| 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. |
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| 3. |
- Milillo, A., et al.
(författare)
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Investigating Mercury's Environment with the Two-Spacecraft BepiColombo Mission
- 2020
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Ingår i: Space Science Reviews. - : Springer Science and Business Media LLC. - 0038-6308 .- 1572-9672. ; 216:5
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Forskningsöversikt (refereegranskat)abstract
- The ESA-JAXA BepiColombo mission will provide simultaneous measurements from two spacecraft, offering an unprecedented opportunity to investigate magnetospheric and exospheric dynamics at Mercury as well as their interactions with the solar wind, radiation, and interplanetary dust. Many scientific instruments onboard the two spacecraft will be completely, or partially devoted to study the near-space environment of Mercury as well as the complex processes that govern it. Many issues remain unsolved even after the MESSENGER mission that ended in 2015. The specific orbits of the two spacecraft, MPO and Mio, and the comprehensive scientific payload allow a wider range of scientific questions to be addressed than those that could be achieved by the individual instruments acting alone, or by previous missions. These joint observations are of key importance because many phenomena in Mercury's environment are highly temporally and spatially variable. Examples of possible coordinated observations are described in this article, analysing the required geometrical conditions, pointing, resolutions and operation timing of different BepiColombo instruments sensors.
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| 4. |
- Herique, A., et al.
(författare)
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Direct observations of asteroid interior and regolith structure : Science measurement requirements
- 2018
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Ingår i: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 62:8, s. 2141-2162
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Tidskriftsartikel (refereegranskat)abstract
- Our knowledge of the internal structure of asteroids is, so far, indirect - relying entirely on inferences from remote sensing observations of the surface, and theoretical modeling of formation and evolution. What are the bulk properties of the regolith and deep interior? And what are the physical processes that shape asteroid internal structures? Is the composition and size distribution observed on the surface representative of the bulk? These questions are crucial to understand small bodies' history from accretion in the early Solar System to the present, and direct measurements are needed to answer these questions for the benefit of science as well as for planetary defense or exploration. Radar is one of the main instruments capable of sounding asteroids to characterize internal structure from sub-meter to global scale. In this paper, we review the science case for direct observation of the deep internal structure and regolith of a rocky asteroid of kilometer size or smaller. We establish the requirements and model dielectric properties of asteroids to outline a possible instrument suite, and highlight the capabilities of radar instrumentation to achieve these observations. We then review the expected science return including secondary objectives contributing to the determination of the gravitational field, the shape model, and the dynamical state. This work is largely inherited from MarcoPolo-R and AIDA/AIM studies.
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| 5. |
- Macfarlane, M. D., et al.
(författare)
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Shape abnormalities of the caudate nucleus correlate with poorer gait and balance: Results from a subset of the ladis study
- 2015
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Ingår i: The American journal of geriatric psychiatry. - : Elsevier BV. - 1064-7481. ; 23:1, s. 59-
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Tidskriftsartikel (refereegranskat)abstract
- Objective Functional deficits seen in several neurodegenerative disorders have been linked with dysfunction in frontostriatal circuits and with associated shape alterations in striatal structures. The severity of visible white matter hyperintensities (WMHs) on magnetic resonance imaging has been found to correlate with poorer performance on measures of gait and balance. This study aimed to determine whether striatal volume and shape changes were correlated with gait dysfunction. Methods Magnetic resonance imaging scans and clinical gait/balance data (scores from the Short Physical Performance Battery [SPPB]) were sourced from 66 subjects in the previously published LADIS trial, performed in nondisabled individuals older than age 65 years with WMHs at study entry. Data were obtained at study entry and at 3-year follow-up. Caudate nuclei and putamina were manually traced using a previously published method and volumes calculated. The relationships between volume and physical performance on the SPPB were investigated with shape analysis using the spherical harmonic shape description toolkit. Results There was no correlation between the severity of WMHs and striatal volumes. Caudate nuclei volume correlated with performance on the SPPB at baseline but not at follow-up, with subsequent shape analysis showing left caudate changes occurred in areas corresponding to inputs of the dorsolateral prefrontal, premotor, and motor cortex. There was no correlation between putamen volumes and performance on the SPPB. Conclusion Disruption in frontostriatal circuits may play a role in mediating poorer physical performance in individuals with WMHs. Striatal volume and shape changes may be suitable biomarkers for functional changes in this population. © 2015 American Association for Geriatric Psychiatry.
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| 6. |
- 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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| 7. |
- Jones, G. H., et al.
(författare)
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The dust halo of Saturn's largest icy moon, Rhea
- 2008
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Ingår i: Science. - : American Association for the Advancement of Science (AAAS). - 0036-8075 .- 1095-9203. ; 319:5868, s. 1380-1384
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Tidskriftsartikel (refereegranskat)abstract
- Saturn's moon Rhea had been considered massive enough to retain a thin, externally generated atmosphere capable of locally affecting Saturn's magnetosphere. The Cassini spacecraft's in situ observations reveal that energetic electrons are depleted in the moon's vicinity. The absence of a substantial exosphere implies that Rhea's magnetospheric interaction region, rather than being exclusively induced by sputtered gas and its products, likely contains solid material that can absorb magnetospheric particles. Combined observations from several instruments suggest that this material is in the form of grains and boulders up to several decimetres in size and orbits Rhea as an equatorial debris disk. Within this disk may reside denser, discrete rings or arcs of material.
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| 8. |
- 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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| 9. |
- Blomberg, Lars G., et al.
(författare)
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MEFISTO - An electric field instrument for BepiColombo/MMO
- 2006
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Ingår i: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 38:4, s. 672-679
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Tidskriftsartikel (refereegranskat)abstract
- MEFISTO, together with the companion instrument WPT, are planning the first-ever in situ measurements of the electric field in the magnetosphere of planet Mercury. The instruments have been selected by JAXA for inclusion in the BepiColombo/MMO payload, as part of the Plasma Wave Investigation coordinated by Kyoto University. The magnetosphere of Mercury was discovered by Mariner 10 in 1974 and will be studied further by Messenger starting in 2011. However, neither spacecraft did or will measure the electric field. Electric fields are crucial in the dynamics of a magnetosphere and for the energy and plasma transport between different regions within the magnetosphere as well as between the magnetosphere and the surrounding regions. The MEFISTO instrument will be capable of measuring electric fields from DC to 3 MHz, and will thus also allow diagnostics of waves at all frequencies of relevance to the Hermean magnetosphere. MEFISTO is a double-probe electric field instrument. The double-probe technique has strong heritage and is well proven on missions such as Viking, Polar, and Cluster. For BepiColombo, a newly developed deployment mechanism is planned which reduces the mass by a factor of about 5 compared to conventional mechanisms for 15 in long booms. We describe the basic characteristics of the instrument and briefly discuss the new developments made to tailor the instrument to flight in Mercury orbit.
