Volume 630, October 2019
Rosetta mission full comet phase results
|Number of page(s)||17|
|Section||Planets and planetary systems|
|Published online||20 September 2019|
Solar wind charge exchange in cometary atmospheres
II. Analytical model
Department of Physics, University of Oslo,
PO Box 1048 Blindern, 0316 Oslo, Norway
2 Swedish Institute of Space Physics, PO Box 812, 981 28 Kiruna, Sweden
3 Luleå University of Technology, Department of Computer Science, 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 Science Directorate, Chemistry & Dynamics Branch, NASA Langley Research Center, Hampton, VA 23666, USA
10 SSAI, Hampton, VA 23666, USA
11 Zernike Institute for Advanced Materials, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
Accepted: 11 January 2019
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.
Key words: comets: general / comets: individual: 67P/Churyumov-Gerasimenko / instrumentation: detectors / waves / solar wind / methods: analytical
© ESO 2019
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