Temporal and Azimuthal Variability in the Io Plasma Torus
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Temporal and Azimuthal Variability in the Io Plasma Torus
Andrew StefflPeter DelamereFran Bagenal
August 9, 2005
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The Cassini UVIS Io torus dataset
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UVIS observing geometry November 12, 2000
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UVIS observations November 12, 2000
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Temporal Variations in the Io Plasma Torus
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Torus Ion Composition vs. Time
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Tvashtar erupted between Feb. 2000 (I24) and Dec. 2000 (G28)
Tvashtar
PrometheusPrometheus
G28 December 2000 (Enhanced Color)C21 July 1999 (“Natural” Color)
Images courtesy NASA/JPL-Caltech
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Galileo Dust Detector data from Krüger et al. 2003
Cassini UVIS data collected from October 1st onwards
Model neutral source—Gaussian peaking 25 days before October 1 x3.5 increase
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• Extend the torus chemistry model of Delamere et al. [2004]• Model includes:
– Electron impact ionization e.g. S + e- → S+ + 2e-
– Recombination e.g. – Charge exchange e.g. or– Radiative cooling e.g. S++ + e- → S++ + e- + ν– Coulomb collisions between the ion and electron populations
• Energy from pickup ions alone can’t produce the observed torus composition– Need an additional energy source– Add small population (~0.23% of total Ne) of hot electrons (~50 eV)
• Five basic model parameters:– Neutral source rate SN
– O/S neutrals ratio O/S– Fraction of hot electrons fh
– Temperature of hot electrons Th
– Radial transport rate τ
Modeling torus chemistry
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Time Variable Torus Chemistry Model
• Allow neutral source (SN) to vary with time– Assume source increase has a Gaussian profile
• Fit for the amplitude (αN) and width (σN) of the Gaussian
• Center Gaussian increase (t0,N) on 5 Sept 2000 based on Galileo Dust Detector profile
– Transport rate proportional to 1/SN
• Model profiles can not match UVIS results unless hot electron fraction (fh) also varies with time– Assume hot electron increase is also Gaussian
– Fit for the amplitude (αh), width (σh), and center of the Gaussian (t0,h)
• O/S ratio and hot electron temperature (Th) are held constant
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Comparison of Observed and Model Composition
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Azimuthal Variations and
Torus Periodicities
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Long-term variations with System III longitude are small…
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But, significant azimuthal variations are present in the UVIS data
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Short-term Azimuthal Variability
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UVIS data show anti-correlation of S II and S IV
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Phase vs. Time
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Lomb-Scargle Periodogram Peaks at 10.07 hours
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Azimuthal Variability (amplitude)
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#1
#2
#3
Amplitude vs. Time
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#1 #2 #3
Sinusoidal fit to derived mixing ratio Data
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Modeling Azimuthal Variability
• Extend the torus chemistry model of Delamere et al. (2004) by including– 24 azimuthal bins
– Azimuthal transport of plasma• Plasma rotation speed is independent of hot electrons
• Can be fixed in System III or allowed to vary (3 km/s)
– Latitudinal averaging i.e. plasma on the centrifugal equator is offset from neutrals on the rotational equator
• To get observed modulation, 2 periods are required– Add 2 azimuthally varying hot electron sources:
• Primary hot electron (~55 eV) source rotates with 10.07 hour period
• Secondary hot electron source remains fixed in System III longitude
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Conclusions• A major volcanic event occurred on Io in September 2000
– Resulted in ~3.3x increase in amount of neutrals supplied to the torus– UVIS observed the torus returning to more “typical” conditions
• The Io torus exhibits significant azimuthal variations in ion composition – Long-term variations with System III longitude only ~5%– Variations of up to 25% seen on timescales of a few days– The Io torus always shows azimuthal variation in composition– Azimuthal variations lag System III rotation period by 1.4%– Amplitude of azimuthal compositional variation appears modulated by
the pattern’s location in System III longitude• Torus chemistry models can match observed torus behavior by
including:– 3.3x increase in neutral source around September 2000– Azimuthally varying source of hot electrons that rotates 1.5% slower– Azimuthally varying source of hot electrons fixed in System III
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Unanswered Questions• Why does the torus exhibit periodicity at 10.07 hours?
– What happened to the System IV periodicity at 10.21 hours? • Perhaps the observed 10.07 hour periodicity is the same phenomenon as System IV,
just at a different period.
– Is the 10.07 hour period related to the neutral source event that preceded the Cassini flyby?
– Does the torus currently exhibit a 10.07 hour periodicity? System IV? Something else?
• What mechanism(s) produces the System III-fixed and subcorotating sources of hot electrons?– Perhaps not too difficult to produce hot electrons that are fixed in System III– It’s not obvious how to produce a source of hot electrons that slips relative to
both System III and the underlying torus plasma.
• Is there some other way to reproduce the UVIS observations without two azimuthally varying sources of hot electrons?– Azimuthally-varying plasma rotation speed can’t do it.– Azimuthally-varying radial transport timescale can’t do it.