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Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P
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, Space Plasma Physics. (Rymd- och plasmafysik, Space Plasma Physics)ORCID iD: 0000-0001-7854-6001
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. (Rymd- och plasmafysik, Space Plasma Physics)ORCID iD: 0000-0003-2926-6761
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. (Rymd- och plasmafysik, Space Plasma Physics)ORCID iD: 0000-0002-5386-8255
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
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2018 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, no 7, p. 5870-5893Article in journal (Refereed) Published
Abstract [en]

Abstract A major point of interest in cometary plasma physics has been the diamagnetic cavity, an unmagnetized region in the innermost part of the coma. Here we combine Langmuir and Mutual Impedance Probe measurements to investigate ion velocities and electron temperatures in the diamagnetic cavity of comet 67P, probed by the Rosetta spacecraft. We find ion velocities generally in the range 2?4 km/s, significantly above the expected neutral velocity 1 km/s, showing that the ions are (partially) decoupled from the neutrals, indicating that ion-neutral drag was not responsible for balancing the outside magnetic pressure. Observations of clear wake effects on one of the Langmuir probes showed that the ion flow was close to radial and supersonic, at least with respect to the perpendicular temperature, inside the cavity and possibly in the surrounding region as well. We observed spacecraft potentials  V throughout the cavity, showing that a population of warm (?5 eV) electrons was present throughout the parts of the cavity reached by Rosetta. Also, a population of cold ( ) electrons was consistently observed throughout the cavity, but less consistently in the surrounding region, suggesting that while Rosetta never entered a region of collisionally coupled electrons, such a region was possibly not far away during the cavity crossings.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2018. Vol. 123, no 7, p. 5870-5893
Keywords [en]
comets, Rosetta, plasma, diamagnetic cavity, ion velocity, electron temperature
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
URN: urn:nbn:se:uu:diva-356424DOI: 10.1029/2018JA025542ISI: 000442664300043OAI: oai:DiVA.org:uu-356424DiVA, id: diva2:1235716
Funder
Swedish National Space Board, 109/12, 168/15, 166/14Swedish Research Council, 621-2013-4191
Note

Article published in Early View on 25 July, 2018

Available from: 2018-07-26 Created: 2018-07-26 Last updated: 2018-11-05Bibliographically approved
In thesis
1. Plasma environment of an intermediately active comet: Evolution and dynamics observed by ESA's Rosetta spacecraft at 67P/Churyumov-Gerasimenko
Open this publication in new window or tab >>Plasma environment of an intermediately active comet: Evolution and dynamics observed by ESA's Rosetta spacecraft at 67P/Churyumov-Gerasimenko
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The subject of this thesis is the evolution and dynamics of the plasma environment of a moderately active comet before, during and after its closest approach to the Sun. For over 2 years in 2014-2016, the European Space Agency’s Rosetta spacecraft followed the comet 67P/Churyumov-Gerasimenko at distances typically between a few tens and a few hundred kilometers from the nucleus, the longest and closest inspection of a comet ever made. Its payload included a suite of five plasma instruments (the Rosetta Plasma Consortium, RPC), providing unprecedented in-situ measurements of the plasma environment in the inner coma of a comet.

In the first two studies, we use spacecraft potential measurements by the Langmuir probe instrument (LAP) to study the evolving cometary plasma environment. The spacecraft potential was mostly negative, often below -10 V and sometimes below -20 V, revealing the presence of warm (around 5-10 eV) coma photoelectrons, not effectively cooled by collisions with the relatively tenuous coma gas. The magnitude of the negative spacecraft potential depends on the electron density and traced heliocentric, cometocentric, seasonal and diurnal variations in cometary outgassing, consistent with production at or inside the cometocentric distance of the spacecraft as the dominant source of the observed plasma.

In the third study, we investigate ion velocities and electron temperatures in the diamagnetic cavity of the comet, combining LAP and Mutual Impedance Probe (MIP) measurements. Ion velocities were generally in the range 2-4 km/s, well above the expected neutral velocity of at most 1 km/s. Thus, the ions were (at least partially) decoupled from the neutrals already inside the diamagnetic cavity, indicating that ion-neutral drag was not responsible for balancing the outside magnetic pressure. The spacecraft potential was around -5 V throughout the cavity, showing that warm electrons were consistently present inside the cavity, at least as far in as Rosetta reached. Also, cold (below about 0.1 eV) electrons were consistently observed throughout the cavity, but less consistently in the surrounding region, suggesting that while Rosetta never entered a region of efficient collisional cooling of electrons, such a region was possibly not far away during the cavity crossings. Also, it reinforces the idea of previous authors that the intermittent nature of the cold electron component was due to filamentation of this cold plasma at or near the cavity boundary, possibly related to an instability of this boundary.

Finally, we report the detection of large-amplitude, quasi-harmonic density-fluctuations with associated magnetic field oscillations in association with asymmetric plasma and magnetic field enhancements previously found in the region surrounding the diamagnetic cavity, occurring predominantly on their descending slopes. Typical frequencies are around 0.1 Hz, i.e. about ten times the water and half the proton gyro-frequency, and the associated magnetic field oscillations, when detected, have wave vectors perpendicular to the background magnetic field. We suggest that they are Ion Bernstein waves, possibly excited by the drift-cyclotron instability resulting from the strong plasma inhomogeneities this region.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2018. p. 90
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1694
Keywords
Rosetta, comet, 67P, RPC-LAP, RPC-MIP, RPC-ICA, RPC-MAG, ROSINA-COPS, Langmuir probe, spacecraft potential, plasma, Churyumov-Gerasimenko, ion composition analyzer, diamagnetic cavity, ion velocity, electron temperature, ion-neutral drag, collisional coupling, wake effects, filamentation, electron cooling, plasma waves, hybrid waves, Bernstein waves, minimum variance, polarization, wavelet, density fluctuations, magnetic field oscillations, drift-cyclotron instability, asymmetric plasma and magnetic field enhancements
National Category
Natural Sciences
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-356426 (URN)978-91-513-0386-4 (ISBN)
Public defence
2018-09-14, Ångström 80101, Lägerhyddsvägen 1, Uppsala, 13:15 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 108/12, 109/12, 135/13, 149/12, 166/14, 168/15,Swedish Research Council, 621-2013-4191
Available from: 2018-08-21 Created: 2018-07-30 Last updated: 2018-08-27

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Publisher's full texthttps://doi.org/10.1029/2018JA025542

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Odelstad, EliasEriksson, Anders I.Johansson, Fredrik L.Vigren, ErikAndré, Mats

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