1. 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, USA
3 University of Bern, Physikalisches Institut, Bern, Switzerland
4Applied Physics Laboratory, Johns Hopkins University, Laurel, USA
5Institute of Space and Astronautical Science,Sagamihara, Japan

2. 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

3. 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
The sensor which provides relevant for this discusison measurements JNA

4. ENA imaging concept A+ A0
surface
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+ B0
surface (B)

5. ENA imaging of the Moon. Chandrayaan-1 results
ENA relative flux
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

6. 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
Talking on ENA diagnistic relevant to the lander

7. H+
S3+ S+

8. Sputtered ENAs (1) Sputtered yields are very high (> 10) for
heavy projectiles on water ices
(Baragiola et al., 2003)
BUT

9. 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

10. 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
ENA sensor
Backscattering ENA
Ice block
Ion beam

11. 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

12. H+ beams Sputtering Sputtering Backscattering
No corrections for energy - dependent sensitivity

13. O+ beams Sputtered Recolled H
No corrections for energy - dependent sensitivity

14. 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)

15. ENA imaging of Ganymede (1)
4 populations

16. Plasma precipitation regions
Jia et al., 2011, MHD - model

17. ENA imaging of Ganymede (2)

18. 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