Cassini UVIS system
scans: the distribution
of atomic hydrogen
and atomic oxygen
Cassini PSG meeting 30/01/08
Henrik Melin, Don E. Shemansky
Planetary and Space Science Division
Space Environment Technologies
Pasadena, CA
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Cassini UVIS
pre SOI
system scans:
OI 1304 Å
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Atomic Oxygen 1304Å: Group 1
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Atomic Oxygen 1304Å: Group 2
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Atomic Oxygen 1304Å: Group 3
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Atomic Oxygen 1304Å: Group 4
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Atomic Oxygen 1304Å: Group 5
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Oxygen 1304Å: East-West cut across center
Rendered at 1x1 Rs resolution
Text
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Cassini UVIS
pre SOI
system scans:
Lyman-alpha maps
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Atomic Hydrogen Lyman-alpha: Group 1
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Atomic Hydrogen Lyman-alpha: Group 2
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Atomic Hydrogen Lyman-alpha: Group 3
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Atomic Hydrogen Lyman-alpha: Group 4
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Atomic Hydrogen Lyman-alpha: Group 5
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Atomic Hydrogen Ly-alpha profiles:
East - West cut across center
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Atomic Hydrogen Ly-alpha profiles: North - South
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Post-SOI Hydrogen
System Scans
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Atomic Hydrogen Lyman-alpha Post SOI 2004
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Atomic Hydrogen Lyman-alpha Post SOI 2005
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Source of magnetospheric hydrogen
The hot-tail population of cold photoelectrons colissionally dissociate molecular
hydrogen to produce hot atomic hydrogen on
sunlit side of the planet, some of which have
enough kinetic energy to escape the
atmosphere.
Shemansky & Hall 1992
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A simple model
• 2D - Equatorial plane of Saturn
• Gaussian velocity distribution
(fwhm = 20 km/s)
• Random velocity vector
• Preferential production in afternoon
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Equatorial plane of Saturn
SUN
Cassini
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Comparing the model (solid black line) to data
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Summary: oxygen
• Distribution is wide (10 Rs), inconsistent
with a narrow Enceladus torus.
• Distribution is patchy - consistent with a
stochastic source.
• The peak is not always at 4 Rs - additional
sources in the inner magnetosphere?
• Variability on the order of weeks, cannot be
driven by chemistry (slow), must be dynamic
redistribution.
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The source of hydrogen
• Very large distribution, extending beyond
40Rs.
• Distribution consistent with the sunlit side
of Saturn as the dominant source.
• Cool population of electrons - tail end (high
E) cause electron impact dissociation of H2
to produce hot hydrogen.
• This provides a heating source for the upper
atmosphere.
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