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.

SM13B-0337: Modeling the Temporal and Azimuthal Variability of the Io Plasma Torus Observed by Cassini UVISA.J. Steffl* (SwRI), P.A.Delamere (CU/LASP), F. Bagenal (CU/LASP)

*E-mail: steffl@boulder.swri.edu

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?

– It’s not too difficult to produce a pattern of hot electrons that is fixed in System III, but it’s not obvious how to produce a pattern 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.

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 Jupiter’s 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 >850Å 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.

UVIS DATA

Sn,0 2.0 x 1028 s-1

O/S 1.7fh 0.23%Te,hot 55 eV 70 days

n 2.3

t0,n DOY 249

n 30 days

h 0.08%

t0,h DOY 279

h 60 days

hIII 0.025

h,III 20˚hIV 0.25

h,IV 300˚

12.2˚/day

v 0 km/s

Sn,0 2.0 x 1028 s-1

O/S 1.4fh 0.23%Te,hot 55 eV 70 days

n 2.3

t0,n DOY 249

n 30 days

h 0.08%

t0,h DOY 279

h 60 days

hIII 0.25

h,III 280˚hIV 0.40

h,IV 150˚

12.2˚/day

v 3 km/s

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.

Model Fit to Temporal Variations UVIS-Observed Azimuthal Variability

Fig 1AFig 1C

Fig 1B

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Modeling Torus ChemistryWe start with the torus chemistry model of Delamere and Bagenal [2003]. This model calculates the mass and energy flux of neutrals and ions in a 1 cm3 box. It includes the effects of electron impact ionization, recombination, charge exchange, radiative cooling, and Coulomb collisions between the various ion and electron populations. Mass is supplied via a neutral source (atoms of O and S are supplied at rate, SN, in a ratio of O/S), while it is lost when ions become neutralized (fast neutral) or via radial transport of plasma (which is proportional to 1/ SN). Energy is supplied via the “pickup energy” from freshly ionized neutrals and via a small fraction (fh, ~0.2%) of the electron population that is heated to suprathermal temperatures (Th). Energy is lost primarily through radiative cooling, but also through fast neutrals and radial transport. See poster SM13B-0336 by Bagenal and Delamere for more information.

To successfully model the observed temporal variation of the torus composition requires a time-variable neutral source. We model this as a Gaussian increase in SN, with amplitude, αN, and width, σN. A corresponding Gaussian increase in fh is also required.

The azimuthal variations are modeled by extending the model to include 24 boxes, equally spaced in System III longitude. Plasma flows between the longitudinal boxes with a speed of v. The hot electron fraction, fh, varies sinusoidally with System III longitude. The local fh is additionally modulated by its longitude in a coordinate system that drifts º/day, relative to System III longitude. Thus:

€

fh (t,λ III ) = fh,0 + α he−( t−t0 )2 /σ h

2

( ) × 1+ α h,λ IIIcos(λ III − φh,λ III

)( )

× 1+ α h,λ IVcos(λ III − φh,λ IV

− ΔΩt)( )

€

Sn (t) = Sn,0 1+ α ne−( t−t0,n )2 /σ n

2

( )

Fig 2A

Fig 2B Fig 2C

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 Jupiter’s 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.

Fig 3A

Fig 3B Fig 3C

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.

Fig. 4ATable 4

Fig. 4B

Fig. 5ATable 5

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 v=3 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.