Modeling the Temporal and Azimuthal Variability of
the Io Plasma Torus Observed by Cassini UVIS
Andrew J. Steffl*, Peter A. Delamere, Fran Bagenal
University of Colorado/ LASP
We present results of our efforts to model the temporal and azimuthal
variability of the Io plasma torus during the Cassini encounter with Jupiter. We
extend the torus neutral cloud theory model of Delamere et al. (2004) to
include azimuthal variations. The temporal variation in torus composition
observed by Cassini UVIS can be modeled by supposing a factor of ~3
increase in the amount of material supplied to the extended neutral clouds
around Io on 5 September 2000. The 10.07-hour periodicity in the UVIS data
and the observed azimuthal variability and modulation can be reproduced by
models that assume two independent and azimuthally varying sources hot
electrons— one that remains fixed in System III longitude and a second source
that slips 12.2 degrees/day relative to System III.
*now
at Southwest Research Institute, Boulder, CO
UVIS observations November 12, 2000
Torus Spectral Image and Composition vs. time
• Spectral images to the left show the torus as seen by Cassini
UVIS at 4 different magnetic (System III) longitudes
– The UVIS slit is parallel to the Jovian equator
– Jovian north is to the left and the dusk side is up
• Minimize line-of-sight effects by analyzing only the dawn and
dusk ansa
• Use spectral model described in Steffl et al. [2004b] to derive
torus ion composition, electron temperature and column
density
• Torus ion mixing ratios (Nion/Ne) for a 45 day period during
approach shown at right
• Torus composition changes dramatically during this period
– S+ falls off while S3+ increases
– O+ and S++ remain relatively constant
– Modeled by 3x increase in neutral source [Delamere et al. 2004]
• High frequency (near Jovian rotation frequency) variations in
torus composition seen throughout observing period
Torus Ion Composition vs. Time
Modeling torus chemistry
• Start with the torus chemistry model of Delamere and Bagenal [2003]
• Model includes:
–
–
–
–
–
Electron impact ionization
e.g. S + e- → S+ + 2eRecombination
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
O/S neutrals ratio
Fraction of hot electrons
Temperature of hot electrons
Radial transport timescale
SN
O/S
fh
Th
τ
Time Variable Torus Chemistry Model
• Allow neutral source (SN) to vary with time
– Assume source increase has a Gaussian profile

Sn (t)  Sn,0 1  n e(tt0,n )
2

/ n2
• Fit for the amplitude (αN) and width (σN) of the Gaussian
• Center Gaussian increase (t0,N) on 5 Sept 2000 (DOY 249) based on Galileo Dust
Detector profile

– Transport rate proportional to 1/SN

• Models with   Ntorus can not match UVIS observations
• Model profiles can not match UVIS observations unless hot electron
fraction (fh) also varies with time
tt
f
(t)

f


e
h
h,0
h
– Assume
hot electron increase is also Gaussian

2
0,h
/ h2
– 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
 O mixing ratio could be due to
– Offset between UVIS data and model predicted
a variable O/S ratio in the neutral source
– Hot electron temperature is a relatively insensitive model parameter
Comparison of Observed and Model Composition
Short-term Azimuthal Variability
Phase vs. Time
Amplitude vs. Time
#2
#1
#3
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 (v) is an independent parameter
• Latitudinal averaging i.e. plasma on the centrifugal equator is offset from
neutrals on the rotational equator
• The observed modulation requires two periods
– Add 2 azimuthally-varying (sinusoidal variation) 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

f h (t, III )  f h,0   h e
(tt 0 )2 / h2
 1 
h,  III
cos( III   h, III )
1  h,IV cos( III   h,IV  t)
Conclusions
• Cubic centimeter models of torus chemistry have been quite successful at
reproducing conditions in the Io torus during the Cassini UVIS
observations.
• A major volcanic event occurred on Io on or around September 2000
– Resulted in ~3.3x increase in amount of neutrals supplied to the torus
– ~25% increase in hot electrons supplied to the torus
– UVIS observed the torus returning to more “typical” conditions
• The Io torus exhibits significant azimuthal variations in ion composition
–
–
–
–
Variations of up to 25% (amplitude) seen on timescales of a few days
The Io torus always shows azimuthal variation in composition
The azimuthal variation in composition lags System III rotation period by 1.4%
The amplitude of azimuthal compositional variation appears to be modulated
by the pattern’s location in System III longitude
• The azimuthal and temporal variations seen by UVIS are reprodeuced by
models that include:
– A primary, azimuthally-varying source of hot electrons rotating 1.4% slower
than the System III rotation period.
– A secondary, azimuthally-varying source of hot electrons that remains fixed in
System III longitude.
Sn,0
O/S
fh
Te,hot

2.0 x 1028 s-1
1.7
0.23%
55 eV
70 days
n
2.3
t0,n
DOY 249
n
30 days
h
0.08%
t0,h
DOY 279
h
60 days
h
III
0.025
h,III
h IV
20˚
h,IV
300˚

12.2˚/day
v
0 km/s
0.25
Azimuthal Model Examples
• Both models include the above temporal variability
• 17 adjustable model parameters
–
–
–
–
5 for basic model (Sn,0, O/S, fh, Te,hot, 
6 for time-variability (t0,n, n, n, t0,h, h, h)
5 for azimuthal variation (h III, h,III, h IV, h,III, and 
1 for deviation of plasma from rigid corotation (v)
• Model on the left assumes torus plasma is fixed in System III
– The primary subcorotating pattern of hot electrons has an amplitude of 25%
– The secondary pattern of hot electrons remains source
• Model on the right assumes torus plasma subcorotates by 3 km/s
– Thomas et al (2001) and Brown (1994) report ~3 km/s subcorotation of plasma
– Adding plasma subcorotation requires an increase in the amplitude of the
azimuthal variation in hot electrons
• Subcorotating source has an amplitude of 40%
• Corotating source has an amplitude of 25%
Sn,0
O/S
fh
Te,hot

2.0 x 1028 s-1
1.4
0.23%
55 eV
70 days
n
2.3
t0,n
DOY 249
n
30 days
h
0.08%
t0,h
DOY 279
h
60 days
h
III
0.25
h,III
h IV
280˚
h,IV
150˚

12.2˚/day
v
3 km/s
0.40
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.