Cassini UVIS Io Plasma Torus Observations

The Tail Wagging the Dog: Hot Electrons as the Driver of Azimuthal Variations in

the Io Plasma TorusA.J. Steffl (SwRI), P.A. Delamere

(CU/LASP), F. Bagenal (CU/LASP)MOP 2007, San Antonio, TX

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Cassini UVIS Io Plasma Torus Observations

• 45-day observing period (1 October 2000 to 14 November 2000) • 1.904x106 s of integration time• Individual integrations of 1000s• Duty cycle of roughly 53%• Fit spectra from ansae, extract composition

Temporal Changes in Torus Composition

Sinusoidal Variations of the Torus Dusk Ansa

Phase of Azimuthal Variations with Time

Slope yields a 10.07-hour period; System III period is 9.925 hours

Lomb-Scargle Periodogram

Periodogram peak at 10.07 hours

Note about System IV

Brown (1995) Thomas et al. (2001)

System IV is not caused by the plasma rotation speed

• Start with the torus chemistry model of Delamere and Bagenal [2003]

• Model includes:– Electron impact ionization e.g. S + e- → S+ +

2e-

– Charge exchange e.g. O + O+ → O++ O*

– Recombination e.g. S++++ e- → S++ + ν– Radiative cooling e.g. S++ + e- → S++ + e- + ν– Coulomb collisions e.g. O++ S++ → O++ S++

• Mass & Energy Sources:– Extended neutral clouds– Pickup energy– Hot electron population (“hot” = 50 eV)

• Loss Processes:– Fast Neutrals– Outward Radial Transport– Radiation (UV photons)

Modeling Torus Chemistry 1

• 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 timescale

fh is, by far, the most sensitive model parameter

Modeling Torus Chemistry 2

Azimuthal Model• Extend the basic model by

including 24 azimuthal bins

• Azimuthal transport of plasma at speed v (3 km/s)

• Neutrals centered about the rotational equator

• Plasma centered about centrifugal equator– 6.6º offset from rotational

equator

– All species are Gaussians about equator with scale height set by temperature

– Ion-Neutral reactions most greatest at

Azimuthal Variation in Hot Electrons

•A small change in fh rapidly produces a large change in torus composition

•Add a sinusoidal azimuthal variation in hot electrons

€

fh (t,λ III ) = 1+α h,λ IVcos(λ III −φh,λ IV

− ΔΩt)( )

Model Phase vs. Time

Sinusoidal Variations of the Torus Dusk Ansa

Amplitude of Azimuthal Variations with Time29 Days = 1/fSystem III -

1/f“System IV”

Results for System IV-only Hot Electron Model

Dual Period Hot Electron Model

• Add a second azimuthal variation in hot electrons– Fixed in System III

IV = 50%

III = 40%

III = 290º

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fh (t,λ III ) = 1+α h,λ IVcos(λ III −φh,λ IV

− ΔΩt)( )

× 1+α h,λ IIIcos(λ III −φh,λ III

)( )

When aligned System IV and System III variations produce large azimuthal variation in composition

When anti-aligned, the torus is nearly azimuthally uniform.

Conclusions• Cassini UVIS observed significant azimuthal variations in torus composition

• Azimuthal variations have a period of 10.07 hours– Slower than System III; faster than traditional System IV

• Amplitude of azimuthal variations exhibits a 29-day cycle

• A primary (50% variation) azimuthal variation in hot electrons rotates

• A secondary (40% variation) variation in hot electrons remains fixed in System III – Peak at III=290°