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Titan’s ionosphere and dust: – as seen by a space weather station
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, Department of Physics and Astronomy, Space Plasma Physics.ORCID iD: 0000-0001-9621-211X
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. , p. 69
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1562
Keywords [en]
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: urn:nbn:se:uu:diva-329490ISBN: 978-91-513-0076-4 (print)OAI: oai:DiVA.org:uu-329490DiVA, id: diva2:1141839
Public defence
2017-11-03, Ångström 2005, Lägerhyddsvägen 1, Uppsala, 09:00 (English)
Opponent
Supervisors
Funder
Swedish National Space Board, Dnr 130/11:2Available from: 2017-10-12 Created: 2017-09-16 Last updated: 2017-10-18
List of papers
1. Negative ion densities in the ionosphere of Titan-Cassini RPWS/LP results
Open this publication in new window or tab >>Negative ion densities in the ionosphere of Titan-Cassini RPWS/LP results
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2013 (English)In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 84, p. 153-162Article in journal (Refereed) Published
Abstract [en]

The Cassini spacecraft Radio and Plasma Wave Science (RPWS) Langmuir Probe (LP) provides in-situ measurements of Titan's ionosphere. We present here data from 47 deep flybys in the time period October 2004 July 2012 of charge densities of positive and negative ions as well as electrons. These densities have been mapped with respect to altitude and solar zenith angle (SZA) in an altitude range of 880-1400 km. The inferred electron number densities are consistent with earlier presented observational results. Negative ion charge densities exhibit a trend that exponentially increases towards lower altitudes within the covered altitude range. This is especially evident on the nightside of Titan (SZA > 110 degrees). The negative ion charge densities at the lowest traversed altitudes (near 960 km) are inferred to be in the range 300-2500 cm(-3). The results show that very few free electrons (n(e)/n(i)similar to 0.1-0.7) exist in the deepest regions (880-1050 km) of Titan's nightside ionosphere. Instead the deep nightside part of Titan's ionosphere is dominated by both negatively and positively charged heavy (> 100 amu) organic ions. We therefore believe a dust/aerosol-ion plasma exists here, similar to what is found in noctilucent clouds in Earth's mesosphere. 

Keywords
Titan, Ionospheric physics, Cassini, Negative ions, Dust-plasma interaction
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-206997 (URN)10.1016/j.pss.2013.05.021 (DOI)000322805100016 ()
Available from: 2013-09-09 Created: 2013-09-09 Last updated: 2017-12-06Bibliographically approved
2. Ion and aerosol precursor densities in Titan's ionosphere: A multi-instrument case study
Open this publication in new window or tab >>Ion and aerosol precursor densities in Titan's ionosphere: A multi-instrument case study
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2016 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 10, p. 10075-10090Article in journal (Refereed) Published
Abstract [en]

The importance of the heavy ions and dust grains for the chemistry and aerosol formation in Titan's ionosphere has been well established in the recent years of the Cassini mission. In this study we combine independent in situ plasma (Radio Plasma and Wave Science Langmuir Probe (RPWS/LP)) and particle (Cassini Plasma Science Electron Spectrometer, Cassini Plasma Science Ion Beam Spectrometer, and Ion and Neutral Mass Spectrometer) measurements of Titan's ionosphere for selected flybys (T16, T29, T40, and T56) to produce altitude profiles of mean ion masses including heavy ions and develop a Titan-specific method for detailed analysis of the RPWS/LP measurements (applicable to all flybys) to further constrain ion charge densities and produce the first empirical estimate of the average charge of negative ions and/or dust grains. Our results reveal the presence of an ion-ion (dusty) plasma below similar to 1100 km altitude, with charge densities exceeding the primary ionization peak densities by a factor >= 2 in the terminator and nightside ionosphere (n(e)/n(i) <= 0.1). We suggest that ion-ion (dusty) plasma may also be present in the dayside ionosphere below 900 km (n(e)/n(i) < 0.5 at 1000 km altitude). The average charge of the dust grains (>= 1000 amu) is estimated to be between -2.5 and -1.5 elementary charges, increasing toward lower altitudes.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-312118 (URN)10.1002/2016JA022980 (DOI)000388965900050 ()
Available from: 2017-01-09 Created: 2017-01-04 Last updated: 2017-11-29Bibliographically approved
3. Titan’s ionosphere: A survey of solar EUV influences
Open this publication in new window or tab >>Titan’s ionosphere: A survey of solar EUV influences
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2017 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 7, p. 7491-7503Article in journal (Refereed) Published
Abstract [en]

Effects of solar EUV on positive ions and heavy negative charge carriers (molecular ions, aerosol, and/or dust) in Titan’s ionosphere are studied over the course of almost 12 years, including 78 flybys below 1400 km altitude between TA (October 2004) and T120 (June 2016). The Radio and Plasma Wave Science/Langmuir Probe-measured ion charge densities (normalized by the solar zenith angle) show statistically significant variations with respect to the solar EUV flux. Dayside charge densities increase by a factor of ≈2 from solar minimum to maximum, while nightside charge densities are found to anticorrelate with the EUV flux and decrease by a factor of ≈3–4. The overall EUV dependence of the ion charge densities suggest inapplicability of the idealized Chapman theory below 1200 km in Titan’s ionosphere. Nightside charge densities are also found to vary along Titan’s orbit, with higher values in the sunward magnetosphere of Saturn compared to the magnetotail.

National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-329486 (URN)10.1002/2017JA023987 (DOI)000407627100038 ()
Funder
Swedish National Space Board, Dnr 130/11:2, Dnr 166/14, Dnr 135/13Swedish Research Council, 621-2013-4191
Available from: 2017-09-16 Created: 2017-09-16 Last updated: 2017-11-16Bibliographically approved
4. Photoionization modeling of Titan’s dayside ionosphere
Open this publication in new window or tab >>Photoionization modeling of Titan’s dayside ionosphere
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2017 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 850, no 2, article id L26Article in journal (Refereed) Published
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:nbn:se:uu:diva-329489 (URN)10.3847/2041-8213/aa998d (DOI)000417541800001 ()
Funder
Swedish National Space Board, 130/11:2, 166/14, 135/13Swedish Research Council, 621-2013-4191
Available from: 2017-09-16 Created: 2017-09-16 Last updated: 2018-01-31Bibliographically approved

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