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

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3. Results of model calculations

Interstellar atoms in the heliosphere can be divided into four different populations depending on which region of the interface they originate from. Indeed, after charge-exchange with the protons, parameters of the newly created (secondary) atoms strongly depend on the local plasma parameters. The heliospheric interface can be divided into 4 regions with very different plasma properties: the supersonic solar wind up to the TS (region 1), the compressed and heated solar wind in the region between the TS and HP (region 2), the compressed interstellar medium between the HP and the BS (region 3) and the unperturbed interstellar medium (region 4). In correspondence with these regions, we divide the interstellar neutrals into 4 populations.

Fig. 2 displays number densities of all populations of interstellar atoms as a function of heliocentric distance in the direction anti-parallel to the interstellar flow vector (the upwind direction). Curves 1 on the figure correspond to calculations including electron impact ionization. The comparison of these curves with dashed lines (where the electron impact ionization is not taken into account) shows that the main effect of the electron impact ionization appears in the compressed solar wind (in the region between the TS and the BS). Indeed, according to the Baranov-Malama model, the solar wind plasma is mostly heated in this region. It is assumed in the model that electrons and protons have the same temperature. The temperature reaches [FORMULA] K in the compressed solar wind. For primary interstellar atoms (population 4, Fig. 2a) the effect of additional filtration due to electron impact ionization is about 9%. For secondary interstellar population (population 3, Fig. 2b) the effect is even larger, about 15% of additional filtration. These two interstellar populations are the densest in the heliosphere, and thus their filtration is the most important number. Figs. 2c and 2d show number densities of hot oxygen born between the TS and the HP (population 2), and energetic oxygen atoms born in the supersonic solar wind (population 1). It is seen from the comparison of curve 1 and curve 2 in Fig. 2c that the number density of population 2 increases due to the electron impact. At the HP the increase is about factor of two. As a matter of fact, electron impact ionization increases the number density of oxygen ions, and reverse charge exchange [FORMULA] leads to additional hot oxygen neutrals.

[FIGURE] Fig. 2. Neutral oxygen number density in the upwind direction for primary interstellar atoms - population 4 (A), secondary interstellar atoms - population 3 (B), "subsonic solar" atoms - population 2 (C) and " supersonic solar" atoms - population 1 (D). Plotted values are normalized to the oxygen atom number density in unperturbed interstellar medium. Curves 1 correspond to calculations for oxygen where electron impact ionization has been taken into account, curves 2 are the results without electron impact, curves 3 are the relevant normalized densities of hydrogen populations. Positions of the TS, HP, BS are shown.

Fig. 2 shows also the distributions of interstellar hydrogen populations in the heliospheric interface. It can be seen when comparing O and H curves that the "oxygen wall" (the increase in density of population 3, Fig. 2b) is less pronounced than the "hydrogen wall". However, the ratio of oxygen number density of population 3 at the TS (the TS is about at 100 AU Upwind) to the interstellar number density is almost the same as for hydrogen. It is also interesting to note that due to the larger oxygen mass the maximum density in the oxygen "wall" is closer to the HP than for hydrogen. Nevertheless, the density of primary interstellar atoms (population 4) in the outer heliosphere (Fig. 2) and the filtration factor (the ratio of atom number density in the out r heliosphere to atom number density in the LIC) of oxygen are larger than for hydrogen. The filtration factors are equal to 0.7 and 0.475 for oxygen and hydrogen correspondently. This is due to the smaller charge-exchange cross section for oxygen. Larger primary oxygen penetration into the heliosphere with almost the same penetration for the secondary interstellar atoms could lead to significant difference between the properties of the interstellar oxygen and hydrogen in the heliosphere, because the secondary atoms are more heated and decelerated. Hopefully this difference will be measured in future experiments.

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© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999