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  • 1.
    Becker, T. M.
    et al.
    Southwest Res Inst, San Antonio, TX 78228 USA..
    Retherford, K. D.
    Southwest Res Inst, San Antonio, TX 78228 USA..
    Roth, Lorenz
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Space and Plasma Physics.
    Hendrix, A. R.
    Planetary Sci Inst, Tucson, AZ USA..
    McGrath, M. A.
    SETI Inst, Mountain View, CA USA..
    Saur, J.
    Univ Cologne, Inst Geophys & Meteorol, Cologne, Germany..
    The Far-UV Albedo of Europa From HST Observations2018In: Journal of Geophysical Research - Planets, ISSN 2169-9097, E-ISSN 2169-9100, Vol. 123, no 5, p. 1327-1342Article in journal (Refereed)
    Abstract [en]

    We present an analysis of Europa's far-UV spectral albedo using observations during the 1999-2015 time period made by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. Disk-integrated observations show that the far-UV spectrum in the similar to 130 to 170-nm range is relatively flat or slightly blue (increasing albedo with decreasing wavelength) for the studied hemispheres: the leading, trailing, and anti-Jovian hemispheres. At Lyman- (121.6nm), the albedo of the trailing hemisphere continues the blue trend, but it reddens for the leading hemisphere. Also at this wavelength, the albedo of the leading hemisphere, which is higher than the trailing hemisphere at near-UV and visible wavelengths, is lower than the trailing hemisphere, exhibiting spectral inversion. We find no evidence of a sharp water-ice absorption edge at 165nm on any hemisphere of Europa, which is intriguing since such an absorption feature has been observed on the icy Saturnian satellites. Plain Language Summary We used observations spanning from 1999 to 2015 obtained by the Space Telescope Imaging Spectrograph on the Hubble Space Telescope to study the surface reflectance of Europa at far-ultraviolet (UV) wavelengths. We find that Europa has a low reflectance in the UV and that there is little variation in the surface brightness at most of the UV wavelengths. When observed at visible wavelengths, one of Europa's hemispheres is brighter than the other, but at the UV wavelength of 121.6nm, the hemisphere brightness is reversed. We also find that Europa looks different from the icy moons of Saturn at far-UV wavelengths.

  • 2. Blöcker, A.
    et al.
    Saur, J.
    Roth, Lorenz
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Europa's plasma interaction with an inhomogeneous atmosphere: Development of Alfvén winglets within the Alfvén wings2016In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 10, p. 9794-9828Article in journal (Refereed)
    Abstract [en]

    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.

  • 3.
    Blöcker, Aljona
    et al.
    KTH, School of Electrical Engineering (EES). Univ Cologne, Inst Geophys & Meteorol, Cologne, Germany.
    Saur, Joachim
    Univ Cologne, Inst Geophys & Meteorol, Cologne, Germany..
    Roth, Lorenz
    KTH, School of Electrical Engineering (EES).
    Strobel, Darrell F.
    Johns Hopkins Univ, Dept Earth & Planetary Sci, Baltimore, MD 21218 USA.;Johns Hopkins Univ, Dept Phys & Astron, Baltimore, MD 21218 USA..
    MHD Modeling of the Plasma Interaction With Io's Asymmetric Atmosphere2018In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, no 11, p. 9286-9311Article in journal (Refereed)
    Abstract [en]

    Io's atmosphere, with an average equatorial column density of >= 10(20) m(-2), exhibits significant density variations with latitude and longitude. We apply a 3-D magnetohydrodynamic model to investigate the effects of atmospheric asymmetries, both locally from volcanic plumes and globally, on the plasma and magnetic field environment of Io. The model takes into account collisions between ions and neutrals, plasma production and loss due to electron impact ionization and dissociative recombination, and the ionospheric Hall effect. Our simulation results show that volcanic plumes influence the plasma interaction locally, generating Alfven winglets within Io's global Alfven wing. Signals from individual plumes can however barely be probed by magnetic field measurements during spacecraft flybys at Io. In contrast, the surface number density, scale height, the longitudinal and latitudinal variations of the global atmosphere are crucial factors for modeling and understanding magnetic field and plasma perturbations. Comparing our model results with the magnetic field data from the 124 and 127 flybys of the Galileo spacecraft, we find that the measured perturbations can be primarily caused by the plasma interaction with the longitudinally asymmetric atmosphere. This implies that a significant magnetic induction signal from a partially molten magma ocean is not necessarily required to explain the Galileo magnetometer data.