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| 10. |
- 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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| 11. |
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| 12. |
- Salvado, G., et al.
(författare)
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The protective gene dose effect of the APOE epsilon 2 allele on gray matter volume in cognitively unimpaired individuals
- 2022
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Ingår i: Alzheimers & Dementia. - : Wiley. - 1552-5260 .- 1552-5279. ; 18:7, s. 1383-1395
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Tidskriftsartikel (refereegranskat)abstract
- Introduction: Harboring two copies of the apolipoprotein E (APOE) epsilon 2 allele strongly protects against Alzheimer's disease (AD). However, the effect of this genotype on gray matter (GM) volume in cognitively unimpaired individuals has not yet been described. Methods: Multicenter brain magnetic resonance images (MRIs) from cognitively unimpaired epsilon 2 homozygotes were matched (1:1) against all other APOE genotypes for relevant confounders (n = 223). GM volumes of epsilon 2 genotypic groups were compared to each other and to the reference group (APOE epsilon 3/epsilon 3). Results: Carrying at least one epsilon 2 allele was associated with larger GM volumes in brain areas typically affected by AD and also in areas associated with cognitive resilience. APOE epsilon 2 homozygotes, but not APOE epsilon 2 heterozygotes, showed larger GM volumes in areas related to successful aging. Discussion: In addition to the known resistance against amyloid-beta deposition, the larger GM volumes in key brain regions may confer APOE epsilon 2 homozygotes additional protection against AD-related cognitive decline.
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| 13. |
- Shebanits, Oleg, 1984-, et al.
(författare)
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On Ion Drifts and Neutral Winds in Titan’s Thermosphere
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Saturn’s largest moon Titan hosts an atmosphere with complex organic chemistry initiated partly in the ionosphere. The nightside chemistry may be influenced by the ion transport from the dayside ionosphere. In turn, ion transport (ion drifts) may be affected by the neutral winds, which cannot be measured directly by Cassini. In this study we derive the ion drifts along the spacecraft trajectories based on analysis of in-situ measurements of electron and ion fluxes, mean masses of positive and negative ions and the magnetic field. Data from Titan flybys TA to T100 (Oct 2005 - Apr 2014) is included in this study, 55 flybys are below 1400 km and 48 are below 1200 km altitude. Based on the measurements, three regions of the ionosphere were defined by altitude: 1) above 1600 km, ions are ?×?-drifting (frozen into the magnetic field), 2) 1100-1600 km, dynamo-region, ions drift partly in opposite directions (perpendicular to B) and 3) below 1100 km (upper limit depends on convection electric field strength), ions are following neutrals.
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| 14. |
- Ulusen, D., et al.
(författare)
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Comparisons of Cassini flybys of the Titan magnetospheric interaction with an MHD model : Evidence for organized behavior at high altitudes
- 2012
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 217:1, s. 43-54
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Tidskriftsartikel (refereegranskat)abstract
- Recent papers suggest the significant variability of conditions in Saturn's magnetosphere at the orbit of Titan. Because of this variability, it was expected that models would generally have a difficult time regularly comparing to data from the Titan flybys. However, we find that in contrast to this expectation, it appears that there is underlying organization of the interaction features roughly above similar to 1800 km (1.7 Rt) altitude by the average external field due to Saturn's dipole moment. In this study, we analyze Cassini's plasma and magnetic field data collected at 9 Titan encounters during which the external field is close to the ideal southward direction and compare these observations to the results from a 2-fluid (1 ion, 1 electron) 7-species MHD model simulations obtained under noon SLT conditions. Our comparative analysis shows that under noon SLT conditions the Titan plasma interaction can be viewed in two layers: an outer layer between 6400 and 1800 km where interaction features observed in the magnetic field are in basic agreement with a purely southward external field interaction and an inner layer below 1800 km where the magnetic field measurements show strong variations and deviate from the model predictions. Thus the basic features inferred from the Voyager 1 flyby seem to be generally present above similar to 1800 km in spite of the ongoing external variations from SLT excursions, time variability and magnetospheric current systems as long as a significant southward external field component is present. At around similar to 1800 km kinetic effects (such as mass loading and heavy ion pickup) and below 1800 km ionospheric effects (such as drag of ionospheric plasma due to coupling with neutral winds and/or magnetic memory of Titan's ionosphere) complicate what is observed.
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| 15. |
- Westlake, J. H., et al.
(författare)
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The observed composition of ions outflowing from Titan
- 2012
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 39, s. L19104-
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Tidskriftsartikel (refereegranskat)abstract
- We report on Cassini Ion and Neutral Mass Spectrometer (INMS) observations above Titan's exobase at altitudes of 2225 km to 3034 km. We observe significant densities of CH5+, HCNH+ and C2H5+ that require ion-molecule reactions to be produced in the quantities observed. The measured composition and ion velocity (about 0.8-1.5 km/s) suggest that the observed ions must have been created deep inside Titan's ionosphere (below the exobase) and then transported to the detection altitude. Plasma motion from below Titan's exobase to large distances can be driven by a combination of thermal pressure and magnetic forces. The observed outward flows may link the main ionosphere with the more distant wake and provide a source of hydrocarbon ions in the Saturnian system. Citation: Westlake, J. H., et al. (2012), The observed composition of ions outflowing from Titan, Geophys. Res. Lett., 39, L19104, doi: 10.1029/2012GL053079.
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| 16. |
- 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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| 17. |
- Edberg, Niklas J. T., et al.
