Abstract

1
SM13B-0337 Modeling the Temporal and Azimuthal
Variability of the Io Plasma Torus Observed by
Cassini UVIS A.J. Steffl (SwRI), P.A.Delamere
(CU/LASP), F. Bagenal (CU/LASP) E-mail
steffl_at_boulder.swri.edu
Abstract Observations by the Cassini Ultraviolet
Imaging Spectrograph (UVIS) showed remarkable
temporal and azimuthal variability in the
composition of the Io plasma torus. Among the
observed variations are a factor of 2 decrease
in the amount of S II (and corresponding increase
in the amount of S IV) present in the torus over
a 45-day period and a nearly sinusoidal azimuthal
variation in composition with a period some 1.5
longer than the System III rotation period of
Jupiter. Unexpectedly, the amplitude of this
azimuthal variation in composition is modulated
by its phase relative to System III longitude.
Here, we present results from our efforts to
model the observed compositional variability of
the Io torus using a neutral cloud theory model
with additional heating supplied by a small
fraction (0.2) of hot (50 eV) electrons. The
long-term variation in composition is found to be
caused by a factor of 3 increase in the amount
of neutral material supplied to the torus,
perhaps resulting from increased volcanic
activity on Io. Superimposed on this temporal
variation, the observed azimuthal variability and
its modulation with System III longitude can be
reproduced by introducing two azimuthally-varying
sources of hot electrons a primary source that
slips 12.2/day relative to System III and a
secondary source that remains fixed in System III
longitude.
UVIS DATA
Fig 1B
Fig 1A
Fig 1C
Figure 1A shows the UVIS observing geometry for
November 12, 2000. The Cassini spacecraft was
oriented so that the long axis of the UVIS slit
was parallel with Jupiters equator plane. Figure
1B shows images of the UVIS EUV channel at four
different sub-spacecraft System III longitudes.
Jovian north is to the left and the dusk ansa is
up. The emissions gt850Å at the center of the
images are from the Jovian aurora. Spectra are
extracted from each ansa of the torus. Figure 1C
shows a spectrum from the dusk ansa of the Io
plasma torus, obtained on October 2, 2000. Major
spectral features have been labeled by the ion
primarily responsible for the emission. The EUV
torus spectrum is composed almost entirely of
emissions from ionized sulfur and oxygen.
Model Fit to Temporal Variations
UVIS-Observed Azimuthal Variability
Fig 3A
Fig 3C
Fig 3B
Fig 2B
Fig 2C
Fig 2A
Figure 3A--Relative ion mixing ratios, electron
temperature, and electron column density for a
typical 3-day period, obtained from UVIS
observations of the dusk ansa. Values have been
normalized to the average value over the 3-day
period. A quantities show a nearly sinusoidal
variation with time. Note the strong
anti-correlation between S II and S IV. The
period of variation is within a few percent of
Jupiters System III rotation period (9.975
hours), implying that UVIS is observing sections
of a spatially-varying torus (i.e. azimuthal
variation) rotating in and out of the field of
view. A simple sinusoidal function (with the
System III rotation period) has been fit to each
quantity. Figure 3B--The phase (i.e. the
location of the peak) of the sinusoidal function
is shown versus time. For visual clarity, the top
half of the figure is a copy of the bottom half.
All four ion species show a roughly linear
increase in phase with time. The slope of the
increase implies that the pattern of azimuthal
variation in plasma composition rotates with a
period of 10.07 hours, 1.5 slower than the
System III rotation period of Jupiter. Figure
3C--The amplitude of the sinusoidal function
versus time. The amplitude of S II and S IV
varies between 4-25, while the amplitude of the
major ion species, O II and S III, remains small
and relatively constant. The peaks in the S II
and S IV emissions are separated by 29 days--the
synodic period between a 10.07-hour period and
the System III rotation period of Jupiter.
The model fit to the azimuthally-averaged mixing
ratios derived from UVIS observations on
2000-10-1 through 2000-11-14 and 2001-01-14 is
shown in Figure 2A. The model includes a 2.3x
increase in the neutral source rate, centered on
DOY 249, and a Gaussian increase in hot
electrons, centered on DOY 279. The model curves
closely match the UVIS data for the three sulfur
ion species. The model curve for oxygen deviates
somewhat from the UVIS data, suggesting the O/S
ratio was not constant (i.e. greater fraction of
S during the neutral source increase) . Figure 2B
shows the temperature, scale height, number
density, and column density (integrated over a
flux tube) for the 5 ion species included in the
model. Figure 2C shows these quantities for the
thermal electron population.

• Conclusions
• The long baseline of observations by Cassini UVIS
  provides a (so far) unique opportunity to study
  temporal and spatial variability in the Io plasma
  torus.
• The Io torus exhibited significant temporal and
  azimuthal variations in composition during the
  UVIS observations.
• The medium-term (months) observed change in
  plasma composition is well modeled by assuming a
  factor of 2.3 increase in the amount of neutral
  atoms supplied to the torus around DOY 249,
  followed by a gradual decay to normal levels
• The Io torus exhibits significant azimuthal
  variation in plasma composition.
• The pattern of azimuthal variation rotates 1.5
  slower than the System III rotation period.
• The amplitude of azimuthal variation is modulated
  by its position relative to System III longitude.
  
• The azimuthal behavior of the torus can be
  modeled by assuming that the amount of hot
  electrons in the torus is a function of both
  System III longitude and a coordinate system that
  slips 12.2/day relative to System III.
• Unanswered Questions
• Why does the torus exhibit periodicity at 10.07
  hours and not at the previously-observed System
  IV period of 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?
• Observations by the New Horizons spacecraft may
  answer some of these this questions when it flies
  past Jupiter in Feb. 2007.
• What mechanism(s) produces the two separate
  modulations of hot electrons?
• Its not too difficult to produce a pattern of
  hot electrons that is fixed in System III, but
  its not obvious how to produce a pattern of hot
  electrons that slips relative to both System III
  and the underlying torus plasma.

Fig. 4B
Table 4
Table 5
Fig. 4A
Fig. 5A
We model the azimuthal behavior of the Io torus
by modulating the hot electron fraction with both
System III longitude (?III) and the longitude in
a 10.07-hour coordinate system (?IV). The model
fits to the phase versus time (Fig. 3B) and
amplitude versus time (Fig. 3C) for the ion
species S II and S IV are shown in Figure 4A. The
model parameters used are given in Table 4. Given
all the simplifications made, the model does a
remarkable job of matching the observed behavior
of the Io plasma torus. Figure 4B shows the
UVIS-derived (plot symbols) and model mixing
ratios (solid line) for two two-day periods. When
the two hot electron modulations are in phase
(DOY 279), they constructively interfere and the
azimuthal variation in composition is greatest.
15 days later, when the two modulations are 180
out of phase, the torus plasma composition is
most azimuthally-uniform. Although the model does
a good job fitting S II and S IV, it is somewhat
poorer at fitting the behavior of O II and S III.
This could be a result of errors in the reaction
rates used by the model.
The model in panel 4 assumed that the torus
plasma was rigidly corotating with the System III
coordinate system. However, several observations
(Brown 1994 Thomas et al. 2001) have shown that
the torus plasma actually deviates from
corotation by about ?v3 km/s. If this effect is
included, the model can still match the general
behavior of S II and S IV, but less well than in
the case of rigid corotation. The model
parameters used are given in Table 5. Since the
plasma is slipping relative to both System III
and the 10.07-hour period, the amplitudes of the
hot electron modulation must be significantly
greater.