Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko

Observations between 3.6 and 2.0 AU

¹ Swedish Institute of Space Physics, Box 812, 981 28 Kiruna, Sweden
e-mail: hans.nilsson@irf.se
² Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Rymdcampus 1, 981 28 Kiruna, Sweden
³ Aalto University, School of Electrical Engineering, Department of Radio Science and Engineering, PO Box 13000, 00076 Aalto, Finland
⁴ Belgian Institute for Space Aeronomy, avenue Circulaire 3, 1180 Brussels, Belgium
⁵ Swedish Institute of Space Physics, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, Sweden
⁶ TU – Braunschweig, Institute for Geophysics and extraterrestrial Physics, Mendelssohnstr. 3, 38106 Braunschweig, Germany
⁷ Southwest Research Institute, 6220 Culebra Rd., San Antonio, TX 78238, USA
⁸ Imperial College London, Exhibition Road, London SW7 2AZ, UK
⁹ Wigner Research Centre for Physics, 1121 Konkoly Thege street 29–33, Budapest, Hungary
¹⁰ Institut de Recherche en Astrophysique et Planétologie, 31028 Toulouse, France
¹¹ University of Helsinki, Department of Physics, PO Box 64, University of Helsinki, 00014 Helsinki, Finland
¹² Finnish Meteorological Institute, PO BOX 503, 00101 Helsinki, Finland
¹³ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace (LPC2E), UMR 7328 CNRS – Université d’ Orléans, France
¹⁴ Space Research Institute, Austrian Academy of Sciences, Schmiedlstraße 6, 8042 Graz, Austria
¹⁵ Rosetta Science Ground Segment, SRE-OOR, Office A006, European Space Astronomy Centre, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain

Received: 20 March 2015
Accepted: 6 May 2015

Abstract

Context. The Rosetta spacecraft is escorting comet 67P/Churyumov-Gerasimenko from a heliocentric distance of >3.6 AU, where the comet activity was low, until perihelion at 1.24 AU. Initially, the solar wind permeates the thin comet atmosphere formed from sublimation.

Aims. Using the Rosetta Plasma Consortium Ion Composition Analyzer (RPC-ICA), we study the gradual evolution of the comet ion environment, from the first detectable traces of water ions to the stage where cometary water ions accelerated to about 1 keV energy are abundant. We compare ion fluxes of solar wind and cometary origin.

Methods. RPC-ICA is an ion mass spectrometer measuring ions of solar wind and cometary origins in the 10 eV–40 keV energy range.

Results. We show how the flux of accelerated water ions with energies above 120 eV increases between 3.6 and 2.0 AU. The 24 h average increases by 4 orders of magnitude, mainly because high-flux periods become more common. The water ion energy spectra also become broader with time. This may indicate a larger and more uniform source region. At 2.0 AU the accelerated water ion flux is frequently of the same order as the solar wind proton flux. Water ions of 120 eV–few keV energy may thus constitute a significant part of the ions sputtering the nucleus surface. The ion density and mass in the comet vicinity is dominated by ions of cometary origin. The solar wind is deflected and the energy spectra broadened compared to an undisturbed solar wind.

Conclusions. The flux of accelerated water ions moving from the upstream direction back toward the nucleus is a strongly nonlinear function of the heliocentric distance.