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Diving Deep into Saturn's Equatorial Ionosphere with Cassini: Insights from the Grand Finale
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy. Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0003-3038-3359
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Description
Abstract [en]

In the summer of 2017, the Cassini mission concluded its nearly 13 years orbiting Saturn with a series of daring dives between the rings and the upper reaches of Saturn's atmosphere. This last phase of the mission, called the Grand Finale, revealed a highly variable equatorial ionosphere dominated by a large influx of ring material from Saturn's D ring. The papers included in this thesis utilize data gathered during these proximal orbits to gain insights into the nature and effects of the infalling ring material.

Initially, we derive upper limits for the effective recombination coefficient in Saturn's equatorial ionosphere at altitudes below 2500 km, where photochemical equilibrium can be assumed, to constrain the composition of the positive ion species. Our inceptive results indicate that ion species with low recombination coefficients are dominant.

We follow up on this by developing a photochemical model, incorporating grain charging, to investigate the effects of the ring influx on the plasma composition. The model results at an altitude of 1700 km yield vastly different abundances of two types of neutral species when compared to those derived from measurements, ultimately representing the difficulty of reconciling the observed H+ and H3+ densities with our and other model results.

Exploring the nature of narrow decreases in the ionospheric H2+ densities reveals a time shift in the ion data. After correcting for this, the decreases line up very well with calculated shadows for substructures in Saturn's C ring. We can further estimate the optical depths of these substructures and investigate at which altitudes photochemical equilibrium for H2+ is applicable.

The direct measurement of heavier neutral species during the proximal orbits is complicated by the high spacecraft speed. We devise a method to utilize helium ion chemistry to independently derive the mixing ratios of these heavier neutrals in Saturn's ionosphere. Our results show considerable variability, which may suggest temporal and/or spatial changes in the ring influx. A comparison with other studies indicates that potentially only the most volatile ring-sourced species significantly ablate to enter the gas phase in this region of Saturn's ionosphere.

Finally, we compare the fixed-bias Langmuir probe electron densities and the light ion densities. They exhibit a strong positive correlation for most parts of the proximal orbits even on short timescales. We find three distinct regions in the proximal orbits, which can provide further insight into the ionospheric composition, connection to the rings, and measurement uncertainties.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2023. , p. 59
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 2313
Keywords [en]
Saturn, Cassini, Grand Finale, Ionosphere, Photochemistry, Planetary Rings, Planetary Science, Space Plasma, Space Physics
National Category
Fusion, Plasma and Space Physics
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
URN: urn:nbn:se:uu:diva-512834ISBN: 978-91-513-1910-0 (print)OAI: oai:DiVA.org:uu-512834DiVA, id: diva2:1801819
Public defence
2023-11-23, Sonja Lyttkens (101121), Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 143/18Available from: 2023-11-02 Created: 2023-10-03 Last updated: 2023-11-02
List of papers
1. Constraining the Positive Ion Composition in Saturn's Lower Ionosphere with the Effective Recombination Coefficient
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
2. Empirical Photochemical Modeling of Saturn's Ionization Balance Including Grain Charging
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
3. Identifying Shadowing Signatures of C Ring Ringlets and Plateaus in Cassini Data from Saturn's Ionosphere
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
4. Utilizing Helium Ion Chemistry to Derive Mixing Ratios of Heavier Neutral Species in Saturn's Equatorial Ionosphere
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
5. Electron to Light Ion Density Ratios during Cassini's Grand Finale: Addressing Open Questions About Saturn's Low-Latitude Ionosphere
Open this publication in new window or tab >>Electron to Light Ion Density Ratios during Cassini's Grand Finale: Addressing Open Questions About Saturn's Low-Latitude Ionosphere
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(English)Manuscript (preprint) (Other academic)
Keywords
Saturn, Cassini, Grand Finale, Ionosphere, Photochemistry, Planetary Rings, Planetary Science, Space Plasma, Space Physics
National Category
Fusion, Plasma and Space Physics
Research subject
Physics with specialization in Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-512832 (URN)
Funder
Swedish National Space Board, 143/18
Available from: 2023-10-03 Created: 2023-10-03 Last updated: 2023-10-10

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