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Publications (10 of 14) Show all publications
Edberg, N. J. T., Vigren, E., Snowden, D., Regoli, L. H., Shebanits, O., Wahlund, J.-E., . . . Cui, J. (2018). Titan's Variable Ionosphere During the T118 and T119 Cassini Flybys. Geophysical Research Letters, 45(17), 8721-8728
Open this publication in new window or tab >>Titan's Variable Ionosphere During the T118 and T119 Cassini Flybys
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2018 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 45, no 17, p. 8721-8728Article in journal (Refereed) Published
Abstract [en]

We report on unusual dynamics in Titan's ionosphere as a significant difference in ionospheric electron density is observed between the T118 and T119 Cassini nightside flybys. Two distinct nightside electron density peaks were present during T118, at 1,150 and 1,200km, and the lowest density ever observed in Titan's ionosphere at altitudes 1,000-1,350km was during T118. These flybys were quite similar in geometry, Saturn local time, neutral density, extreme ultraviolet flux, and ambient magnetic field conditions. Despite this, the Radio and Plasma Waves/Langmuir Probe measured a density difference up to a factor of 6 between the passes. The overall difference was present and similar during both inbound and outbound legs. By ruling out other factors, we suggest that an exceptionally low rate of particle impact ionization in combination with dynamics in the ionosphere is the explanation for the observations. Plain Language Summary Using the Cassini satellite in orbit around Saturn, we make measurements during two close passes of the moon Titan. We observe how the electron density in the uppermost part of the moon's atmosphere-the ionosphere-changes drastically from one pass to the next. We also observe unexpectedly high peaks of electron density in a specific altitude range during the first pass. The findings are attributed to low influx of charged particles from Saturn's magnetosphere as well as to increased dynamics of the plasma in the ionosphere. The study emphasizes the complexity of the physical process at play at the moon and aims at gaining further understanding of this environment.

Place, publisher, year, edition, pages
American Geophysical Union (AGU), 2018
National Category
Fusion, Plasma and Space Physics Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-363432 (URN)10.1029/2018GL078436 (DOI)000445727500003 ()
Funder
Swedish Research Council, 621-2013-4191Swedish National Space Board, 135/13
Available from: 2018-10-18 Created: 2018-10-18 Last updated: 2018-10-18Bibliographically approved
Desai, R. T., Coates, A. J., Wellbrock, A., Vuitton, V., Crary, F. J., Gonzalez-Caniulef, D., . . . Sittler, E. C. (2017). Carbon Chain Anions and the Growth of Complex Organic Molecules in Titan's Ionosphere. Astrophysical Journal Letters, 844(2), Article ID L18.
Open this publication in new window or tab >>Carbon Chain Anions and the Growth of Complex Organic Molecules in Titan's Ionosphere
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2017 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 844, no 2, article id L18Article in journal (Refereed) Published
Abstract [en]

Cassini discovered a plethora of neutral and ionized molecules in Titan's ionosphere including, surprisingly, anions and negatively charged molecules extending up to 13,800 u q-1. In this Letter, we forward model the Cassini electron spectrometer response function to this unexpected ionospheric component to achieve an increased mass resolving capability for negatively charged species observed at Titan altitudes of 950-1300 km. We report on detections consistently centered between 25.8 and 26.0 u q-1 and between 49.0-50.1 u q(-1) which are identified as belonging to the carbon chain anions, CN-/C3N- and/or C2H-/C4H-, in agreement with chemical model predictions. At higher ionospheric altitudes, detections at 73-74 u q-1 could be attributed to the further carbon chain anions C5N-/C6H- but at lower altitudes and during further encounters extend over a higher mass/charge range. This, as well as further intermediary anions detected at > 100 u, provide the first evidence for efficient anion chemistry in space involving structures other than linear chains. Furthermore, at altitudes below < 1100 km, the low-mass anions (< 150 u q-1) were found to deplete at a rate proportional to the growth of the larger molecules, a correlation that indicates the anions are tightly coupled to the growth process. This study adds Titan to an increasing list of astrophysical environments where chain anions have been observed and shows that anion chemistry plays a role in the formation of complex organics within a planetary atmosphere as well as in the interstellar medium.