  • 4.
    Dahlgren, Hanna
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Schlatter, Nicola Manuel
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ivchenko, Nickolay
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    Roth, Lorenz
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Karlsson, Alexander
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Relation of anomalous F region radar echoes in the high-latitude ionosphere to auroral precipitation2017In: Annales Geophysicae, ISSN 0992-7689, E-ISSN 1432-0576, Vol. 35, no 3, p. 475-479Article in journal (Refereed)
    Abstract [en]

    Non-thermal echoes in incoherent scatter radar observations are occasionally seen in the high-latitude ionosphere. Such anomalous echoes are a manifestation of plasma instabilities on spatial scales matching the radar wavelength. Here we investigate the occurrence of a class of spatially localized anomalous echoes with an enhanced zero Doppler frequency feature and their relation to auroral particle precipitation. The ionization profile of the E region is used to parametrize the precipitation, with nmE and hmE being the E region peak electron density and the altitude of the peak, respectively. We find the occurrence rate of the echoes to generally increase with nmE and decrease with hmE, thereby indicating a correlation between the echoes and high-energy flux precipitation of particles with a high characteristic energy. The highest occurrence rate of > 20% is found for hmE = 109 km and nmE D 10(11.9) m(-3), averaged over the radar observation volume.

  • 5.
    Grodent, Denis
    et al.
    Univ Liege, STAR Inst, Lab Phys Atmospher & Planetaire, Liege, Belgium..
    Bonfond, B.
    Univ Liege, STAR Inst, Lab Phys Atmospher & Planetaire, Liege, Belgium..
    Yao, Z.
    Univ Liege, STAR Inst, Lab Phys Atmospher & Planetaire, Liege, Belgium..
    Gerard, J-C
    Radioti, A.
    Univ Liege, STAR Inst, Lab Phys Atmospher & Planetaire, Liege, Belgium..
    Dumont, M.
    Univ Liege, STAR Inst, Lab Phys Atmospher & Planetaire, Liege, Belgium..
    Palmaerts, B.
    Univ Liege, STAR Inst, Lab Phys Atmospher & Planetaire, Liege, Belgium..
    Adriani, A.
    INAF, Ist Astrofis & Planetol Spaziali, Rome, Italy..
    Badman, S. V.
    Univ Lancaster, Phys Dept, Lancaster, England..
    Bunce, E. J.
    Univ Leicester, Dept Phys & Astron, Leicester, Leics, England..
    Clarke, J. T.
    Boston Univ, Ctr Space Phys, Boston, MA 02215 USA..
    Connerney, J. E. P.
    NASA, Goddard Space Flight Ctr, Planetary Magnetospheres Lab, Solar Syst Explorat Div, Greenbelt, MD USA..
    Gladstone, G. R.
    Southwest Res Inst, Dept Space Sci, San Antonio, TX USA..
    Greathouse, T.
    Southwest Res Inst, Dept Space Sci, San Antonio, TX USA..
    Kimura, T.
    RIKEN, Wako, Saitama, Japan..
    Kurth, W. S.
    Univ Iowa, Dept Phys & Astron, Iowa City, IA 52242 USA..
    Mauk, B. H.
    Johns Hopkins Univ, Appl Phys Lab, Laurel, MD USA..
    McComas, D. J.
    Princeton Univ, Dept Astrophys Sci, Princeton, NJ 08544 USA..
    Nichols, J. D.
    Univ Leicester, Dept Phys & Astron, Leicester, Leics, England..
    Orton, G. S.
    CALTECH, Jet Prop Lab, Pasadena, CA USA..
    Roth, Lorenz
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Saur, J.
    Univ Cologne, Inst Geophys & Meteorol, Cologne, Germany..
    Valek, P.
    Southwest Res Inst, Dept Space Sci, San Antonio, TX USA.;Univ Texas San Antonio, Dept Phys & Astron, San Antonio, TX USA..
    Jupiter's Aurora Observed With HST During Juno Orbits 3 to 72018In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 123, no 5, p. 3299-3319Article in journal (Refereed)
    Abstract [en]

    A large set of observations of Jupiter's ultraviolet aurora was collected with the Hubble Space Telescope concurrently with the NASA-Juno mission, during an eight-month period, from 30 November 2016 to 18 July 2017. These Hubble observations cover Juno orbits 3 to 7 during which Juno in situ and remote sensing instruments, as well as other observatories, obtained a wealth of unprecedented information on Jupiter's magnetosphere and the connection with its auroral ionosphere. Jupiter's ultraviolet aurora is known to vary rapidly, with timescales ranging from seconds to one Jovian rotation. The main objective of the present study is to provide a simplified description of the global ultraviolet auroral morphology that can be used for comparison with other quantities, such as those obtained with Juno. This represents an entirely new approach from which logical connections between different morphologies may be inferred. For that purpose, we define three auroral subregions in which we evaluate the auroral emitted power as a function of time. In parallel, we define six auroral morphology families that allow us to quantify the variations of the spatial distribution of the auroral emission. These variations are associated with changes in the state of the Jovian magnetosphere, possibly influenced by Io and the Io plasma torus and by the conditions prevailing in the upstream interplanetary medium. This study shows that the auroral morphology evolved differently during the five similar to 2week periods bracketing the times of Juno perijove (PJ03 to PJ07), suggesting that during these periods, the Jovian magnetosphere adopted various states.

