1. Temporal and Azimuthal Variability in the Io Plasma Torus
Andrew Steffl
Peter Delamere
Fran Bagenal
August 9, 2005

2. The Cassini UVIS Io torus dataset

3. UVIS observing geometry November 12, 2000

4. UVIS observations November 12, 2000

5. Temporal Variations in the Io Plasma Torus

6. Torus Ion Composition vs. Time

7. Tvashtar erupted between Feb. 2000 (I24) and Dec. 2000 (G28)
Prometheus
Prometheus
C21 July 1999 (“Natural” Color)
G28 December 2000 (Enhanced Color)
Images courtesy NASA/JPL-Caltech

8. 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

9. Modeling torus chemistry
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

10. 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

11. Comparison of Observed and Model Composition

12. Azimuthal Variations and Torus Periodicities

13. Long-term variations with System III longitude are small…

14. But, significant azimuthal variations are present in the UVIS data

15. Short-term Azimuthal Variability

16. UVIS data show anti-correlation of S II and S IV

17. Phase vs. Time

19. Lomb-Scargle Periodogram Peaks at 10.07 hours

21. Azimuthal Variability (amplitude)

22. Amplitude vs. Time #1 #2 #3

23. Sinusoidal fit to derived mixing ratio
Data #1 #2 #3

24. 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 hour period
Secondary hot electron source remains fixed in System III longitude

26. 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

27. Unanswered Questions
Why does the torus exhibit periodicity at hours?
What happened to the System IV periodicity at hours?
Perhaps the observed hour periodicity is the same phenomenon as System IV, just at a different period.
Is the hour period related to the neutral source event that preceded the Cassini flyby?
Does the torus currently exhibit a 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.