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On the ion-neutral coupling in cometary comae
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0003-2926-6761
2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 482, no 2, p. 1937-1941Article in journal (Refereed) Published
Abstract [en]

In a cometary coma, the ion-neutral decoupling distance, sometimes referred to as the ion exobase or collisionopause, can be defined as the cometocentric distance, r(in), where ions, initially moving with the neutral outgassing speed, have a probability of 1/e of not colliding with neutrals on their subsequent journey radially outwards. We present an analytical model for calculating this decoupling distance in the presence of a static radial electric field. We show that for a logarithmically decaying potential, the value of r(in) can even decrease to similar to 15 per cent of its field-free case value. Moreover, already at this distance, the effective ion speed can be expected to markedly exceed the neutral expansion velocity. These analytical results are in line with previous numerical calculations, adapting similar but not identical field profiles. The presence of a non-negligible ambipolar electric field and limited importance of ion-neutral collisional coupling are further supported by observations in the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko by plasma instruments onboard Rosetta that reveal ion speeds several times higher than the neutral expansion velocity.

Place, publisher, year, edition, pages
2019. Vol. 482, no 2, p. 1937-1941
Keywords [en]
molecular processes, comets (general)
National Category
Astronomy, Astrophysics and Cosmology Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:uu:diva-374113DOI: 10.1093/mnras/sty2869ISI: 000454578700037OAI: oai:DiVA.org:uu-374113DiVA, id: diva2:1281747
Available from: 2019-01-23 Created: 2019-01-23 Last updated: 2019-01-23Bibliographically approved

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Vigren, ErikEriksson, Anders I.

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Astronomy, Astrophysics and CosmologyFusion, Plasma and Space Physics

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