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.
Highlights
The Rosetta spacecraft first encountered comet 67P/Churyumov–Gerasimenko (67P) in August 2014
As Galand et al (2016) studied pre-perihelion cases, we focus in this paper on complementary post-perihelion case studies where we apply the same data-based ionospheric model in order to validate the findings of Galand et al (2016) and Heritier et al (2017a), and test the robustness of this model to singular events, such as the impacts of co-rotating interacting regions (CIRs) onto the cometary atmosphere or the ability of the model to reproduce measurements obtained during and after abrupt changes in the cometocentric distances covered by Rosetta
The neutral outflow velocity is estimated from previous ionospheric studies (Galand et al 2016; Heritier et al 2017a) and measurements from Microwave Instruments for the Rosetta Orbiter (MIRO) (Gulkis et al 2007), which give values lying within the 300–800 ms−1 range (Gulkis et al 2015; Lee et al 2015; Marshall et al 2017) for large heliocentric distance conditions. – νe−(r) [s−1] is the electron-impact ionization frequency
Summary
The Rosetta spacecraft first encountered comet 67P/Churyumov–Gerasimenko (67P) in August 2014. While the ionospheric model of Galand et al (2016) used constant neutral and ion velocities, the model was upgraded to be applied to the end of mission study (Heritier et al 2017a) to feature neutral and ion velocity profiles (calibrated to ROSINA–COPS) reproducing the expansion and acceleration of the cometary gas as it moves away from the nucleus This new feature was essential to explain the near-surface plasma densities observed by RPC–MIP and RPC–LAP as most of the acceleration of the outgassing flow takes place in the first few kilometres above the surface (Tenishev et al 2008; Fougere 2014).
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