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Vigren, E. & Eriksson, A. I. (2019). On the ion-neutral coupling in cometary comae. Monthly notices of the Royal Astronomical Society, 482(2), 1937-1941
Open this publication in new window or tab >>On the ion-neutral coupling in cometary comae
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.

Keywords
molecular processes, comets (general)
National Category
Astronomy, Astrophysics and Cosmology Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-374113 (URN)10.1093/mnras/sty2869 (DOI)000454578700037 ()
Available from: 2019-01-23 Created: 2019-01-23 Last updated: 2019-01-23Bibliographically approved
Engelhardt, I. A. A., Eriksson, A. I., Vigren, E., Valliéres, X., Rubin, M., Gilet, N. & Henri, P. (2018). Cold electrons at comet 67P/Churyumov-Gerasimenko. Astronomy and Astrophysics, Article ID A51.
Open this publication in new window or tab >>Cold electrons at comet 67P/Churyumov-Gerasimenko
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2018 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, article id A51Article in journal (Refereed) Published
Abstract [en]

Context. The electron temperature of the plasma is one important aspect of the environment. Electrons created by photoionization or impact ionization of atmospheric gas have energies ∼10 eV. In an active comet coma the gas density is high enough for rapid cooling of the electron gas to the neutral gas temperature (few hundred kelvin). How cooling evolves in less active comets has not been studied before.

Aims. To investigate how electron cooling varied as comet 67P/Churyumov-Gerasimenko changed its activity by three orders of magnitude during the Rosetta mission.

Methods. We use in-situ data from Rosetta plasma and neutral gas sensors. By combining Langmuir probe bias voltage sweeps and Mutual Impedance Probe measurements we determine when cold electrons form at least 25% of the total electron density. We compare the results to what is expected from simple models of electron cooling, using the observed neutral gas density as input.

Results. We demonstrate that the slope of the Langmuir probe sweep can be used as a proxy for cold electron presence. We show statistics of cold electron observations over the 2 year mission period. We find cold electrons at lower activity than expected by a simple model based on free radial expansion and continuous loss of electron energy. Cold electrons are seen mainly when the gas density indicates an exobase may have formed.

Conclusions. Collisional cooling of electrons following a radial outward path is not sufficient for explaining the observations. We suggest the ambipolar electric field is important for the observed cooling. This field keeps electrons in the inner coma for much longer time, giving them time to dissipate energy by collisions with the neutrals. We conclude there is need of better models to describe the plasma environment of comets, including at least two populations of electrons and the ambipolar field.

National Category
Fusion, Plasma and Space Physics
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-348472 (URN)10.1051/0004-6361/201833251 (DOI)000441817100004 ()
Funder
Swedish National Space Board, 171/12, 109/12, 166/14The European Space Agency (ESA)
Available from: 2018-04-18 Created: 2018-04-18 Last updated: 2018-11-12Bibliographically approved
Hajra, R., Henri, P., Myllys, M., Heritier, K. L., Galand, M., Wedlund, C. S., . . . Wattieauxu, G. (2018). Cometary plasma response to interplanetary corotating interaction regions during 2016 June-September: a quantitative study by the Rosetta Plasma Consortium. Monthly notices of the Royal Astronomical Society, 480(4), 4544-4556
Open this publication in new window or tab >>Cometary plasma response to interplanetary corotating interaction regions during 2016 June-September: a quantitative study by the Rosetta Plasma Consortium
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2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 480, no 4, p. 4544-4556Article in journal (Refereed) Published
Abstract [en]

