UVIS Io Torus Paper Summary
• 2 Papers Submitted to Icarus and in review process:
• Cassini UVIS Observations of the Io Plasma Torus: I. Initial
Results by Steffl, Stewart, and Bagenal
• Final revisions complete and resubmitted
• Cassini UVIS Observations of the Io Plasma Torus: II.
Radial Variations by Steffl, Bagenal, and Stewart
• Undergoing final revisions
• Paper submitted to JGR:
• Modeling variability of plasma conditions in the Io torus
during the Cassini era by Delamere, Steffl, and Bagenal
• More papers on the way. Possible subjects include:
• Compositional changes vs. Time
• Periodicities in the torus
• Latitudinal/Longitudinal structure
EUV Io Torus Spectrum Oct 1, 2000
Io Torus Temporal
Luminosity Variations
Io Torus Compositional
Variations
EUV Io Torus Spectrum Oct 1, 2000
Modeling the EUV Spectra
Data Analysis Conclusions
• 1,904 spectral images of the Io torus obtained from October 1,
2000 – November 14, 2000
• Torus EUV emitted power fell by 25% over 45 days
• Both torus and aurora can brighten suddenly, then fade back to
“normal”
• PDusk/PDawn is variable, ranging from 0.74-2.29 with mean of 1.32
and standard deviation of 0.25
• Dawn and dusk ansa have same plasma composition
• Dusk ansa is ~15% hotter than dawn ansa
• Spectral model closely fits data; yields ion mixing ratios and
electron temperature of torus
• Mixing ratio of S II steadily decreases while mixing ratio of S IV
steadily increases
• No change in electron temperature over observing period
Data Analysis Conclusions
• Significant longitudinal variations in plasma composition and
temperature exist in the torus
• Phase of longitudinal variations is not constant
• Long-term longitudinal variations of ansa power and electron
temperature are minor (5%)
• It is dangerous to characterize the torus based on observations
made over only a few days or a narrow range of longitudes
• Chemistry model successfully predicts timescales for torus events
• Increasing hot electrons in the short term (few hours), increases
emissions, but does not change composition
• Long term changes in hot electrons (few days) causes
compositional change in ionization state of torus plasma i.e. S II
→S IV
Torus Chemistry Modeling
Torus Chemistry Model
• Described in Delamere and Bagenal [2003]
• Five model parameters: ttransport rate; neutral source rate, S;
O/Sneutral ratio; Fhot and Thot
• Model includes: ionization, charge exchange, recombination,
radiative cooling, ion-electron coupling and electron-electron
coupling
• Initial model conditions: Te hot= 40 eV; Fe hot= 0.3%; Source=
~0.5 ton/s O/S = ~1.7; t = 40days
• Increasing fraction of hot electrons to 0.5% causes change in
S II and S IV densities in ~4 days, similar to that observed
in plot to the left
• Te increases slightly on timescale of a few hours
Figure 2. Model results for October
plasma conditions showing mixing
ratios for the 5 major ion
species and the total EUV luminosity
as functions of transport time and
neutral source rate. The best is shown
by the square for feh = 0.23%, Teh =
80 eV, and O/S = 1.7. The shaded
regions indicate uncertainties in the
Cassini UVIS analysis.
Figure 3. Model results for January
plasma conditions showing mixing
ratios for the 5 major ion
species and the total EUV luminosity as
functions of transport time and neutral
source rate. The best is shown by the
square for feh = 0.25%, Teh = 46 eV,
and O/S = 1.9. The shaded regions
indicate uncertainties in the Cassini
UVIS analysis.