Keywords
astrobiology, astrochemistry, ISM: molecules, planets and satellites: atmospheres, planets and satellites: individual (Titan)
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-332933 (URN)10.3847/2041-8213/aa7851 (DOI)000406276800001 ()
Funder
Swedish National Space Board, Dnr 130/11:2
Available from: 2017-11-15 Created: 2017-11-15 Last updated: 2017-11-15Bibliographically approved
Holmberg, M. K., Shebanits, O., Wahlund, J.-E., Morooka, M., Vigren, E., Andre, N., . . . Gilbert, L. K. (2017). Density Structures, Dynamics, and Seasonal and Solar Cycle Modulations of Saturn's Inner Plasma Disk. Journal of Geophysical Research - Space Physics, 122(12), 12258-12273
Open this publication in new window or tab >>Density Structures, Dynamics, and Seasonal and Solar Cycle Modulations of Saturn's Inner Plasma Disk
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2017 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 12, p. 12258-12273Article in journal (Refereed) Published
Abstract [en]

We present statistical results from the Cassini Radio and Plasma Wave Science (RPWS) Langmuir probe measurements recorded during the time interval from orbit 3 (1 February 2005) to 237 (29 June 2016). A new and improved data analysis method to obtain ion density from the Cassini LP measurements is used to study the asymmetries and modulations found in the inner plasma disk of Saturn, between 2.5 and 12 Saturn radii (1 RS = 60, 268 km). The structure of Saturn's plasma disk is mapped, and the plasma density peak, n(max), is shown to be located at similar to 4.6 RS and not at the main neutral source region at 3.95 RS. The shift in the location of n(max) is due to that the hot electron impact ionization rate peaks at similar to 4.6 RS. Cassini RPWS plasma disk measurements show a solar cycle modulation. However, estimates of the change in ion density due to varying EUV flux is not large enough to describe the detected dependency, which implies that an additional mechanism, still unknown, is also affecting the plasma density in the studied region. We also present a dayside/nightside ion density asymmetry, with nightside densities up to a factor of 2 larger than on the dayside. The largest density difference is found in the radial region 4 to 5 RS. The dynamic variation in ion density increases toward Saturn, indicating an internal origin of the large density variability in the plasma disk rather than being caused by an external source origin in the outer magnetosphere.

Place, publisher, year, edition, pages
AMER GEOPHYSICAL UNION, 2017
National Category
Fusion, Plasma and Space Physics
Identifiers
urn:nbn:se:uu:diva-342472 (URN)10.1002/2017JA024311 (DOI)000422735500032 ()
Funder
Swedish National Space Board, 166/14, 130/11:2
Available from: 2018-02-26 Created: 2018-02-26 Last updated: 2018-02-26Bibliographically approved
Shebanits, O., Vigren, E., Wahlund, J.-E., Edberg, N. J. T., Cui, J., Mandt, K. & Waite, H. (2017). Photoionization modeling of Titan’s dayside ionosphere. Astrophysical Journal Letters, 850(2), Article ID L26.
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
Shebanits, O., Vigren, E., Wahlund, J.-E., Holmberg, M., Morooka, M., Edberg, N. J. T., . . . Waite, H. (2017). Titan’s ionosphere: A survey of solar EUV influences. Journal of Geophysical Research - Space Physics, 122(7), 7491-7503
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
Shebanits, O. (2017). Titan’s ionosphere and dust: – as seen by a space weather station. (Doctoral dissertation). Uppsala: Acta Universitatis Upsaliensis
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. p. 69
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1562
Keywords
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
Shebanits, O., Wahlund, J.-E. -., Edberg, N. J. T., Crary, F. J., Wellbrock, A., Andrews, D. J., . . . Waite, J. H. (2016). Ion and aerosol precursor densities in Titan's ionosphere: A multi-instrument case study. Journal of Geophysical Research - Space Physics, 121(10), 10075-10090
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
Edberg, N. J. T., Andrews, D. J., Bertucci, C., Gurnett, D. A., Holmberg, M. K. G., Jackman, C. M., . . . Wahlund, J.-E. (2015). Effects of Saturn's magnetospheric dynamics on Titan's ionosphere. Journal of Geophysical Research - Space Physics, 120(10), 8884-8898
Open this publication in new window or tab >>Effects of Saturn's magnetospheric dynamics on Titan's ionosphere
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2015 (English)In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 10, p. 8884-8898Article in journal (Refereed) Published
Abstract [en]

