Solar wind charge exchange in cometary atmospheres. I. Charge-changing and ionization cross sections for He and H particles in H$_2$O

TL;DR

This paper provides detailed charge-changing and ionization cross sections for hydrogen and helium particles in water, crucial for modeling solar wind interactions with cometary atmospheres, especially at low energies relevant to solar wind speeds.

Contribution

It offers polynomial fits for experimentally measured cross sections and Maxwellian-averaged values, enhancing the accuracy of models of solar wind-comet interactions.

Findings

Single-electron capture by He2+ dominates at typical solar wind speeds.

Ionization cross sections peak above 20 keV, influencing ion production.

Local heating may enhance previously negligible processes.

Abstract

Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet, mass-loading the solar wind through an effective conversion of fast light solar wind ions into slow heavy cometary ions. To understand these processes and place them in the context of a solar wind plasma interacting with a neutral atmosphere, numerical or analytical models are necessary. Inputs of these models, such as collision cross sections and chemistry, are crucial. Book-keeping and fitting of experimentally measured charge-changing and ionization cross sections of hydrogen and helium particles in a water gas are discussed, with emphasis on the low-energy/low-velocity range that is characteristic of solar wind bulk speeds ($< 20$ keV u$^{-1}$/$2000$ km s$^{-1}$). We provide polynomial fits for cross sections of charge-changing and ionization reactions, and list the experimental needs for future studies. We calculated Maxwellian-averaged cross sections and fitted them with bivariate polynomials for solar wind temperatures ranging from $10^5$ to $10^6$ K ($12-130$ eV). Single- and double-electron captures by He$^{2+}$ dominate at typical solar wind speeds. Correspondingly, single-electron capture by H$^+$ and single-electron loss by H$^-$ dominate at these speeds, resulting in the production of energetic neutral atoms (ENAs). Ionization cross sections all peak at energies above $20$ keV and are expected to play a moderate role in the total ion production. The effect of solar wind Maxwellian temperatures is found to be maximum for cross sections peaking at higher energies, suggesting that local heating at shock structures in cometary and planetary environments may favor processes previously thought to be negligible.