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Vigren, E., Dreyer, J., Eriksson, A. I., Johansson, F. L., Morooka, M. & Wahlund, J.-E. (2022). Empirical Photochemical Modeling of Saturn's Ionization Balance Including Grain Charging. The Planetary Science Journal, 3(2), Article ID 49.
Open this publication in new window or tab >>Empirical Photochemical Modeling of Saturn's Ionization Balance Including Grain Charging
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2022 (English)In: The Planetary Science Journal, E-ISSN 2632-3338, Vol. 3, no 2, article id 49Article in journal (Refereed) Published
Abstract [en]

We present a semianalytical photochemical model of Saturn's near-equatorial ionosphere and adapt it to two regions (similar to 2200 and similar to 1700 km above the 1 bar level) probed during the inbound portion of Cassini's orbit 292 (2017 September 9). The model uses as input the measured concentrations of molecular hydrogen, hydrogen ion species, and free electrons, as well as the measured electron temperature. The output includes upper limits, or constraints, on the mixing ratios of two families of molecules, on ion concentrations, and on the attachment rates of electrons and ions onto dust grains. The model suggests mixing ratios of the two molecular families that, particularly near similar to 1700 km, differ notably from what independent measurements by the Ion Neutral Mass Spectrometer suggest. Possibly connected to this, the model suggests an electron-depleted plasma with a level of electron depletion of around 50%. This is in qualitative agreement with interpretations of Radio Plasma Wave Science/Langmuir Probe measurements, but an additional conundrum arises in the fact that a coherent photochemical equilibrium scenario then relies on a dust component with typical grain radii smaller than 3 angstrom.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2022
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-495353 (URN)10.3847/PSJ/ac4eee (DOI)000911845800001 ()
Funder
Swedish National Space Board
Available from: 2023-01-30 Created: 2023-01-30 Last updated: 2023-10-03Bibliographically approved
Dreyer, J., Vigren, E., Johansson, F., Shebanits, O., Morooka, M., Wahlund, J.-E., . . . Waite, J. H. (2022). Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn's Ionosphere. The Planetary Science Journal, 3(7), Article ID 168.
Open this publication in new window or tab >>Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn's Ionosphere
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2022 (English)In: The Planetary Science Journal, E-ISSN 2632-3338, Vol. 3, no 7, article id 168Article in journal (Refereed) Published
Abstract [en]

For orbits 288 and 292 of Cassini's Grand Finale, clear dips (sharp and narrow decreases) are visible in the H-2(+) densities measured by the Ion and Neutral Mass Spectrometer (INMS). In 2017, the southern hemisphere of Saturn was shadowed by its rings and the substructures within. Tracing a path of the solar photons through the ring plane to Cassini's position, we can identify regions in the ionosphere that were shadowed by the individual ringlets and plateaus (with increased optical depths) of Saturn's C ring. The calculated shadowed altitudes along Cassini's trajectory line up well with the dips in the H-2(+) data when adjusting the latter based on a detected evolving shift in the INMS timestamps since 2013, illustrating the potential for verification of instrument timings. We can further estimate the mean optical depths of the ringlets/plateaus by comparing the dips to inbound H-2(+) densities. Our results agree well with values derived from stellar occultation measurements. No clear dips are visible for orbits 283 and 287, whose periapsides were at higher altitudes. This can be attributed to the much longer chemical lifetime of H2+ at these higher altitudes, which in turn can be further used to estimate a lower limit for the flow speed along Cassini's trajectory. The resulting estimate of similar to 0.3 km s(-1) at an altitude of similar to 3400 km is in line with prior suggestions. Finally, the ringlet and plateau shadows are not associated with obvious dips in the electron density, which is expected due to their comparatively long chemical (recombination) lifetime.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2022
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-497057 (URN)10.3847/PSJ/ac7790 (DOI)000914478200001 ()
Funder
Swedish National Space Board, 143/18E. och K.G. Lennanders StipendiestiftelseSwedish National Space Board, Dnr 195/20
Available from: 2023-02-23 Created: 2023-02-23 Last updated: 2023-10-03Bibliographically approved
Johansson, F., Vigren, E., Waite, J. H., Miller, K., Eriksson, A., Edberg, N. J. T. & Dreyer, J. (2022). Implications from secondary emission from neutral impact on Cassini plasma and dust measurements. Monthly notices of the Royal Astronomical Society, 515(2), 2340-2350
Open this publication in new window or tab >>Implications from secondary emission from neutral impact on Cassini plasma and dust measurements
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2022 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 515, no 2, p. 2340-2350Article in journal (Refereed) Published
Abstract [en]

