OH in Saturn's magnetosphere: Observations and implications

Richardson, J. D.; Eviatar, A.; McGrath, M. A.; Vasyliūnas, V. M.

doi:10.1029/98je01127

Cited by 89 publications

(150 citation statements)

References 29 publications

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“…At Saturn, the magnetospheric plasma source, originating from icy moons and ring grains, is believed to be on a lesser scale than at Jupiter by more than an order of magnitude (e.g. Richardson, 1992;Richardson et al, 1998). However, as shown directly by Voyager observations, it is still sufficient to cause significant sub-corotation of the equatorial magnetospheric plasma at radial distances beyond ∼5 R S (Richardson 1986(Richardson , 1995Richardson and Sittler, 1990), such that the same coupling current system is required to flow.…”

Section: Introductionmentioning

confidence: 97%

“…The physics of this region is dominated by plasma production and pick-up from neutral gas sources originating from ring grains and the icy surfaces of moons, followed by centrifugally-driven outward radial transport associated with small-scale motions not illustrated in the figure (e.g. Johnson et al, 1989;Pospieszalska and Johnson, 1991;Richardson, 1992;Richardson et al, 1998). No detailed self-consistent dynamical models of these processes have yet been proposed for Saturn as they have for Jupiter (e.g.…”

Section: Equatorial Flowsmentioning

confidence: 99%

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Saturn's polar ionospheric flows and their relation to the main auroral oval

2004

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Abstract. We consider the flows and currents in Saturn's polar ionosphere which are implied by a three-component picture of large-scale magnetospheric flow driven both by planetary rotation and the solar wind interaction. With increasing radial distance in the equatorial plane, these components consist of a region dominated by planetary rotation where planetary plasma sub-corotates on closed field lines, a surrounding region where planetary plasma is lost down the dusk tail by the stretching out of closed field lines followed by plasmoid formation and pinch-off, as first described for Jupiter by Vasyliunas, and an outer region driven by the interaction with the solar wind, specifically by reconnection at the dayside magnetopause and in the dawn tail, first discussed for Earth by Dungey. The sub-corotating flow on closed field lines in the dayside magnetosphere is constrained by Voyager plasma observations, showing that the plasma angular velocity falls to around half of rigid corotation in the outer magnetosphere, possibly increasing somewhat near the dayside magnetopause, while here we provide theoretical arguments which indicate that the flow should drop to considerably smaller values on open field lines in the polar cap. The implied ionospheric current system requires a four-ring pattern of field-aligned currents, with distributed downward currents on open field lines in the polar cap, a narrow ring of upward current near the boundary of open and closed field lines, and regions of distributed downward and upward current on closed field lines at lower latitudes associated with the transfer of angular momentum from the planetary atmosphere to the sub-corotating planetary magnetospheric plasma. Recent work has shown that the upward current associated with sub-corotation is not sufficiently intense to produce significant auroral acceleration and emission. Here we suggest that the observed auroral oval at Saturn instead corresponds to the ring of upward current bounding the region of open and closed field lines. Estimates indicate that auroras of brightness from a few kR to a few tens of kR can be produced byCorrespondence to: S. W. H. Cowley (swhc1@ion.le.ac.uk) precipitating accelerated magnetospheric electrons of a few keV to a few tens of keV energy, if the current flows in a region which is sufficiently narrow, of the order of or less than ∼1000 km (∼1 • latitude) wide. Arguments are also given which indicate that the auroras should typically be significantly brighter on the dawn side of the oval than at dusk, by roughly an order of magnitude, and should be displaced somewhat towards dawn by the down-tail outflow at dusk associated with the Vasyliunas cycle. Model estimates are found to be in good agreement with data derived from high quality images newly obtained using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, both in regard to physical parameters, as well as local time effects. The implication of this picture is that the form, position, and brightness of Saturn's main auroral o...

