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On magnetospheric electron impact ionisation and dynamics in Titan's ram-side and polar ionosphere - a Cassini case study
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. (Space Plasma Physics)
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Swedish Institute of Space Physics, Uppsala Division. (Space Plasma Physics)
Department of Physics, Imperial College London.
2007 (English)In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 25, no 11, 2359-2369 p.Article 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.

Place, publisher, year, edition, pages
2007. Vol. 25, no 11, 2359-2369 p.
Keyword [en]
ionosphere (Planetary ionospheres; Plasma; temperature and density); Magnetospheric physics Magnetosphere-ionosphere interactions)
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:uu:diva-14309ISI: 000251998900009OAI: oai:DiVA.org:uu-14309DiVA: diva2:42079
Available from: 2008-01-29 Created: 2008-01-29 Last updated: 2017-12-11Bibliographically approved
In thesis
1. On the Formation and Structure of the Ionosphere of Titan
Open this publication in new window or tab >>On the Formation and Structure of the Ionosphere of Titan
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. 64 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 926
Keyword
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:nbn:se:uu:diva-172148 (URN)978-91-554-8344-9 (ISBN)
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

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http://www.ann-geophys.net/25/2359/2007/angeo-25-2359-2007.pdf

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Ågren, KarinWahlund, Jan-Erik

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