We investigate the role of secondary electron and ion emission from impact of gas molecules on the Cassini Langmuir probe (RPWS-LP or LP) measurements in the ionosphere of Saturn. We add a model of the emission currents, based on laboratory measurements and data from comet 1P/Halley, to the equations used to derive plasma parameters from LP bias voltage sweeps. Reanalysing several hundred sweeps from the Cassini Grand Finale orbits, we find reasonable explanations for three open conundrums from previous LP studies of the Saturn ionosphere. We find an explanation for the observed positive charging of the Cassini spacecraft, the possibly overestimated ionospheric electron temperatures, and the excess ion current reported. For the sweeps analysed in detail, we do not find (indirect or direct) evidence of dust having a significant charge-carrying role in Saturn's ionosphere. We also produce an estimate of H2O number density from the last six revolutions of Cassini through Saturn's ionosphere in greater detail than reported by the Ion and Neutral Mass Spectrometer. Our analysis reveals an ionosphere that is highly structured in latitude across all six final revolutions, with mixing ratios varying with two orders of magnitude in latitude and one order of magnitude between revolutions and altitude. The result is generally consistent with an empirical photochemistry model balancing the production of H+ ions with the H+ loss through charge transfer with e.g. H2O, CH4, and CO2, for which water vapour appears as the likeliest dominant source of the signal in terms of yield and concentration.

Place, publisher, year, edition, pages
Oxford University Press (OUP), 2022
Keywords
planets and satellites: atmospheres, plasmas, space vehicles: instruments, methods: data analysis, methods: observational
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-482038 (URN)10.1093/mnras/stac1856 (DOI)000834368400005 ()
Funder
Swedish National Space Board, 143/18
Available from: 2022-08-19 Created: 2022-08-19 Last updated: 2022-08-19Bibliographically approved
Edberg, N. J. T., Johansson, F. L., Eriksson, A. I., Vigren, E., Henri, P. & De Keyser, J. (2022). Radial distribution of plasma at comet 67P: Implications for cometary flyby missions. Astronomy and Astrophysics, 663, Article ID A42.
Open this publication in new window or tab >>Radial distribution of plasma at comet 67P: Implications for cometary flyby missions
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2022 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 663, article id A42Article in journal (Refereed) Published
Abstract [en]

Context. The Rosetta spacecraft followed comet 67P/Churyumov-Gerasimenko (67P) for more than two years at a slow walking pace (similar to 1 m s(-1)) within 1500 km from the nucleus. During one of the radial movements of the spacecraft in the early phase of the mission, the radial distribution of the plasma density could be estimated, and the ionospheric density was found to be inversely proportional to the cometocentric distance r from the nucleus (a 1/r distribution). Aims. This study aims to further characterise the radial distribution of plasma around 67P throughout the mission and to expand on the initial results. We also aim to investigate how a 1/r distribution would be observed during a flyby with a fast (similar to 10's km s(-1)) spacecraft, such as the upcoming Comet Interceptor mission, when there is also an asymmetry introduced to the outgassing over the comet surface. Methods. To determine the radial distribution of the plasma, we used data from the Langmuir probe and Mutual Impedance instruments from the Rosetta Plasma Consortium during six intervals throughout the mission, for which the motion of Rosetta was approximately radial with respect to the comet. We then simulated what distribution a fast flyby mission would actually observe during its passage through a coma when there is a 1/r plasma density distribution as well as a sinusoidal variation with a phase angle (and then a sawtooth variation) multiplied to the outgassing rate. Results. The plasma density around comet 67P is found to roughly follow a 1/r dependence, although significant deviations occur in some intervals. If we normalise all data to a common outgassing rate (or heliocentric distance) and combine the intervals to a radial range of 10-1500 km, we find a 1/r(1.19) average distribution. The simulated observed density from a fast spacecraft flying through a coma with a 1/r distribution and an asymmetric outgassing can, in fact, appear anywhere in the range from a 1/r distribution to a 1/r(2) distribution, or even slightly outside of this range. Conclusions. The plasma density is distributed in such a way that it approximately decreases in a manner that is inversely proportional to the cometocentric distance. This is to be expected from the photoionisation of a collision-less, expanding neutral gas at a constant ionisation rate and expansion speed. The deviation from a pure 1/r distribution is in many cases caused by asymmetric outgassing over the surface, additional ionisation sources being present, electric fields accelerating plasma, and changing upstream solar wind conditions. A fast flyby mission can observe a radial distribution that deviates significantly from a 1/r trend if the outgassing is not symmetric over the surface. The altitude profile that will be observed depends very much on the level of outgassing asymmetry, the flyby velocity, the comet rotation rate, and the rotation phase. It is therefore essential to include data from both the inbound and outbound legs, as well as to compare plasma density to neutral density to get a more complete understanding of the radial distribution of the plasma.

