| 1. |
- Arridge, Christopher S., et al.
(författare)
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Uranus Pathfinder : exploring the origins and evolution of Ice Giant planets
- 2012
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Ingår i: Experimental astronomy. - : Springer Science and Business Media LLC. - 0922-6435 .- 1572-9508. ; 33:2-3, s. 753-791
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Tidskriftsartikel (refereegranskat)abstract
- The "Ice Giants" Uranus and Neptune are a different class of planet compared to Jupiter and Saturn. Studying these objects is important for furthering our understanding of the formation and evolution of the planets, and unravelling the fundamental physical and chemical processes in the Solar System. The importance of filling these gaps in our knowledge of the Solar System is particularly acute when trying to apply our understanding to the numerous planetary systems that have been discovered around other stars. The Uranus Pathfinder (UP) mission thus represents the quintessential aspects of the objectives of the European planetary community as expressed in ESA's Cosmic Vision 2015-2025. UP was proposed to the European Space Agency's M3 call for medium-class missions in 2010 and proposed to be the first orbiter of an Ice Giant planet. As the most accessible Ice Giant within the M-class mission envelope Uranus was identified as the mission target. Although not selected for this call the UP mission concept provides a baseline framework for the exploration of Uranus with existing low-cost platforms and underlines the need to develop power sources suitable for the outer Solar System. The UP science case is based around exploring the origins, evolution, and processes at work in Ice Giant planetary systems. Three broad themes were identified: (1) Uranus as an Ice Giant, (2) An Ice Giant planetary system, and (3) An asymmetric magnetosphere. Due to the long interplanetary transfer from Earth to Uranus a significant cruise-phase science theme was also developed. The UP mission concept calls for the use of a Mars Express/Rosetta-type platform to launch on a Soyuz-Fregat in 2021 and entering into an eccentric polar orbit around Uranus in the 2036-2037 timeframe. The science payload has a strong heritage in Europe and beyond and requires no significant technology developments.
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| 2. |
- Roth, Lorenz, et al.
(författare)
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A phenomenological model of Io’s UV aurora based on HST/STIS observations
- 2014
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Ingår i: Icarus. - : Elsevier. - 0019-1035 .- 1090-2643. ; 228, s. 386-406
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Tidskriftsartikel (refereegranskat)abstract
- We have carried out a comprehensive analysis of a large set of spatially resolved observations of Io's OI 1304. Å, OI] 1356. Å, SI 1479. Å and SI] 1900. Å aurora taken by the Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope (HST) between 1997 and 2001. We find that the variability of the observed morphologies can be solely explained by the changes of the plasma and magnetic field environment of the Io torus and by the viewing perspective. The variations in brightness are strongly correlated with the periodic variations of the ambient electron density. Based on these findings we develop a phenomenological model for the spatial distribution of the oxygen and sulfur emissions in Io's vicinity. Taking into account Io's position with respect to the plasma torus, the orientation of Jupiter's magnetic field and the viewing perspective of the observation, the model calculates the auroral morphology and brightness. By fitting the model parameters to the observations we find that the model is able to reproduce the main features in all images obtained over a period of five years with one parameter set for each emission multiplet. The spatial distribution of the OI] 1356. Å, OI 1304. Å, SI 1479. Å, and SI] 1900. Å multiplets are shown to be very similar. In contrast to previous investigations, the model results reveal that the majority of the radiation from the bound atmosphere is emitted within 100. km above the surface. The equatorial aurora spots extend far into the wake region explaining observed features in the downstream region. The relative brightness of two the equatorial spots is best explained by our model if the emission on the day-side flank of Io is higher by a factor of ~1.5 with respect to the nightside flank. The measured brightness during an observation in eclipse is significantly lower than expected from the fitted model. The day-night asymmetry and the brightness decrease in eclipse support the idea of a wide collapse of Io's atmosphere in shadow. Since our phenomenological aurora model is able to reproduce the main features of the observed morphology by taking into account the variations of the magnetospheric parameters, it can be applied to predict the emission for future UV aurora observations for a given time and position of the observer.
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| 3. |
- Roth, Lorenz, et al.
