Cometary ion dynamics at 67P: a collisional test-particle approach with Rosetta data comparison
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Author(s)
Type
Journal Article
Abstract
The Rosetta spacecraft escorted comet 67P/Churyumov–Gerasimenko for two years, gathering a rich and variable data set.
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 ionization 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are no longer valid, and plasma density is overestimated.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 × 1026 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. 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.
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 ionization 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are no longer valid, and plasma density is overestimated.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 × 1026 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. 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.
Date Issued
2025-08-01
Date Acceptance
2025-07-11
Citation
Monthly Notices of the Royal Astronomical Society, 2025, 541 (4), pp.3590-3605
ISSN
0035-8711
Publisher
Oxford University Press (OUP)
Start Page
3590
End Page
3605
Journal / Book Title
Monthly Notices of the Royal Astronomical Society
Volume
541
Issue
4
Copyright Statement
© The Author(s) 2025. Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
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Publication Status
Published
Article Number
staf1162
Date Publish Online
2025-07-16