Place, publisher, year, edition, pages
EDP Sciences, 2022
Keywords
plasmas, comets: general
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-481382 (URN)10.1051/0004-6361/202243776 (DOI)000822878100004 ()
Funder
Swedish National Space Board
Available from: 2022-08-11 Created: 2022-08-11 Last updated: 2022-08-11Bibliographically approved
Vigren, E., Eriksson, A. I., Johansson, F. L., Marschall, R., Morooka, M. & Rubin, M. (2021). A Case for a Small to Negligible Influence of Dust Charging on the Ionization Balance in the Coma of Comet 67P. The Planetary Science Journal, 2(4), Article ID 156.
Open this publication in new window or tab >>A Case for a Small to Negligible Influence of Dust Charging on the Ionization Balance in the Coma of Comet 67P
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2021 (English)In: The Planetary Science Journal, E-ISSN 2632-3338, Vol. 2, no 4, article id 156Article in journal (Refereed) Published
Abstract [en]

A recent work aided by Rosetta in situ measurements set constraints on the dust-to-gas mass emission ratio and the size distribution of dust escaping the nucleus of comet 67P/Churyumov-Gerasimenko near perihelion. Here we use this information along with other observables/parameters as input into an analytical model aimed at estimating the number density of electrons attached to dust particles near the position of Rosetta. These theoretical estimates are compared to in situ measurements of the degree of ionization. The comparison proposes that Rosetta, while near perihelion, was typically not in electron-depleted regions of the inner coma of 67P. Our work suggests a typical level of electron depletion probably below 10% and possibly below 1%. In line with previous studies, we find, again with certain assumptions and other observables/parameters as input, that the observed negative spacecraft charging to a few tens of volts does not significantly impact the detection of charged dust grains, with a possible exception for grains with radii less than similar to 10 nm.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2021
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-497011 (URN)10.3847/PSJ/ac134f (DOI)000912770700001 ()
Funder
Swedish National Space Board, 168/15The European Space Agency (ESA), 4000118957/16/ES/JD
Available from: 2023-02-24 Created: 2023-02-24 Last updated: 2023-02-24Bibliographically approved
Dreyer, J., Vigren, E., Morooka, M., Wahlund, J.-E., Buchert, S. C., Johansson, F. L. & Waite, J. H. (2021). Constraining the Positive Ion Composition in Saturn's Lower Ionosphere with the Effective Recombination Coefficient. The Planetary Science Journal, 2(1), Article ID 39.
Open this publication in new window or tab >>Constraining the Positive Ion Composition in Saturn's Lower Ionosphere with the Effective Recombination Coefficient
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2021 (English)In: The Planetary Science Journal, E-ISSN 2632-3338, Vol. 2, no 1, article id 39Article in journal (Refereed) Published
Abstract [en]

The present study combines Radio and Plasma Wave Science/Langmuir Probe and Ion and Neutral Mass Spectrometer data from Cassini's last four orbits into Saturn's lower ionosphere to constrain the effective recombination coefficient α300 from measured number densities and electron temperatures at a reference electron temperature of 300 K. Previous studies have shown an influx of ring material causes a state of electron depletion due to grain charging, which will subsequently affect the ionospheric chemistry. The requirement to take grain charging into account limits the derivation of α300 to upper limits. Assuming photochemical equilibrium and using an established method to calculate the electron production rate, we derive upper limits for α300 of ≲ 3 × 10−7 cm3 s−1 for altitudes below 2000 km. This suggests that Saturn's ionospheric positive ions are dominated by species with low recombination rate coefficients like HCO+. An ionosphere dominated by water group ions or complex hydrocarbons, as previously suggested, is incompatible with this result, as these species have recombination rate coefficients > 5 × 10−7 cm3 s−1 at an electron temperature of 300 K.

