Solar wind charge exchange in cometary atmospheres. I. Charge-changing and ionization cross sections for He and H particles in H2O
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A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
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Date
2019-10-01
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Language
en
Pages
22
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Astronomy & Astrophysics, Volume 630
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.Description
Funding Information: cA knolw edgemen. The work at University of Oslo was funded by the Norwegian Research Council grant No. 240 000. The work at NASA/SSAI was supported by NASA Astrobiology Institute grant NNX15AE05G and by the NASA HIDEE Program. Work at the Royal Belgian Institute for Space Aeronomy was supported by the Belgian Science Policy Office through the Solar-Terrestrial Centre of Excellence. Work at Umeå University was funded by SNSB grant 201/15 and SNSA grant 108/18. Work at Imperial College London was supported by STFC of UK under grant ST/N000692/1 and by ESA under contract No. 4000119035/16/ES/JD. C.S.W. is grateful to S. Barabash (IRF Kiruna, Sweden) for suggesting to investigate electron stripping processes at a comet. The authors thank the ISSI International Team “Plasma Environment of comet 67P after Rosetta” for fruitful discussions and collaborations. C.S.W. acknowledges the Cross sections in ion-atom collisions (http://cdfe.sinp.msu.ru/ services/cccs/HTM/main.htm) database hosted by the Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University (Russia) and maintained by N.V. Novikov and Ya. A. Teplova, which was instrumental in finding cross-section datasets for H2, H, O2, and O targets. C.S.W. is indebted to M.S.W. and L.S.W. for cynosural advice throughout this obstinate quest for the best experimental cross sections. Datasets of the Rosetta mission can be freely accessed from ESA’s Planetary Science Archive (http://archives.esac. esa.int/psa). Publisher Copyright: © ESO 2019.
Keywords
Plasmas, comets: general, comets: individual: 67P / Churyumov-Gerasimenko, instrumentation: detectors, solar wind: charge-exchange processes, Methods: data analysis: cross sections
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Citation
Wedlund, C S, Bodewits, D, Alho, M, Hoekstra, R, Behar, E, Gronoff, G, Gunell, H, Nilsson, H, Kallio, E & Beth, A 2019, ' Solar wind charge exchange in cometary atmospheres. I. Charge-changing and ionization cross sections for He and H particles in H2O ', Astronomy & Astrophysics, vol. 630, A35 . https://doi.org/10.1051/0004-6361/201834848