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Constraining the Positive Ion Composition in Saturn's Lower Ionosphere with the Effective Recombination Coefficient
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
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0003-2647-8259
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0001-9958-0241
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.ORCID iD: 0000-0002-2107-5859
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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 Society American Astronomical Society, 2021. Vol. 2, no 1, article id 39
National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:uu:diva-458301DOI: 10.3847/psj/abd6e9ISI: 000912867900001OAI: oai:DiVA.org:uu-458301DiVA, id: diva2:1609608
Funder
Swedish National Space Board, 143/18Swedish National Space Board, 168/15Available from: 2021-11-08 Created: 2021-11-08 Last updated: 2024-01-15Bibliographically approved
In thesis
1. Diving Deep into Saturn's Equatorial Ionosphere with Cassini: Insights from the Grand Finale
Open this publication in new window or tab >>Diving Deep into Saturn's Equatorial Ionosphere with Cassini: Insights from the Grand Finale
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
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
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-512834 (URN)978-91-513-1910-0 (ISBN)
Public defence
2023-11-23, Sonja Lyttkens (101121), Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, 143/18
Available from: 2023-11-02 Created: 2023-10-03 Last updated: 2023-11-02

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Dreyer, JoshuaVigren, ErikMorooka, MichikoWahlund, Jan-ErikBuchert, Stephan C.Johansson, Fredrik L.

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Dreyer, JoshuaVigren, ErikMorooka, MichikoWahlund, Jan-ErikBuchert, Stephan C.Johansson, Fredrik L.
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