Four interplanetary corotating interaction regions (CIRs) were identified during 2016 June-September by the Rosetta Plasma Consortium (RPC) monitoring in situ the plasma environment of the comet 67P/Churyumov-Gerasimenko (67P) at heliocentric distances of similar to 3-3.8 au. The CIRs, formed in the interface region between low- and high-speed solar wind streams with speeds of similar to 320-400 km s(-1) and similar to 580-640 km s(-1), respectively, are characterized by relative increases in solar wind proton density by factors of similar to 13-29, in proton temperature by similar to 7-29, and in magnetic field by similar to 1-4 with respect to the pre-CIR values. The CIR boundaries are well defined with interplanetary discontinuities. Out of 10 discontinuities, four are determined to be forward waves and five are reverse waves, propagating at similar to 5-92 per cent of the magnetosonic speed at angles of similar to 20 degrees-87 degrees relative to ambient magnetic field. Only one is identified to be a quasi-parallel forward shock with magnetosonic Mach number of similar to 1.48 and shock normal angle of similar to 41 degrees. The cometary ionosphere response was monitored by Rosetta from cometocentric distances of similar to 4-30 km. A quiet time plasma density map was developed by considering dependences on cometary latitude, longitude, and cometocentric distance of Rosetta observations before and after each of the CIR intervals. The CIRs lead to plasma density enhancements of similar to 500-1000 per cent with respect to the quiet time reference level. Ionospheric modelling shows that increased ionization rate due to enhanced ionizing (>12-200 eV) electron impact is the prime cause of the large cometary plasma density enhancements during the CIRs. Plausible origin mechanisms of the cometary ionizing electron enhancements are discussed.

Keywords
methods: data analysis, methods: observational, Sun: rotation, solar wind, comets: general, comets: individual: 67P/Churyumov-Gerasimenko
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-371053 (URN)10.1093/mnras/sty2166 (DOI)000449617100022 ()
Available from: 2018-12-19 Created: 2018-12-19 Last updated: 2018-12-19Bibliographically approved
Hajra, R., Henri, P., Vallieres, X., More, J., Gilet, N., Wattieaux, G., . . . Rubin, M. (2018). Dynamic unmagnetized plasma in the diamagnetic cavity around comet 67P/Churyumov-Gerasimenko. Monthly notices of the Royal Astronomical Society, 475(3), 4140-4147
Open this publication in new window or tab >>Dynamic unmagnetized plasma in the diamagnetic cavity around comet 67P/Churyumov-Gerasimenko
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2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 475, no 3, p. 4140-4147Article in journal (Refereed) Published
Abstract [en]

The Rosetta orbiter witnessed several hundred diamagnetic cavity crossings (unmagnetized regions) around comet 67P/Churyumov-Gerasimenko during its two year survey of the comet. The characteristics of the plasma environment inside these diamagnetic regions are studied using in situ measurements by the Rosetta Plasma Consortium instruments. Although the unmagnetized plasma density has been observed to exhibit little dynamics compared to the very dynamical magnetized cometary plasma, we detected several localized dynamic plasma structures inside those diamagnetic regions. These plasma structures are not related to the direct ionization of local cometary neutrals. The structures are found to be steepened, asymmetric plasma enhancements with typical rising-to-descending slope ratio of similar to 2.8 (+/- 1.9), skewness similar to 0.43 (+/- 0.36), mean duration of similar to 2.7 (+/- 0.9) min and relative density variation Delta N/N of similar to 0.5 (+/- 0.2), observed close to the electron exobase. Similar steepened plasma density enhancements were detected at the magnetized boundaries of the diamagnetic cavity as well as outside the diamagnetic region. The plausible scalelength and propagation direction of the structures are estimated from simple plasma dynamics considerations. It is suggested that they are large-scale unmagnetized plasma enhancements, transmitted from the very dynamical outer magnetized region to the inner magnetic field-free cavity region.

Place, publisher, year, edition, pages
OXFORD UNIV PRESS, 2018
Keywords
methods: data analysis, methods: observational, comets: general, comets: individual: 67P/Churyumov-Gerasimenko
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-353112 (URN)10.1093/mnras/sty094 (DOI)000427141900087 ()
Available from: 2018-06-11 Created: 2018-06-11 Last updated: 2018-06-11Bibliographically approved
Graham, D. B., Vaivads, A., Khotyaintsev, Y. V., Eriksson, A., André, M., Malaspina, D. M., . . . Plaschke, F. (2018). Enhanced Escape of Spacecraft Photoelectrons Caused by Langmuir and Upper Hybrid Waves. Journal of Geophysical Research - Space Physics, 123(9), 7534-7553
Open this publication in new window or tab >>Enhanced Escape of Spacecraft Photoelectrons Caused by Langmuir and Upper Hybrid Waves
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2018 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, no 9, p. 7534-7553Article in journal (Refereed) Published
Abstract [en]

