SOLAR-WIND FLOW WITH HYDROGEN PICKUP

The interaction of the solar wind with neutral interstellar hydrogen i s reconsidered. A detailed derivation of the well-known one-fluid hydr odynamic description of the solar wind modified by interstellar pickup ions is presented, based on a semikinetic guiding center description. This analysis clarifies several of the underlying assumptions implici t in Holzer's (1972) model. A brief discussion of stationary solar win d solutions modified by the interstellar neutral gas is given, and, on the basis of this analysis, we deduce that the termination shock may be interpreted as a surface of strong discontinuity with an attached w eak discontinuity. Our principal result addresses the stability of the supersonic solar wind in the presence of mass, momentum, and energy l oading. It is found that beyond 25 AU a gradient catastrophe can devel op, which leads to the formation of the termination shock in the outer heliosphere. The location of the termination shock may be interpreted as due primarily to the properties of the interstellar neutrals (such as the value of their number density) rather than as due entirely to the balancing of the solar wind dynamic pressure and interstellar plas ma pressure. Examples of recent multidimensional computations of the i nteraction of the solar wind with the local interstellar medium that i nclude interstellar neutrals in a fully self-consistent manner are pre sented, and these provide support for the simpler one-dimensional anal ysis.