Context.
The Rosetta spacecraft provided us with a unique opportunity to study comet 67P
/
Churyumov-Gerasimenko from a close
perspective and over a two-year 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 di
ff
erent drivers of the cometary ionosphere and assess their variability over time and over the
di
ff
erent 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
two-year 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
four 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.