  • 6.
    Hue, V
    et al.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Greathouse, T. K.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Bonfond, B.
    Univ Liege, STAR Inst, LPAP, Liege, Belgium..
    Saur, J.
    Univ Cologne, Inst Geophys & Meteorol, Cologne, Germany..
    Gladstone, G. R.
    Southwest Res Inst, San Antonio, TX 78238 USA.;Univ Texas San Antonio, Dept Phys & Astron, San Antonio, TX USA..
    Roth, Lorenz
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Davis, M. W.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Gerard, J-C
    Univ Liege, STAR Inst, LPAP, Liege, Belgium..
    Grodent, D. C.
    Univ Liege, STAR Inst, LPAP, Liege, Belgium..
    Kammer, J. A.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Szalay, J. R.
    Princeton Univ, Dept Astrophys Sci, Princeton, NJ 08544 USA..
    Versteeg, M. H.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Bolton, S. J.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Connerney, J. E. P.
    Space Res Corp, Annapolis, MD USA.;NASA, Goddard Space Flight Ctr, Greenbelt, MD USA..
    Levin, S. M.
    Jet Prop Lab, Pasadena, CA USA..
    Hinton, P. C.
    Southwest Res Inst, San Antonio, TX 78238 USA.;Univ Colorado, Lab Atmosphere & Space Phys, Boulder, CO 80309 USA..
    Bagenal, F.
    Southwest Res Inst, San Antonio, TX 78238 USA.;Univ Colorado, Lab Atmosphere & Space Phys, Boulder, CO 80309 USA..
    Juno-UVS Observation of the Io Footprint During Solar Eclipse2019In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, no 7, p. 5184-5199Article in journal (Refereed)
    Abstract [en]

    The two main ultraviolet-signatures resulting from the Io-magnetosphere interaction are the local auroras on Io's atmosphere, and the Io footprints on Jupiter. We study here how Io's daily eclipses affect the footprint. Previous observations showed that its atmosphere collapses in eclipse. While remote observers can observe Io's local auroras briefly when Io disappears behind Jupiter, Juno is able to follow the Io footprint in the unlit hemisphere. Theoretical models of the variability of the energy flux fed into the Alfven wings, ultimately powering the footprints, are not sufficiently constrained by observations. For the first time, we use observations of Io's footprint from the Ultraviolet Spectrograph (UVS) on Juno recorded as Io went into eclipse. We benchmark the trend of the footprint brightness using observations by UVS taken over Io's complete orbit and find that the footprint emitted power variation with Jupiter's rotation shows fairly consistent trends with previous observations. Two exploitable data sets provided measurements when Io was simultaneously in eclipse. No statistically significant changes were recorded as Io left and moved into eclipse, respectively, suggesting either that (i) Io's atmospheric densities within and outside eclipse are large enough to produce a saturated plasma interaction, that is, in the saturated state, changes in Io's atmospheric properties to first order do not control the total Alfvenic energy flux, (ii) the atmospheric collapse during the Juno observations was less than previously observed, or (iii) additional processes of the Alfven wings in addition to the Poynting flux generated at Io control the footprint luminosity.

  • 7. Musacchio, Fabrizio
    et al.
    Saur, Joachim
    Roth, Lorenz
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Retherford, Kurt D.
    McGrath, Melissa A.
    Feldman, Paul D.
    Strobel, Darrell F.
    Morphology of Ganymede's FUV auroral ovals2017In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 3, p. 2855-2876Article in journal (Refereed)
    Abstract [en]

