Constraining ion transport in the diamagnetic cavity of comet 67P
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Published version
Author(s)
Type
Journal Article
Abstract
The European Space Agency Rosetta mission escorted comet 67P for a 2-yr section of its six and a half-year orbit around the
Sun. By perihelion in 2015 August, the neutral and plasma data obtained by the spacecraft instruments showed the comet had
transitioned to a dynamic object with large-scale plasma structures and a rich ion environment. One such plasma structure is
the diamagnetic cavity: a magnetic field-free region formed by interaction between the unmagnetized cometary plasma and
the impinging solar wind. Within this region, unexpectedly high ion bulk velocities have been observed, thought to have been
accelerated by an ambipolar electric field. We have developed a 1D numerical model of the cometary ionosphere to constrain
the impact of various electric field profiles on the ionospheric density profile and ion composition. In the model, we include
three ion species: H2O+, H3O+, and NH+
4 . The latter, not previously considered in ionospheric models including acceleration, is
produced through the protonation of NH3 and only lost through ion–electron dissociative recombination, and thus particularly
sensitive to the time-scale of plasma loss through transport. We also assess the importance of including momentum transfer
when assessing ion composition and densities in the presence of an electric field. By comparing simulated electron densities to
Rosetta Plasma Consortium data sets, we find that to recreate the plasma densities measured inside the diamagnetic cavity near
perihelion, the model requires an electric field proportional to r−1 of around 0.5–2 mV m−1 surface strength, leading to bulk ion
speeds at Rosetta of 1.2–3.0 km s−1.
Sun. By perihelion in 2015 August, the neutral and plasma data obtained by the spacecraft instruments showed the comet had
transitioned to a dynamic object with large-scale plasma structures and a rich ion environment. One such plasma structure is
the diamagnetic cavity: a magnetic field-free region formed by interaction between the unmagnetized cometary plasma and
the impinging solar wind. Within this region, unexpectedly high ion bulk velocities have been observed, thought to have been
accelerated by an ambipolar electric field. We have developed a 1D numerical model of the cometary ionosphere to constrain
the impact of various electric field profiles on the ionospheric density profile and ion composition. In the model, we include
three ion species: H2O+, H3O+, and NH+
4 . The latter, not previously considered in ionospheric models including acceleration, is
produced through the protonation of NH3 and only lost through ion–electron dissociative recombination, and thus particularly
sensitive to the time-scale of plasma loss through transport. We also assess the importance of including momentum transfer
when assessing ion composition and densities in the presence of an electric field. By comparing simulated electron densities to
Rosetta Plasma Consortium data sets, we find that to recreate the plasma densities measured inside the diamagnetic cavity near
perihelion, the model requires an electric field proportional to r−1 of around 0.5–2 mV m−1 surface strength, leading to bulk ion
speeds at Rosetta of 1.2–3.0 km s−1.
Date Issued
2024-05
Date Acceptance
2024-03-22
Citation
Monthly Notices of the Royal Astronomical Society, 2024, 530 (1), pp.66-81
ISSN
0035-8711
Publisher
Oxford University Press
Start Page
66
End Page
81
Journal / Book Title
Monthly Notices of the Royal Astronomical Society
Volume
530
Issue
1
Copyright Statement
© 2024 The Author(s). 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.
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.
License URL
Identifier
https://academic.oup.com/mnras/article/530/1/66/7634366
Subjects
67P/CHURYUMOV-GERASIMENKO
Astronomy & Astrophysics
COLD ELECTRONS
comets: individual: comet 67P/CG
DISSOCIATIVE RECOMBINATION
EVOLUTION
MODEL
NUMBER DENSITY
Physical Sciences
PLASMA
plasmas
ROSETTA
RPC-ICA
Science & Technology
STORAGE-RING
Publication Status
Published
Date Publish Online
2024-03-23