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Edberg, N. J. T., Eriksson, A. I., Vigren, E., Nilsson, H., Gunell, H., Götz, C., . . . De Keyser, J. (2024). Scale size of cometary bow shocks. Astronomy and Astrophysics, 682, Article ID A51.
Open this publication in new window or tab >>Scale size of cometary bow shocks
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2024 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 682, article id A51Article in journal (Refereed) Published
Abstract [en]

Context. In past decades, several spacecraft have visited comets to investigate their plasma environments. In the coming years, Comet Interceptor will make yet another attempt. This time, the target comet and its outgassing activity are unknown and may not be known before the spacecraft has been launched into its parking orbit, where it will await a possible interception. If the approximate outgassing rate can be estimated remotely when a target has been identified, it is desirable to also be able to estimate the scale size of the plasma environment, defined here as the region bound by the bow shock.

Aims. This study aims to combine previous measurements and simulations of cometary bow shock locations to gain a better understanding of how the scale size of cometary plasma environments varies. We compare these data with models of the bow shock size, and we furthermore provide an outgassing rate-dependent shape model of the bow shock. We then use this to predict a range of times and cometocentric distances for the crossing of the bow shock by Comet Interceptor, together with expected plasma density measurements along the spacecraft track.

Methods. We used data of the location of cometary bow shocks from previous spacecraft missions, together with simulation results from previously published studies. We compared these results with an existing model of the bow shock stand-off distance and expand on this to provide a shape model of cometary bow shocks. The model in particular includes the cometary outgassing rate, but also upstream solar wind conditions, ionisation rates, and the neutral flow velocity.

Results. The agreement between the gas-dynamic model and the data and simulation results is good in terms of the stand-off distance of the bow shock as a function of the outgassing rate. For outgassing rates in the range of 1027–1031–s-1, the scale size of cometary bow shocks can vary by four orders of magnitude, from about 102 km to 106 km, for an ionisation rate, flow velocity, and upstream solar wind conditions typical of those at 1 AU. The proposed bow shock shape model shows that a comet plasma environment can range in scale size from the plasma environment of Mars to about half of that of Saturn.

Conclusions. The model-data agreement allows for the planning of upcoming spacecraft comet encounters, such as that of Comet Interceptor, when a target has been identified and its outgassing rate is determined. We conclude that the time a spacecraft can spend within the plasma environment during a flyby can range from minutes to days, depending on the comet that is visited and on the flyby speed. However, to capture most of the comet plasma environment, including pick-up ions and upstream plasma waves, and to ensure the highest possible scientific return, measurements should still start well upstream of the expected bow shock location. From the plasma perspective, the selected target should preferably be an active comet with the lowest possible flyby velocity.

Place, publisher, year, edition, pages
EDP Sciences, 2024
Keywords
comets: general, plasmas
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-525518 (URN)10.1051/0004-6361/202346566 (DOI)001163661400003 ()
Funder
Swedish Research Council, 2020-03962
Available from: 2024-03-25 Created: 2024-03-25 Last updated: 2024-03-25Bibliographically approved
Vigren, E., Eriksson, A., Edberg, N. J. T. & Snodgrass, C. (2023). A potential aid in the target selection for the comet interceptor mission. Planetary and Space Science, 237, Article ID 105765.
Open this publication in new window or tab >>A potential aid in the target selection for the comet interceptor mission
2023 (English)In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 237, article id 105765Article in journal (Refereed) Published
Abstract [en]

The upcoming Comet Interceptor mission involves a parking phase around the Sun-Earth L2 point before transferring to intercept the orbit of a long period comet, interstellar object or a back-up target in the form of a short-period comet. The target is not certain to be known before the launch in 2029. During the parking phase there may thus arise a scenario wherein a decision needs to be taken of whether to go for a particular comet or whether to discard that option in the hope that a better target will appear within a reasonable time frame later on. We present an expectation value-based formalism that could aid in the associated decision making provided that outlined requirements for its implementation exist.