The spacecraft potential is often used to infer rapid changes in the thermal plasma density. The variations in spacecraft potential associated with large-amplitude Langmuir and upper hybrid waves are investigated with the Magnetospheric Multiscale (MMS) mission. When large-amplitude Langmuir and upper hybrid waves are observed, the spacecraft potential increases. The changes in spacecraft potential are shown to be due to enhanced photoelectron escape from the spacecraft when the wave electric fields reach large amplitude. The fluctuations in spacecraft potential follow the envelope function of the Langmuir and upper hybrid waves. Comparison with the high-resolution electron moments shows that the changes in spacecraft potential associated with the waves are not due to density perturbations. Indeed, using the spacecraft potential as a density probe leads to unphysically large density fluctuations. In addition, the changes in spacecraft potential are shown to increase as density decreases: larger spacecraft potential changes are observed in the magnetosphere, than in the magnetosheath and solar wind. These results show that external electric fields can lead to unphysical results when the spacecraft potential is used as a density probe. The results suggest that fluctuations in the spacecraft potential alone cannot be used to determine whether nonlinear processes associated with Langmuir and upper hybrid waves, such as the ponderomotive force and three-wave decay, are occurring.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2018
Keywords
Langmuir waves, photoelectron current, spacecraft potential, upper hybrid waves
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-369520 (URN)10.1029/2018JA025874 (DOI)000448376600029 ()
Funder
Swedish National Space Board, 175/15Swedish National Space Board, 128/17
Available from: 2018-12-17 Created: 2018-12-17 Last updated: 2018-12-17Bibliographically approved
Madsen, B., Wedlund, C. S., Eriksson, A., Goetz, C., Karlsson, T., Gunell, H., . . . Miloch, W. J. (2018). Extremely Low-Frequency Waves Inside the Diamagnetic Cavity of Comet 67P/Churyumov-Gerasimenko. Geophysical Research Letters, 45(9), 3854-3864
Open this publication in new window or tab >>Extremely Low-Frequency Waves Inside the Diamagnetic Cavity of Comet 67P/Churyumov-Gerasimenko
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2018 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 9, p. 3854-3864Article in journal (Refereed) Published
Abstract [en]

The European Space Agency/Rosetta mission to comet 67P/Churyumov‐Gerasimenko has provided several hundred observations of the cometary diamagnetic cavity induced by the interaction between outgassed cometary particles, cometary ions, and the solar wind magnetic field. Here we present the first electric field measurements of four preperihelion and postperihelion cavity crossings on 28 May 2015 and 17 February 2016, using the dual‐probe electric field mode of the Langmuir probe (LAP) instrument of the Rosetta Plasma Consortium. We find that on large scales, variations in the electric field fluctuations capture the cavity and boundary regions observed in the already well‐studied magnetic field, suggesting the electric field mode of the LAP instrument as a reliable tool to image cavity crossings. In addition, the LAP electric field mode unravels for the first time extremely low‐frequency waves within two cavities. These low‐frequency electrostatic waves are likely triggered by lower‐hybrid waves observed in the surrounding magnetized plasma.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2018
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-357750 (URN)10.1029/2017GL076415 (DOI)000434111700012 ()
Funder
The Research Council of Norway, 240000The Research Council of Norway, 230996Swedish National Space Board, 109/12
Available from: 2018-08-22 Created: 2018-08-22 Last updated: 2018-08-22Bibliographically approved
Wahlund, J.-E., Morooka, M. W., Hadid, L. Z., Persoon, A. M., Farrell, W. M., Gurnett, D. A., . . . Vigren, E. (2018). In situ measurements of Saturn's ionosphere show that it is dynamic and interacts with the rings. Science, 359(6371), 66-68
Open this publication in new window or tab >>In situ measurements of Saturn's ionosphere show that it is dynamic and interacts with the rings
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2018 (English)In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 359, no 6371, p. 66-68Article in journal (Refereed) Published
Abstract [en]

