Astron. Astrophys. 344, 317-321 (1999)

1. Introduction

Our solar system is moving through a partially ionized interstellar cloud. The ionized fraction of this local interstellar cloud (LIC) interacts with the expanding solar wind and forms the LIC-Solar Wind interface (or heliospheric interface). Interstellar atoms penetrate deeply in the heliosphere where they could be observed directly as atoms, pick-up ions or anomalous cosmic rays (ACR) and indirectly through backscattering of the solar radiation. However, gas-dynamical parameters of these neutrals suffer significant modifications during the crossing of the heliospheric interface. In particular, the flux of neutral atoms may decrease during the crossing of the heliospheric interface. This phenomenon, called "filtration" is due to the coupling of the atoms with the plasma in the interface, i.e. some of the atoms acquire the same motion as the ions and are diverted away from the heliosphere. The filtration factor is the ratio between the interstellar atom density inside the heliosphere, at a distance from the Sun large enough for the direct solar wind and EUV ionization to be negligible, and the initial interstellar neutral density outside the heliosphere, in the unperturbed interstellar medium. The filtration factor is then a measurement of the coupling with the plasma. Since such modifications depend on the type of atom through the charge-exchange cross-section, the relative abundances and the velocity distributions of different species inside the heliosphere are different from the original interstellar abundances and velocity distributions.

Interstellar hydrogen and helium are most interesting among the interstellar atoms due to their large cosmic abundances and the availability of observations. There has been recently an increasing interest in heavier elements of the LIC, in particular O, N, Ne, C. This interest in minor species is now growing due to the recent successful detection of pick-up and ACR ions, with the Ulysses and Voyager spacecraft respectively. Oxygen is of particular interest, because it is one of the most perturbed elements due to its large charge exchange cross section with the protons. Very recently, Gruntman & Fahr (1998) proposed a mapping of the heliopause in the oxygen ion reson ance line (83.4 nm). in the heliospheric interface is strongly connected with the distribution of neutral oxygen.

Numerical modeling of interstellar oxygen penetration in the heliosphere (Fahr et al., 1995; Izmodenov et al., 1997; Kausch & Fahr, 1997) shows that the fraction of neutral oxygen penetrating into the heliosphere is rather high. It varies between 60-90% on the upwind side depending on the interstellar parameters and which theoretical model is actually used for the analysis. The above models included the effects of direct charge-exchange , reverse charge exchange reaction and photoionization. Izmodenov et al. (1997) compared oxygen ion fluxes in the vicinity of the Sun deduced from the Ulysses measurements (Gloeckler et al., 1993) with recent nearby stars spectroscopic HST observations of the neutral oxygen to neutral hydrogen ratio in the Local Cloud (Linsky et al., 1995). The comparison has been made on the basis of the solution of the Boltzmann equation for interstellar atoms by means of a Monte-Carlo method. It has been shown that an additional filtration of atomic oxygen is required to reconcile these two types of observations on the basis of the kinetic model.

Electron impact ionization in the heated post-shocked solar wind was considered by Izmodenov et al. as possibly responsible for the stronger filtration of the interstellar oxygen in the heliospheric interface. The influence of electron impact on interstellar oxygen filtering has been taken into account for the first time by Fahr et al. (1995). These authors have done it in a simplified way, by multiplying the charge-exchange cross-section by a number larger than one, this number being taken constant throughout the whole interface, which is equivalent to the implicit assumption that the electron impact ionization rate does not vary as a function of temperature.

Here we calculate explicitly the interstellar oxygen filtration with and without electron impact ionization in the frame of the two-shock heliospheric interface model of Baranov & Malama (1993, 1995, 1996). To compute the interstellar oxygen density in the interface, we use a Monte-Carlo method with the splitting of the trajectories developed by Malama (1991).

© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999
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