| 1. |
- Coustenis, A., et al.
(författare)
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TandEM : Titan and Enceladus mission
- 2009
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Ingår i: Experimental astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 23:3, s. 893-946
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Tidskriftsartikel (refereegranskat)abstract
- TandEM was proposed as an L-class (large) mission in response to ESA's Cosmic Vision 2015-2025 Call, and accepted for further studies, with the goal of exploring Titan and Enceladus. The mission concept is to perform in situ investigations of two worlds tied together by location and properties, whose remarkable natures have been partly revealed by the ongoing Cassini-Huygens mission. These bodies still hold mysteries requiring a complete exploration using a variety of vehicles and instruments. TandEM is an ambitious mission because its targets are two of the most exciting and challenging bodies in the Solar System. It is designed to build on but exceed the scientific and technological accomplishments of the Cassini-Huygens mission, exploring Titan and Enceladus in ways that are not currently possible (full close-up and in situ coverage over long periods of time). In the current mission architecture, TandEM proposes to deliver two medium-sized spacecraft to the Saturnian system. One spacecraft would be an orbiter with a large host of instruments which would perform several Enceladus flybys and deliver penetrators to its surface before going into a dedicated orbit around Titan alone, while the other spacecraft would carry the Titan in situ investigation components, i.e. a hot-air balloon (MontgolfiSre) and possibly several landing probes to be delivered through the atmosphere.
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| 2. |
- Brain, D., et al.
(författare)
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A comparison of global models for the solar wind interaction with Mars
- 2010
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 206:1, s. 139-151
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Tidskriftsartikel (refereegranskat)abstract
- We present initial results from the first community-wide effort to compare global plasma interaction model results for Mars. Seven modeling groups participated in this activity, using MHD, multi-fluid, and hybrid assumptions in their simulations. Moderate solar wind and solar EUV conditions were chosen, and the conditions were implemented in the models and run to steady state. Model output was compared in three ways to determine how pressure was partitioned and conserved in each model, the location and asymmetry of plasma boundaries and pathways for planetary ion escape, and the total escape flux of planetary oxygen ions. The two participating MHD models provided similar results, while the five sets of multi-fluid and hybrid results were different in many ways. All hybrid results, however, showed two main channels for oxygen ion escape (a pickup ion 'plume' in the hemisphere toward which the solar wind convection electric field is directed, and a channel in the opposite hemisphere of the central magnetotail), while the MHD models showed one (a roughly symmetric channel in the central magnetotail). Most models showed a transition from an upstream region dominated by plasma dynamic pressure to a magnetosheath region dominated by thermal pressure to a low altitude region dominated by magnetic pressure. However, calculated escape rates for a single ion species varied by roughly an order of magnitude for similar input conditions, suggesting that the uncertainties in both the current and integrated escape over martian history as determined by models are large. These uncertainties are in addition to those associated with the evolution of the Sun, the martian dynamo, and the early atmosphere, highlighting the challenges we face in constructing Mars' past using models.
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| 3. |
- Waite, J. H., Jr., et al.
(författare)
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Chemical interactions between Saturn's atmosphere and its rings
- 2018
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Ingår i: Science. - : AMER ASSOC ADVANCEMENT SCIENCE. - 0036-8075 .- 1095-9203. ; 362:6410
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Tidskriftsartikel (refereegranskat)abstract
- The Pioneer and Voyager spacecraft made close-up measurements of Saturn's ionosphere and upper atmosphere in the 1970s and 1980s that suggested a chemical interaction between the rings and atmosphere. Exploring this interaction provides information on ring composition and the influence on Saturn's atmosphere from infalling material. The Cassini Ion Neutral Mass Spectrometer sampled in situ the region between the D ring and Saturn during the spacecraft's Grand Finale phase. We used these measurements to characterize the atmospheric structure and material influx from the rings. The atmospheric He/H-2 ratio is 10 to 16%. Volatile compounds from the rings (methane; carbon monoxide and/or molecular nitrogen), as well as larger organic-bearing grains, are flowing inward at a rate of 4800 to 45,000 kilograms per second.
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| 4. |
- Cravens, T. E., et al.
(författare)
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Dynamical and magnetic field time constants for Titan's ionosphere : Empirical estimates and comparisons with Venus
- 2010
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 115:8, s. A08319-
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Tidskriftsartikel (refereegranskat)abstract
- Plasma in Titan's ionosphere flows in response to forcing from thermal pressure gradients, magnetic forces, gravity, and ion-neutral collisions. This paper takes an empirical approach to the ionospheric dynamics by using data from Cassini instruments to estimate pressures, flow speeds, and time constants on the dayside and nightside. The plasma flow speed relative to the neutral gas speed is approximately 1 m s(-1) near an altitude of 1000 km and 200 m s(-1) at 1500 km. For comparison, the thermospheric neutral wind speed is about 100 m s(-1). The ionospheric plasma is strongly coupled to the neutrals below an altitude of about 1300 km. Transport, vertical or horizontal, becomes more important than chemistry in controlling ionospheric densities above about 1200-1500 km, depending on the ion species. Empirical estimates are used to demonstrate that the structure of the ionospheric magnetic field is determined by plasma transport (including neutral wind effects) for altitudes above about 1000 km and by magnetic diffusion at lower altitudes. The paper suggests that a velocity shear layer near 1300 km could exist at some locations and could affect the structure of the magnetic field. Both Hall and polarization electric field terms in the magnetic induction equation are shown to be locally important in controlling the structure of Titan's ionospheric magnetic field. Comparisons are made between the ionospheric dynamics at Titan and at Venus.
