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

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

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

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)

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

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

H+ S3+ S+

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

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

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

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

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

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

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)

ENA imaging of Ganymede (1) 4 populations

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

ENA imaging of Ganymede (2)

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