(författare)
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Extreme densities in Titan's ionosphere during the T85 magnetosheath encounter
- 2013
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Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 40:12, s. 2879-2883
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Tidskriftsartikel (refereegranskat)abstract
- We present Cassini Langmuir probe measurements of the highest electron number densities ever reported from the ionosphere of Titan. The measured density reached 4310cm(-3) during the T85 Titan flyby. This is at least 500cm(-3) higher than ever observed before and at least 50% above the average density for similar solar zenith angles. The peak of the ionospheric density is not reached on this flyby, making the maximum measured density a lower limit. During this flyby, we also report that an impacting coronal mass ejection (CME) leaves Titan in the magnetosheath of Saturn, where it is exposed to shocked solar wind plasma for at least 2 h 45 min. We suggest that the solar wind plasma in the magnetosheath during the CME conditions significantly modifies Titan's ionosphere by an addition of particle impact ionization by precipitating protons.
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| 18. |
- 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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| 19. |
- Moore, L., et al.
(författare)
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Models of Saturn's Equatorial Ionosphere Based on In Situ Data From Cassini's Grand Finale
- 2018
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Ingår i: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 45:18, s. 9398-9407
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Tidskriftsartikel (refereegranskat)abstract
- We present new models of Saturn's equatorial ionosphere based on the first in situ measurements of its upper atmosphere. The neutral spectrum measured by Cassini's Ion and Neutral Mass Spectrometer, which includes substantial methane, ammonia, and organics in addition to the anticipated molecular hydrogen, helium, and water, serves as input for unexpectedly complex ionospheric chemistry. Heavy molecular ions are found to dominate Saturn's equatorial low-altitude ionosphere, with a mean ion mass of 11Da. Key molecular ions include H3O+ and HCO+; other abundant heavy ions depend upon the makeup of the mass 28 neutral species, which cannot be uniquely determined. Ion and Neutral Mass Spectrometer neutral species lead to generally good agreement between modeled and observed plasma densities, though poor reproduction of measured H+ and H-3(+) variability and an overabundance of modeled H-3(+) potentially hint at missing physical processes in the model, including a loss process that affects H-3(+) but not H+. Plain Language Summary Cassini's Grand Finale enabled the first-ever direct measurements of Saturn's upper atmosphere. Here we use Cassini's unique measurements to construct new models of the plasma in this important boundary region that separates the dense lower atmosphere from space. Based on the complex array of observed gases, we find that heavy molecular ions are dominant near Saturn's equator. This surprising result demonstrates that the chemistry in Saturn's equatorial upper atmosphere is substantially more complex than anticipated. The presence of these unexpected ions potentially represents a new method of monitoring Saturn's ionosphere remotely. Furthermore, as other Cassini measurements indicate that the complex chemistry is likely driven by an influx of ring-derived material, such observations may even help to track the evolution of Saturn's rings as they lose mass to its atmosphere.
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| 20. |
- Wahlund, Jan Erik, et al.
(författare)
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On the amount of heavy molecular ions in Titan's ionosphere
- 2009
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Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 57:14-15, s. 1857-1865
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Tidskriftsartikel (refereegranskat)abstract
- We present observational evidence that the ionosphere of Titan below an altitude of 1150 km is a significant source of heavy (> 100 amu) molecular organic species. This study is based on measurements by five instruments (RPWS/LP, RPWS/E, INMS, CAPS/ELS, CAPS/IBS) onboard the Cassini spacecraft during three flybys (T17, T18, T32) of Titan. The ionospheric peaks encountered at altitudes of 950-1300 km had densities in the range 900-3000 cm(-3). Below these peaks the number densities of heavy positively charged ions reached 100-2000 cm(-3) and approached 50-70% of the total ionospheric density with an increasing trend toward lowest measured altitudes. Simultaneously measured negatively charged ion densities were in the range 50-150 cm(-3). These results imply that similar to 10(5)similar to 10(6) heavy positively charged ions/m(3)/s are continuously recombining into heavy neutrals and supply the atmosphere of Titan. The ionosphere may in this way produce 0.1-1 Mt/yr of heavy organic compounds and is therefore a sizable source for aerosol formation. We also predict that Titan's ionosphere is dominated by heavy (> 100 amu) molecular ions below 950 km.
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| 21. |
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| 22. |
- Boström, R., Eriksson, A.I., Åhlén, L., Blomberg, L., Gustafsson, G., Holback, B., Holmgren, G., Holtet, J., Jansson, S.E., Lindqvist, P.A., Lybekk, B., Mälkki, A., Riihelä, P. and Wahlund, J.E.
(författare)
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The Rosetta Dual Langmuir Probe Instrument for Measurements of Plasma Density, Temperature and Flow Velocity.
- 2001
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Ingår i: ESA SP-1165.
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 23. |
- Edberg, Niklas J. T., et al.
(författare)
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Solar cycle modulation of Titan's ionosphere
- 2013
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Ingår i: Journal of Geophysical Research-Space Physics. - : American Geophysical Union (AGU). - 2169-9380. ; 118:8, s. 5255-5264
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Tidskriftsartikel (refereegranskat)abstract
- During the six Cassini Titan flybys T83-T88 (May 2012 to November 2012) the electron density in the ionospheric peak region, as measured by the radio and plasma wave science instrument/Langmuir probe, has increased significantly, by 15-30%, compared to previous average. These measurements suggest that a longterm change has occurred in the ionosphere of Titan, likely caused by the rise to the new solar maximum with increased EUV fluxes. We compare measurements from TA, TB, and T5, from the declining phase of solar cycle 23 to the recent T83-T88 measurements during cycle 24, since the solar irradiances from those two intervals are comparable. The peak electron densities normalized to a common solar zenith angle N-norm from those two groups of flybys are comparable but increased compared to the solar minimum flybys (T16-T71). The integrated solar irradiance over the wavelengths 1-80nm, i.e., the solar energy flux, F-e, correlates well with the observed ionospheric peak density values. Chapman layer theory predicts that NnormFek, with k=0.5. We find observationally that the exponent k=0.540.18. Hence, the observations are in good agreement with theory despite the fact that many assumptions in Chapman theory are violated. This is also in good agreement with a similar study by Girazian and Withers (2013) on the ionosphere of Mars. We use this power law to estimate the peak electron density at the subsolar point of Titan during solar maximum conditions and find it to be about 6500cm(-3), i.e., 85-160% more than has been measured during the entire Cassini mission.
