The Heliospheric Hydrogen Distribution: A Multifluid Model

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Abstract
We investigate the self-consistent interaction of the local interstellar cloud (LIC) and solar wind, focusing on its manifestations in the heliospheric hydrogen (H) distribution. This system is modeled hydrodynamically as a fluid proton-electron plasma and three H fluids, each arising from charge exchange production within three distinct plasma environments. Perhaps our most significant finding is that, based on the Dalgarno cross section, thermalizing H-H collisions are crucial to determining the heliospheric H distribution. Hot secondary H atoms produced from charge exchange with the solar wind will be thermalized with the bulk of the cooler LIC H distribution. This thermalization should be complete for the postheliospheric H beyond ~108 downstream from the Sun. Observed along nearby downstream interstellar sightlines, we may expect to see temperatures of order 105 K in the bulk of the postheliospheric-traversal H distribution. Recent observations along the Sirius sightline by Bertin et al. may be explainable in these terms. Based on the Dalgarno cross section for thermalizing H-proton collisions without charge transfer, this interaction should be important as well. This implies that previous charge exchange-only models have underestimated the degree of H-proton coupling and thereby the efficiency of heliospheric filtration of LIC H. The variation among published values of the charge exchange cross section is 40% at 1 eV. We find that this variation will affect predictions of the H density at 50 AU by a similar factor. We performed calculations using the larger of the charge exchange cross sections to conclude that the proton density in the LIC is not likely greater than a few ×10-2 cm-3. We show densities, temperatures, and radial velocities of the three H fluids along the sight lines of some nearby stars for one of these calculations, using an LIC proton density of 0.1 cm-3. Results from a first crude model of the effects of H-H collisions are given in an appendix.