Solar wind charge exchange in cometary atmospheres III. Results from the Rosetta mission to comet 67P/Churyumov-Gerasimenko

Cyril Simon Wedlund, Etienne Behar, Hans Nilsson, Markku Alho, Esa Kallio, Herbert Gunell, Dennis Bodewits, Kevin Heritier, Marina Galand, Arnaud Beth, Martin Rubin, Kathrin Altwegg, Martin Volwerk, Guillaume Gronoff, Ronnie Hoekstra

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Abstract

Solar wind charge-changing reactions are of paramount importance to thephysico-chemistry of the atmosphere of a comet. The ESA/Rosetta missionto comet 67P/Churyumov-Gerasimenko (67P) provides a unique opportunity to study charge-changing processes in situ. To understand the role of these reactions in the evolution of the solar wind plasma, and interpret the complex in-situ measurements made by Rosetta, numerical or analytical models are necessary. We use an extended analytical formalism describing solar wind charge-changing processes at comets along solarwind streamlines. The model is driven by solar wind ion measurements from the Rosetta Plasma Consortium-Ion Composition Analyzer (RPC-ICA) and neutral density observations from the Rosetta Spectrometer for Ion and Neutral Analysis-Comet Pressure Sensor (ROSINA-COPS), as well as charge-changing cross sections of hydrogen and helium particles in a water gas. A mission-wide overview of charge-changing efficiencies at comet 67P is presented. Electron capture cross sections dominate and favor the production of He and H energetic neutral atoms, with fluxes expected to rival those of H+ and He2+ ions. Neutral outgassing rates are retrieved from local RPC-ICA flux measurements, and match ROSINA's estimates very well. From the model, we find that solar wind charge exchange is unable to fully explain the magnitude of the sharp drop of solar wind ion fluxes observed by Rosetta for heliocentric distances below 2.5 AU. This is likely because the model does not take into account the relative ion dynamics and, to a lesser extent, ignore the formation of bow shock-like structures upstream of the nucleus. This work also shows that the ionization by solar EUV radiation and energetic electrons dominates the source of cometary ions, although solar wind contributions may be significant during isolated events.
Original languageEnglish
Article numberA37
Number of pages15
JournalAstronomy & Astrophysics
Volume630
Early online date5 Feb 2019
DOIs
Publication statusPublished - 20 Sep 2019
MoE publication typeA1 Journal article-refereed

Keywords

  • Comets: General
  • comets: individual: 67P / Churyumov-Gerasimenko
  • instrumentation: detectors
  • solar wind, methods: analytical
  • solar wind: charge-exchange processes

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