Ionization rates in the heliosheath and in astrosheaths: spatial dependence and dynamical relevance

Abstract

Context. In the heliosphere, especially in the inner heliosheath, mass-, momentum-, and energy-loading induced by the ionization of neutral interstellar species plays an important, but for some species, especially helium, an underestimated role. Aims. We discuss the implementation of charge exchange and electron impact processes for interstellar neutral hydrogen and helium and their implications for the subsequent modeling. We especially emphasize the importance of electron impact and a more sophisticated numerical treatment of the charge exchange reactions. Moreover, we discuss the nonresonant charge exchange effects. Methods. We discuss rate coeffcients and revise the influence of the cross-sections in the (magneto-)hydrodynamic equations for different reactions and also their representation in the collision integrals. Results. Electron impact is in some regions of the heliosphere, particularly in the heliotail, more effective than charge exchange, and the ionization of neutral interstellar helium contributes about 40% to the mass- and momentum-loading in the heliosheath. The charge exchange cross-sections need to be modeled with higher accuracy, especially in view of the latest developments made in describing them. Conclusions. The ionization of helium and the electron impact ionization of hydrogen need to be taken into account in modeling the heliosheath and, in general, astrosheaths. Moreover, the charge exchange cross-sections need to be handled in a more sophisticated way, either by developing better analytic approximations or by numerically solving the collision integrals