence of electron impact ionization

A fraction of the interstellar atoms crosses the interface between the solar wind and the ionized component of the local interstellar medium (the heliospheric interface), and enters the heliosphere. The number of penetrating atoms depends on the strength of the atom-plasma interaction, i.e. on the atom type. In this paper we consider the penetration of the interstellar oxygen atoms, and we study in detail the influence of the electron impact ionization. It is shown that this process leads to a significant interstellar O-atom density decrease in the heated (by the termination shock) solar wind. A comparison between oxygen and hydrogen shows that in the heliosphere these two elements have abundances, temperatures and velocities that differ from the corresponding interstellar parameters. On the basis of these new model results, we compare the interstellar OI/HI ratio measured by the HST with the ratio inside the heliosphere which can be deduced from SWICS/Ulysses O+ and H+ pickup data.