Place, publisher, year, edition, pages
Elsevier, 2023
Keywords
Comets, Solar system, Probability theory
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-514901 (URN)10.1016/j.pss.2023.105765 (DOI)001071302000001 ()
Funder
Swedish National Space Board, 202100047
Available from: 2023-10-31 Created: 2023-10-31 Last updated: 2023-10-31Bibliographically approved
Kim, K., Edberg, N. J. T., Shebanits, O., Wahlund, J.-E., Vigren, E. & Bertucci, C. (2023). On Current Sheets and Associated Density Spikes in Titan's Ionosphere as Seen From Cassini. Journal of Geophysical Research - Space Physics, 128(3)
Open this publication in new window or tab >>On Current Sheets and Associated Density Spikes in Titan's Ionosphere as Seen From Cassini
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2023 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 128, no 3Article in journal (Refereed) Published
Abstract [en]

The Cassini spacecraft made in-situ measurements of Titan's plasma environment during 126 close encounters between 2004 and 2017. Here we report on observations from the Radio and Plasma Waves System/Langmuir probe instrument (RPWS/LP) from which we have observed, primarily on the outbound leg, a localized increase of the electron density by up to 150 cm−3 with respect to the background. This feature, appearing as an electron density spike in the data, is found during 28 of the 126 flybys. The data from RPWS/LP, the electron spectrometer from the Cassini Plasma Spectrometer package , and the magnetometer is used to calculate electron densities and magnetic field characteristics. The location of these structures around Titan with respect to the nominal corotation direction and the sun direction is investigated. We find that the electron density spikes are primarily observed on the dayside and ramside of Titan. We also observe magnetic field signatures that could suggest the presence of current sheets in most cases. The density spikes are extended along the trajectory of the spacecraft with the horizontal scale of ∼537 ± 160 km and vertical scale ∼399 ± 163 km. We suggest that the density spikes are formed as a result of the current sheet formation.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2023
Keywords
Titan, Cassini, current sheets, Langmuir probe
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-501378 (URN)10.1029/2022JA031118 (DOI)000949118800001 ()
Funder
Swedish Research Council, 2020-03962Swedish National Space Board
Available from: 2023-05-08 Created: 2023-05-08 Last updated: 2023-05-08Bibliographically approved
Dreyer, J., Vigren, E., Johansson, F. L. & Waite, J. H. (2023). Utilizing Helium Ion Chemistry to Derive Mixing Ratios of Heavier Neutral Species in Saturn's Equatorial Ionosphere. Journal of Geophysical Research - Space Physics, 128(6), Article ID e2023JA031488.
Open this publication in new window or tab >>Utilizing Helium Ion Chemistry to Derive Mixing Ratios of Heavier Neutral Species in Saturn's Equatorial Ionosphere
2023 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 128, no 6, article id e2023JA031488Article in journal (Refereed) Published
Abstract [en]

A surprisingly strong influx of organic-rich material into Saturn's upper atmosphere from its rings was observed during the proximal obits of the Grand Finale of the Cassini mission. Measurements by the Ion and Neutral Mass Spectrometer (INMS) gave insights into the composition of the material, but it remains to be resolved what fraction of the inferred heavy volatiles should be attributed as originating from the fragmentation of dust particles in the instrument versus natural ablation of grains in the atmosphere. In the present study, we utilize measured light ion and neutral densities to further constrain the abundances of heavy volatiles in Saturn's ionosphere through a steady-state model focusing on helium ion chemistry. We first show that the principal loss mechanism of He+ in Saturn's equatorial ionosphere is through reactions with species other than H-2. Based on the assumption of photochemical equilibrium at altitudes below 2,500 km, we then proceed by estimating the mixing ratio of heavier volatiles down to the closest approaches for Cassini's proximal orbits 288 and 292. Our derived mixing ratios for the inbound part of both orbits fall below those reported from direct measurements by the INMS, with values of similar to 2 x 10(-4) at closest approaches and order-of-magnitude variations in either direction over the orbits. This aligns with previous suggestions that a large fraction of the neutrals measured by the INMS stems from the fragmentation of infalling dust particles that do not significantly ablate in the considered part of Saturn's atmosphere and are thus unavailable for reactions. Plain Language Summary During the final orbits of the Cassini mission, the spacecraft flew between Saturn's rings and the planets upper atmosphere. The onboard plasma instruments detected a large amount of ring particles falling toward the planet, but direct measurements of the composition of these grains are complicated due to the high spacecraft speed and instrumental effects. In this study, we present an independent method to estimate the abundance of heavier neutral species entering the atmosphere from infalling ring material. This method relies on helium ion chemistry and the measured light ion and neutral densities. Our results generally fall below those inferred from direct measurements. Together with comparisons to other studies, this potentially suggests that a large fraction of the infalling neutral species do not significantly ablate in the considered part of Saturn's atmosphere (and remain bound to the dust grains instead) and are thus unavailable for reactions.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2023
Keywords
Saturn, Cassini, ionosphere, space plasma, ion chemistry, planetary science
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-508434 (URN)10.1029/2023JA031488 (DOI)001011943200001 ()
Funder
Swedish National Space Board, 143/18
Available from: 2023-08-02 Created: 2023-08-02 Last updated: 2023-10-06Bibliographically approved
Vigren, E. (2023). Wait Till You See Them All Together [Letter to the editor]. The American mathematical monthly, 130(9), 868-868
Open this publication in new window or tab >>Wait Till You See Them All Together
2023 (English)In: The American mathematical monthly, ISSN 0002-9890, E-ISSN 1930-0972, Vol. 130, no 9, p. 868-868Article in journal, Letter (Other academic) Published
Place, publisher, year, edition, pages
The Mathematical Association of America, 2023
National Category
Other Mathematics
Identifiers
urn:nbn:se:uu:diva-515655 (URN)001084489900022 ()
Available from: 2023-11-09 Created: 2023-11-09 Last updated: 2023-11-09Bibliographically approved
Vigren, E. & Dieckmann, A. (2022). A New Result in Form of Finite Triple Sums for a Series from Ramanujan's Notebooks. Symmetry, 14(6), Article ID 1090.
Open this publication in new window or tab >>A New Result in Form of Finite Triple Sums for a Series from Ramanujan's Notebooks
2022 (English)In: Symmetry, E-ISSN 2073-8994, Vol. 14, no 6, article id 1090Article in journal (Refereed) Published
Abstract [en]