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| 24. |
- Hill, T. W., et al.
(författare)
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Charged nanograins in the Enceladus plume
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. A05209-
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Tidskriftsartikel (refereegranskat)abstract
- There have been three Cassini encounters with the south-pole eruptive plume of Enceladus for which the Cassini Plasma Spectrometer (CAPS) had viewing in the spacecraft ram direction. In each case, CAPS detected a cold dense population of heavy charged particles having mass-to-charge (m/q) ratios up to the maximum detectable by CAPS (similar to 10(4) amu/e). These particles are interpreted as singly charged nanometer-sized water-ice grains. Although they are detected with both negative and positive net charges, the former greatly outnumber the latter, at least in the m/q range accessible to CAPS. On the most distant available encounter (E3, March 2008) we derive a net (negative) charge density of up to similar to 2600 e/cm(3) for nanograins, far exceeding the ambient plasma number density, but less than the net (positive) charge density inferred from the RPWS Langmuir probe data during the same plume encounter. Comparison of the CAPS data from the three available encounters is consistent with the idea that the nanograins leave the surface vents largely uncharged, but become increasingly negatively charged by plasma electron impact as they move farther from the satellite. These nanograins provide a potentially potent source of magnetospheric plasma and E-ring material.
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| 25. |
- 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
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A10213-
-
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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| 26. |
- Ma, Y. J., et al.
(författare)
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Time-dependent global MHD simulations of Cassini T32 flyby : From magnetosphere to magnetosheath
- 2009
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114:3, s. A03204-
-
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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| 27. |
- Madanian, H., et al.
(författare)
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Solar cycle variations in ion composition in the dayside ionosphere of Titan
- 2016
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 121:8, s. 8013-8037
-
Tidskriftsartikel (refereegranskat)abstract
- One Titanian year spans over two complete solar cycles, and the solar irradiance has a significant effect on ionospheric densities. Solar cycle 24 has been one of the quietest cycles on record. In this paper we show data from the Cassini ion and neutral mass spectrometer (INMS) and the radio and plasma wave science Langmuir probe spanning the time period from early 2005, at the declining phase of solar cycle 23, to late 2015 at the declining phase of solar cycle 24. Densities of different ion species measured by the INMS show a consistent enhancement for high solar activity, particularly near the ionospheric peak. The density enhancement is best seen in primary ion species such as CH3+ rather than heavier ion species such as HCNH+. Unlike at Earth, where the ionosphere and atmosphere thermally expand at high solar activity, at Titan the altitude of the ionospheric peak decreases, indicating that the underlying neutral atmosphere was less extensive. Among the major ion species, CH5+ shows the largest decrease in peak altitude, whereas heavy ions such as C3H5+ show very little decrease. We also calculate the ion production rates using a theoretical model and a simple empirical model using INMS data and show that these effectively predict the increased ion production rates at high solar activity.
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| 28. |
- Richard, M. S., et al.
(författare)
-
Energetics of Titan's ionosphere : Model comparisons with Cassini data
- 2011
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 116, s. A09310-
-
Tidskriftsartikel (refereegranskat)abstract
- Observed electron and ion temperatures in planetary ionospheres are higher than the neutral temperature. Instruments on board the Cassini spacecraft have shown this is also true for Titan. The Radio and Plasma Wave Science Langmuir Probe (RPWS-LP) (Wahlund et al., 2005) has measured electron temperatures above 1000 K. Ionospheric ion temperatures were deduced from a combined analysis of data from the Cassini Plasma Spectrometer and Ion and Neutral Mass Spectrometer (INMS) (Crary et al., 2009). Elevated electron temperatures attributed to heating by suprathermal electrons were predicted by pre-Cassini models (e.g., Gan et al., 1992; Roboz and Nagy, 1994) and observed by the Cassini electron spectrometer. Models of the energetic electrons and ions are presented that include Cassini inputs (i.e., measured neutral densities from INMS). The results are compared between 800 and 1800 km with suprathermal electron fluxes and plasma temperatures measured by Cassini instruments emphasizing the thermal electron temperature. Using only solar inputs, the dayside model agrees well with electron temperatures measured by RPWS-LP (Agren et al., 2009) between 1000 and 1400 km. At higher altitudes energy input from magnetospheric electrons is needed to reproduce the measured temperature. Incorporating typical magnetospheric electron fluxes into the dayside does not noticeably increase ion production near the ionospheric peak; however, effects can be seen near 1350 km. Joule heating effects are shown to be capable of contributing significantly to the ion temperature. Magnetospheric suprathermal electrons are shown to provide sufficient heating for the thermal electron population in the middle to upper ionosphere on the nightside.
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| 29. |
- Robertson, I. P., et al.
(författare)
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Structure of Titan's ionosphere : Model comparisons with Cassini data
- 2009
-
Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 57:14-15, s. 1834-1846
-
Tidskriftsartikel (refereegranskat)abstract
- Solar extreme ultraviolet 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. New insights into Titan's ionosphere are being facilitated by data from several instruments onboard the Cassini Orbiter, although the Ion and Neutral Mass Spectrometer (INMS) measurements will be emphasized here. We present dayside ionosphere models and compare the results with both Radio and Plasma Wave-Langmuir Probe (RPWS/LP) and INMS data, exploring the sensitivity of models to ionospheric chemistry schemes and solar flux variations. Modeled electron densities for the dayside leg of T18 and all of T17 (dayside) had reasonable agreement with the measured RPWS electron densities and INMS total ion densities. Magnetospheric inputs make at best minor contributions to the ionosphere for these flybys, at least for altitudes above about 1000 km. At lower (< 1100 km) altitudes, the total ion densities measured by the INMS are less than the electron densities measured by the RPWS/LP which could be due to heavy (> 100 daltons) ions, which the INMS is not able to detect. Qualitatively, INMS spectra exhibit the same ion species and 12 amu family separations for the dayside ionospheres of T17 and T18 as were seen in the mass spectra measured during T5 (nightside). However, the relative abundance of high-mass (m > 50) ion species is about 10 times less for the dayside T17 and T18 passes than it was for the polar nightside T5 flyby, which can perhaps be explained in several ways including differences in neutral composition, less dissociative recombination on the nightside than on the dayside (due to lower electron densities and affecting heavier ion species more than lighter ones), and transport of longer-lived high-mass species from day-to-night.
