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| Titel |
Interruption of ion acceleration by collisions with neutrals in a cometary
coma: a 1D model applied to 67P/Churyumov-Gerasimenko |
| VerfasserIn |
Erik Vigren, Anders I. Eriksson, Fredrik L. Johansson |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250150510
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| Publikation (Nr.) |
 EGU/EGU2017-14980.pdf |
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| Zusammenfassung |
| We present a 1D model of a cometary ionosphere with the main purpose to investigate the
ability of the neutrals to hamper ion acceleration along electric fields in the radial direction.
Because ion-neutral reaction cross-sections are energy dependent, the distance from a
cometary nucleus within which ions remain collisionally coupled to the neutrals
is dictated not only by the comet’s activity level but also by the electromagnetic
fields in the coma. As electron cooling is inefficient due to low neutral gas density
and density decays with cometocentric distance a significant ambipolar electric
field may develop. In the model we treat charge transfer processes as replacing a
fast ion and a slow neutral with a fast neutral and a slow ion. For a given neutral
background and electric field profile, the model, which in essence is based on the
multiplication principle of probabilities, calculates observables such as the total ion number
density, the H3O+/H2O+ number density ratio, the mean ion drift speed and the ion
energy distribution function, as a function of cometocentric distance. The model is
applied to different conditions encountered by Rosetta during its mission to comet
67P/Churyumov-Gerasimenko. Our findings include i) that the activity, even near perihelion,
was probably not high enough for an efficient ion-neutral coupling all the way to the
spacecraft location, and ii) that model runs using electric field profiles that give ion number
densities and mean flow speeds within limits constrained by RPC observations
tend to give H3O+/H2O+ number density ratios higher than typically observed by
ROSINA/DFMS (e.g., [1]). We discuss also the influence on the results of including the ion
motion in large-amplitude plasma waves into the model. Finally we highlight some
reactions for which determinations of cross-sections at low collision energies would be
valuable.
[1] Fuselier, S.A., et al. 2016, MNRAS, 462, S312 |
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