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Negative ion densities in the ionosphere of Titan-Cassini RPWS/LP results
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.
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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2013 (English)In: Planetary and Space Science, ISSN 0032-0633, E-ISSN 1873-5088, Vol. 84, 153-162 p.Article 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. 

Place, publisher, year, edition, pages
2013. Vol. 84, 153-162 p.
Keyword [en]
Titan, Ionospheric physics, Cassini, Negative ions, Dust-plasma interaction
National Category
Natural Sciences
Identifiers
URN: urn:nbn:se:uu:diva-206997DOI: 10.1016/j.pss.2013.05.021ISI: 000322805100016OAI: oai:DiVA.org:uu-206997DiVA: diva2:646401
Available from: 2013-09-09 Created: 2013-09-09 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Pre-biotic molecules and dynamics in the ionosphere of Titan: a space weather station perspective
Open this publication in new window or tab >>Pre-biotic molecules and dynamics in the ionosphere of Titan: a space weather station perspective
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Saturn’s largest moon Titan (2575 km radius) is the second largest in the Solar system. Titan is the only known moon with a fully developed nitrogen-rich atmosphere with ionosphere extending to ~2000 km altitude, hosting complex organic chemistry. One of the main scientific interests of Titan’s atmosphere and ionosphere is the striking similarity to current theories of those of 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 reviews the first half of the PhD project on the production of pre-biotic molecules in the atmosphere of Titan and early Earth, focusing on the ion densities and dynamics in Titan’s ionosphere derived from the in-situ measurements by the Cassini Langmuir probe.

One of the main results is the detection of significant, up to ~2300 cm-3, charge densities of heavy (up to ~13000 amu) negative ions in Titan’s ionosphere below 1400 km altitude. On the nightside of the ionosphere at altitudes below 1200 km, the heavy negative ion charge densities are comparable to the positive ion densities and are in fact the main negative charge carrier, making this region of the ionosphere exhibit properties of dusty plasma. The overall trend is the exponential increasing of the negative ion charge densities towards lower altitudes.

Another important result is the detection of ion drifts that between 880-1100 km altitudes in Titan’s ionosphere translate to neutral winds of 0.5-5.5 km/s. Ion drifts define three regions by altitude, the top layer (above ~1600 km altitude) where the ions are frozen into the background magnetic field, the dynamo region (1100 – 1600 km altitudes) where the ions are drifting in partly opposing directions due to ion-neutral collisions in the presence of the magnetic and electric fields and the bottom layer (below 1100 km altitude) of the ionosphere, where the ions are coupled to neutrals by collisions.

Place, publisher, year, edition, pages
Department of Physics and Astronomy, 2015. 38 p.
Keyword
Saturn, Titan, Ionosphere, Langmuir Probe
National Category
Fusion, Plasma and Space Physics
Research subject
Space and Plasma Physics
Identifiers
urn:nbn:se:uu:diva-248118 (URN)
Presentation
2015-02-18, Polhemsalen, Ångström laboratory, 14:10 (English)
Opponent
Supervisors
Funder
Swedish National Space Board
Available from: 2015-04-08 Created: 2015-03-27 Last updated: 2015-04-08Bibliographically approved
2. 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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Shebanits, OlegWahlund, Jan-ErikEdberg, Niklas J. T.

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