Plasma source and loss at comet 67P during the Rosetta mission

¹ Department of Physics, Imperial College London, Prince Consort Road, London SW7 2AZ, UK
e-mail: k.heritier15@imperial.ac.uk
² LPC2E, CNRS, Université d’Orléans, 45100 Orléans, France
³ Ångström Laboratory, Swedish Institute of Space Physics, Lägerhyddsvägen 1, 752 37 Uppsala, Sweden
⁴ Physikalisches Institut, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
⁵ Southwest Research Institute, PO Drawer 28510, San Antonio, TX 78228-0510, USA
⁶ Space Sciences, Technologies and Astrophysics Research (STAR) Institute, Université de Liège, 4000 Liège, Belgium

Received: 22 February 2018
Accepted: 3 June 2018

Abstract

Context. The Rosetta spacecraft provided us with a unique opportunity to study comet 67P/Churyumov–Gerasimenko (67P) from a close perspective and over a 2-yr time period. Comet 67P is a weakly active comet. It was therefore unexpected to find an active and dynamic ionosphere where the cometary ions were largely dominant over the solar wind ions, even at large heliocentric distances.

Aims. Our goal is to understand the different drivers of the cometary ionosphere and assess their variability over time and over the different conditions encountered by the comet during the Rosetta mission.

Methods. We used a multi-instrument data-based ionospheric model to compute the total ion number density at the position of Rosetta. In-situ measurements from the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) and the Rosetta Plasma Consortium (RPC)–Ion and Electron Sensor (IES), together with the RPC–LAngmuir Probe instrument (LAP) were used to compute the local ion total number density. The results are compared to the electron densities measured by RPC–Mutual Impedance Probe (MIP) and RPC–LAP.

Results. We were able to disentangle the physical processes responsible for the formation of the cometary ions throughout the 2-yr escort phase and we evaluated their respective magnitudes. The main processes are photo-ionization and electron-impact ionization. The latter is a significant source of ionization at large heliocentric distance (>2 au) and was predominant during the last 4 months of the mission. The ionosphere was occasionally subject to singular solar events, temporarily increasing the ambient energetic electron population. Solar photons were the main ionizer near perihelion at 1.3 au from the Sun, during summer 2015.