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| 30. |
- Shebanits, O., et al.
(författare)
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Saturn's near-equatorial ionospheric conductivities from in situ measurements
- 2020
-
Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 10:1
-
Tidskriftsartikel (refereegranskat)abstract
- Cassini's Grand Finale orbits provided for the first time in-situ measurements of Saturn's topside ionosphere. We present the Pedersen and Hall conductivities of the top near-equatorial dayside ionosphere, derived from the in-situ measurements by the Cassini Radio and Wave Plasma Science Langmuir Probe, the Ion and Neutral Mass Spectrometer and the fluxgate magnetometer. The Pedersen and Hall conductivities are constrained to at least 10(-5)-10(-4) S/m at (or close to) the ionospheric peak, a factor 10-100 higher than estimated previously. We show that this is due to the presence of dusty plasma in the near-equatorial ionosphere. We also show the conductive ionospheric region to be extensive, with thickness of 300-800 km. Furthermore, our results suggest a temporal variation (decrease) of the plasma densities, mean ion masses and consequently the conductivities from orbit 288 to 292.
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| 31. |
- Stephan, K., et al.
(författare)
-
Regions of interest on Ganymede's and Callisto's surfaces as potential targets for ESA's JUICE mission
- 2021
-
Ingår i: Planetary and Space Science. - : Elsevier. - 0032-0633 .- 1873-5088. ; 208
-
Tidskriftsartikel (refereegranskat)abstract
- The JUpiter Icy moons Explorer (JUICE) will investigate Ganymede's and Callisto's surfaces and subsurfaces from orbit to explore the geologic processes that have shaped and altered their surfaces by impact, tectonics, possible cryovolcanism, space weathering due to micrometeorites, radiation and charged particles as well as explore the structure and properties of the icy crust and liquid shell (Grasset et al., 2013). The best possible synergy of the JUICE instruments is required to answer the major science objective of this mission and to fully exploit the po-tential of the JUICE mission. Therefore, the JUICE team is aiming to define high priority targets on both Gany-mede's and Callisto's surfaces to support the coordination of the planning activities by the individual instrument teams. Based on the science objectives of the JUICE mission and the most recent knowledge of Ganymede's and Callisto's geologic evolution we propose a collection of Regions of Interest (RoIs), which characterize surface features and terrain types representing important traces of geologic processes, from past and/or present cryovolcanic and tectonic activity to space weathering processes, which are crucial to understand the geologic evolution of Ganymede and Callisto. The proposed evaluation of RoIs is based on their scientific importance as well as on the opportunities and conditions to observe them during the currently discussed mission profile.
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| 32. |
- Ulusen, D., et al.
(författare)
-
Investigation of the force balance in the Titan ionosphere : Cassini T5 flyby model/data comparisons
- 2010
-
Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 210:2, s. 867-880
-
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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| 33. |
- Westlake, J. H., et al.
(författare)
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Titan's ionospheric composition and structure : Photochemical modeling of Cassini INMS data
- 2012
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. E01003-
-
Tidskriftsartikel (refereegranskat)abstract
- Titan's upper atmosphere produces an ionosphere at high altitudes from photoionization and electron impact that exhibits complex chemical processes in which hydrocarbons and nitrogen-containing molecules are produced through ion-molecule reactions. The structure and composition of Titan's ionosphere has been extensively investigated by the Ion and Neutral Mass Spectrometer (INMS) onboard the Cassini spacecraft. We present a detailed study using linear correlation analysis, 1-D photochemical modeling, and empirical modeling of Titan's dayside ionosphere constrained by Cassini measurements. The 1-D photochemical model is found to reproduce the primary photoionization products of N(2) and CH(4). The major ions, CH(5)(+), C(2)H(5)(+), and HCNH(+) are studied extensively to determine the primary processes controlling their production and loss. To further investigate the chemistry of Titan's ionosphere we present an empirical model of the ion densities that calculates the ion densities using the production and loss rates derived from the INMS data. We find that the chemistry included in our model sufficiently reproduces the hydrocarbon species as observed by the INMS. However, we find that the chemistry from previous models appears insufficient to accurately reproduce the nitrogen-containing organic compound abundances observed by the INMS. The major ion, HCNH(+), is found to be overproduced in both the empirical and 1-D photochemical models. We analyze the processes producing and consuming HCNH(+) in order to determine the cause of this discrepancy. We find that a significant chemical loss process is needed. We suggest that the loss process must be with one of the major components, namely C(2)H(2), C(2)H(4), or H(2).
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| 34. |
- Andersson, L., Wahlund, J.E., Clemmons, J., Gustavsson, B. and Eliasson, L.
(författare)
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Electromagnetic waves and bursty electron acceleration: implications from Freja.
- 2001
-
Ingår i: Annales Geophysicae. ; 20:2, s. 139-150
-
Tidskriftsartikel (refereegranskat)abstract
- Dispersive Alfven wave activity is identified in four dayside auroral oval events measured by the Freja satellite. The events are characterized by ion injection, bursty electron precipitation below about 1 keV, transverse ion heating and broadband extreme
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| 35. |
- Andre, M., Behlke, R., Wahlund, J.E., Vaivads, A., Eriksson, A., Tjulin, A., Carozzi, T. D., Cully, C., Gustafsson, G., Sundkvist, D., Khotyaintsev, Y., Cornilleau-Wehrlin, N., Rezeau, L., Maksimovic, M., Lucek, E., Balogh, A., Dunlop, M., Lindqvist, P.A.
(författare)
-
Multi-spacecraft observations of broadband waves near the lower hybrid frequency at the Earthward edge of the magnetopause.
- 2001
-
Ingår i: Annales Geophysicae. ; 19:6, s. 1471-1481
-
Tidskriftsartikel (refereegranskat)abstract
- Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have a
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| 36. |
- Andre, M., et al.
