2. Self-consistent two-shock model of the LISM-solar wind interaction
In this paper we start with the self-consistent "supersonic" interface model computed by Baranov and Malama (1993). It is based on an iterative method combining a gas- dynamical model characterized by two shock waves, a solar wind termination shock (TS) and an interstellar bow shock (BS), and one contact discontinuity, the heliopause (HP) (see Fig. 1), with a Monte-Carlo simulation for the neutral H flow through the plasma field. This axisymmetric model assumes that the interstellar medium is three-fluid with neutral (H-atoms) and plasma (electrons and protons) components. The boundary conditions for the proton density, the bulk velocity and the Mach number of the solar wind at the Earth's orbit are taken as:
The same quantities for the unperturbed interstellar medium are:
The corresponding temperature in the LISM is 6700 K. The temperature and the velocity of the unperturbed interstellar flow are in agreement with the heliospheric neutral helium parameters derived by Witte et al (1993) and the LIC parameters deduced from ground-based and Space Telescope stellar spectra (Lallement and Bertin, 1992, Linsky et al, 1993).
The number density of H atoms in the unperturbed LISM is either
or b) .
Case a corresponds to the pure plasma-plasma interaction, because interstellar neutrals are absent. It does not correspond to the actual situation, which is better represented by case b). However, it is interesting to consider the case a) and the comparison between a) and b), for both the structure (and size) of the interface (fig. 1), and for the neutral oxygen filtering (see Sect. 4).
The computational domain is delimited by a sphere of radius as the inner boundary, and, as the outer boundary, by a surface whose section is the contour labelled CDB in Fig. 1 on the upwind side, and a surface Z= cste= on the downwind side. Here is the polar angle counted from the upwind axis. This outer boundary extends up to (0)=400AU on the upwind side and up to on the downwind side. The surface is chosen in such a way that the LISM perturbations outside the computational region are negligible. Fig. 1 shows the structure of the interface in the XOZ plane, where the OZ axis coincides with the axis of symmetry and is antiparallel to the velocity vector of the LISM's (the Sun is at the center of the coordinate system). In this figure, 'BS' is the bow shock formed in the LISM due to the deceleration of the plasma component. 'HP' is the heliopause (contact or tangential discontinuity) separating the LISM plasma compressed by the bow shock BS, from the solar wind compressed by the termination shock TS. Solid lines (resp. dashed lines) are for the case b (resp. a). Hydrogen atoms which penetrate the solar wind flow through the whole BS-HP-TS plasma structure. Shown in Fig. 2 are the proton and H density distributions, the bulk velocities and the temperatures issued from this model, which are now used in the present study as background properties for the inflowing neutral oxygen. Fig. 2a displays the LISM protons density, Fig. 2b the proton velocity component parallel to the Z-axis, Fig. 2c the proton temperature, and Fig. 2d the number density of the neutral H atoms, for three directions: the upwind direction (Curve 1, ), the "sidewind" or direction perpendicular to the flow axis (curve 2, ) and the downwind direction ( curve 3, ). These distributions are discussed in details in Baranov & Malama (1993). Note that, at variance with a pure plasma/plasma interaction, the LISM, upstream from the bow shock, as well as the solar wind, upstream from the terminal shock, both have space-varying characteristics showing that they "feel" the shock before crossing it. This is due to resonance charge exchange processes with the neutrals and the 'pick-up' of the 'new' protons.
© European Southern Observatory (ESO) 1997