The Post-Cassini View of the Io Plasma Torus

Andrew Steffl, Adam Shinn (Southwest Research Institute – Boulder)

With thanks to Fran Bagenal, Peter Delamere, Nick Schneider and the UVIS team

MOP 2011, Boston

• Introduction• The Io Plasma Torus in 2 Minutes• The Role of Hot electrons• Longitudinal Variations

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations1. Io’s Volcanoes

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations2. Atmospheric

Losses

IPT in 2 minutes

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations3. Torus emitting in the FUV

Bagenal & Delamere (2010)

Mass & Energy Flow Through the Torus

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations

UVIS EUV Spectrum of the Dusk Ansa

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations

Sinusoidal Variations of the Torus Dusk Ansa

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations

Phase of Azimuthal Variations with Time

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

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations

Lomb-Scargle Periodogram

Periodogram peak at 10.07 hours

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations

Note about System IV

Brown (1995)Thomas et al. (2001)

System IV is not caused by the local plasma rotation speed

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations

Sinusoidal Variations of the Torus Dusk Ansa

Introduction

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Hot ElectronsLongitudinal Variations

29 Days = 1/fSystem III - 1/f“System IV”

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations

Dual Hot Electron Model• Two variations of hot

electrons– One drifting relative to

System III– One fixed in System III

• IV = 50%• III = 40%• III = 290º

€

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

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

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations

Conclusions

Frank & Patterson 2000

IPT in 2 minutes

Hot ElectronsLongitudinal Variations

Introduction

Hess et al., 2011 (submitted)

Mechanism for producing a System III-fixed variation in hot electrons

Conclusions• Most of the neutral material picked up into the Io

plasma torus comes from the extended neutral clouds

• Hot electrons are required to supply the Io plasma torus with energy

• Cassini UVIS observed significant longitudinal variations in the Io torus composition– The variations drift at the System IV period– The amplitude varies on the beat period of System IV

and System III• Models with 2 longitudinal variations of

hot electrons can qualitatively match the torus behavior– Hess et al. 2011 propose a mechanism for

producing hot electrons with a peak at 290º System III

– What produces the System IV component?

Introduction

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Hot ElectronsLongitudinal Variations

€

A = 0.5cos ω III t( ) + 0.5cos ω IV t( )

€

A = 1+0.5cos ω III t( )( ) × 0.5cos ω IV t( )( )

€

A = 1+0.5cos ω III t( )( ) × 1+0.5cos ω IV t( )( )

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations

System III

System IV – System III

System IV

System IV

System IV

System IIISystem

IV – System III

System IV + System III

System IV + System III

Introduction

IPT in 2 minutes

Hot ElectronsLongitudinal Variations