Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede

Barth, C. A.; Hord, C. W.; Stewart, A. I. F.; Pryor, W. R.; Simmons, K. E.; McClintock, W. E.; Ajello, J. M.; Naviaux, K. L.; Aiello, John

doi:10.1029/97gl01927

Cited by 82 publications

(63 citation statements)

References 24 publications

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“…Global Ly-α emissions can potentially originate from sunlight scattered off atomic hydrogen in the atmosphere, which is likely also the source of the extended Ly-α emissions detected at Ganymede (27,45). Ganymede's global Ly-α emissions have peak brightnesses above 500 R and are thus considerably brighter than the global Ly-α brightnesses we find for Europa.…”

Section: G-jmentioning

confidence: 99%

Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora

Roth

Retherford

Saur

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

113

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Significance Images of Europa’s UV aurora taken by the Hubble Space Telescope in December 2012 have revealed local hydrogen and oxygen emissions in intensity ratios that identify the source as electron impact excitation of water molecules. The existence of water vapor plumes as a source for the detected localized water vapor and the possible accessibility of subsurface liquid water reservoirs at these locations have important implications for the exploration of Europa’s potentially habitable environments. The observations reported here tested whether orbital position near the apocenter is an essential requirement for plume activity. Only an upper limit on the amount of water vapor was obtained. Orbital position is therefore not a sufficient condition for detecting plumes and they may be episodic events.

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Section: G-jmentioning

confidence: 99%

Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora

Roth

Retherford

Saur

et al. 2014

Proc. Natl. Acad. Sci. U.S.A.

113

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“…We have inferred densities of 3 and 90 AMU cm −3 for G8 and G28 respectively from which we calculated an ionospheric scale height of 465 km, which is much smaller that the estimate of 1000 km by Gurnett et al (1996) but closer to the estimate of 600 km by Eviatar et al (2001a). Similarly, the plasma temperature obtained through the magnetospheric model by Eviatar et al (2001a) is 360 K, whereas Barth et al (1997) found a temperature of 450 K. The reason for this discrepancy of our values and previous estimates can be found in the location of Ganymede with respect to the Jovian current sheet. The calculations we performed did not take this into account.…”

Section: Discussionmentioning

confidence: 99%

“…The observed density ratio is n(1.5)/n(2) ≈ 27 for which β ≈ 20 or T i ≈ 360 K. The temperature inferred for the exosphere is of the order of 0.03 eV, a temperature that falls well outside the specifications of the PLS instrument (Frank et al, 1992). Barth et al (1997) arrive at a slightly higher exospheric temperature of 450 K, using Galileo ultraviolet observations. The pickup ions discussed above are at higher energies than the thermal plasma on the closed field lines.…”

Section: Plsmentioning

confidence: 99%

“…Gary, 1991;Huddleston and Johnstone, 1992;Volwerk et al, 2001;Cowee et al, 2007;Delva et al, 2008a, for application in different space environments). Basically, part of the energy in the ring distribution of the pick-up ions can be released into cyclotron wave energy if the energy of the ring well exceeds the thermal energy of the background plasma, which is indeed the case with an exospheric ion temperature of ∼450 K (Barth et al, 1997) and a pick-up velocity of 50 to 100 km s −1 . The free energy in the ring (E fr ) is given by…”

Section: Ion Cyclotron Wavesmentioning

confidence: 99%

“…Similarly, plasma wave observations have shown typical magnetospheric emissions (Gurnett et al, 1996). Other aspects of Ganymede's magnetosphere include auroral emissions (Barth et al, 1997;Hall et al, 1998;Eviatar et al, 2001a) and the presence of polar caps (Khurana et al, 2007), high-latitude regions from which field lines emerge but do not return to Ganymede's surface.…”

Section: Introductionmentioning

confidence: 99%

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ULF waves in Ganymede's upstream magnetosphere

et al. 2013

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Abstract. Ganymede's mini-magnetosphere, embedded in Jupiter's larger one, sustains ULF (ultra-low frequency) waves that are analyzed here using data from two Galileo flybys that penetrate deeply into the upstream closed field line region. The magnetometer data are used to identify field line resonances, magnetopause waves and ion cyclotron waves. The plasma densities that are inferred from the interpretation of these waves are compared with the observations made by other plasma and wave experiments on Galileo and with numerical and theoretical models of Ganymede's magnetosphere.

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Detection of a hydrogen corona at Callisto

et al. 2017

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In December 2001, the Space Telescope Imaging Spectrograph of the Hubble Space Telescope obtained far‐ultraviolet spectral images of Jupiter's moon Callisto. The leading and trailing hemispheres were observed in the spectral range 1190 Å to 1720 Å when the moon was at eastern and western elongations, respectively. We analyzed the hydrogen Lyman α (1216 Å) signal in the two observations and found that faint atmospheric emissions extending up to several moon radii away are present in addition to the solar flux reflected off the surface. We show that the detected atmospheric Lyman α emissions are consistent with an escaping hydrogen corona with a vertical column density in the range of (6–12) ×1011 cm−2. The derived hydrogen abundance is about 2 times higher when the moon is at eastern elongation, possibly related to increased water sublimation when the visibly darker leading hemisphere is illuminated by the Sun. The detected hemispheric difference is larger than the measurement uncertainties providing first evidence for variations in Callisto's neutral atmosphere.

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Galileo ultraviolet spectrometer observations of atomic hydrogen in the atmosphere of Ganymede

Cited by 82 publications

References 24 publications

Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora

Orbital apocenter is not a sufficient condition for HST/STIS detection of Europa’s water vapor aurora

ULF waves in Ganymede's upstream magnetosphere

Detection of a hydrogen corona at Callisto

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