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On the Formation and Structure of the Ionosphere of Titan
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

We present results on the ionospheric structure around Titan observed during numerous deep (<1000 km) flybys by the Cassini spacecraft. Our results are based on measurements by the radio and plasma wave science instrument, in particular the Langmuir probe. In addition, data from the magnetometer and electron spectrometer have contributed.

The ionosphere of Titan is created when the atmosphere of the moon becomes ionised. There are several mechanisms that contribute to this, the most important of which are considered to be photoionisation by EUV from the Sun with associated photoelectron ionisation, and particle impact ionisation by electrons and ions from Saturn’s corotating magnetosphere.

We investigate the influence of the solar zenith angle on the electron number density at the ionospheric peak. The results show on average four times more plasma on the dayside compared to the nightside, with typical densities of 2500 – 3500 cm-3 and 400 – 1000 cm-3, respectively. In a complementary study, we make a case study of a nightside flyby and show that the altitude structure of the deep ionosphere is reproducible by a simple electron impact ionisation model. Taken together, this leads to the conclusion that solar photons are the main ionisation source of the dayside ionosphere. However, magnetospheric particle precipitation also contributes and can explain the electron densities seen on the nightside.

As Titan does not exhibit any large intrinsic magnetic field, the fact that it is embedded in the magnetosphere of Saturn means that the Kronian field drapes around the moon and gives rise to an induced magnetosphere. We show that there are currents of the order of 10 – 100 nA m-2 flowing in the ionosphere of the moon. Associated with the currents are perpendicular electric fields ranging from 0.5 to 3 µV m-1.

Finally, we investigate measurements obtained during T70, the deepest Titan flyby performed to date. We show that there is a substantial amount of negative ions present below an altitude of 900 km. This confirms previous result by the electron spectrometer, showing negative ions at higher altitudes in Titan’s ionosphere.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. , p. 64
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 926
Keyword [en]
Titan, Cassini, space physics, ionisation, electron density, ionosphere, negative ions, electric currents, electric fields, solar zenith angle, Langmuir probe
National Category
Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
URN: urn:nbn:se:uu:diva-172148ISBN: 978-91-554-8344-9 (print)OAI: oai:DiVA.org:uu-172148DiVA, id: diva2:514763
Public defence
2012-05-25, Polhemsalen, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2012-05-04 Created: 2012-04-02 Last updated: 2012-08-01Bibliographically approved
List of papers
1. On magnetospheric electron impact ionisation and dynamics in Titan's ram-side and polar ionosphere - a Cassini case study
Open this publication in new window or tab >>On magnetospheric electron impact ionisation and dynamics in Titan's ram-side and polar ionosphere - a Cassini case study
2007 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 25, no 11, p. 2359-2369Article in journal (Refereed) Published
Abstract [en]

We present data from the sixth Cassini flyby of Titan (T5), showing that the magnetosphere of Saturn strongly interacts with the moon's ionosphere and exo-ionosphere. A simple electron ionisation model provides a reasonable agreement with the altitude structure of the ionosphere. Furthermore, we suggest that the dense and cold exo-ionosphere (from the exobase at 1430 km and outward to several Titan radii from the surface) can be explained by magnetospheric forcing and other transport processes whereas exospheric ionisation by impacting low energy electrons seems to play a minor role.

Keyword
ionosphere (Planetary ionospheres; Plasma; temperature and density); Magnetospheric physics Magnetosphere-ionosphere interactions)
National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-14309 (URN)000251998900009 ()
Available from: 2008-01-29 Created: 2008-01-29 Last updated: 2017-12-11Bibliographically approved
2. On the ionospheric structure of Titan
Open this publication in new window or tab >>On the ionospheric structure of Titan
Show others...
2009 (English)In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 57, no 14-15, p. 1821-1827Article in journal (Refereed) Published
Abstract [en]

In this study we present data from 17 Titan flybys showing that solar photons are the main ionisation source of Titan's dayside atmosphere. This is the first comprehensive study of Solar Zenith Angle (SZA) dependence of the electron number density and electron temperature at the ionospheric peak. The results show on average four times more plasma on the dayside compared to the nightside, with typical dayside electron densities of around 2500-3500 cm(-3) and corresponding nightside densities of around 400-1000 cm(-3). We identify a broad transition region between SZA 50 degrees and 100 degrees, where the ionosphere of Titan changes from being entirely sunlit to being in the shadow of the moon. For SZA <50 degrees the ionisation peak altitude increases with increasing SZA, whereas the transition region and the nightside show more scattered ionospheric peak altitudes. Typical electron temperatures at the ionospheric peak are 0.03-0.06 eV (approximate to 350-700 K) for both dayside and nightside. (C) 2009 Elsevier Ltd. All rights reserved.

Keyword
Titan, Ionospheric physics, Ionosphere-magnetosphere interaction, Cassini, SZA
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-138100 (URN)10.1016/j.pss.2009.04.012 (DOI)000273099100019 ()
Available from: 2010-12-16 Created: 2010-12-16 Last updated: 2017-12-11Bibliographically approved
3. Detection of currents and associated electric fields in Titan's ionosphere from Cassini data
Open this publication in new window or tab >>Detection of currents and associated electric fields in Titan's ionosphere from Cassini data
Show others...
2011 (English)In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, no 4, p. A04313-Article in journal (Refereed) Published
Abstract [en]

We present observations from three Cassini flybys of Titan using data from the radio and plasma wave science, magnetometer and plasma spectrometer instruments. We combine magnetic field and cold plasma measurements with calculated conductivities and conclude that there are currents of the order of 10 to 100 nA m (2) flowing in the ionosphere of Titan. The currents below the exobase (similar to 1400 km) are principally field parallel and Hall in nature, while the Pedersen current is negligible in comparison. Associated with the currents are perpendicular electric fields ranging from 0.5 to 3 mu V m (1).

National Category
Physical Sciences
Identifiers
urn:nbn:se:uu:diva-153575 (URN)10.1029/2010JA016100 (DOI)000289854600004 ()
Available from: 2011-05-16 Created: 2011-05-16 Last updated: 2017-12-11Bibliographically approved
4. Detection of negative ions in the deep ionosphere of Titan during the Cassini T70 flyby
Open this publication in new window or tab >>Detection of negative ions in the deep ionosphere of Titan during the Cassini T70 flyby
2012 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 39, p. L10201-Article in journal (Refereed) Published
Abstract [en]

We present radio and plasma wave science (RPWS) Langmuir probe (LP) observations that give evidence for a population of heavy, negative ions at altitudes below 900 km in Titan's ionosphere during the Cassini T70 flyby. The negative ion density in this region is comparable to, or higher than, the electron density of 760 cm(-3). Both positive and negative ions are moving with a velocity of at least a few hundred m s(-1) relative to Titan. We show two limiting cases where we have analysed RPWS/LP ion measurements. The data can be interpreted as either that a population of negative ions with density comparable to the electron density is present, moving at a very high (>2 km s(-1)) velocity, or that the ion population is moving at a few hundred m s(-1), but with a density an order of magnitude larger than the electron density in the same region.

Keyword
Negative ions, Cassini, Titan, Langmuir probe
National Category
Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-173067 (URN)10.1029/2012GL051714 (DOI)000304261500006 ()
Available from: 2012-04-18 Created: 2012-04-18 Last updated: 2017-12-07Bibliographically approved

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