THE HELIOSPHERIC HYDROGEN DISTRIBUTION - A MULTIFLUID MODEL

We investigate the self-consistent interaction of the local interstell ar cloud (LIC) and solar wind, focusing on its manifestations in the h eliospheric hydrogen (H) distribution. This system is modeled hydrodyn amically as a fluid proton-electron plasma and three H fluids, each ar ising from charge exchange production within three distinct plasma env ironments. Perhaps our most significant finding is that, based on the Dalgarno cross section, thermalizing H-H collisions are crucial to det ermining the heliospheric H distribution. Hot secondary H atoms produc ed from charge exchange with the solar wind will be thermalized with t he bulk of the cooler LIC H distribution. This thermalization should b e complete for the postheliospheric H beyond similar to 10(8) downstre am from the Sun. Observed along nearby downstream interstellar sightli nes, we may expect to see temperatures of order 10(5) K in the bulk of the postheliospheric-traversal H distribution. Recent observations al ong the Sirius sightline by Bertin et al. may be explainable in these terms. Based on the Dalgarno cross section for thermalizing H-proton c ollisions without charge transfer, this interaction should be importan t as well. This implies that previous charge exchange-only models have underestimated the degree of PI-proton coupling and thereby the effic iency of heliospheric filtration of LIC H. The variation among publish ed 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 th e charge exchange cross sections to conclude that the proton density i n the LIC is not likely greater than a few x 10(-2) cm(-3). We show de nsities, temperatures, and radial velocities of the three H fluids alo ng 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.