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Deca, J., Henri, P., Divin, A., Eriksson, A., Galand, M., Beth, A., . . . Horanyi, M. (2019). Building a Weakly Outgassing Comet from a Generalized Ohm's Law. Physical Review Letters, 123(5), Article ID 055101.
Öppna denna publikation i ny flik eller fönster >>Building a Weakly Outgassing Comet from a Generalized Ohm's Law
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2019 (Engelska)Ingår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 123, nr 5, artikel-id 055101Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

When a weakly outgassing comet is sufficiently close to the Sun, the formation of an ionized coma results in solar wind mass loading and magnetic field draping around its nucleus. Using a 3D fully kinetic approach, we distill the components of a generalized Ohm's law and the effective electron equation of state directly from the self-consistently simulated electron dynamics and identify the driving physics in the various regions of the cometary plasma environment. Using the example of space plasmas, in particular multispecies cometary plasmas, we show how the description for the complex kinetic electron dynamics can be simplified through a simple effective closure, and identify where an isotropic single-electron fluid Ohm's law approximation can be used, and where it fails.

Ort, förlag, år, upplaga, sidor
AMER PHYSICAL SOC, 2019
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Identifikatorer
urn:nbn:se:uu:diva-393346 (URN)10.1103/PhysRevLett.123.055101 (DOI)000479003800007 ()
Forskningsfinansiär
The European Space Agency (ESA), 4000119035/16/ES/JD
Tillgänglig från: 2019-09-27 Skapad: 2019-09-27 Senast uppdaterad: 2019-09-27Bibliografiskt granskad
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
Öppna denna publikation i ny flik eller fönster >>On the ion-neutral coupling in cometary comae
2019 (Engelska)Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 482, nr 2, s. 1937-1941Artikel i tidskrift (Refereegranskat) 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.

Nyckelord
molecular processes, comets (general)
Nationell ämneskategori
Astronomi, astrofysik och kosmologi Fusion, plasma och rymdfysik
Identifikatorer
urn:nbn:se:uu:diva-374113 (URN)10.1093/mnras/sty2869 (DOI)000454578700037 ()
Tillgänglig från: 2019-01-23 Skapad: 2019-01-23 Senast uppdaterad: 2019-01-23Bibliografiskt granskad
Morooka, M., Wahlund, J.-E., Hadid, L. Z., Eriksson, A. I., Edberg, N. J. T., Vigren, E., . . . Perry, M. (2019). Saturn's Dusty Ionosphere. Journal of Geophysical Research - Space Physics, 124(3), 1679-1697
Öppna denna publikation i ny flik eller fönster >>Saturn's Dusty Ionosphere
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2019 (Engelska)Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 3, s. 1679-1697Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

Measurements of electrons and ions in Saturn's ionosphere down to 1,500-km altitudes as well as the ring crossing region above the ionosphere obtained by the Langmuir probe onboard the Cassini spacecraft are presented. Five nearly identical deep ionosphere flybys during the Grand Finale orbits and the Final plunge orbit revealed a rapid increase in the plasma densities and discrepancies between the electrons and ions densities (N-e and N-i) near the closest approach. The small N-e/N-i ratio indicates the presence of a dusty plasma, a plasma which charge carrier is dominated by negatively charged heavy particles. Comparison of the Langmuir probe obtained density with the light ion density obtained by the Ion and Neutral Mass Spectrometer confirmed the presence of heavy ions. An unexpected positive floating potential of the probe was also observed when N-e/N-i << 1. This suggests that Saturn's ionosphere near the density peak is in a dusty plasma state consisting of negatively and positively charged heavy cluster ions. The electron temperature (T-e) characteristics in the ionosphere are also investigated and unexpectedly high electron temperature value, up to 5000 K, has been observed below 2,500-km altitude in a region where electron-neutral collisions should be prominent. A well-defined relationship between T-e and N-e/N-i ratio was found, implying that the electron heating at low altitudes is related to the dusty plasma state of the ionosphere.