    We study the morphology of Ganymede's FUV aurora by analyzing spectral images obtained over the past two decades by the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The observations cover the eastern and western elongation as well as various magnetic latitudes of Ganymede within the Jovian plasma sheet. We find both asymmetries in the spatial distribution of auroral brightness on the observed moon disk and temporal variation correlated to Ganymede's changing magnetic latitude. The total disk brightness is on average 1.42 +/- 0.07 times brighter on the leading side (95.4 +/- 2.1R) than on the trailing side (67.2 +/- 2.9R). The brightness ratio of the sub-Jovian hemisphere to the anti-Jovian hemisphere is 1.81 +/- 0.06 on the leading side and 1.41 +/- 0.14 on the trailing side, respectively. Inside the Jovian current sheet, the brightness of the auroral ovals increases by a factor of 1.45 +/- 0.02 on the leading side and decreases by a factor of 0.80 +/- 0.02 on the trailing side. At the current sheet center, the auroral ovals shift 4.1 degrees +/- 0.7 degrees latitude toward Ganymede's planetographic equator on the leading side and 2.9 degrees +/- 1.5 degrees toward the poles on the trailing side. Both effects, the variation of brightness and the movement of the ovals are correlated to a stronger interaction of Jupiter's magnetospheric plasma with Ganymede's minimagnetosphere inside the current sheet. Finally, we calculate the latitudinal difference of the northern and southern ovals from Ganymede's magnetic equator. The result suggests a farther westward orientation of Ganymede's dipole magnetic moment at approximately 47 degrees + 58 degrees/-43 degrees west longitude compared to previous estimates.

  • 8. Paganini, L.
    et al.
    Villanueva, G. L.
    Roth, Lorenz
    KTH, School of Electrical Engineering and Computer Science (EECS), Electrical Engineering, Space and Plasma Physics.
    Mandell, A. M.
    Hurford, T. A.
    Retherford, K. D.
    Mumma, M. J.
    A measurement of water vapour amid a largely quiescent environment on Europa2019In: Nature Astronomy, E-ISSN 2397-3366Article in journal (Refereed)
    Abstract [en]

    Previous investigations proved the existence of local density enhancements in Europa’s atmosphere, advancing the idea of a possible origination from water plumes. These measurement strategies, however, were sensitive either to total absorption or atomic emissions, which limited the ability to assess the water content. Here we present direct searches for water vapour on Europa spanning dates from February 2016 to May 2017 with the Keck Observatory. Our global survey at infrared wavelengths resulted in non-detections on 16 out of 17 dates, with upper limits below the water abundances inferred from previous estimates. On one date (26 April 2016) we measured 2,095 ± 658 tonnes of water vapour at Europa’s leading hemisphere. We suggest that the outgassing of water vapour on Europa occurs at lower levels than previously estimated, with only rare localized events of stronger activity.

  • 9. Plainaki, C.
    et al.
    Cassidy, T. A.
    Shematovich, V. I.
    Milillo, A.
    Wurz, P.
    Vorburger, A.
    Roth, Lorenz
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Galli, A.
    Rubin, M.
    Blöcker, A.
    Brandt, P. C.
    Crary, F.
    Dandouras, I.
    Jia, X.
    Grassi, D.
    Hartogh, P.
    Lucchetti, A.
    McGrath, M.
    Mangano, V.
    Mura, A.
    Orsini, S.
    Paranicas, C.
    Radioti, A.
    Retherford, K. D.
    Saur, J.
    Teolis, B.
    Towards a Global Unified Model of Europa’s Tenuous Atmosphere2018In: Space Science Reviews, ISSN 0038-6308, E-ISSN 1572-9672, Vol. 214, no 1, article id 40Article, review/survey (Refereed)
    Abstract [en]

    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.

  • 10.
    Retherford, K. D.
    et al.
    Southwest Res Inst, San Antonio, TX 78238 USA.;Univ Texas San Antonio, San Antonio, TX 78249 USA..
    Roth, Lorenz
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Becker, T. M.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Feaga, L. M.
    Univ Maryland, College Pk, MD 20742 USA..
    Tsang, C. C. C.
    Southwest Res Inst, Boulder, CO 80302 USA..
    Jessup, K. L.
    Southwest Res Inst, Boulder, CO 80302 USA..
    Grava, C.
    Southwest Res Inst, San Antonio, TX 78238 USA..
    Io's Atmosphere Silhouetted by Jupiter Ly alpha2019In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 158, no 4, article id 154Article in journal (Refereed)
    Abstract [en]