We use the Cassini Radio and Plasma Wave Science/Langmuir probe measurements of the electron density from the first 110 flybys of Titan to study how Saturn's magnetosphere influences Titan's ionosphere. The data is first corrected for biased sampling due to varying solar zenith angle and solar energy flux (solar cycle effects). We then present results showing that the electron density in Titan's ionosphere, in the altitude range 1600-2400km, is increased by about a factor of 2.5 when Titan is located on the nightside of Saturn (Saturn local time (SLT) 21-03h) compared to when on the dayside (SLT 09-15 h). For lower altitudes (1100-1600km) the main dividing factor for the ionospheric density is the ambient magnetospheric conditions. When Titan is located in the magnetospheric current sheet, the electron density in Titan's ionosphere is about a factor of 1.4 higher compared to when Titan is located in the magnetospheric lobes. The factor of 1.4 increase in between sheet and lobe flybys is interpreted as an effect of increased particle impact ionization from approximate to 200eV sheet electrons. The factor of 2.5 increase in electron density between flybys on Saturn's nightside and dayside is suggested to be an effect of the pressure balance between thermal plus magnetic pressure in Titan's ionosphere against the dynamic pressure and energetic particle pressure in Saturn's magnetosphere.

Keywords
Titan, Saturn's magnetosphere, Cassini, ionosphere
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-272411 (URN)10.1002/2015JA021373 (DOI)000366135200052 ()
Funder
Swedish Research Council
Available from: 2016-01-25 Created: 2016-01-13 Last updated: 2017-11-30Bibliographically approved
Vigren, E., Galand, M., Yelle, R. V., Wellbrock, A., Coates, A. J., Snowden, D., . . . Mandt, K. (2015). Ionization balance in Titan's nightside ionosphere. Icarus (New York, N.Y. 1962), 248, 539-546
Open this publication in new window or tab >>Ionization balance in Titan's nightside ionosphere
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2015 (English)In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 248, p. 539-546Article in journal (Refereed) Published
Abstract [en]

Based on a multi-instrumental Cassini dataset we make model versus observation comparisons of plasma number densities, n(p) = (n(e)n(1))(1/2) (n(e) and n(1) being the electron number density and total positive ion number density, respectively) and short-lived ion number densities (N+, CH2+, CH3+, CH4+) in the southern hemisphere of Titan's nightside ionosphere over altitudes ranging from 1100 and 1200 km and from 1100 to 1350 km, respectively. The n(p) model assumes photochemical equilibrium, ion-electron pair production driven by magnetospheric electron precipitation and dissociative recombination as the principal plasma neutralization process. The model to derive short-lived-ion number densities assumes photochemical equilibrium for the short-lived ions, primary ion production by electron-impact ionization of N-2 and CH4 and removal of the short-lived ions through reactions with CH4. It is shown that the models reasonably reproduce the observations, both with regards to tip and the number densities of the short-lived ions. This is contrasted by the difficulties in accurately reproducing ion and electron number densities in Titan's sunlit ionosphere. (C) 2014 Elsevier Inc. All rights reserved.

Keywords
Titan, Ionospheres, Titan, atmosphere
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:uu:diva-247497 (URN)10.1016/j.icarus.2014.11.012 (DOI)000348411000037 ()
Available from: 2015-03-19 Created: 2015-03-19 Last updated: 2017-12-04Bibliographically approved
Shebanits, O. (2015). Pre-biotic molecules and dynamics in the ionosphere of Titan: a space weather station perspective. (Licentiate dissertation). Department of Physics and Astronomy
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. p. 38
Keywords
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
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0001-9621-211x

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