Ion acoustic waves at comet 67P/Churyumov-Gerasimenko

Observations and computations^⋆

¹ Royal Belgian Institute for Space Aeronomy (BIRA-IASB), Avenue Circulaire 3, 1180 Brussels, Belgium
e-mail: herbert.gunell@physics.org
² Swedish Institute of Space Physics, Box 812, 981 28 Kiruna, Sweden
³ Department of Physics, Umeå University, 901 87 Umeå, Sweden
⁴ Swedish Institute of Space Physics, Ångström Laboratory, Lägerhyddsvägen 1, 75121 Uppsala, Sweden
⁵ Department of Physics and Astronomy, Uppsala University, 75120 Uppsala, Sweden
⁶ LPC2E, CNRS, 45071 Orléans, France
⁷ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁸ Institut für Geophysik und extraterrestrische Physik, TU Braunschweig, Mendelssohnstr. 3, 38106 Braunschweig, Germany
⁹ Department of Physics, University of Oslo, Box 1048 Blindern, 0316 Oslo, Norway
¹⁰ Laboratoire de Chimie Quantique et Photophysique, Université Libre de Bruxelles, 50 Avenue F. D. Roosevelt, 1050 Brussels, Belgium

Received: 28 September 2016
Accepted: 11 January 2017

Abstract

Context. On 20 January 2015 the Rosetta spacecraft was at a heliocentric distance of 2.5 AU, accompanying comet 67P/Churyumov-Gerasimenko on its journey toward the Sun. The Ion Composition Analyser (RPC-ICA), other instruments of the Rosetta Plasma Consortium, and the ROSINA instrument made observations relevant to the generation of plasma waves in the cometary environment.

Aims. Observations of plasma waves by the Rosetta Plasma Consortium Langmuir probe (RPC-LAP) can be explained by dispersion relations calculated based on measurements of ions by the Rosetta Plasma Consortium Ion Composition Analyser (RPC-ICA), and this gives insight into the relationship between plasma phenomena and the neutral coma, which is observed by the Comet Pressure Sensor of the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis instrument (ROSINA-COPS).

Methods. We use the simple pole expansion technique to compute dispersion relations for waves on ion timescales based on the observed ion distribution functions. These dispersion relations are then compared to the waves that are observed. Data from the instruments RPC-LAP, RPC-ICA and the mutual impedance probe (RPC-MIP) are compared to find the best estimate of the plasma density.

Results. We find that ion acoustic waves are present in the plasma at comet 67P/Churyumov-Gerasimenko, where the major ion species is H₂O⁺. The bulk of the ion distribution is cold, k_BT_i = 0.01 eV when the ion acoustic waves are observed. At times when the neutral density is high, ions are heated through acceleration by the solar wind electric field and scattered in collisions with the neutrals. This process heats the ions to about 1 eV, which leads to significant damping of the ion acoustic waves.

Conclusions. In conclusion, we show that ion acoustic waves appear in the H₂O⁺ plasmas at comet 67P/Churyumov-Gerasimenko and how the interaction between the neutral and ion populations affects the wave properties.