Ort, förlag, år, upplaga, sidor
American Geophysical Union (AGU), 2019
Nyckelord
Saturn's ionosphere, dusty plasma, Langmuir probe, plasma density, electron temperature
Nationell ämneskategori
Fusion, plasma och rymdfysik
Identifikatorer
urn:nbn:se:uu:diva-387566 (URN)10.1029/2018JA026154 (DOI)000466087900016 ()
Forskningsfinansiär
Vetenskapsrådet, 621-2013-4191Vetenskapsrådet, 2.2.1-312/16Rymdstyrelsen, Dnr 174/15 135/13 162/14
Tillgänglig från: 2019-06-24 Skapad: 2019-06-24 Senast uppdaterad: 2019-06-24Bibliografiskt granskad
Edberg, N. J. T., Eriksson, A., Vigren, E., Johansson, F., Goetz, C., Nilsson, H., . . . Henri, P. (2019). The Convective Electric Field Influence on the Cold Plasma and Diamagnetic Cavity of Comet 67P. Astronomical Journal, 158(2), Article ID 71.
Öppna denna publikation i ny flik eller fönster >>The Convective Electric Field Influence on the Cold Plasma and Diamagnetic Cavity of Comet 67P
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2019 (Engelska)Ingår i: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 158, nr 2, artikel-id 71Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We studied the distribution of cold electrons (<1 eV) around comet 67P/Churyumov-Gerasimenko with respect to the solar wind convective electric field direction. The cold plasma was measured by the Langmuir Probe instrument and the direction of the convective electric field E-conv = -nu x B was determined from magnetic field (B) measurements inside the coma combined with an assumption of a purely radial solar wind velocity nu. We found that the cold plasma is twice as likely to be observed when the convective electric field at Rosetta's position is directed toward the nucleus (in the -E(conv )hemisphere) compared to when it is away from the nucleus (in the +E-conv hemisphere). Similarly, the diamagnetic cavity, in which previous studies have shown that cold plasma is always present, was also found to be observed twice as often when in the -E-conv hemisphere, linking its existence circumstantially to the presence of cold electrons. The results are consistent with hybrid and Hall magnetohydrodynamic simulations as well as measurements of the ion distribution around the diamagnetic cavity.

Ort, förlag, år, upplaga, sidor
IOP PUBLISHING LTD, 2019
Nyckelord
comets: individual (67P), magnetic fields, plasmas, space vehicles: instruments
Nationell ämneskategori
Fusion, plasma och rymdfysik Astronomi, astrofysik och kosmologi
Identifikatorer
urn:nbn:se:uu:diva-391291 (URN)10.3847/1538-3881/ab2d28 (DOI)000476604700001 ()
Forskningsfinansiär
Vetenskapsrådet, 621-2013-4191Vetenskapsrådet, 621-2014-5526Rymdstyrelsen, 109/12Rymdstyrelsen, 135/13
Tillgänglig från: 2019-08-22 Skapad: 2019-08-22 Senast uppdaterad: 2019-08-22Bibliografiskt granskad
Vigren, E., Edberg, N. J. T., Eriksson, A., Galand, M., Henri, P., Johansson, F., . . . Vallieres, X. (2019). The Evolution of the Electron Number Density in the Coma of Comet 67P at the Location of Rosetta from 2015 November through 2016 March. Astrophysical Journal, 881(1), Article ID 6.
Öppna denna publikation i ny flik eller fönster >>The Evolution of the Electron Number Density in the Coma of Comet 67P at the Location of Rosetta from 2015 November through 2016 March
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2019 (Engelska)Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 881, nr 1, artikel-id 6Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

A comet ionospheric model assuming the plasma moves radially outward with the same bulk speed as the neutral gas and not being subject to severe reduction through dissociative recombination has previously been tested in a series of case studies associated with the Rosetta mission at comet 67P/Churyumov-Gerasimenko. It has been found that at low activity and within several tens of kilometers from the nucleus such models (which originally were developed for such conditions) generally work well in reproducing observed electron number densities, in particular when plasma production through both photoionization and electron-impact ionization is taken into account. Near perihelion, case studies have, on the contrary, shown that applying similar assumptions overestimates the observed electron number densities at the location of Rosetta. Here we compare Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Comet Pressure sensor-driven model results with Rosetta Plasma Consortium/Mutual Impedance Probe-derived electron number densities for an extended time period (2015 November through 2016 March) during the postperihelion phase with southern summer/spring. We observe a gradual transition from a state when the model grossly overestimates (by more than a factor of 10) the observations to being in reasonable agreement during 2016 March.