    We report results from a new technique for mapping Io's SO2 vapor distribution. The Space Telescope Imaging Spectrograph (STIS) instrument on the Hubble Space Telescope observed Io during four Jupiter transit events to obtain medium resolution far-UV spectral images near the Ly alpha wavelength of 121.6 nm. Jupiter's bright Ly alpha dayglow provides a bright, mostly uniform background light source for opacity measurements, much like during a stellar occultation or transiting exoplanet event. Peaks in the photoabsorption cross-sections for sulfur dioxide occur near 122 nm, with resulting absorptions raising the altitude where a tangential line-of-sight opacity of similar to 1 occurs. This method of measuring column densities along lines of sight above the limb uses detailed image simulations and complements Ly alpha reflectance imaging and other methods for measuring Io's SO2 gas on the disk. Our reported near-terminator limb observations with STIS confirm the findings from previous Ly alpha disk reflectance imaging that Io's polar SO2 density is an order of magnitude lower than found at the equator. We provide constraints for additional attenuation by atmospheric hydrogen atoms produced by charge exchange reactions between magnetospheric protons and Io's atmosphere. Searches for plume-related features provided no definitive enhancements within the signal quality, ruling out unusually high levels of activity for Pele and Tvashtar.

  • 11. Rhoden, A. R.
    et al.
    Hurford, T. A.
    Roth, Lorenz
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. Southwest Research Institute, United States .
    Retherford, K.
    Linking Europa's plume activity to tides, tectonics, and liquid water2015In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 253, p. 169-178Article in journal (Refereed)
    Abstract [en]

    Although the surface is geologically young (30-80. Myr), there is little information as to whether tidally-driven surface processes are ongoing. However, a recent detection of water vapor near Europa's south pole suggests that it may be geologically active. Initial observations indicated that Europa's plume eruptions are time-variable and may be linked to its tidal cycle. Saturn's moon, Enceladus, which shares many similar traits with Europa, displays tidally-modulated plume eruptions, which bolstered this interpretation. However, additional observations of Europa at the same time in its orbit failed to yield a plume detection, casting doubt on the tidal control hypothesis. The purpose of this study is to analyze the timing of plume eruptions within the context of Europa's tidal cycle to determine whether such a link exists and examine the inferred similarities and differences between plume activity on Europa and Enceladus. To do this, we determine the locations and orientations of hypothetical tidally-driven fractures that best match the temporal variability of the plumes observed at Europa. Specifically, we identify model faults that are in tension at the time in Europa's orbit when a plume was detected and in compression at times when the plume was not detected. We find that tidal stress driven solely by eccentricity is incompatible with the observations unless additional mechanisms are controlling the eruption timing or restricting the longevity of the plumes. The addition of obliquity tides, and corresponding precession of the spin pole, can generate a number of model faults that are consistent with the pattern of plume detections. The locations and orientations of these hypothetical source fractures are robust across a broad range of precession rates and spin pole directions. Analysis of the stress variations across the fractures suggests that the plumes would be best observed earlier in the orbit (true anomaly ~120°). Our results indicate that Europa's plumes, if confirmed, differ in many respects from the Enceladean plumes and that either active fractures or volatile sources are rare.

  • 12.
    Roth, Lorenz
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Constraints on water vapor and sulfur dioxide at Ceres: Exploiting the sensitivity of the Hubble Space Telescope2018In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 305, p. 149-159Article in journal (Refereed)
    Abstract [en]

    Far-ultraviolet observations of dwarf-planet (1) Ceres were obtained on several occasions in 2015 and 2016 by the Cosmic Origins Spectrograph (COS) and the Space Telescope Imaging Spectrograph (STIS), both on board the Hubble Space Telescope (HST). We report a search for neutral gas emissions at hydrogen, oxygen and sulfur lines around Ceres from a potential teneous exosphere. No detectable exosphere emissions are present in any of the analyzed HST observations. We apply analytical models to relate the derived upper limits for the atomic species to a water exosphere (for H and O) and a sulfur dioxide exosphere (for S and O), respectively. The H and O upper limits constrain the H2O production rate at the surface to (2-4) x 10(26) molecules s(-1) or lower, similar to or slightly larger than previous detections and upper limits. With low fluxes of energetic protons measured in the solar wind prior to the HST observations and the obtained non-detections, an assessment of the recently suggested sputter-generated water exosphere during solar energetic particle events is not possible. Investigating a sulfur dioxide-based exosphere, we find that the O and S upper limits constrain the SO2 density at the surface to values similar to 10(10) times lower than the equilibrium vapor pressure density. This result implies that SO2 is not present on Ceres' sunlit surface, contrary to previous findings in HST ultraviolet reflectance spectra but in agreement with the absence of SO2 infrared spectral features as observed by the Dawn spacecraft.

