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

A fraction of the interstellar atoms crosses the interface between the sola r wind and the ionized component of the local interstellar medium (the heli ospheric interface), and enters the heliosphere. The number of penetrating atoms depends on the strength of the atom-plasma interaction, i.e. on the a tom 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 dens ity decrease in the heated (by the termination shock) solar wind. A compari son between oxygen and hydrogen shows that in the heliosphere these two ele ments have abundances, temperatures and velocities that differ from the cor responding interstellar parameters. On the basis of these new model results , we compare the interstellar OI/HI ratio measured by the HST with the rati o inside the heliosphere which can be deduced from SWICS/Ulysses O+ and Hpickup data.