Solar wind charge exchange in cometary atmospheres: II. Analytical model
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A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
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Date
2019-10-01
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Language
en
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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 because they mass-load the solar wind through an effective conversion of fast, light solar wind ions into slow, heavy cometary ions. The ESA/Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) provided a unique opportunity to study charge-changing processes in situ. Aims. To understand the role of charge-changing reactions in the evolution of the solar wind plasma and to interpret the complex in situ measurements made by Rosetta, numerical or analytical models are necessary. Methods. An extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines is presented. It is based on a thorough book-keeping of available charge-changing cross sections of hydrogen and helium particles in a water gas. Results. After presenting a general 1D solution of charge exchange at comets, we study the theoretical dependence of charge-state distributions of (He2+, He+, He0) and (H+, H0, H-) on solar wind parameters at comet 67P. We show that double charge exchange for the He2+-H2O system plays an important role below a solar wind bulk speed of 200 km s-1, resulting in the production of He energetic neutral atoms, whereas stripping reactions can in general be neglected. Retrievals of outgassing rates and solar wind upstream fluxes from local Rosetta measurements deep in the coma are discussed. Solar wind ion temperature effects at 400 km s-1 solar wind speed are well contained during the Rosetta mission. Conclusions. As the comet approaches perihelion, the model predicts a sharp decrease of solar wind ion fluxes by almost one order of magnitude at the location of Rosetta, forming in effect a solar wind ion cavity. This study is the second part of a series of three on solar wind charge-exchange and ionization processes at comets, with a specific application to comet 67P and the Rosetta mission.Description
Funding Information: cA knowledgements. The work at University of Oslo was funded by the Norwegian Research Council “Rosetta” grant No. 240000. 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. Work at Imperial College London was supported by STFC of UK under grant ST/K001051/1 and ST/N000692/1, ESA, under contract No.4000119035/16/ES/JD. The work at NASA/SSAI was supported by NASA Astrobiology Institute grant NNX15AE05G and by the NASA HIDEE Program. C.S.W. would like to thank S. Barabash (IRF Kiruna, Sweden) for useful impetus on the work leading to the present study and 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. thanks M.S.W. and L.S.W. for help in structuring this immense workload and for unwavering encouragements throughout these two years of work. Dataset 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
Comets: general, Comets: individual: 67P/Churyumov-Gerasimenko, Instrumentation: detectors, Methods: analytical, Solar wind, Waves
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Citation
Wedlund, C S, Behar, E, Kallio, E, Nilsson, H, Alho, M, Gunell, H, Bodewits, D, Beth, A, Gronoff, G & Hoekstra, R 2019, ' Solar wind charge exchange in cometary atmospheres : II. Analytical model ', Astronomy & Astrophysics, vol. 630, A36 . https://doi.org/10.1051/0004-6361/201834874, https://doi.org/10.1051/0004-6361/201834874