Io plasma torus electrons: Voyager 1

Sittler, E. C.; Strobel, D. F.

doi:10.1029/ja092ia06p05741

Cited by 162 publications

(153 citation statements)

References 67 publications

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“…If the plasma electrons are substantially cooled near 10, the ionization rate will greatly decrease. This scenario is supported by the apparent low electron temperature observed at 10 by the IUE spacecraft (Ballester et al, 1987) and by the dip in electron temperature near lo's flux tube observed by the Voyager spacecraft (Sittler and Strobel, 1987).…”

Section: Sputtered Coronasupporting

confidence: 63%

Sodium in Io's Extended Atmosphere

Schneider¹

1988

PASP

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This dissertation combines several new observations of the Io sodium cloud to create a consistent picture of the extended Io atmosphere and its interaction with the Jovian plasma torus. I used the LPL echelle spectrograph to obtain three types of high-resolution spectra of the extended sodium cloud at the sodium D lines (5890 Â, 5896 Â). The first class of observations made use of the mutual satellite eclipses of 1985 to probe the density profile of the atmosphere in the range 1.4 to 10 Io radii, a previously unstudied region. The second type of observation examined the sodium emission in the immediate vicinity of Io, allowing an accurate measurement of the velocity structure around Io. The final method employed a high-sensitivity detector to study faint jets of high-speed sodium farther out in the extended cloud. The synthesis of these three data sets results in a better understanding of how sodium is distributed about Io as a function of position and velocity.The extended atmosphere of Io is composed of many kinematically distinct components. The distribution in space is linked to their characteristic velocities, with low-energy sodium confined near Io and faster atoms (10 to 100 km sec -1 ) prevalent beyond ~ 25 Io radii. The sodium density falls rapidly near Io and more slowly outside 5.6 Io radii, the effective limit of the gravity of Io.The data indicate that the atmosphere is collisionally thick near the surface but becomes thin by an altitude of ~ 700 km. The upper limit of the exobase location is derived from reliable sodium density measurements made during the satellite eclipses. The lower limit is indirectly inferred from the velocity distribution of sodium near Io and the nature of high-speed jets far from Io.The high-speed jets reveal a new type of close interaction between the corotating plasma and the atmosphere of Io. The morphology and brightness of the jets require a two-reaction process in which atmospheric sodium is ionized, accelerated to high speeds, and then charge-exchanges with other sodium atoms. These processes must occur near the atmospheric exobase, indicating that the atmosphere of Io is not completely protected from the plasma flow.

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Section: Sputtered Coronasupporting

confidence: 63%

Sodium in Io's Extended Atmosphere

Schneider¹

1988

PASP

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“…However, in-situ measurements of the electron distribution in the Io plasma torus made by the Voyager and Galileo spacecrafts suggest that the electron distribution function in the Io torus may actually be nonthermal or at least have a non-thermal, high-energy tail (Sittler andStrobel 1987, Frank andPaterson 2000). Rather than examining the changes in the torus spectrum due to an arbitrary, non-thermal electron distribution, we have focused our efforts on modeling the effects of a kappa electron distribution function (Vasyliunas 1968).…”

Section: Torus Spectral Emissions Modelmentioning

confidence: 99%

Cassini UVIS observations of the Io plasma torus.II. Radial variations

Steffl

Bagenal

Stewart

2004

Icarus

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ABSTRACT.On January 14, 2001, shortly after the Cassini spacecraft's closest approach to Jupiter, the Ultraviolet Imaging Spectrometer (UVIS) made a radial scan through the midnight sector of Io plasma torus. The Io torus has not been previously observed at this local time. The UVIS data consist of 2-D spectrally dispersed images of the Io plasma torus in the wavelength range of 561Å-1912Å. We developed a spectral emissions model that incorporates the latest atomic physics data contained in the CHIANTI database in order to derive the composition of the torus plasma as a function of radial distance. Electron temperatures derived from the UVIS torus spectra are generally less than those observed during the Voyager era. We find the torus ion composition derived from the UVIS spectra to be significantly different from the composition during the Voyager era. Notably, the torus contains substantially less oxygen, with a total oxygen-to-sulfur ion ratio of 0.9. The average ion charge state has increased to 1.7. We detect S V in the Io torus at the 3σ level. S V has a mixing ratio of 0.5%. The spectral emission model used in can approximate the effects of a non-thermal distribution of electrons. The ion composition derived using a kappa distribution of electrons is identical to that derived using a Maxwellian electron distribution; however, the kappa distribution model requires a higher electron column density to match the observed brightness of the spectra. The derived value of the kappa parameter decreases with radial distance and is consistent with the value of κ=2.4 at 8 R J derived by the Ulysses URAP instrument (Meyer-Vernet et al., 1995). The observed radial profile of electron column density is consistent with a flux tube content, NL 2 , that is proportional to r -2 .

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“…The standard electron parameters used are based on Voyager measurements (42). Galileo observations revealed time-variable differing plasma sheet conditions (e.g., ref.…”

Section: Aurora Modelmentioning

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.

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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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Io plasma torus electrons: Voyager 1

Cited by 162 publications

References 67 publications

Sodium in Io's Extended Atmosphere

Sodium in Io's Extended Atmosphere

Cassini UVIS observations of the Io plasma torus.II. Radial variations

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

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