| 1. |
- Taylor, S. A., et al.
(författare)
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Modeling, Analysis, and Interpretation of Photoelectron Energy Spectra at Enceladus Observed by Cassini
- 2018
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 123:1, s. 287-296
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Tidskriftsartikel (refereegranskat)abstract
- The Electron Spectrometer (ELS) of the Cassini Plasma Spectrometer has observed photoelectrons produced in the plume of Enceladus. These photoelectrons are observed during Enceladus encounters in the energetic particle shadow where the spacecraft is largely shielded from penetrating radiation by the moon. We present a complex electron spectrum at Enceladus including evidence of two previously unidentified electron populations at 6–10 eV and 10–16 eV. We estimate that the proportion of “hot” (>15 eV) to “cold” (<15 eV) electrons during the Enceladus flybys is ≈ 0.1–0.5%. We have constructed a model of photoelectron production in the plume and compared it with ELS Enceladus flyby data by scaling and energy shifting according to spacecraft potential. We suggest that the complex structure of the electron spectrum observed can be explained entirely by photoelectron production in the plume ionosphere.
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| 2. |
- Desai, R. T., et al.
(författare)
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Carbon Chain Anions and the Growth of Complex Organic Molecules in Titan's Ionosphere
- 2017
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Ingår i: Astrophysical Journal Letters. - : American Astronomical Society. - 2041-8205 .- 2041-8213. ; 844:2
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Ma, Y. J., et al.
(författare)
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The importance of thermal electron heating in Titan's ionosphere : Comparison with Cassini T34 flyby
- 2011
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A10213-
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Tidskriftsartikel (refereegranskat)abstract
- We use a new magnetohydrodynamic (MHD) model to study the effects of thermal-electron heating in Titan's ionosphere. This model improves the previously used multispecies MHD model by solving both the electron and ion pressure equations instead of a single plasma pressure equation. This improvement enables a more accurate evaluation of ion and electron temperatures inside Titan's ionosphere. The model is first applied to an idealized case, and the results are compared in detail with those of the single-pressure MHD model to illustrate the effects of the improvement. Simulation results show that the dayside ionosphere thermal pressure is larger than the upstream pressure during normal conditions, when Titan is located in the dusk region; thus Saturn's magnetic field is shielded by the highly conducting ionosphere, similar to the interaction of Venus during solar maximum conditions. This model is also applied to a special flyby of Titan, the T34 flyby, which occurred near the dusk region. It is shown that better agreement with the magnetometer data can be achieved using the two-fluid MHD model with the inclusion of the effects of thermal electron heating. The model results clearly demonstrate the importance of thermal-electron heating in Titan's ionosphere.
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| 4. |
- Ma, Y. J., et al.
(författare)
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Time-dependent global MHD simulations of Cassini T32 flyby : From magnetosphere to magnetosheath
- 2009
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114:3, s. A03204-
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Tidskriftsartikel (refereegranskat)abstract
- When the Cassini spacecraft flew by Titan on 13 June 2007, at 13.6 Saturn local time, Titan was directly observed to be outside Saturn's magnetopause. Cassini observations showed dramatic changes of magnetic field orientation as well as other plasma flow parameters during the inbound and outbound segments. In this paper, we study Titan's ionospheric responses to such a sudden change in the upstream plasma conditions using a sophisticated multispecies global MHD model. Simulation results of three different cases (steady state, simple current sheet crossing, and magnetopause crossing) are presented and compared against Cassini Magnetometer, Langmuir Probe, and Cassini Plasma Spectrometer observations. The simulation results provide clear evidence for the existence of a fossil field that was induced in the ionosphere. The main interaction features, as observed by the Cassini spacecraft, are well reproduced by the time-dependent simulation cases. Simulation also reveals how the fossil field was trapped during the interaction and shows the coexistence of two pileup regions with opposite magnetic orientation, as well as the formation of a pair of new Alfven wings and tail disconnection during the magnetopause crossing process.
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| 5. |
- Wellbrock, A., et al.
