4 Conclusion

In this work we have presented the results obtained by combining the hydrogen distribution model obtained by Izmodenov et al. (2001) which takes into account effects of the heliospheric interface with the full radiative transfer model described by Quémerais (2000). The main change applied to the interplanetary background model was to allow for the existence of three hydrogen populations, each with distinct local density, bulk velocity and projected temperatures.

This model has been applied to compute interplanetary background line profiles as seen by an observer at 1 AU from the sun. This study was compared to previous results of Quémerais (2000) obtained for a hot model of hydrogen distribution.

The most conspicuous difference between the hot model and three-population model computations is the existence for the latter type of model of a hot component (apparent temperature around more than $1.5 \times 10^5$ K) which does not exist in the case of the hot model. At 1 AU in the upwind direction, this component amounts to less than 5% of the total intensity. In the downwind direction, it represents up to 15% of the total intensity. Detecting this component would give a very strong proof of the existence and structure of the heliospheric interface. Use of the hydrogen cell data from the SWAN instrument will be made to try and detect this feature. High signal to noise measurements from the Hubble Space Telescope (Clarke et al. 1998) could also be useful here.

Other effects on the line profile as seen from one AU are harder to use to discriminate between the two types of hydrogen distributions. The main reason is that time-dependent variations of the solar parameters (radiation pressure for instance) induce changes in the hydrogen distribution in the inner heliosphere which are similar to those due to the interface. Thus a full model of the hydrogen distribution should include time-dependent variations of the solar parameters. Bzowski et al. (2001) developed an initial study for the hot model. We need to quantify the changes induced by solar cycle variations of the solar parameters. Once this is done, we are confident that we will be able to discriminate between the different models from actual interplanetary line profile data.

Acknowledgements

This work was supported partly by INTAS grant 2001-0270. V.I. was also supported by the International Space Science Institute in Bern, and RFBR grants 01-02-17551, 02-02-06011 and 01-01-00759.