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标题：Solar wind charge exchange in cometary atmospheres
其他标题：I. Charge-changing and ionization cross sections for He and H particles in H2O
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作者：Cyril Simon Wedlund ; Dennis Bodewits ; Markku Alho 等
期刊名称：Astronomy & Astrophysics
印刷版ISSN：0004-6361
电子版ISSN：1432-0746
出版年度：2019
卷号：630
页码：1-22
DOI：10.1051/0004-6361/201834848
语种：English
出版社：EDP Sciences
摘要：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 105to 106K (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.
关键词：Key wordsencomets: generalcomets: individual: 67P/Churyumov-Gerasimenkoinstrumentation: detectorssolar windmethods: data analysisplasmas