Interstellar hydrogen ionization in the heliosheath

The expanding solar wind plasma undergoes a shock transition at the interstellar boundary of the solar system and fills the inner heliosheath, the region between the termination shock and the heliopause. The nonequilibrium heliosheath plasma is a main source of energetic neutral atoms that allow remote probing of the heliospheric interface region. Global models of the heliosphere interaction with the interstellar medium often disregard solar photoionization and electron impact ionization of interstellar gas in the heliosheath. When ionization is included, it is commonly treated in a simplified manner and complexity of heliospheric interactions obscures its effect on the properties of the plasma. This work concentrates on physical estimates of ionization and shows that it may lead to significant mass loading of plasma flows in the heliosheath. In turn, mass loading would slow down plasma flows and deplete populations of nonthermal energetic protons. The magnitude of the effect depends on poorly understood and largely unknown energy transfer to electrons at the termination shock and beyond. The presented estimates show that inclusion of ionization is indispensable for global heliospheric modeling and for interpretation of heliosphere imaging in fluxes of energetic neutral atoms.