Ort, förlag, år, upplaga, sidor
IOP PUBLISHING LTD, 2019
Nyckelord
comets: individual (67P), molecular processes
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Identifikatorer
urn:nbn:se:uu:diva-393332 (URN)10.3847/1538-4357/ab29f7 (DOI)000479099500006 ()
Forskningsfinansiär
Vetenskapsrådet, 621-2013-4191Vetenskapsrådet, 621-2014-5526Rymdstyrelsen, 109/02Rymdstyrelsen, 114/13Rymdstyrelsen, 135/13Rymdstyrelsen, 166/14
Tillgänglig från: 2019-09-27 Skapad: 2019-09-27 Senast uppdaterad: 2019-09-27Bibliografiskt granskad
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.
Öppna denna publikation i ny flik eller fönster >>Cold electrons at comet 67P/Churyumov-Gerasimenko
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2018 (Engelska)Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, artikel-id A51Artikel i tidskrift (Refereegranskat) 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.

Nationell ämneskategori
Fusion, plasma och rymdfysik
Forskningsämne
Fysik med inriktning mot rymd- och plasmafysik
Identifikatorer
urn:nbn:se:uu:diva-348472 (URN)10.1051/0004-6361/201833251 (DOI)000441817100004 ()
Forskningsfinansiär
Rymdstyrelsen, 171/12, 109/12, 166/14The European Space Agency (ESA)
Tillgänglig från: 2018-04-18 Skapad: 2018-04-18 Senast uppdaterad: 2018-11-12Bibliografiskt granskad
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
Öppna denna publikation i ny flik eller fönster >>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 (Engelska)Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 480, nr 4, s. 4544-4556Artikel i tidskrift (Refereegranskat) 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.

Nyckelord
methods: data analysis, methods: observational, Sun: rotation, solar wind, comets: general, comets: individual: 67P/Churyumov-Gerasimenko
Nationell ämneskategori
Fusion, plasma och rymdfysik Astronomi, astrofysik och kosmologi
Identifikatorer
urn:nbn:se:uu:diva-371053 (URN)10.1093/mnras/sty2166 (DOI)000449617100022 ()
Tillgänglig från: 2018-12-19 Skapad: 2018-12-19 Senast uppdaterad: 2018-12-19Bibliografiskt granskad
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
Öppna denna publikation i ny flik eller fönster >>Dynamic unmagnetized plasma in the diamagnetic cavity around comet 67P/Churyumov-Gerasimenko
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2018 (Engelska)Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 475, nr 3, s. 4140-4147Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
OXFORD UNIV PRESS, 2018
Nyckelord
methods: data analysis, methods: observational, comets: general, comets: individual: 67P/Churyumov-Gerasimenko
Nationell ämneskategori
Fusion, plasma och rymdfysik
Identifikatorer
urn:nbn:se:uu:diva-353112 (URN)10.1093/mnras/sty094 (DOI)000427141900087 ()
Tillgänglig från: 2018-06-11 Skapad: 2018-06-11 Senast uppdaterad: 2018-06-11Bibliografiskt granskad
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
Öppna denna publikation i ny flik eller fönster >>Enhanced Escape of Spacecraft Photoelectrons Caused by Langmuir and Upper Hybrid Waves
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2018 (Engelska)Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, nr 9, s. 7534-7553Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
AMER GEOPHYSICAL UNION, 2018
Nyckelord
Langmuir waves, photoelectron current, spacecraft potential, upper hybrid waves
Nationell ämneskategori
Fusion, plasma och rymdfysik Astronomi, astrofysik och kosmologi
Identifikatorer
urn:nbn:se:uu:diva-369520 (URN)10.1029/2018JA025874 (DOI)000448376600029 ()
Forskningsfinansiär
Rymdstyrelsen, 175/15Rymdstyrelsen, 128/17
Tillgänglig från: 2018-12-17 Skapad: 2018-12-17 Senast uppdaterad: 2018-12-17Bibliografiskt granskad
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
Öppna denna publikation i ny flik eller fönster >>Extremely Low-Frequency Waves Inside the Diamagnetic Cavity of Comet 67P/Churyumov-Gerasimenko
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2018 (Engelska)Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, nr 9, s. 3854-3864Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
AMER GEOPHYSICAL UNION, 2018
Nationell ämneskategori
Fusion, plasma och rymdfysik
Identifikatorer
urn:nbn:se:uu:diva-357750 (URN)10.1029/2017GL076415 (DOI)000434111700012 ()
Forskningsfinansiär
Norges forskningsråd, 240000Norges forskningsråd, 230996Rymdstyrelsen, 109/12
Tillgänglig från: 2018-08-22 Skapad: 2018-08-22 Senast uppdaterad: 2018-08-22Bibliografiskt granskad
Organisationer
Identifikatorer
ORCID-id: ORCID iD iconorcid.org/0000-0003-2926-6761

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