(författare)
-
Multi-spacecraft observations of broadband waves near the lower hybrid frequency at the Earthward edge of the magnetopause
- 2001
-
Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 19:12-okt, s. 1471-1481
-
Tidskriftsartikel (refereegranskat)abstract
- Broadband waves around the lower hybrid frequency (around 10 Hz) near the magnetopause are studied, using the four Cluster satellites. These waves are common at the Earthward edge of the boundary layer, consistent with earlier observations, and can have amplitudes at least up to 5 mV/m. These waves are similar on all four Cluster satellites, i.e. they are likely to be distributed over large areas of the boundary. The strongest electric fields occur during a few seconds, i.e. over distances of a few hundred km in the frame of the moving magnetopause, a scale length comparable to the ion gyroradius. The strongest magnetic oscillations in the same frequency range are typically found in the boundary layer, and across the magnetopause. During an event studied in detail, the magnetopause velocity is consistent with a large-scale depression wave, i.e. an inward bulge of magnetosheath plasma, moving tailward along the nominal magnetopause boundary. Preliminary investigations indicate that a rather flat front side of the large-scale wave is associated with a rather static small-scale electric field, while a more turbulent backside of the large-scale wave is associated with small-scale time varying electric field wave packets.
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| 37. |
- Cedres, N., et al.
(författare)
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Association of Cerebrovascular and Alzheimer Disease Biomarkers With Cholinergic White Matter Degeneration in Cognitively Unimpaired Individuals
- 2022
-
Ingår i: Neurology. - : Ovid Technologies (Wolters Kluwer Health). - 0028-3878 .- 1526-632X. ; 99:15
-
Tidskriftsartikel (refereegranskat)abstract
- Background and Objectives Several pathologic processes might contribute to the degeneration of the cholinergic system in aging. We aimed to determine the contribution of amyloid, tau, and cerebrovascular biomarkers toward the degeneration of cholinergic white matter (WM) projections in cognitively unimpaired individuals. Methods The contribution of amyloid and tau pathology was assessed through CSF levels of the A beta(42/40) ratio and phosphorylated tau (p-tau). CSF A beta(38) levels were also measured. Cerebrovascular pathology was assessed using automatic segmentations of WM lesions (WMLs) on MRI. Cholinergic WM projections (i.e., cingulum and external capsule pathways) were modeled using tractography based on diffusion tensor imaging data. Sex and APOE epsilon 4 carriership were also included in the analysis as variables of interest. Results We included 203 cognitively unimpaired individuals from the H70 Gothenburg Birth Cohort Studies (all individuals aged 70 years, 51% female). WM lesion burden was the most important contributor to the degeneration of both cholinergic pathways (increase in mean square error [IncMSE] = 98.8% in the external capsule pathway and IncMSE = 93.3% in the cingulum pathway). Levels of A beta(38) and p-tau also contributed to cholinergic WM degeneration, especially in the external capsule pathway (IncMSE = 28.4% and IncMSE = 23.4%, respectively). The A beta(42/40) ratio did not contribute notably to the models (IncMSE<3.0%). APOE epsilon 4 carriers showed poorer integrity in the cingulum pathway (IncMSE = 21.33%). Women showed poorer integrity of the external capsule pathway (IncMSE = 21.55%), which was independent of amyloid status as reflected by the nonsignificant differences in integrity when comparing amyloid-positive vs amyloid-negative women participants (T-201 = -1.55; p = 0.123). Discussion In cognitively unimpaired older individuals, WMLs play a central role in the degeneration of cholinergic pathways. Our findings highlight the importance of WM lesion burden in the elderly population, which should be considered in the development of prevention programs for neurodegeneration and cognitive impairment.
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| 38. |
- Cravens, T. E., et al.
(författare)
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Model-data comparisons for Titan's nightside ionosphere
- 2009
-
Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 199:1, s. 174-188
-
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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| 39. |
- Cui, J., et al.
(författare)
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Diurnal variations of Titan's ionosphere
- 2009
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 114:6, s. A06310-
-
Tidskriftsartikel (refereegranskat)abstract
- We present our analysis of the diurnal variations of Titan's ionosphere (between 1000 and 1300 km) based on a sample of Ion Neutral Mass Spectrometer (INMS) measurements in the Open Source Ion (OSI) mode obtained from eight close encounters of the Cassini spacecraft with Titan. Although there is an overall ion depletion well beyond the terminator, the ion content on Titan's nightside is still appreciable, with a density plateau of similar to 700 cm(-3) below similar to 1300 km. Such a plateau is a combined result of significant depletion of light ions and modest depletion of heavy ones on Titan's nightside. We propose that the distinctions between the diurnal variations of light and heavy ions are associated with their different chemical loss pathways, with the former primarily through "fast'' ion-neutral chemistry and the latter through "slow'' electron dissociative recombination. The strong correlation between the observed night-to-day ion density ratios and the associated ion lifetimes suggests a scenario in which the ions created on Titan's dayside may survive well to the nightside. The observed asymmetry between the dawn and dusk ion density profiles also supports such an interpretation. We construct a time-dependent ion chemistry model to investigate the effect of ion survival associated with solid body rotation alone as well as superrotating horizontal winds. For long-lived ions, the predicted diurnal variations have similar general characteristics to those observed. However, for short-lived ions, the model densities on the nightside are significantly lower than the observed values. This implies that electron precipitation from Saturn's magnetosphere may be an additional and important contributor to the densities of the short-lived ions observed on Titan's nightside.
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| 40. |
- Edberg, Niklas J. T., et al.
(författare)
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Spatial distribution of low-energy plasma around comet 67P/CG from Rosetta measurements
- 2015
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 42:11, s. 4263-4269
-
Tidskriftsartikel (refereegranskat)abstract
- We use measurements from the Rosetta plasma consortium Langmuir probe and mutual impedance probe to study the spatial distribution of low-energy plasma in the near-nucleus coma of comet 67P/Churyumov-Gerasimenko. The spatial distribution is highly structured with the highest density in the summer hemisphere and above the region connecting the two main lobes of the comet, i.e., the neck region. There is a clear correlation with the neutral density and the plasma to neutral density ratio is found to be approximate to 1-210(-6), at a cometocentric distance of 10km and at 3.1AU from the Sun. A clear 6.2h modulation of the plasma is seen as the neck is exposed twice per rotation. The electron density of the collisionless plasma within 260km from the nucleus falls off with radial distance as approximate to 1/r. The spatial structure indicates that local ionization of neutral gas is the dominant source of low-energy plasma around the comet.
