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Photoionization modelling of Titan’s dayside ionosphere
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. (Space Plasma Physics)
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

Previous modeling studies of Titan’s dayside ionosphere predicts electron numberdensities roughly a factor of 2 higher than observed by the RPWS/Langmuir probe. The issuecan equivalently be described as that the ratio between the calculated electron productionrates and the square of the observed electron number densities result in roughly a factor of4 higher effective recombination coefficient than expected from the ion composition and theelectron temperature. Here we make an extended reassessment of Titan’s dayside ionizationbalance focusing on 34 flybys between TA and T120. Using a re-calibrated dataset and bytaking the presence of negative ions into account we arrive at lower effective recombinationcoefficients compared with earlier studies. The values are still higher than expected from theion composition and the electron temperature, but by a factor of ~2 − 3 instead of a factorof ~4. We have also investigated whether the derived effective recombination coefficientsdisplay dependencies on parameters such as the solar zenith angle, the integrated solar EUVintensity (< 80 nm) and the corotational plasma ram direction and found statisticallysignificant trends which may be explained by a declining photoionization against thebackground ionization by magnetospheric particles (SZA, RAM) and altered photochemistry(EUV). We find that a series of flybys that occurred during solar minimum (2008) and withsimilar flyby geometries are associated with enhanced values of the effective recombinationcoefficient compared with the remaining dataset, which also suggests a chemistry dependenton the sunlight conditions.

National Category
Fusion, Plasma and Space Physics
Identifiers
URN: urn:nbn:se:uu:diva-329489OAI: oai:DiVA.org:uu-329489DiVA: diva2:1141838
Available from: 2017-09-16 Created: 2017-09-16 Last updated: 2017-09-16
In thesis
1. Titan’s ionosphere and dust: – as seen by a space weather station
Open this publication in new window or tab >>Titan’s ionosphere and dust: – as seen by a space weather station
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Titan, the largest moon of Saturn, is the only known moon with a fully developed nitrogen-rich atmosphere, its ionosphere is detectable as high as 2200 km above its surface and hosts complex organic chemistry. Titan’s atmosphere and ionosphere has striking similarities to current theories of these regions around Earth 3.5 billion years ago. The Cassini spacecraft has been in orbit around Saturn since 2004 and carries a wide range of instruments for investigating Titan’s ionosphere, among them the Langmuir probe, a “space weather station”, manufactured and operated by the Swedish Institute of Space Physics, Uppsala.

This thesis presents studies of positive ions, negative ions and negatively charged dust grains (also called aerosols) in Titan’s ionosphere using the in-situ measurements by the Cassini Langmuir probe, supplemented by the data from particle mass spectrometers. One of the main results is the detection of significant (up to about 4000 cm-3) charge densities of heavy (up to about 13800 amu/charge) negative ions and dust grains in Titan’s ionosphere below 1400 km altitude. The dust is found to be the main negative charge carrier below about 1100 km on the nightside/terminator ionosphere, forming a dusty plasma (also called “ion-ion” plasma). A new analysis method is developed using a combination of simultaneous observations by multiple instruments for a case study of four flybys of Titan’s ionosphere, further constraining the ionospheric plasma charge densities. This allows to predict a dusty plasma in the dayside ionosphere below 900 km altitude (thus declaring it a global phenomenon), as well as to empirically estimate the average charge of the negative ions and dust grains to between -2.5 and -1.5 elementary charges. The complete Cassini dataset spans just above 13 years, allowing to study effects of the solar activity on Titan’s ionosphere. From solar minimum to maximum, the increase in the solar EUV flux increases the densities by a factor of ~2 in the dayside ionosphere and, surprisingly, decreases by a factor of ~3-4 in the nightside ionosphere. The latter is proposed to be an effect of the ionospheric photochemistry modified by higher solar EUV flux. Modelling photoionization also reveals an EUV trend (as well as solar zenith angle and corotational plasma ram dependencies) in the loss rate coefficient.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 69 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1562
Keyword
Titan, Cassini, Ionosphere, Dusty plasma, Ion-ion plasma, Langmuir probe, aerosols, tholins
National Category
Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-329490 (URN)978-91-513-0076-4 (ISBN)
Public defence
2017-11-03, Ångström 2005, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, Dnr 130/11:2
Available from: 2017-10-12 Created: 2017-09-16 Last updated: 2017-10-18

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