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| 5. |
- Luhmann, J. G., et al.
(författare)
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Investigating magnetospheric interaction effects on Titan's ionosphere with the Cassini orbiter Ion Neutral Mass Spectrometer, Langmuir Probe and magnetometer observations during targeted flybys
- 2012
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 219:2, s. 534-555
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Tidskriftsartikel (refereegranskat)abstract
- In the similar to 6 years since the Cassini spacecraft went into orbit around Saturn in 2004, roughly a dozen Titan flybys have occurred for which the Ion Neutral Mass Spectrometer (INMS) measured that moon's ionospheric density and composition. For these, and for the majority of the similar to 60 close flybys probing to altitudes down to similar to 950 km, Langmuir Probe electron densities were also obtained. These were all complemented by Cassini magnetometer observations of the magnetic fields affected by the Titan plasma interaction. Titan's ionosphere was expected to differ from those of other unmagnetized planetary bodies because of significant contributions from particle impact due to its magnetospheric environment. However, previous analyses of these data clearly showed the dominance of the solar photon source, with the possible exception of the nightside. This paper describes the collected ionospheric data obtained in the period between Cassini's Saturn Orbit Insertion in 2004 and 2009, and examines some of their basic characteristics with the goal of searching for magnetospheric influences. These influences might include effects on the altitude profiles of impact ionization by magnetospheric particles at the Titan orbit location, or by locally produced pickup ions freshly created in Titan's upper atmosphere. The effects of forces on the ionosphere associated with both the draped and penetrating external magnetic fields might also be discernable. A number of challenges arise in such investigations given both the observed order of magnitude variations in the magnetospheric particle sources and the unsteadiness of the magnetospheric magnetic field and plasma flows at Titan's (similar to 20Rs (Saturn Radius)) orbit. Transterminator flow of ionospheric plasma from the dayside may also supply some of the nightside ionosphere, complicating determination of the magnetospheric contribution. Moreover, we are limited by the sparse sampling of the ionosphere during the mission as the Titan interaction also depends on Saturn Local Time as well as possible intrinsic asymmetries and variations of Titan's neutral atmosphere. We use organizations of the data by key coordinate systems of the plasma interaction with Titan's ionosphere to help interpret the observations. The present analysis does not find clear characteristics of the magnetosphere's role in defining Titan's ionosphere. The observations confirm the presence of an ionosphere produced mainly by sunlight, and an absence of expected ionospheric field signatures in the data. Further investigation of the latter, in particular, may benefit from numerical experiments on the inner boundary conditions of 3D models including the plasma interaction and features such as neutral winds.
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| 6. |
- Ulusen, D., et al.
(författare)
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Investigation of the force balance in the Titan ionosphere : Cassini T5 flyby model/data comparisons
- 2010
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 210:2, s. 867-880
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Tidskriftsartikel (refereegranskat)abstract
- Cassini's Titan flyby on 16 April, 2005 (T5) is the only encounter when the two main ionizing sources of the moon's atmosphere, solar radiation and corotating plasma, align almost anti-parallel. In this paper a single-fluid multi-species 3D MHD model of the magnetospheric plasma interaction for T5 conditions is analyzed. Model results are compared to observations to investigate the ionospheric dynamics at Titan as well as to understand the deviations from a typical solar wind interaction, such as Venus' interaction with the solar wind. Model results suggest that for the T5 interaction configuration, corotating plasma is the dominant driver determining the global interaction features at high altitudes. In the lower ionosphere below similar to 1500 km altitude - where the control of the ionospheric composition transfers from dynamic to chemical processes - magnetic and thermal pressure gradients oppose each other locally, complicating the ionospheric dynamics. Model results also imply that the nightside ionosphere - produced only by the impact ionization in the model - does not provide enough thermal pressure to balance the incident plasma dynamic pressure. As a result, the induced magnetic barrier penetrates into the ionosphere by plasma convection down to similar to 1000 km altitude and by magnetic diffusion below this altitude. Moreover, strong horizontal drag forces due to ion-neutral collisions and comparable drag forces estimated from possible neutral winds in the lower ionosphere below similar to 1400 km altitude oppose over local regions, implying that the Titan interaction must be treated as a 3D problem. Ion and electron densities calculated from the model generally agree with the Cassini Ion Neutral Mass Spectrometer and Langmuir probe measurements; however, there are significant differences between the calculated and measured magnetic fields. We discuss possible explanations for the discrepancy in the magnetic field predictions.
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