(författare)
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Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa's water vapor aurora
- 2014
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 111:48, s. E5123-E5132
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Tidskriftsartikel (refereegranskat)abstract
- We report far-ultraviolet observations of Jupiter's moon Europa taken by Space Telescope Imaging Spectrograph (STIS) of the Hubble Space Telescope (HST) in January and February 2014 to test the hypothesis that the discovery of a water vapor aurora in December 2012 by local hydrogen (H) and oxygen (O) emissions with the STIS originated from plume activity possibly correlated with Europa's distance from Jupiter through tidal stress variations. The 2014 observations were scheduled with Europa near the apocenter similar to the orbital position of its previous detection. Tensile stresses on south polar fractures are expected to be highest in this orbital phase, potentially maximizing the probability for plume activity. No local H and O emissions were detected in the new STIS images. In the south polar region where the emission surpluses were observed in 2012, the brightnesses are sufficiently low in the 2014 images to be consistent with any H2O abundance from(0-5)x10(15) cm(-2). Large high-latitude plumes should have been detectable by the STIS, independent of the observing conditions and geometry. Because electron excitation of water vapor remains the only viable explanation for the 2012 detection, the new observations indicate that although the same orbital position of Europa for plume activity may be a necessary condition, it is not a sufficient condition. However, the December 2012 detection of coincident HI Lyman-alpha and OI 1304-angstrom emission surpluses in an similar to 200-km high region well separated above Europa's limb is a firm result and not invalidated by our 2014 STIS observations.
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| 4. |
- Roth, Lorenz, et al.
(författare)
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Simulation of Io’s auroral emission : Constraints on the atmosphere in eclipse
- 2011
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Ingår i: Icarus. - : Elsevier BV. - 0019-1035 .- 1090-2643. ; 214:2, s. 495-509
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Tidskriftsartikel (refereegranskat)abstract
- We study the morphology of Io's aurora by comparing simulation results of a three-dimensional (3D) two-fluid plasma model to observations by the high-resolution Long-Range Reconnaissance Imager (LORRI) on-board the New Horizons spacecraft and by the Hubble Space Telescope Advanced Camera for Surveys (HST/ACS). In 2007, Io's auroral emission in eclipse has been observed simultaneously by LORRI and ACS and the observations revealed detailed features of the aurora, such as a huge glowing plume at the Tvashtar paterae close to the North pole. The auroral radiation is generated in Io's atmosphere by collisions between impinging magnetospheric electrons and various neutral gas components. We calculate the interaction of the magnetospheric plasma with Io's atmosphere-ionosphere and simulate the auroral emission. Our aurora model takes into account not only the direct influence of the atmospheric distribution on the morphology and intensity of the emission, but also the indirect influence of the atmosphere on the plasma environment and thus on the exciting electrons. We find that the observed morphology in eclipse can be explained by a smooth (non-patchy) equatorial atmosphere with a vertical column density that corresponds to ∼10% of the column density of the sunlit atmosphere. The atmosphere is asymmetric with two times higher density and extension on the downstream hemisphere. The auroral emission from the Tvashtar volcano enables us to constrain the plume gas content for the first time. According to our model, the observed intensity of the Tvashtar plume implies a mean column density of ∼5×1015cm-2 for the plume region.
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| 5. |
- Saur, Joachim, et al.
(författare)
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HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS OBSERVATIONS OF EUROPA’S ATMOSPHERIC ULTRAVIOLET EMISSION AT EASTERN ELONGATION
- 2011
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 738:2
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Tidskriftsartikel (refereegranskat)abstract
- We report results of a Hubble Space Telescope (HST) campaign with the Advanced Camera for Surveys to observe Europa at eastern elongation, i.e., Europa's leading side, on 2008 June 29. With five consecutive HST orbits, we constrain Europa's atmospheric OI 1304 angstrom and OI 1356 angstrom emissions using the prism PR130L. The total emissions of both oxygen multiplets range between 132 +/- 14 and 226 +/- 14 Rayleigh. An additional systematic error with values on the same order as the statistical errors may be due to uncertainties in modeling the reflected light from Europa's surface. The total emission also shows a clear dependence of Europa's position with respect to Jupiter's magnetospheric plasma sheet. We derive a lower limit for the O-2 column density of 6 x 10(18) m(-2). Previous observations of Europa's atmosphere with the Space Telescope Imaging Spectrograph in 1999 of Europa's trailing side show an enigmatic surplus of radiation on the anti-Jovian side within the disk of Europa. With emission from a radially symmetric atmosphere as a reference, we searched for an anti-Jovian versus sub-Jovian asymmetry with respect to the central meridian on the leading side and found none. Likewise, we searched for departures from a radially symmetric atmospheric emission and found an emission surplus centered around 90 degrees west longitude, for which plausible mechanisms exist. Previous work about the possibility of plumes on Europa due to tidally driven shear heating found longitudes with strongest local strain rates which might be consistent with the longitudes of maximum UV emissions. Alternatively, asymmetries in Europa's UV emission can also be caused by inhomogeneous surface properties, an optically thick atmospheric contribution of atomic oxygen, and/or by Europa's complex plasma interaction with Jupiter's magnetosphere.
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