We consider a function g(r, x, u) with x, u is an element of C and r is an element of N, which, over a symmetric domain, equals the sum of an infinite series as noted in the 16th Entry of Chapter 3 in Ramanujan's second notebook. The function attracted new attention since it was established to be closely connected to the theory of labelled trees. However, to the best of our knowledge, a closed-form solution allowing, e.g., the rapid computation of g (r, x, u) in Mathematica without explicit use of recursions has been lacking until now. Our proposed formula transforms the part depending on the variable u into a more symmetric form, which then appears inside a finite triple sum consisting of binomials and Stirling numbers of the second kind.

Place, publisher, year, edition, pages
MDPI AG, 2022
Keywords
Ramanujan, Stirling numbers, combinatorics
National Category
Discrete Mathematics
Identifiers
urn:nbn:se:uu:diva-480279 (URN)10.3390/sym14061090 (DOI)000817651500001 ()
Available from: 2022-07-08 Created: 2022-07-08 Last updated: 2022-07-08Bibliographically approved
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
Vigren, E., Eriksson, A. ,. & Bergman, S. (2022). On positively charged dust in the coma of comet 67P. Monthly notices of the Royal Astronomical Society, 513(1), 536-540
Open this publication in new window or tab >>On positively charged dust in the coma of comet 67P
2022 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 513, no 1, p. 536-540Article in journal (Refereed) Published
Abstract [en]

Moment analysis of ion spectrograms measured by the Ion Composition Analyser (ICA) in the coma of comet 67P typically produces an ion number density estimate markedly lower than the number density of free electrons as measured by the Mutual Impedance Probe and the dual Langmuir Probe. While there are good reasons to distrust the ion density moment estimate in these circumstances, the issue cannot yet be considered fully understood and it is of interest to see whether any natural non-instrumental cause is possible. An obvious such cause would be whether a significant fraction of the positive charge density resides in positively charged dust grains that are not measured by the ICA. Here, we show that this scenario is highly unlikely, even near perihelion where photoemission is the strongest. In our semi-analytical grain charging model, we balance the current contributions to grains of photoelectron emission and electron attachment so as to find the expected charge state for a grain of a given radius. The charge state is affected by the ambient electron number density, the electron temperature, and the heliocentric distance. While at times the bulk of the dust population around comet 67P could be charged positive, dust charging, including photoelectron emission, should have a negligible influence on the overall ionization balance in the cometary coma simply because the dust particles are not ubiquitous enough.

Place, publisher, year, edition, pages
Oxford University Press (OUP), 2022
Keywords
comets: general, comets: individual: 67P
National Category
Astronomy, Astrophysics and Cosmology Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-474114 (URN)10.1093/mnras/stac869 (DOI)000785870200006 ()
Available from: 2022-05-09 Created: 2022-05-09 Last updated: 2022-05-09Bibliographically approved
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ORCID iD: ORCID iD iconorcid.org/0000-0003-2647-8259

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