  • 13.
    Roth, Lorenz
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Ivchenko, Nickolay
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Retherford, K. D.
    Cunningham, N. J.
    Feldman, P. D.
    Saur, J.
    Spencer, J. R.
    Strobel, D. F.
    Constraints on an exosphere at Ceres from Hubble Space Telescope observations2016In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 6, p. 2465-2472Article in journal (Refereed)
    Abstract [en]

    We report far ultraviolet observations of Ceres obtained with the Cosmic Origin Spectrograph (COS) of the Hubble Space Telescope in the search for atomic emissions from an exosphere. The derived brightnesses at the oxygen lines at 1304 Å and 1356 Å are consistent with zero signals within the 1σ propagated statistical uncertainties. The OI 1304 Å brightness of 0.12 ± 0.20 Rayleighs can be explained by solar resonant scattering from an atomic oxygen column density of (8.2 ± 13.4) × 1010 cm-2. Assuming that O is produced by photodissociation of H2O, we derive an upper limit for H2O abundance and compare it to previous observations. Our upper limit is well above the expected O brightness for a tenuous sublimated H2O exosphere, but it suggests that H2O production with a rate higher than 4 × 1026 molecules s-1 was not present at the time of the COS observation. Additionally, we derive an extremely low geometric albedo of ∼1% in the 1300 Å to 1400 Å range.

  • 14.
    Roth, Lorenz
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Retherford, Kurt D.
    Ivchenko, Nickolay
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Schlatter, Nicola
    KTH.
    Strobel, Darrell F.
    Becker, Tracy M.
    Grava, Cesare
    DETECTION OF A HYDROGEN CORONA IN HST Ly alpha IMAGES OF EUROPA IN TRANSIT OF JUPITER2017In: Astronomical Journal, ISSN 0004-6256, E-ISSN 1538-3881, Vol. 153, no 2, article id 67Article in journal (Refereed)
    Abstract [en]

    We report far-ultraviolet observations of Europa in transit of Jupiter obtained with the Space Telescope Imaging Spectrograph of the Hubble Space Telescope on six occasions between 2014 December and 2015 March. Absorption of Jupiter's bright hydrogen Ly alpha dayglow is detected in a region several moon radii above the limb in all observations. The observed extended absorption provides the first detection of an atomic hydrogen corona around Europa. Molecular constituents in Europa's global sputtered atmosphere are shown to be optically thin to Lya. The observations are consistent with a radially escaping H corona with maximum densities at the surface in the range of (1.5-2.2) x 10(3) cm(-3), confirming the abundances predicted by Monte Carlo simulations. In addition, we search for anomalies around the limb of Europa from absorption by localized high H2O abundances from active plumes. No significant local absorption features are detected. We find that an H2O plume with line-of-sight column density in the order of 10(16) cm(-2), as inferred by Roth et al. would not be detectable based on the statistical fluctuations of the transit measurements, and hence is not excluded or further constrained. The presence of plumes with line-of-sight column densities of >2 x 10(17) cm(-2) can be excluded at a 3-sigma level during five of our six observations.

  • 15.
    Roth, Lorenz
    et al.
    KTH, School of Electrical Engineering (EES). Southwest Research Institute, United States .
    Retherford, Kurt D.
    Saur, Joachim
    Strobel, Darrell F.
    Feldman, Paul D.
    McGrath, Melissa A.
    Nimmo, Francis
    Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa's water vapor aurora2014In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 111, no 48, p. E5123-E5132Article in journal (Refereed)
    Abstract [en]

    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.

  • 16.
    Roth, Lorenz
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics. Southwest Research Institute, United States.
    Saur, J.
    Retherford, K. D.
    Strobel, D. F.
    Feldman, P. D.
    McGrath, M. A.
    Spencer, J. R.
    Blöcker, A.
    Ivchenko, Nickolay
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Europa's far ultraviolet oxygen aurora from a comprehensive set of HST observations2016In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 121, no 3, p. 2143-2170Article in journal (Refereed)
    Abstract [en]

    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.

  • 17.
    Roth, Lorenz
    et al.
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Saur, Joachim
    Retherford, Kurt D.
    Bloecker, Aljona
    Strobel, Darrell F.
    Feldman, Paul D.
    Constraints on Io's interior from auroral spot oscillations2017In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 2, p. 1903-1927Article in journal (Refereed)
    Abstract [en]

