4. Comparison of OI/HI interstellar and heliospheric ratios
The most accurate comparison between interstellar and heliospheric oxygen makes use of Hubble Space Telescope (HST) nearby star observations on one hand, and Ulysses SWICS pick-up data on the other. The most appropriate HST data in the solar environment are those of Linsky et al. (1995). These authors have measured both neutral oxygen and neutral hydrogen column densities along the line-of-sight towards the star Capella. Towards this target located at 12 pc, only one velocity cloud component produces interstellar absorption lines, the one corresponding to our Local Cloud (Lallement & Bertin, 1992). As a consequence, relative abundances deduced from column density calculations towards this star directly apply to the LIC (and not to companion clouds) and are the most representative of the extra-heliospheric gas. According to the Linsky et al. data the neutral oxygen to neutral hydrogen ratio OI/HI is known with a rather small uncertainty of the order 10%:
Recently, on the basis of SWICS/Ulysses oxygen pick-up ion data, Gloeckler & Geiss (1999) have concluded that the neutral oxygen and hydrogen number densities in the outer heliosphere are equal to and respectively.
The neutral oxygen to neutral hydrogen ratio in the interstellar medium is linked to the ratio in the heliosphere by:
Here is the filtration factor.
with the mean value .
From the results presented in Sect. 3 we conclude that and . Thus the ratio is in perfect agreement with the mean value from Gloeckler & Geiss (1998) and Linsky et al. (1995) results. Now, how sensitive is this calculated ratio to the ionization fraction of the circumsolar gas? A parametric study of the interstellar hydrogen filtration has been done by Izmodenov et al. (1999) which answers this question in case of H (i.e. models and filtrations were computed for various values of the ionization fraction). A similar research could be done for interstellar oxygen. However, Izmodenov et al. (1999) have shown that a large increase (from to ) of the ionization fraction of the circumsolar gas changes the hydrogen filtration factor by only 20%. For this reason we think that the ratio will not be very sensitive to interstellar ionization. Now, the OI/HI filtration ratio (3) has a rather large uncertainty (about 50%). As a consequence it seems impossible at the present time to use the interstellar oxygen as a very good diagnostics of the interstellar proton number density and the interstellar magnetic field, as it has been done for hydrogen (Izmodenov et al., 1999). New and more precise experimental data are needed to do it.
In the present calculations we have used two-shock heliospheric interface model by Baranov & Malama (1993, 1995, 1996). The possible existence of a one-shock heliospheric interface is also discussed in the literature. Indeed, in the case of a rather strong interstellar magnetic field ( for ) the speed of the fast magneto-sonic wave in the LIC is larger than the relative Sun-LIC velocity. In this case the interstellar flow is sub-magnetosonic flow and there is no bow shock. Gayley et al. (1997) have modified the equation of state of the gas to simulate the effect of an interstellar magnetic field. For their one-shock model the authors also have qualitatively the same picture: the hydrogen wall, the filtration in the heliospheric interface. However, there are quantitative differences with the two-shock models. Moreover, Williams et al. (1997), Baranov et al. (1998) have shown that there are non-negligible differences between kinetic and multi-fluid models. Thus, using one-shock or two-shock multifluid model (versus kinetic Monte-Carlo model) could lead to some quantitative (but not qualitative!!!) changes.
© European Southern Observatory (ESO) 1999
Online publication: March 10, 1999