Place, publisher, year, edition, pages
American Astronomical SocietyAmerican Astronomical Society, 2021
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-458301 (URN)10.3847/psj/abd6e9 (DOI)000912867900001 ()
Funder
Swedish National Space Board, 143/18Swedish National Space Board, 168/15
Available from: 2021-11-08 Created: 2021-11-08 Last updated: 2024-01-15Bibliographically approved
Kalha, C., Fernando, N. K., Bhatt, P., Johansson, F., Lindblad, A., Rensmo, H., . . . Regoutz, A. (2021). Hard x-ray photoelectron spectroscopy: a snapshot of the state-of-the-art in 2020. Journal of Physics: Condensed Matter, 33(23), Article ID 233001.
Open this publication in new window or tab >>Hard x-ray photoelectron spectroscopy: a snapshot of the state-of-the-art in 2020
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2021 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 33, no 23, article id 233001Article, review/survey (Refereed) Published
Abstract [en]

Hard x-ray photoelectron spectroscopy (HAXPES) is establishing itself as an essential technique for the characterisation of materials. The number of specialised photoelectron spectroscopy techniques making use of hard x-rays is steadily increasing and ever more complex experimental designs enable truly transformative insights into the chemical, electronic, magnetic, and structural nature of materials. This paper begins with a short historic perspective of HAXPES and spans from developments in the early days of photoelectron spectroscopy to provide an understanding of the origin and initial development of the technique to state-of-the-art instrumentation and experimental capabilities. The main motivation for and focus of this paper is to provide a picture of the technique in 2020, including a detailed overview of available experimental systems worldwide and insights into a range of specific measurement modi and approaches. We also aim to provide a glimpse into the future of the technique including possible developments and opportunities.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP)IOP Publishing, 2021
Keywords
photoelectron spectroscopy, hard x-ray photoelectron spectroscopy, photoemission spectroscopy
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-444641 (URN)10.1088/1361-648X/abeacd (DOI)000649914800001 ()33647896 (PubMedID)
Funder
Swedish Energy AgencySwedish Research Council, 2014-6463Swedish Research Council, 2018-05336
Available from: 2021-06-09 Created: 2021-06-09 Last updated: 2024-01-15Bibliographically approved
Gunell, H., Goetz, C., Odelstad, E., Beth, A., Hamrin, M., Henri, P., . . . Wieser, G. S. (2021). Ion acoustic waves near a comet nucleus: Rosetta observations at comet 67P/Churyumov-Gerasimenko. Annales Geophysicae, 39(1), 53-68
Open this publication in new window or tab >>Ion acoustic waves near a comet nucleus: Rosetta observations at comet 67P/Churyumov-Gerasimenko
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2021 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 39, no 1, p. 53-68Article in journal (Refereed) Published
Abstract [en]

Ion acoustic waves were observed between 15 and 30 km from the centre of comet 67P/Churyumov-Gerasimenko by the Rosetta spacecraft during its close flyby on 28 March 2015. There are two electron populations: one cold at k(B)T(e) approximate to 0.2 eV and one warm at k(B)T(e) approximate to 2 eV. The ions are dominated by a cold (a few hundredths of electronvolt) distribution of water group ions with a bulk speed of (3-3.7) km s(-1). A warm k(B)T(e) approximate to 6 eV ion population, which also is present, has no influence on the ion acoustic waves due to its low density of only 0.25 % of the plasma density. Near closest approach the propagation direction was within 50 degrees from the direction of the bulk velocity. The waves, which in the plasma frame appear below the ion plasma frequency f(pi) approximate to 2 kHz, are Doppler-shifted to the spacecraft frame where they cover a frequency range up to approximately 4 kHz. The waves are detected in a region of space where the magnetic field is piled up and draped around the inner part of the ionised coma. Estimates of the current associated with the magnetic field gradient as observed by Rosetta are used as input to calculations of dispersion relations for current-driven ion acoustic waves, using kinetic theory. Agreement between theory and observations is obtained for electron and ion distributions with the properties described above. The wave power decreases over cometocentric distances from 24 to 30 km. The main difference between the plasma at closest approach and in the region where the waves are decaying is the absence of a significant current in the latter. Wave observations and theory combined supplement the particle measurements that are difficult at low energies and complicated by spacecraft charging.

Place, publisher, year, edition, pages
Copernicus PublicationsCOPERNICUS GESELLSCHAFT MBH, 2021
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-437448 (URN)10.5194/angeo-39-53-2021 (DOI)000611357600001 ()
Funder
Swedish National Space Board, 96/15Swedish National Space Board, 108/18
Available from: 2021-03-12 Created: 2021-03-12 Last updated: 2024-01-15Bibliographically approved
Bergman, S., Wieser, G. S., Wieser, M., Johansson, F. L., Vigren, E., Nilsson, H., . . . Williamson, H. (2021). Ion bulk speeds and temperatures in the diamagnetic cavity of comet 67P from RPC-ICA measurements. Monthly notices of the Royal Astronomical Society, 503(2), 2733-2745
Open this publication in new window or tab >>Ion bulk speeds and temperatures in the diamagnetic cavity of comet 67P from RPC-ICA measurements
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2021 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 503, no 2, p. 2733-2745Article in journal (Refereed) Published
Abstract [en]