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| 41. |
- Farrell, W. M., et al.
(författare)
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Modification of the plasma in the near-vicinity of Enceladus by the enveloping dust
- 2010
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 37:20, s. L20202-
-
Tidskriftsartikel (refereegranskat)abstract
- The plasma near Saturn's equator is quasi-corotating, but those fluid elements entering the near-vicinity of the moon Enceladus become uniquely modified. Besides the solid body, the Moon has a surrounding dust envelop that we show herein to be detected similar to 20 Enceladus radii (1 R-E = 252 km) both north and south of the body. Previous reports indicate that corotating plasma slows down substantially in the near-vicinity of Enceladus. We show herein that the commencement of this plasma slow down matches closely with Cassini's entry into the dense portions of the enveloping dust in the northern hemisphere above the Moon. We also examine in detail the source of the dust about 400 km above the south polar fissures. We find that a large positive potential must exist between the south pole of the moon and the spacecraft to account for ions streaming away from the pole on connecting magnetic field lines.
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| 42. |
- Gamier, P., et al.
(författare)
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Deriving the characteristics of warm electrons (100-500 eV) in the magnetosphere of Saturn with the Cassini Langmuir probe
- 2014
-
Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 104, s. 173-184
-
Tidskriftsartikel (refereegranskat)abstract
- Though Langmuir probes (LP) are designed to investigate cold plasma regions (e.g. ionospheres), a recent analysis revealed a strong sensitivity of the Cassini LP measurements to hundreds of eV electrons. These warm electrons impact the surface of the probe and generate a significant current of secondary electrons, that impacts both the DC level and the slope of the current-voltage curve of the LP (for negative potentials) through energetic contributions that may be modeled with a reasonable precision. We show here how to derive information about the incident warm electrons from the analysis of these energetic contributions, in the regions where the cold plasma component is small with an average temperature in the range similar to [100-500] eV. First, modeling the energetic contributions (based on the incident electron flux given by a single anode of the CAPS spectrometer) allows us to provide information about the pitch angle anisotropies of the incident hundreds of eV electrons. The modeling reveals indeed sometimes a large variability of the estimated maximum secondary electron yield (which is a constant for a surface material) needed to reproduce the observations. Such dispersions give evidence for strong pitch angle anisotropies of the incident electrons, and using a functional form of the pitch angle distribution even allows us to derive the real peak angle of the distribution. Second, rough estimates of the total electron temperature may be derived in the regions where the warm electrons are dominant and thus strongly influence the LP observations, i.e. when the average electron temperature is in the range similar to [100-500] eV. These regions may be identified from the LP observations through large positive values of the current-voltage slope at negative potentials. The estimated temperature may then be used to derive the electron density in the same region, with estimated densities between similar to 0.1 and a few particles/cm(3) (cc). The derived densities are in better agreement with the CAPS measurements than the values derived from the proxy technique (Morooka et al., 2009) based on the floating potential of the LP. Both the electron temperature and the density estimates lie outside the classical capabilities of the LP, which are essentially n(e) > 5 cc and T-e <5 eV at Saturn. This approximate derivation technique may be used in the regions where the cold plasma component is small with an average temperature in the range similar to [100-500] eV, which occurs often in the L range 6.4-9.4 R-S when Cassini is off the equator, but may occur anywhere in the magnetosphere. This technique may be all the more interesting since the CAPS instrument was shut down, and, though it cannot replace the CAPS instrument, the technique can provide useful information about the electron moments, with probably even better estimates than CAPS in some cases (when the plasma is strongly anisotropic). Finally, a simple modeling approach allows us to predict the impact of the energetic contributions on LP measurements in any plasma environment whose characteristics (density, temperature, etc.) are known. LP observations may thus be influenced by warm electrons in several planetary plasma regions in the solar system, and ambient magnetospheric electron density and temperature could be estimated in some of them (e.g. around several galilean satellites) through the use of Langmuir probes.
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| 43. |
- Gustafsson, Georg, et al.
(författare)
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First results of electric field and density observations by Cluster EFW based on initial months of operation
- 2001
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 19:12-okt, s. 1219-1240
-
Tidskriftsartikel (refereegranskat)abstract
- Highlights are presented from studies of the electric field data from various regions along the CLUSTER orbit. They all point towards a very high coherence for phenomena recorded on four spacecraft that are separated by a few hundred kilometers for structures over the whole range of apparent frequencies from I mHz to 9 kHz. This presents completely new opportunities to study spatial-temporal plasma phenomena from the magnetosphere out to the solar wind. A new probe environment was constructed for the CLUSTER electric field experiment that now produces data of unprecedented quality. Determination of plasma flow in the solar wind is an example of the capability of the instrument.
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| 44. |
- Hadid, Lina Z, et al.
(författare)
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Ring Shadowing Effects on Saturn's Ionosphere : Implications for Ring Opacity and Plasma Transport
- 2018
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Ingår i: Geophysical Research Letters. - : AMER GEOPHYSICAL UNION. - 0094-8276 .- 1944-8007. ; 45:19, s. 10084-10092
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Tidskriftsartikel (refereegranskat)abstract
- We present new results obtained by the Radio and Plasma Wave Science Langmuir probe on board Cassini during the Grand Finale. The total direct current sampled by the Langmuir probe at negative bias voltage is used to study the effect of the ring shadows on the structure of the Kronian topside ionosphere. The D and C rings and the Cassini Division are confirmed to be optically thin to extreme ultraviolet solar radiation. However, different responses from the opaque A and B rings are observed. The edges of the A ring shadow are shown to match the A ring boundaries, unlike the B ring, which indicates variable responses to the B ring shadow. We show that the variable responses are due to the ionospheric plasma, more precisely to the longer chemical lifetime of H+ compared to H-2(+) and H-3(+), suggesting that the plasma is transported from the sunlit region to the shadowed one in the ionosphere. Plain Language Summary As Saturn's northern hemisphere experienced summer during the Grand Finale, the planet's northern dayside hemisphere and its rings were fully illuminated by the Sun. However, the southern hemisphere was partly obscured because of the shadows cast by the A and B rings. Using the in situ measurements of the Langmuir probe part of the Radio and Plasma Wave Science investigation on board the Cassini spacecraft, we study for the first time the effect of the ring shadows on Saturn's ionosphere. From the ring shadows signatures on the total ion current collected by the Langmuir probe, we show that the A and B rings are optically thicker (to the solar extreme ultraviolet radiation) than the inner C and D rings and the Cassini Division to the solar extreme ultraviolet radiation. Moreover, we reproduce the boundaries of the A ring and the outer edge of the B ring. Furthermore, observed variations with respect to the inner edge of the B ring imply a delayed response of the ionospheric H+ because of its long lifetime and suggest that the ionospheric plasma is transported from an unshadowed region to a shadowed one in the ionosphere.
