Solar wind charge exchange in cometary atmospheres. I. Charge-changing and ionization cross sections for He and H particles in H2O

Cyril Simon Wedlund, Dennis Bodewits, Markku Alho, Ronnie Hoekstra, Etienne Behar, Guillaume Gronoff, Herbert Gunell, Hans Nilsson, Esa Kallio, Arnaud Beth

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Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet, mass-loading the solarwind through an effective conversion of fast light solar wind ions into slow heavy cometary ions. To understand these processes and place the min the context of a solar wind plasma interacting with a neutralatmosphere, 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 bulkspeeds ($<20$ 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.
Original languageEnglish
Number of pages22
JournalAstronomy & Astrophysics
Early online date11 Feb 2019
Publication statusPublished - Oct 2019
MoE publication typeA1 Journal article-refereed


  • Plasmas
  • comets: general
  • comets: individual: 67P / Churyumov-Gerasimenko
  • instrumentation: detectors
  • solar wind: charge-exchange processes
  • Methods: data analysis: cross sections

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