Solar wind charge exchange in cometary atmospheres

I. Charge-changing and ionization cross sections for He and H particles in H₂O

¹ Department of Physics, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway
e-mail: c.s.wedlund@fys.uio.no
² Physics Department, Auburn University, Auburn, AL 36849, USA
³ Department of Electronics and Nanoengineering, School of Electrical Engineering, Aalto University, PO Box 15500, 00076 Aalto, Finland
⁴ Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
⁵ Swedish Institute of Space Physics, PO Box 812, 981 28 Kiruna, Sweden
⁶ Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Kiruna 981 28, Sweden
⁷ Science directorate, Chemistry & Dynamics branch, NASA Langley Research Center, Hampton, VA 23666 Virginia, USA
⁸ SSAI, Hampton, VA 23666 Virginia, USA
⁹ Royal Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180 Brussels, Belgium
¹⁰ Department of Physics, Umeå University, 901 87 Umeå, Sweden
¹¹ Department of Physics, Imperial College London, Prince Consort Road, London SW7 2AZ, UK

Received: 13 December 2018
Accepted: 11 February 2019

Abstract

Context. 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.

Aims. 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.

Methods. 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).

Results. We provide polynomial fits for cross sections of charge-changing and ionization reactions, and list the experimental needs for future studies. To take into account the energy distribution of the solar wind, we calculated Maxwellian-averaged cross sections and fitted them with bivariate polynomials for solar wind temperatures ranging from 10⁵ to 10⁶ K (12–130 eV).

Conclusions. Single- and double-electron captures by He²⁺ 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. However, 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. This study is the first part in a series of three on charge exchange and ionization processes at comets, with a specific application to comet 67P/Churyumov-Gerasimenko and the Rosetta mission.