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| 45. |
- Jacobs, Heidi I. L., et al.
(författare)
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The association between white matter hyperintensities and executive decline in mild cognitive impairment is network dependent
- 2012
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Ingår i: Neurobiology of Aging. - : Elsevier BV. - 1558-1497 .- 0197-4580. ; 33:1, s. 1-201
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Tidskriftsartikel (refereegranskat)abstract
- White matter hyperintensities (WMH) in Mild Cognitive Impairment (MCI) have been associated with impaired executive functioning, although contradictory findings have been reported. The aim of this study was to examine whether WMH location influenced the relation between WMH and executive functioning in MCI participants (55-90 years) in the European multicenter memory-clinic-based DESCRIPA study, who underwent MRI scanning at baseline (N = 337). Linear mixed model analysis was performed to test the association between WMH damage in three networks (frontal-parietal, frontal-subcortical and frontal-parietal-subcortical network) and change in executive functioning over a 3-year period. WMH in the frontal-parietal and in the frontal-parietal-subcortical network were associated with decline in executive functioning. However, the frontal-subcortical network was not associated with change in executive functioning. Our results suggest that parietal WMH are a significant contributor to executive decline in MCI and that investigation of WMH in the cerebral networks supporting cognitive functions provide a new way to differentiate stable from cognitive declining MCI individuals. (C) 2012 Elsevier Inc. All rights reserved.
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| 46. |
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| 47. |
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| 48. |
- Ma, Ying-Juan, et al.
(författare)
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3D global multi-species Hall-MHD simulation of the Cassini T9 flyby
- 2007
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 34:24, s. L24S10-
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Tidskriftsartikel (refereegranskat)abstract
- The wake region of Titan is an important component of Titan's interaction with its surrounding plasma and therefore a thorough understanding of its formation and structure is of primary interest. The Cassini spacecraft passed through the distant downstream region of Titan on 18: 59: 30 UT Dec. 26, 2005, which is referred to as the T9 flyby and provided a great opportunity to test our understanding of the highly dynamic wake region. In this paper we compare the observational data (from the magnetometer, plasma analyzer and Langmuir probe) with numerical results using a 7-species Hall MHD Titan model. There is a good agreement between the observed and modeled parameters, given the uncertainties in plasma measurements and the approximations inherent in the Hall MHD model. Our simulation results also show that Hall MHD model results fit the observations better than the non-Hall MHD model for the flyby, consistent with the importance of kinetic effects in the Titan interaction. Based on the model results, we also identify various regions near Titan where Hall MHD models are applicable.
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| 49. |
- Mandt, Kathleen E., et al.
(författare)
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Ion densities and composition of Titan's upper atmosphere derived from the Cassini Ion Neutral Mass Spectrometer : Analysis methods and comparison of measured ion densities to photochemical model simulations
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. E10006-
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Tidskriftsartikel (refereegranskat)abstract
- The Cassini Ion Neutral Mass Spectrometer (INMS) has measured both neutral and ion species in Titan's upper atmosphere and ionosphere and the Enceladus plumes. Ion densities derived from INMS measurements are essential data for constraining photochemical models of Titan's ionosphere. The objective of this paper is to present an optimized method for converting raw data measured by INMS to ion densities. To do this, we conduct a detailed analysis of ground and in-flight calibration to constrain the instrument response to ion energy, the critical parameter on which the calibration is based. Data taken by the Cassini Radio Plasma Wave Science Langmuir Probe and the Cassini Plasma Spectrometer Ion Beam Spectrometer are used as independent measurement constraints in this analysis. Total ion densities derived with this method show good agreement with these data sets in the altitude region (similar to 1100-1400 km) where ion drift velocities are low and the mass of the ions is within the measurement range of the INMS (1-99 Daltons). Although ion densities calculated by the method presented here differ slightly from those presented in previous INMS publications, we find that the implications for the science presented in previous publications is mostly negligible. We demonstrate the role of the INMS ion densities in constraining photochemical models and find that (1) cross sections having high resolution as a function of wavelength are necessary for calculating the initial photoionization products and (2) there are disagreements between the measured ion densities representative of the initial steps in Titan photochemistry that require further investigation.
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| 50. |
- Menietti, J. D., et al.
(författare)
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Survey of Saturn Z-mode emission
- 2015
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Ingår i: Journal of Geophysical Research - Space Physics. - 2169-9380 .- 2169-9402. ; 120:8, s. 6176-6187
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Tidskriftsartikel (refereegranskat)abstract
- Because of the role of Z-mode emission in the diffusive scattering and resonant acceleration of electrons, we conduct a survey of intensity in the Saturn inner magnetosphere. Z mode is primarily observed as 5kHz narrowband emission in the lower density regions where the ratio of cyclotron to plasma frequency, f(c)/f(p)>1 to which we limit this study. This occurs at Saturn along the inner edge of the Enceladus torus near the equator and at higher latitudes. We present profiles and parametric fits of intensity as a function of frequency, radius, latitude, and local time. The magnetic field intensity levels are lower than chorus, but the electric field intensities are comparable. We conclude that Z-mode wave-particle interactions may make a significant contribution to electron acceleration in the inner magnetosphere of Saturn, supplementing acceleration produced by chorus emission.
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