Ionization rates in the heliosheath and in astrosheaths

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 coefficients 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.

Helium Energetic Neutral Atoms from the Heliosphere: Perspectives for Future Observations

“…The primary source of information on the cross sections is Volume 1 of the ORNL CFADC "Redbooks" (Barnett 1990). It is the same source as used by Scherer et al (2014). However, for some reactions other sources are also used, as given in the references in Table 3.…”

A detailed calculation of neutral hydrogen ionization frequencies used in turbulence transport models in the heliosphere

“…which, following Scherer et al (2014), leads to ν pi = 8 × 10 −8 (r o /r) 2 . This is very similar to the 1 AU value for the photoionization frequency of 9×10 −8 Hz employed by Whang (1998).…”

“…1). The cross-section we use for this particular interaction is reported by Scherer et al (2014), with a value of ∼2×10 −15 cm −2 taken for a relative collision energy of ∼930 eV. This value is identical to that employed by Usmanov et al (2012).…”

“…A possible source of energy for this phenomenon, suggested by Williams et al (1995), is the formation of pickup ions from interstellar neutral hydrogen, which could generate turbulence (Lee & Ip 1987), the cascade of which would then deposit this energy as heat into the solar wind core protons. As neutral hydrogen particles enter the heliosphere, they become ionized either by charge exchange mechanisms with solar wind protons, by solar ultraviolet light, or by electron impacts (see, e.g., Zank 1999;Fahr et al 2000;Isenberg et al 2003;Isenberg 2005;Scherer et al 2014). This introduces an unstable distribution of pickup protons into the solar wind, which in turn are quickly scattered, generating wavemodes propagating parallel to the heliospheric magnetic field (Smith et al 2001).…”

Cosmic rays in astrospheres

“…This would then also require including helium as a new species in the Euler energy equations (including different species, see Scherer et al 2014), which would violate our assumption of a single fluid. The interaction of other species concerning the energy loss by Coulomb collisions is, however, already included in the cooling functions, so that we can continue with the single-fluid equations consisting of protons including the cooling term for a first analysis.…”