    The morphology of Io's aurora is dominated by bright spots near the equator that oscillate up and down in approximate correlation with the oscillating orientation of the Jovian magnetospheric field. Analyzing Hubble Space Telescope images, we find that the auroral spots oscillate in phase with the time-variable Jovian magnetic field at Io and that the amplitude of the spot oscillations is reduced by 15% (+/- 5%) with respect to the amplitude of the magnetic field oscillation. We investigate the effects of Io's plasma interaction and magnetic induction in the moon's interior on the magnetic field topology and the aurora oscillations using a magnetohydrodynamic (MHD) simulation and an analytical induction model. The results from the MHD simulation suggest that the plasma interaction has minor effects on the oscillations, while the magnetic induction generally reduces magnetic field oscillations near the surface. However, the analytical model shows that induction in any near-surface layer for which the skin depth is larger than the thickness-like a conductive magma ocean-would induce a phase shift, in conflict with the observations. Under the assumption that the spot oscillations represent the magnetic field oscillation, we constrain the conductance of a near-surface layer to 1 x 10(3) S or lower. A magma ocean with conductances of 10(4) S or higher as derived from Galileo magnetometer measurements would cause overly strong attenuation of the amplitude in addition to the irreconcilable phase shift. The observed weakly attenuated, in-phase spot oscillation is consistent with induction in a deep, highly conductive layer like Io's metallic core.

  • 18. Roth, Lorenz
    et al.
    Saur, Joachim
    Retherford, Kurt D.
    Feldman, Paul D.
    Strobel, Darrell F.
    A phenomenological model of Io’s UV aurora based on HST/STIS observations2014In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 228, p. 386-406Article in journal (Refereed)
    Abstract [en]

    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. 

  • 19. Roth, Lorenz
    et al.
    Saur, Joachim
    Retherford, Kurt D.
    Strobel, Darrell F.
    Feldman, Paul D.
    McGrath, Melissa A.
    Nimmo, Francis
    Transient Water Vapor at Europa’s South Pole2014In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 343, no 6167, p. 171-174Article in journal (Refereed)
  • 20. Roth, Lorenz
    et al.
    Saur, Joachim
    Retherford, Kurt D.
    Strobel, Darrell F.
    Spencer, John R.
    Simulation of Io’s auroral emission: Constraints on the atmosphere in eclipse2011In: Icarus (New York, N.Y. 1962), ISSN 0019-1035, E-ISSN 1090-2643, Vol. 214, no 2, p. 495-509Article in journal (Refereed)
    Abstract [en]

    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.

  • 21. Rousselot, P.
    et al.
    Opitom, C.
    Jehin, E.
    Hutsemékers, D.
    Manfroid, J.
    Villarreal, M. N.
    Li, J. -Y
    Castillo-Rogez, J.
    Russell, C. T.
    Vernazza, P.
    Marsset, M.
    Roth, Lorenz
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Dumas, C.
    Yang, B.
    Prettyman, T. H.
    Mousis, O.
    Search for water outgassing of (1) Ceres near perihelion2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 628, article id A22Article in journal (Refereed)
    Abstract [en]

    Context. (1) Ceres is the largest body in the main asteroid belt and one of the most intriguing objects in the solar system, in part because of the discovery of water outgassing by the Herschel Space Observatory (HSO) and its still-debated origin. Ceres was the target of NASA's Dawn spacecraft for 3.5 yr, which achieved a detailed characterization of the dwarf planet. The possible influence of the local flux of solar energetic particles (SEP) on the production of a Cerean exosphere and water vapor has been suggested, in addition to the sublimation of water ice that depends on the temperature, meaning the heliocentric distance. Aims. We used the opportunity of both the perihelion passage of (1) Ceres in April 2018, and the presence of Dawn in its vicinity (for measuring the SEP flux in real time) to check the influence of heliocentric distance and SEP flux on water outgassing. Methods. We searched for OH emission lines near the limb of Ceres in the near-UV with the UVES spectrograph mounted on the 8-m ESO Very Large Telescope. Two spectra were recorded when Ceres was close to its perihelion, in February 2018, and with Dawn spacecraft orbiting Ceres. It was possible to simultaneously measure energetic particles around Ceres at the time of our observations. Results. Our observations did not permit detection of OH emission lines to a very high sensitivity level. This level is estimated to correspond to a global water production rate of QH2O ∼ 2 × 1026 molecules s-1, similar to the water production rate derived from HSO observations. The solar energetic particles flux measured around Ceres was negligible at the time of these observations. Conclusions. Our observations support the idea that heliocentric distance (i.e., the sublimation of water ice) does not play a major role in the water emission from Ceres. This production rate could be either related to SEP events or to other mechanisms, possibly of endogenic origin.

