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Detection of negative ions in the deep ionosphere of Titan during the Cassini T70 flyby
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.
2012 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 39, L10201- p.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.

Place, publisher, year, edition, pages
2012. Vol. 39, L10201- p.
Keyword [en]
Negative ions, Cassini, Titan, Langmuir probe
National Category
Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
URN: urn:nbn:se:uu:diva-173067DOI: 10.1029/2012GL051714ISI: 000304261500006OAI: oai:DiVA.org:uu-173067DiVA: diva2:516399
Available from: 2012-04-18 Created: 2012-04-18 Last updated: 2017-12-07Bibliographically 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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Ågren, KarinEdberg, NiklasWahlund, Jan-Erik

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