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Cold and warm electrons at comet 67P/Churyumov-Gerasimenko
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0003-2926-6761
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, Department of Physics and Astronomy.ORCID iD: 0000-0002-0957-3847
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0003-3725-4920
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
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2017 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, article id A15Article in journal (Refereed) Published
Abstract [en]

Context. Strong electron cooling on the neutral gas in cometary comae has been predicted for a long time, but actual measurements of low electron temperature are scarce. Aims. Our aim is to demonstrate the existence of cold electrons in the inner coma of comet 67P/Churyumov-Gerasimenko and show filamentation of this plasma.

Methods. In situ measurements of plasma density, electron temperature and spacecraft potential were carried out by the Rosetta Langmuir probe instrument, LAP. We also performed analytical modelling of the expanding two-temperature electron gas.

Results. LAP data acquired within a few hundred km from the nucleus are dominated by a warm component with electron temperature typically 5-10 eV at all heliocentric distances covered (1.25 to 3.83 AU). A cold component, with temperature no higher than about 0.1 eV, appears in the data as short (few to few tens of seconds) pulses of high probe current, indicating local enhancement of plasma density as well as a decrease in electron temperature. These pulses first appeared around 3 AU and were seen for longer periods close to perihelion. The general pattern of pulse appearance follows that of neutral gas and plasma density. We have not identified any periods with only cold electrons present. The electron flux to Rosetta was always dominated by higher energies, driving the spacecraft potential to order -10 V.

Conclusions. The warm (5-10 eV) electron population observed throughout the mission is interpreted as electrons retaining the energy they obtained when released in the ionisation process. The sometimes observed cold populations with electron temperatures below 0.1 eV verify collisional cooling in the coma. The cold electrons were only observed together with the warm population. The general appearance of the cold population appears to be consistent with a Haser-like model, implicitly supporting also the coupling of ions to the neutral gas. The expanding cold plasma is unstable, forming filaments that we observe as pulses.

Place, publisher, year, edition, pages
2017. Vol. 605, article id A15
Keywords [en]
comets: general, plasmas, space vehicles: instruments
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:uu:diva-337755DOI: 10.1051/0004-6361/201630159ISI: 000412231200111OAI: oai:DiVA.org:uu-337755DiVA, id: diva2:1173367
Funder
Swedish National Space Board, 109/12; 171/12; 135/13; 166/14; 168/15Swedish Research Council, 621-2013-4191
Note

Funding: The results presented here are only possible thanks to the combined efforts over 20 yr by many groups and individuals involved in Rosetta, including but not restricted to the ESA project teams at ESTEC, ESOC and ESAC and all people involved in designing, building, testing and operating RPC and LAP. We thank Kathrin Altwegg for discussions of the pulses in LAP and COPS. Rosetta is a European Space Agency (ESA) mission with contributions from its member states and the National Aeronautics and Space Administration (NASA). The work on RPC-LAP data was funded by the Swedish National Space Board under contracts 109/12, 171/12, 135/13, 166/14 and 168/15, and by Vetenskapsradet under contract 621-2013-4191. This work has made use of the AMDA and RPC Quicklook database, provided by a collaboration between the Centre de Donnees de la Physique des Plasmas CDPP (supported by CNRS, CNES, Observatoire de Paris and Universite Paul Sabatier, Toulouse), and Imperial College London (supported by the UK Science and Technology Facilities Council).

Available from: 2018-01-12 Created: 2018-01-12 Last updated: 2018-04-18Bibliographically approved
In thesis
1. Plasma and Dust around Icy Moon Enceladus and Comet 67P/Churyumov-Gerasimenko
Open this publication in new window or tab >>Plasma and Dust around Icy Moon Enceladus and Comet 67P/Churyumov-Gerasimenko
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Saturn's moon Enceladus and comet 67P/Churyumov-Gerasimenko both are examples of icy solar system objects from which gas and dust flow into space. At both bodies, the gas becomes partly ionized and the dust grains get charged. Both bodies have been visited by spacecraft carrying similar Langmuir probe instruments for observing the plasma and the charged dust. As it turns out, the conditions at Enceladus and the comet are different and we emphasize different aspects of their plasma environments. At Enceladus, we concentrate on the characteristic plasma regions and charged dust. At the comet, we investigate the plasma and in particular plasmavariations and cold electrons.

At Enceladus, internal frictional heating leads to gas escaping from cracks in the ice from the south pole region. This causes a plume of gas, which becomes partially ionized, and dust, becoming charged. We have investigated the plasma and charged nanodust in this region by the use of the Langmuir probe (LP) of the Radio and Plasma Wave Science (RPWS) instrument on Cassini. The dust charge density can be calculated from the quasineutrality condition, the difference between ion and electron density measurements from LP. We found support for this method by comparing to measurements of larger dust grains by the RPWS electric antennas. We use the LP method to find that the plasma and dust environment of Enceladus can be divided into at least three regions. In addition to the well known plume, these are the plume edge and the trail region.

At the comet, heat from the Sun sublimates ice to gas dragging dust along as it flows out into space. When the neutral gas molecules are ionized, by photoionization and electron impact ionization, we get a plasma. Models predict that the electron temperature just after ionization is around 10 eV, but that collisions with the neutral gas should cool the electron gas to below 0.1 eV. We used the Langmuir probe instrument (LAP) on Rosetta to estimate plasma temperatures and show a co-existence of cold and warm electrons in the plasma. We find that the cold plasma often is observed as brief pulses not only in the LAP data but also in the measurements of magnetic field, plasma density and ion energy by other Rosetta plasma instruments. We interpret these pulses as filaments of plasma propagating outwards from a diamagnetic cavity, as predicted by hybrid simulations. The gas production rate of comet 67P varied by more than three orders of magnitude during the Rosetta mission (up to March 2016). We therefore have an excellent opportunity to investigate how the electron cooling in a cometary coma evolves with activity. We used a method combining LAP and the Mutual Impedance Probe (MIP) for deriving the presence of cold electrons. We show that cold electrons were present intermittently during a large part of the mission and as far out as 3 AU. Models suggest only negligible cooling and we suggest that the ambipolar field keeps the electrons close to the nucleus and giving them more time to lose energy by collision.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 94
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1673
National Category
Fusion, Plasma and Space Physics
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-348856 (URN)978-91-513-0346-8 (ISBN)
Public defence
2018-06-11, 2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2018-05-18 Created: 2018-04-18 Last updated: 2018-10-08

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Eriksson, Anders I.Engelhardt, Ilka. A. D.André, MatsBoström, RolfEdberg, Niklas J. T.Johansson, Fredrik L.Odelstad, EliasVigren, ErikWahlund, Jan-Erik

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Eriksson, Anders I.Engelhardt, Ilka. A. D.André, MatsBoström, RolfEdberg, Niklas J. T.Johansson, Fredrik L.Odelstad, EliasVigren, ErikWahlund, Jan-ErikWedlund, Cyril SimonBroiles, ThomasMandt, K.Galand, M.
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Swedish Institute of Space Physics, Uppsala DivisionDepartment of Physics and Astronomy
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