The interaction of the solar wind and stellar winds with the partially ionized interstellar medium

Plasmas in the laboratory and interstellar and interplanetary space are frequently partially ionized. Thus, the solar wind and stellar winds often interact with an interplanetary medium that is an admixture of protons, electrons, other charged ions, and neutral atoms. For example, the very local interstellar medium surrounding our heliosphere may be less than 50% ionized, with the dominant constituent being neutral hydrogen (H). As a result, the composition of the solar wind in the outer heliosphere beyond some 10–15 AU is dominated by neutral interstellar H. Our understanding of the complex physics describing the interaction of the solar wind with the partially ionized local interstellar medium (ISM) has advanced significantly in the last 5 years with the development of very sophisticated models which treat the coupling of neutral atoms and plasma self-consistently. A number of major predictions have emerged from these models, such as the existence of a large wall of heated neutral hydrogen upstream of the heliosphere. Remarkably, in the ensuing years, this prediction has been confirmed by high-resolution Hubble Space Telescope Lyman-α spectroscopic data. Subsequent models now consider the interaction of various stellar winds with the ISM, and Hubble observations provide supporting data. An introductory review of the basic physics, and associated observations, of the interaction of the solar wind and stellar winds with the interstellar medium is presented for this exciting and rapidly developing field.