Hydrogen Lyman-alpha Absorption Predictions by Boltzmann Models of the Heliosphere

摘要

We use self-consistent kinetic/hydrodynamic models of the heliosphere topredict H I Lyman-alpha absorption profiles for various lines of sight throughthe heliosphere. These results are compared with Lyman-alpha absorption linesof six nearby stars observed by the Hubble Space Telescope. The directions ofthese lines of sight range from nearly upwind (36 Oph) to nearly downwind(Epsilon Eri). Only three of the Lyman-alpha spectra (36 Oph, Alpha Cen, andSirius) actually show evidence for the presence of heliospheric absorption,which is blended with the ubiquitous interstellar absorption, but the otherthree spectra still provide useful upper limits for the amount of heliosphericabsorption for those lines of sight. Most of our models use a Boltzmannparticle code for the neutrals, allowing us to estimate neutral velocitydistributions throughout the heliosphere, from which we compute modelLyman-alpha absorption profiles. In comparing these models with the data, wefind they predict too much absorption in sidewind and downwind directions,especially when higher Mach numbers are assumed for the interstellar wind.Models created assuming different values of the interstellar temperature andproton density fail to improve the agreement. Somewhat surprisingly, a modelthat uses a multi-fluid treatment of the neutrals rather than the Boltzmannparticle code is more consistent with the data, and we speculate as to why thismay be the case.