Solar wind charge exchange in cometary atmospheres

III. Results from the Rosetta mission to comet 67P/Churyumov-Gerasimenko

¹ Department of Physics, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway
e-mail: c.s.wedlund@fys.uio.no
² Swedish Institute of Space Physics, PO Box 812, 981 28 Kiruna, Sweden
³ Department of Computer Science, Luleå University of Technology, Electrical and Space Engineering, Kiruna 981 28, Sweden
⁴ Department of Electronics and Nanoengineering, School of Electrical Engineering, Aalto University, PO Box 15500, 00076 Aalto, Finland
⁵ Royal Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180 Brussels, Belgium
⁶ Department of Physics, Umeå University, 901 87 Umeå, Sweden
⁷ Physics Department, Auburn University, Auburn, AL 36849, USA
⁸ Department of Physics, Imperial College London, Prince Consort Road, London SW7 2AZ, UK
⁹ Space Research and Planetary Sciences, University of Bern, 3012 Bern, Switzerland
¹⁰ Space Research Institute, Austrian Academy of Sciences, Schmiedlstraße 6, 8042 Graz, Austria
¹¹ Science directorate, Chemistry & Dynamics branch, NASA Langley Research Center, Hampton, VA 23666 Virginia, USA
¹² SSAI, Hampton, VA 23666 Virginia, USA
¹³ Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

Received: 14 December 2018
Accepted: 5 February 2019

Abstract

Context. Solar wind charge-changing reactions are of paramount importance to the physico-chemistry of the atmosphere of a comet. The ESA/Rosetta mission to comet 67P/Churyumov-Gerasimenko (67P) provides a unique opportunity to study charge-changing processes in situ.

Aims. 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.

Methods. We used an extended analytical formalism describing solar wind charge-changing processes at comets along solar wind streamlines. The model is driven by solar wind ion measurements from the Rosetta Plasma Consortium-Ion Composition Analyser (RPC-ICA) and neutral density observations from the Rosetta Spectrometer for Ion and Neutral Analysis-Comet Pressure Sensor (ROSINA-COPS), as well as by charge-changing cross sections of hydrogen and helium particles in a water gas.

Results. 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 (ENAs), with fluxes expected to rival those of H⁺ and He²⁺ ions.

Conclusions. Neutral outgassing rates are retrieved from local RPC-ICA flux measurements and match ROSINA estimates very well throughout the mission. From the model, we find that solar wind charge exchange is unable to fully explain the magnitude of the sharp drop in solar wind ion fluxes observed by Rosetta for heliocentric distances below 2.5 AU. This is likely because the model does not take the relative ion dynamics into account and to a lesser extent because it ignores the formation of bow-shock-like structures upstream of the nucleus. This work also shows that the ionization by solar extreme-ultraviolet radiation and energetic electrons dominates the source of cometary ions, although solar wind contributions may be significant during isolated events.