The ionized upper layer of Saturn's atmosphere, its ionosphere, provides a closure of currents mediated by the magnetic field to other electrically charged regions (for example, rings) and hosts ion-molecule chemistry. In 2017, the Cassini spacecraft passed inside the planet's rings, allowing in situ measurements of the ionosphere. The Radio and Plasma Wave Science instrument detected a cold, dense, and dynamic ionosphere at Saturn that interacts with the rings. Plasma densities reached up to 1000 cubic centimeters, and electron temperatures were below 1160 kelvin near closest approach. The density varied between orbits by up to two orders of magnitude. Saturn's A- and B-rings cast a shadow on the planet that reduced ionization in the upper atmosphere, causing a north-south asymmetry.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-341494 (URN)10.1126/science.aao4134 (DOI)000419324700067 ()29229651 (PubMedID)
Available from: 2018-02-21 Created: 2018-02-21 Last updated: 2018-02-21Bibliographically approved
Odelstad, E., Eriksson, A. I., Johansson, F. L., Vigren, E., Henri, P., Gilet, N., . . . André, M. (2018). Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P. Journal of Geophysical Research - Space Physics, 123(7), 5870-5893
Open this publication in new window or tab >>Ion Velocity and Electron Temperature Inside and Around the Diamagnetic Cavity of Comet 67P
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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
Keywords
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:nbn:se:uu:diva-356424 (URN)10.1029/2018JA025542 (DOI)000442664300043 ()
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
Heritier, K. L., Altwegg, K., Berthelier, J.-J. -., Beth, A., Carr, C. M., De Keyser, J., . . . Vigren, E. (2018). On the origin of molecular oxygen in cometary comae [Letter to the editor]. Nature Communications, 9, Article ID 2580.
Open this publication in new window or tab >>On the origin of molecular oxygen in cometary comae
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2018 (English)In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 9, article id 2580Article in journal, Letter (Other academic) Published
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-366612 (URN)10.1038/s41467-018-04972-5 (DOI)000437101700010 ()29968720 (PubMedID)
Available from: 2018-11-26 Created: 2018-11-26 Last updated: 2018-11-26Bibliographically approved
Engelhardt, I. A. A., Eriksson, A., Stenberg Wieser, G., Goetz, C., Rubin, M., Henri, P., . . . Valliéres, X. (2018). Plasma Density Structures at Comet 67P/Churyumov-Gerasimenko. Monthly notices of the Royal Astronomical Society, 477(1), 1296-1307
Open this publication in new window or tab >>Plasma Density Structures at Comet 67P/Churyumov-Gerasimenko
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2018 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 477, no 1, p. 1296-1307Article in journal (Refereed) Published
Abstract [en]

We present Rosetta RPC case study from four events at various radial distance, phase angle and local time from autumn 2015, just after perihelion of comet 67P/Churyumov-Gerasimenko. Pulse like (high amplitude, up to minutes in time) signatures are seen with several RPC instruments in the plasma density (LAP, MIP), ion energy and flux (ICA) as well as magnetic field intensity (MAG). Furthermore the cometocentric distance relative to the electron exobase is seen to be a good organizing parameter for the measured plasma variations. The closer Rosetta is to this boundary, the more pulses are measured. This is consistent with the pulses being filaments of plasma originating from the diamagnetic cavity boundary as predicted by simulations. 

National Category
Fusion, Plasma and Space Physics
Research subject
Physics with specialization in Space and Plasma Physics; Physics
Identifiers
urn:nbn:se:uu:diva-347003 (URN)10.1093/mnras/sty765 (DOI)000432660300090 ()
Funder
Swedish National Space Board, 171/12Swedish National Space Board, 109/12
Available from: 2018-04-18 Created: 2018-04-18 Last updated: 2018-08-20Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0003-2926-6761

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