| 1. |
- Plainaki, C., et al.
(författare)
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Towards a Global Unified Model of Europa’s Tenuous Atmosphere
- 2018
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Ingår i: Space Science Reviews. - : Springer Netherlands. - 0038-6308 .- 1572-9672. ; 214:1
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Forskningsöversikt (refereegranskat)abstract
- Despite the numerous modeling efforts of the past, our knowledge on the radiation-induced physical and chemical processes in Europa’s tenuous atmosphere and on the exchange of material between the moon’s surface and Jupiter’s magnetosphere remains limited. In lack of an adequate number of in situ observations, the existence of a wide variety of models based on different scenarios and considerations has resulted in a fragmentary understanding of the interactions of the magnetospheric ion population with both the moon’s icy surface and neutral gas envelope. Models show large discrepancy in the source and loss rates of the different constituents as well as in the determination of the spatial distribution of the atmosphere and its variation with time. The existence of several models based on very different approaches highlights the need of a detailed comparison among them with the final goal of developing a unified model of Europa’s tenuous atmosphere. The availability to the science community of such a model could be of particular interest in view of the planning of the future mission observations (e.g., ESA’s JUpiter ICy moons Explorer (JUICE) mission, and NASA’s Europa Clipper mission). We review the existing models of Europa’s tenuous atmosphere and discuss each of their derived characteristics of the neutral environment. We also discuss discrepancies among different models and the assumptions of the plasma environment in the vicinity of Europa. A summary of the existing observations of both the neutral and the plasma environments at Europa is also presented. The characteristics of a global unified model of the tenuous atmosphere are, then, discussed. Finally, we identify needed future experimental work in laboratories and propose some suitable observation strategies for upcoming missions.
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| 2. |
- Roth, Lorenz, et al.
(författare)
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Europa's far ultraviolet oxygen aurora from a comprehensive set of HST observations
- 2016
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Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 121:3, s. 2143-2170
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Tidskriftsartikel (refereegranskat)abstract
- We analyze a large set of far ultraviolet oxygen aurora images of Europa's atmosphere taken by Hubble's Space Telescope Imaging Spectrograph (HST/STIS) in 1999 and on 19 occasions between 2012 and 2015. We find that both brightness and aurora morphology undergo systematic variations correlated to the periodically changing plasma environment. The time variable morphology seems to be strongly affected by Europa's interaction with the magnetospheric plasma. The brightest emissions are often found in the polar region where the ambient Jovian magnetic field line is normal to Europa's disk. Near the equator, where bright spots are found at Io, Europa's aurora is faint suggesting a general difference in how the plasma interaction shapes the aurora at Io and Europa. The dusk side is consistently brighter than the dawnside with only few exceptions, which cannot be readily explained by obvious plasma physical or known atmospheric effects. Brightness ratios of the near-surface OI] 1356 Å to OI 1304 Å emissions between 1.5 and 2.8 with a mean ratio of 2.0 are measured, confirming that Europa's bound atmosphere is dominated by O2. The 1356/1304 ratio decreases with increasing altitude in agreement with a more extended atomic O corona, but O2 prevails at least up to altitudes of ∼900 km. Differing 1356/1304 line ratios on the plasma upstream and downstream hemispheres are explained by a differing O mixing ratio in the near-surface O2 atmosphere of ∼5% (upstream) and ≲1% (downstream), respectively. During several eclipse observations, the aurora does not reveal any signs of systematic changes compared to the sunlit images suggesting no or only weak influence of sunlight on the aurora and an optically thin atmosphere.
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| 3. |
- Blöcker, A., et al.
(författare)
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Europa's plasma interaction with an inhomogeneous atmosphere : Development of Alfvén winglets within the Alfvén wings
- 2016
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Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 121:10, s. 9794-9828
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Tidskriftsartikel (refereegranskat)abstract
- We apply a three-dimensional magnetohydrodynamic (MHD) model to study the influence of inhomogeneities in Europa's atmosphere, as, for example, water vapor plumes, on Europa's plasma interaction with the Jovian magnetosphere. In our model we have included electromagnetic induction in a subsurface water ocean, collisions between ions and neutrals, plasma production and loss due to electron impact ionization, and dissociative recombination. We present a systematic study of the plasma interaction when a local inhomogeneity in the neutral density is present within a global sputtering generated atmosphere. We show that an inhomogeneity near the north or south pole affects the plasma interaction in a way that a pronounced north-south asymmetry is generated. We find that an Alfvén winglet develops within Europa's main Alfvén wing on that side where the inhomogeneity is located. In addition to the MHD model we apply an analytic model based on the model of Saur et al. (2007) to understand the role of steep gradients and discontinuities in the interaction. We compare our model results with the measured magnetic field data from three flybys of the Galileo spacecraft at Europa which included Alfvén wing crossings. Our analysis suggests that the magnetic field might be influenced by atmospheric inhomogeneities during the E26 flyby. The findings of this work will aid in the search for plumes at Europa in future plasma and field observations.
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