(författare)
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Observations of a Solar Energetic Particle Event from Inside and Outside the Coma of Comet 67P
- 2022
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Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 127:12
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Tidskriftsartikel (refereegranskat)abstract
- We analyze observations of an SEP event at Rosetta’s target comet 67P/Churyumov-Gerasimenko during March 6th-10th 2015. The comet was 2.15AU from the Sun, with the Rosetta spacecraft approximately 70km from the nucleus placing it deep inside the comet’s coma and allowing us to study its response. The Eastern flank of an ICME also encountered Rosetta on March 6th and 7th. Rosetta’s RPC data indicate increases in ionization rates, and cometary water group pickup ions exceeding 1keV. Increased charge exchange reactions between solar wind ions and cometary neutrals also indicate increased upstream neutral populations consistent with enhanced SEP induced surface activity. In addition, the most intense parts of the event coincide with observations interpreted as an infant cometary bow shock, indicating that the SEPs may have enhanced the formation and/or intensified the observations. These solar transient events may also have pushed the cometopause closer to the nucleus.We track and discuss characteristics of the SEP event using remote observations by SOHO, WIND and GOES at the Sun, in-situ measurements at STEREO A, Mars and Rosetta, and ENLIL modeling. Based on its relatively prolonged duration, gradual and anisotropic nature and broad angular spread in the heliosphere, we determine the main particle acceleration source to be a distant ICME which emerged from the Sun on March 6th 2015 and was detected locally in the Martian ionosphere but was never encountered by 67P directly. The ICME’s shock produced SEPs for several days which traveled to the in-situ observation sites via magnetic field line connections.
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| 6. |
- Coates, A. J., et al.
(författare)
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Recent Results from Titan's Ionosphere
- 2011
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Ingår i: Space Science Reviews. - : Springer Science and Business Media LLC. - 0038-6308 .- 1572-9672. ; 162:1-4, s. 85-111
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Forskningsöversikt (refereegranskat)abstract
- Titan has the most significant atmosphere of any moon in the solar system, with a pressure at the surface larger than the Earth's. It also has a significant ionosphere, which is usually immersed in Saturn's magnetosphere. Occasionally it exits into Saturn's magnetosheath. In this paper we review several recent advances in our understanding of Titan's ionosphere, and present some comparisons with the other unmagnetized objects Mars and Venus. We present aspects of the ionospheric structure, chemistry, electrodynamic coupling and transport processes. We also review observations of ionospheric photoelectrons at Titan, Mars and Venus. Where appropriate, we mention the effects on ionospheric escape.
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| 7. |
- Cravens, T. E., et al.
(författare)
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Model-data comparisons for Titan's nightside ionosphere
- 2009
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 199:1, s. 174-188
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Tidskriftsartikel (refereegranskat)abstract
- Solar and X-ray radiation and energetic plasma from Saturn's magnetosphere interact with the upper atmosphere producing an ionosphere at Titan. The highly coupled ionosphere and upper atmosphere system mediates the interaction between Titan and the external environment. A model of Titan's nightside ionosphere will be described and the results compared with data from the Ion and Neutral Mass Spectrometer (INMS) and the Langmuir probe (LP) part of the Radio and Plasma Wave (RPWS) experiment for the T5 and T21 nightside encounters of the Cassini Orbiter with Titan. Electron impact ionization associated with the precipitation of magnetospheric electrons into the upper atmosphere is assumed to be the source of the nightside ionosphere, at least for altitudes above 1000 km. Magnetospheric electron fluxes measured by the Cassini electron spectrometer (CAPS ELS) are used as an input for the model. The model is used to interpret the observed composition and structure of the T5 and T21 ionospheres. The densities of many ion species (e.g., CH5+ and C2H5+) measured during T5 exhibit temporal and/or spatial variations apparently associated with variations in the fluxes of energetic electrons that precipitate into the atmosphere from Saturn's magnetosphere.
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| 8. |
- Edberg, Niklas J. T., et al.
(författare)
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Structured ionospheric outflow during the Cassini T55-T59 Titan flybys
- 2011
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Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 59:8, s. 788-797
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Tidskriftsartikel (refereegranskat)abstract
- During the final three of the five consecutive and similar Cassini Titan flybys T55-T59 we observe a region characterized by high plasma densities (electron densities of 1-8 cm(-3)) in the tail/nightside of Titan. This region is observed progressively farther downtail from pass to pass and is interpreted as a plume of ionospheric plasma escaping Titan, which appears steady in both location and time. The ions in this plasma plume are moving in the direction away from Titan and are a mixture of both light and heavy ions with composition revealing that their origin are in Titan's ionosphere, while the electrons are more isotropically distributed. Magnetic field measurements indicate the presence of a current sheet at the inner edge of this region. We discuss the mechanisms behind this outflow, and suggest that it could be caused by ambipolar diffusion, magnetic moment pumping or dispersive Alfven waves.
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| 9. |
- Shebanits, Oleg, et al.
(författare)
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Ion and aerosol precursor densities in Titan's ionosphere : A multi-instrument case study
- 2016
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 121:10, s. 10075-10090
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 10. |
- Vigren, Erik, et al.
(författare)
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Ionization balance in Titan's nightside ionosphere
- 2015
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 248, s. 539-546
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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