Modeling the Plasma Composition of 67P/C-G at different Heliocentric Distances

TL;DR

This study models the plasma composition of comet 67P/Churyumov-Gerasimenko at various distances from the Sun, comparing predictions with in situ measurements to understand ion dynamics and composition changes.

Contribution

It introduces a multifluid chemical-hydrodynamical model to estimate ionospheric composition at different heliocentric distances, validated against Rosetta's plasma measurements.

Findings

Modeled ion densities match in situ data within factors of 1-3.

H3O+ ions dominate at the spacecraft location across most intervals.

Protonation-generated ions are dominant near perihelion at low cometocentric distances.

Abstract

The Rosetta spacecraft accompanied the comet 67P/C-G for nearly 2 years, collecting valuable data on the neutral and ion composition of the coma. The Rosetta Plasma Consortium (RPC) provided continuous measurements of the in situ plasma density while ROSINA-COPS monitored the neutral composition. In this work, we aim to estimate the composition of the cometary ionosphere at different heliocentric distances of the comet. Lauter et al. (2020) derived the temporal evolution of the volatile sublimation rates for 50 separated time intervals on the orbit of 67P/C-G using the COPS and DFMS data. We use these sublimation rates as inputs in a multifluid chemical-hydrodynamical model for 36 of the time intervals for heliocentric distances < 3 au. We compare the total ion densities obtained from our models with the local plasma density measured by the RPC instruments. We find that at the location of the spacecraft, our modeled ion densities match with the in situ measured plasma density within factors of 1 - 3 for many of the time intervals. We obtain the cometocentric distance variation of the ions H2O+ and H3O+ and the ion groups created respectively by the ionization and protonation of neutral species. We see that H3O+ is dominant at the spacecraft location for nearly all the time intervals while ions created due to protonation are dominant at low cometocentric distances for the intervals near perihelion. We also discuss our ion densities in the context of their detection by DFMS.