A quasi-steady 1-D hydrodynamic model, with mass addition, has been used to study the various modes of interaction of the solar wind with a medium-bright, H,O-dominated comet (such as P/Hrilley) approaching the Sun.
At large heliocentric distances (d > 5 AU) the solar wind penetrates unimpeded on to the surface. As the comet moves further in, mass loading of the solar wind by heavy ions from the fledgling cometary atmosphere causes it to slow down, thereby causing a significant enhancement of the interplanetary field. Still further in at d = 3.14 AU, as the mass loading reaches a critical value, a collisionless standing shock is formed in the solar wind upstream of the nucleus. As d decreases further, the distance of this shock from the nucleus increases. The cometary atmosphere becomes dense enough to stand off the solar wind ahead of the nuclear surface and form a well defined tangential discontinuity surface (or 'ionopause') only when d reaches the value 2.65 AU. When d < 2.65 AU an 'inner' shock could, in principle, also form within the cometary ionosphere, although its existence would depend on the detailed thermodynamics of the cometary ionosphere. Resolution of this question is beyond the scope of the present analysis.
The conclusions of the present study would be qualitatively valid for other comets having sizes, surface optical properties and chemical compositions, different from those adopted here. The heliocentric distances at which the various transitions take place from the one mode of solar wind interaction to another, would, of course, be different, with all these distances being smaller for less active comets.