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Section: Introductionmentioning

confidence: 97%

Section: Equatorial Flowsmentioning

confidence: 99%

Saturn's polar ionospheric flows and their relation to the main auroral oval

2004

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“…By combining all these observations, coverage of the torus is fairly complete. The OH density contours are from the model of Richardson et al [1998], who show that the three sets of observations taken together imply peak OH densities of about 700 cm -3 near 4.5 Rs and 30 cm -3 near the rings.…”

Section: Neutralsmentioning

confidence: 99%

“…A model has been developed which is a composite of the 0-D and 2-D models described earlier [Richardson et al, 1998]. The magnetosphere is divided into a grid with grid size 0.…”

Section: Equations Of the Formmentioning

confidence: 99%

Thermal plasma and neutral gas in Saturn's magnetosphere

Richardson¹

1998

Reviews of Geophysics

121

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Abstract. Saturn's magnetosphere contains plasma and neutral particles from Saturn's atmosphere, the rings, the inner icy satellites, and Titan. This paper reviews the observations of plasma and neutrals near Saturn. Plasma conditions were observed during the Pioneer 11 and Voyager 1 and 2 flybys of Saturn. Neutral H was observed by the Voyagers; neutral OH has been observed by the Hubble Space Telescope. The attempts which have been made to understand the physical processes behind the data are also discussed. Saturn's magnetosphere is dominated by neutrals, with a neutral to plasma density ratio of about 10. The neutrals are probably produced by bombardment of the moons by micrometeorites and heavy ions, although there is a discrepancy between predicted sputtering rates and the amount of sputtering necessary to produce the observed neutral densities. Transport of plasma out of the magnetosphere must be very fast, of the order of a few days. Models of the processes occurring in Saturn's magnetosphere are used to determine the densities of neutral and plasma species which cannot be directly observed. What happens to this plasma? The largest portion recombines back into neutral atoms and goes flying out of the magnetosphere. A significant fraction is transported to the boundary of the magnetosphere and escapes down the magnetotail into the solar wind. Of the rest, some smashes into the rings and moons, knocking off neutrals which then form more plasma, and some enters Titan's and Saturn's atmospheres, providing an energy source to drive atmospheric processes and auroral displays.Most of our knowledge of Saturn's magnetosphere comes from the brief flyby encounters of Pioneer 11 (1979) and Voyager 1 (1980) and 2 (1981). Recent observation• using the Hubble space telescope (HST) also give valuable clues as to the neutral density structure. This paper describes the observations from Saturn's magnetosphere, the current understanding of the

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“…The OH observations with HST are sparse, and building sufficient statistics requires summing data from several observation periods. The initial models of OH and its parent, H 2 O, identified a radial and vertical (normal to the equatorial plane) structure to the neutrals but assumed azimuthal symmetry [Richardson et al, 1998;Jurac et al, 2001;Johnson et al, 2006].…”

Section: Introductionmentioning

confidence: 99%

Cassini INMS observations of neutral molecules in Saturn's E‐ring

et al. 2010

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[1] In 2008, the Cassini ion neutral mass spectrometer (INMS) investigation made in situ measurements of neutral species near Saturn's equatorial plane within 0.5 Saturn radii (R S ) of the orbit of Enceladus. After removing the large background and modeling to interpret instrumental effects, the data provide rough constraints on the neutral distribution and composition. These data show an azimuthal asymmetry in the neutral densities and provide measurements used to compare to simulations of neutral H 2 O emitted from Enceladus. Far from Enceladus, the neutral water densities, at a few times 10 3 molecules/cm 3 , are near the detection limit of INMS. Near Enceladus, but outside of the plumes and north of the equatorial plane, the INMS detects particles within 5000 km of Enceladus, with the density increasing to approximately 10 5 molecules/cm 3 at the equatorial plane. The observations also show CO 2 in the form of its dissociated product, CO. On the basis of the spatial distribution of CO 2 counts, the scale height of the neutral cloud above and below the equatorial plane is less than 7000 km. Far from Enceladus, the concentration of CO 2 with respect to H 2 O increases, a consequence of the predicted decline in H 2 O density. Relatively high counts at 2 amu are infrequently observed. These measurements indicate H 2 released during ice-grain impacts and provide a constraint on the frequency and distribution of small ice grains.

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OH in Saturn's magnetosphere: Observations and implications

Cited by 89 publications

References 29 publications

Saturn's polar ionospheric flows and their relation to the main auroral oval

Saturn's polar ionospheric flows and their relation to the main auroral oval

Thermal plasma and neutral gas in Saturn's magnetosphere

Cassini INMS observations of neutral molecules in Saturn's E‐ring

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