  • 22. Saur, J.
    et al.
    Duling, S.
    Roth, Lorenz
    KTH, School of Electrical Engineering (EES).
    Jia, X.
    Strobel, D. F.
    Feldman, P. D.
    Christensen, U. R.
    Retherford, K. D.
    McGrath, M. A.
    Musacchio, F.
    Wennmacher, A.
    Neubauer, F. M.
    Simon, S.
    Hartkorn, O.
    The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals2015In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, no 3, p. 1715-1737Article in journal (Refereed)
    Abstract [en]

    We present a new approach to search for a subsurface ocean within Ganymede through observations and modeling of the dynamics of its auroral ovals. The locations of the auroral ovals oscillate due to Jupiter's time-varying magnetospheric field seen in the rest frame of Ganymede. If an electrically conductive ocean is present, the external time-varying magnetic field is reduced due to induction within the ocean and the oscillation amplitude of the ovals decreases. Hubble Space Telescope (HST) observations show that the locations of the ovals oscillate on average by 2.0 ±1.3. Our model calculations predict a significantly stronger oscillation by 5.8 ± 1.3 without ocean compared to 2.2±1.3 if an ocean is present. Because the ocean and the no-ocean hypotheses cannot be separated by simple visual inspection of individual HST images, we apply a statistical analysis including a Monte Carlo test to also address the uncertainty caused by the patchiness of observed emissions. The observations require a minimum electrical conductivity of 0.09 S/m for an ocean assumed to be located between 150 km and 250 km depth or alternatively a maximum depth of the top of the ocean at 330 km. Our analysis implies that Ganymede's dynamo possesses an outstandingly low quadrupole-to-dipole moment ratio. The new technique applied here is suited to probe the interior of other planetary bodies by monitoring their auroral response to time-varying magnetic fields. Key Points New technique to search for a subsurface ocean in Ganymede with a telescope Ocean affects auroral oscillation caused by time-varying external magnetic field HST observations reveal weak auroral oscillation and imply existence of ocean

  • 23. Saur, Joachim
    et al.
    Feldman, Paul D.
    Roth, Lorenz
    Nimmo, Francis
    Strobel, Darrell F.
    Retherford, Kurt D.
    McGrath, Melissa A.
    Schilling, Nico
    Gerard, Jean-Claude
    Grodent, Denis
    HUBBLE SPACE TELESCOPE/ADVANCED CAMERA FOR SURVEYS OBSERVATIONS OF EUROPA’S ATMOSPHERIC ULTRAVIOLET EMISSION AT EASTERN ELONGATION2011In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 738, no 2Article in journal (Refereed)
    Abstract [en]

    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.

  • 24.
    Saur, Joachim
    et al.
    Univ Cologne, Inst Geophys & Meteorol, Albertus Magnus Pl, D-50923 Cologne, Germany..
    Fischer, Christian
    Univ Cologne, Inst Geophys & Meteorol, Albertus Magnus Pl, D-50923 Cologne, Germany..
    Wennmacher, Alexandre
    Univ Cologne, Inst Geophys & Meteorol, Albertus Magnus Pl, D-50923 Cologne, Germany..
    Feldman, Paul D.
    Johns Hopkins Univ, Dept Phys & Astron, Baltimore, MD 21218 USA..
    Roth, Lorenz
    KTH, School of Electrical Engineering and Computer Science (EECS), Space and Plasma Physics.
    Strobel, Darrell F.
    Johns Hopkins Univ, Dept Phys & Astron, Baltimore, MD 21218 USA.;Johns Hopkins Univ, Dept Earth & Planetary Sci, Baltimore, MD 21218 USA..
    Reiners, Ansgar
    Georg August Univ, Inst Astrophys, Friedrich Hund Pl 1, D-37077 Gottingen, Germany..
    The UV Spectrum of the Ultracool Dwarf LSR J1835+3259 Observed with the Hubble Space Telescope2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 859, no 1Article in journal (Refereed)
    Abstract [en]

    An interesting question about ultracool dwarfs recently raised in the literature is whether their emission is purely internally driven or partially powered by external processes similar to planetary aurora known from the solar system. In this work, we present Hubble Space Telescope observations of the energy fluxes of the M8.5 ultracool dwarf LSR J1835+3259 throughout the ultraviolet (UV). The obtained spectra reveal that the object is generally UV-fainter compared with other earlier-type dwarfs. We detect the Mg II doublet at 2800 angstrom and constrain an average flux throughout the near-UV. In the far-UV without Ly alpha, the ultracool dwarf is extremely faint with an energy output at least a factor of 250 smaller as expected from auroral emission physically similar to that on Jupiter. We also detect the red wing of the Lya emission. Our overall finding is that the observed UV spectrum of LSR J1835 vertical bar 3259 resembles the spectrum of mid/late-type M-dwarf stars relatively well, but it is distinct from a spectrum expected from Jupiter-like auroral processes.

1 - 24 of 24
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