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Energetic Neutral Atom Imaging of the Ganymede - Jovian Magnetosphere Interaction

S. Barabash1, Y. Futaana1, M. Wieser1, X. Jia2 , P. Wurz3, P. Brandt4, K. Asamura5 1Swedish Institute of Space Physics, Kiruna, Sweden

2Dept. of Atmospheric, Oceanic, and Space Science, U. of Michigan, Ann Arbor, USA3 University of Bern, Physikalisches Institut, Bern, Switzerland

4Applied Physics Laboratory, Johns Hopkins University, Laurel, USA5Institute of Space and Astronautical Science,Sagamihara, Japan

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Energetic neutral atom imaging of Ganymede will be performed:Particle Environment Package (PEP), a particle sensor suite including an

ENA imager, was selected to fly onboard JUICE

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PEP configuration (baseline)

• 6 sensors selected (Core configuration)

• 3 units• Combines first-ever at Jupiter

energetic neutral atom imaging with in-situ 3D plasma measurements

• Performs first-ever high resolution gas mass spectroscopy at icy moons to identify surface constituents

• JNA: Jovian Neutrals Analyzer

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ENA imaging concept

• Energetic Neutral Atoms (ENAs) propagate unaffected by electromagnetic forces

• For E >> Eesc (0.64 eV, oxygen) the gravitation bending of trajectories is negligible

• ENA can be used for remote diagnostic of the region of the origin

• Relevant sources:• Backscattering of precipitating ions• Sputtering of the Ganymede surface

materials• ENA flux

A+ A0

surface

A+ B0

surface (B)

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jena ~ Fion (Eb ) surface f (θ) g(E,Eb )

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ENA imaging of the Moon. Chandrayaan-1 results

• Chandrayaan-1/SARA experiment showed that imaging backscattered ENAs is a new and powerful diagnostic tool to investigate plasma - surface interactions

• High backscattered hydrogen flux (20% vs. < 1% expected)

• Mini-magnetospheres screening-off a fraction of the solar wind is well visible on ENA images

Magnetic anomaly

ENA relative flux

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ENA production at Ganymede

• Focus on plasma - surface interaction: charge - exchange ENAs are only relevant for global imaging

• Plasma at the surface• Co-rotating plasma precipitating

along open field lines• Energetic particles from the rad. belts

• Two main mechanisms to produce ENAs• Backscattering (H, O, S) • Sputtering (mostly H and O)

• Imaging of the precipitating plasma via ENAs• Geometry of the precipitation regions:

open/closed field line region• Dynamics of the precipitation• Correlation with albedo features

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H+

S3+

S+

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Sputtered ENAs (1)

Sputtered yields are very high (> 10) for

heavy projectiles on water ices

(Baragiola et al., 2003)

BUT

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1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

1.E+01

1 10 100 1000 10000

Energy, eV

Norm. spectrum

30 keV H -> O, Eb=3.4 eV

30 keV H-> O, Eb= 1 meV

Sputtered ENAs (2)

• Spectrum of the sputtered ENA is: Thomson - Siegmund (pure metal) but with very low very binding energy (Shematovich, private communication)

• Estimations of the fluxes are difficult

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f ion ≈102cm−2s−1sr−1ev−1 at 10 keV

ΔE ≈ 3 keV

Yield ≈10 for heavy

Fneutrals ≈ Yeild f ion ΔE

2π ≈ 3⋅106cm−2s−1sr−1

Fneutralsmeasurable (E >10eV ) ≈ P Fneutrals

P < ≈10−2

Fneutralsmeasurable (E >10eV ) <≈ 3⋅104 cm−2s−1sr−1

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Laboratory experiment on particle irradiation of ice (1)

• To understand the sputtering and backscattering processes dedicated ice experiments on particle irradiation of H2O ices were conducted using a PEP / JNA prototype at the ion facilities at U. of Bern

Ice block

ENA sensor

Ion beam

Backscattering ENA

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Laboratory experiment on particle irradiation of ice (2)

• Typical conditions:• “Dirty” water ice T= 150 K• Pressure 10-5 mbar

• ø5mm beams H+ and O+• Energy range from 1 keV to 43 keV• Masses analyzed: H, O, and “heavy”

neutrals • ENA spectra in 9 energy bands centered at

from 10 eV to 2560 eV

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H+ beams

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Backscattering

Sputtering

Sputtering

No corrections for energy - dependent sensitivity

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O+ beams

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No corrections for energy - dependent sensitivity

Recolled H

Sputtered

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Summary of the results

• Backscattered H-ENA spectrum (1 keV H+) is Maxwellian (T=290 eV). Similar to the regolith but T is 2-3 times higher.

• Sputtered H-ENA spectrum (33 keV O+) is different from both Maxwellian and Thompson-Siegmuind. Cut-off at 2-3 keV much lowers than predicted 8 keV.

• The backscattered coefficient 27% (similar to 19% observed from the Moon)

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ENA imaging of Ganymede (1)

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Plasma precipitation regions

Jia et al., 2011, MHD - model

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ENA imaging of Ganymede (2)

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Relevance of the ENA imaging from an orbiter for lander

• ENA imaging from an orbiter (JUICE):• Provide remote diagnostic of the plasma (ion) environment at the

lander surface• Temporal variations and correlations with the surface properties

variations measured locally at the lander• Define energy and particle inputs (fluences) to the local areas around

the lander