Y Comets are constantly interacting with the solar wind. When the comet activity is high enough, this leads to the creation of a magnetic field free region around the nucleus known as the diamagnetic cavity. It has been suggested that the ion-neutral drag force is balancing the magnetic pressure at the cavity boundary, but after the visit of Rosetta to comet 67P/Churyumov-Gerasimenko the coupling between ions and neutrals inside the cavity has been debated, at least for moderately active comets. In this study, we use data from the ion composition analyser to determine the bulk speeds and temperatures of the low-energy ions in the diamagnetic cavity of comet 67P. The low-energy ions are affected by the negative spacecraft potential, and we use the Spacecraft Plasma Interaction Software to model the resulting influence on the detected energy spectra. We find bulk speeds of 5-10 km s(-1) with a most probable speed of 7 km s(-1), significantly above the velocity of the neutral particles. This indicates that the collisional coupling between ions and neutrals is not strong enough to keep the ions at the same speed as the neutrals inside the cavity. The temperatures are in the range 0.7-1.6 eV, with a peak probability at 1.0 eV. We attribute the major part of the temperature to the fact that ions are born at different locations in the coma, and hence are accelerated over different distances before reaching the spacecraft.

Place, publisher, year, edition, pages
Oxford University PressOxford University Press (OUP), 2021
Keywords
plasmas, methods: data analysis, methods: numerical, comets: individual: 67P/Churyumov-Gerasimenko
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-446194 (URN)10.1093/mnras/stab584 (DOI)000648999700079 ()
Funder
Swedish National Space Board, 130/16Swedish National Space Board, 96/15
Available from: 2021-06-22 Created: 2021-06-22 Last updated: 2024-01-15Bibliographically approved
Stephenson, P., Galand, M., Feldman, P. D., Beth, A., Rubin, M., Bockelee-Morvan, D., . . . Eriksson, A. (2021). Multi-instrument analysis of far-ultraviolet aurora in the southern hemisphere of comet 67P/Churyumov-Gerasimenko. Astronomy and Astrophysics, 647, Article ID A119.
Open this publication in new window or tab >>Multi-instrument analysis of far-ultraviolet aurora in the southern hemisphere of comet 67P/Churyumov-Gerasimenko
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2021 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 647, article id A119Article in journal (Refereed) Published
Abstract [en]

Aims: We aim to determine whether dissociative excitation of cometary neutrals by electron impact is the major source of far-ultraviolet (FUV) emissions at comet 67P/Churyumov-Gerasimenko in the southern hemisphere at large heliocentric distances, both during quiet conditions and impacts of corotating interaction regions observed in the summer of 2016.

Methods: We combined multiple datasets from the Rosetta mission through a multi-instrument analysis to complete the first forward modelling of FUV emissions in the southern hemisphere of comet 67P and compared modelled brightnesses to observations with the Alice FUV imaging spectrograph. We modelled the brightness of OI1356, OI1304, Lyman-beta, CI1657, and CII1335 emissions, which are associated with the dissociation products of the four major neutral species in the coma: CO2, H2O, CO, and O-2. The suprathermal electron population was probed by the Ion and Electron Sensor of the Rosetta Plasma Consortium and the neutral column density was constrained by several instruments: the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA), the Microwave Instrument for the Rosetta Orbiter and the Visual InfraRed Thermal Imaging Spectrometer.

Results: The modelled and observed brightnesses of the FUV emission lines agree closely when viewing nadir and dissociative excitation by electron impact is shown to be the dominant source of emissions away from perihelion. The CII1335 emissions are shown to be consistent with the volume mixing ratio of CO derived from ROSINA. When viewing the limb during the impacts of corotating interaction regions, the model reproduces brightnesses of OI1356 and CI1657 well, but resonance scattering in the extended coma may contribute significantly to the observed Lyman-beta and OI1304 emissions. The correlation between variations in the suprathermal electron flux and the observed FUV line brightnesses when viewing the comet's limb suggests electrons are accelerated on large scales and that they originate in the solar wind. This means that the FUV emissions are auroral in nature.

Place, publisher, year, edition, pages
EDP SciencesEDP SCIENCES S A, 2021
Keywords
comets: individual: 67P/CG, ultraviolet: planetary systems, planets and satellites: aurorae
National Category
Astronomy, Astrophysics and Cosmology Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-441164 (URN)10.1051/0004-6361/202039155 (DOI)000631657600002 ()
Funder
Swedish National Space Board, 108/18
Available from: 2021-05-05 Created: 2021-05-05 Last updated: 2024-01-15Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-5386-8255

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