Cometary ion dynamics at 67P: A collisional test-particle approach with Rosetta data comparison

TL;DR

This study models comet 67P's ion dynamics using a 3D collisional test-particle approach, comparing results with Rosetta data to understand plasma transport regimes and the influence of electric fields.

Contribution

It introduces a collisional test-particle model incorporating hybrid simulation fields to analyze ion transport near comet 67P, highlighting the transition between different plasma regimes.

Findings

Model aligns with Rosetta plasma density data at low outgassing.

Enhanced ion transport explained by ambipolar electric fields near perihelion.

Collisional cooling of electrons is crucial for accurate ion dynamics modeling.

Abstract

The Rosetta spacecraft escorted comet 67P/Churyumov-Gerasimenko for two years, gathering a rich and variable dataset. Amongst the data from the Rosetta Plasma Consortium (RPC) suite of instruments are measurements of the total electron density from the Mutual Impedance Probe (MIP) and Langmuir Probe (LAP). At low outgassing, the plasma density measurements can be explained by a simple balance between the production through ionisation and loss through transport. Ions are assumed to travel radially at the outflow speed of the neutral gas. Near perihelion, the assumptions of this field-free chemistry-free model are no longer valid, and plasma density is overestimated. This can be explained by enhanced ion transport by an ambipolar electric field inside the diamagnetic cavity, where the interplanetary magnetic field does not reach. In this study, we explore the transition between these two regimes, at intermediate outgassing ($5.4 \times10^{26}~\mathrm{s^{-1}}$), when the interaction between the cometary and solar wind plasma influences the transport of the ions. We use a 3D collisional test-particle model, adapted from Stephenson et al. 2022 to model the cometary ions with input electric and magnetic fields from a hybrid simulation for 2.5-3 au. The total plasma density from this model is then compared to data from MIP/LAP and to the field-free chemistry-free model. In doing so, we highlight the limitations of the hybrid approach and demonstrate the importance of modelling collisional cooling of the electrons to understand the ion dynamics close to the nucleus.