Interaction of the solar wind with the neutral component of the interstellar gas

A model is constructed to represent the interaction between the solar wind and the neutral component of the interstellar gas. It is found that the neutral gas has several important effects on the solar-wind expansion beyond the orbit of the earth and that it should be possible to infer the presence of the neutral gas from observations of the solar wind made by a space probe traveling into the outer solar system. The effects include a deceleration and heating of the supersonic solar wind, a cooling of and pressure reduction in the subsonic solar wind, and a tightening of the spiral magnetic field in the supersonic solar wind. Although in principle the neutral gas can lead to a shock-free transition from supersonic to subsonic flow, it appears that in practice a shock is present. The minimum distance to the shock is estimated to be less than 50 AU and probably more than 25 AU.