Gabriella Stenberg Wieser

During the first Earth gravity assist maneuver of JUICE on 20 August 2024 the spacecraft passed through the Earth plasmasphere for about 2 h. Before closest approach at a distance of 2.1 R _E the Jovian Electron and Ion spectrometer JEI of the PEP instrument suite was switched on for 40 min in an ion mode test configuration. The high plasma density of about 3000/cm ³ (observed by the RPWI plasma wave instrument) led to a negative charging of the spacecraft which allowed a rare observation of the cold and dense plasmaspheric ion populations. Since the ions are only corotating with the Earth at a velocity of about 1 km s ⁻¹ at this distance the observed ion speed is dominated by the spacecraft velocity of about 8 km s ⁻¹ . For this reason ions with different mass appear at different energies in the energy spectrum observed by the JEI sensor. In addition the spacecraft potential leads to specific filtering of ion masses in the observed angular distribution. By calculating the sensor response function for these specific observing conditions it is possible to quantify densities of the different ion species. This response calculation depends critically on the response of the JEI channel electron multipliers to the ion velocity and mass. Since the sensor was operated with a low post-acceleration further laboratory calibrations may be needed for this specific setup. Nevertheless we can already conclude from the observations that ions with mass > 4 amu q ⁻¹ contribute at least 25 % to the observed total ion density. A flux peak observed in the energy spectrum at 15 eV can only be explained by the presence of heavy molecular ions with mass ∼ 30 amu q ⁻¹ . Molecular ions have only been rarely detected in the outer Earth plasmasphere. The observations indicate that the JEI sensor can also be used to achieve ion composition measurements in the exospheres of the Jovian moons.

The Juice flyby of Earth in August 2024 gave us the first chance to evaluate the performance of the Jovian Plasma Dynamics and Composition analyzer (JDC) in environments similar to those expected at Jupiter. JDC is one of the sensors belonging to the Particle Environment Package (PEP) on the Juice spacecraft. It measures positive and negative ions as well as electrons in the energy range 1 eV q ⁻¹ to 35 keV q ⁻¹ . One of the most challenging observations at the final destination is the one of the low-energy ion populations in the tenuous ionospheres of Jupiter's icy moons. During the Juice flyby of Earth we discovered that the energies of the positive ions observed by JDC were not easy to interpret due to a problem with the energy sweep. Using measurements made on ground, we were able to reconstruct the observed energies and construct a new sweeping scheme that solves the problem and that will greatly improve future observations. We also used a simulation to explain the effects of the spacecraft velocity and spacecraft potential on the recorded positive ion fluxes when Juice passed through the Earth's plasmasphere. The study highlights the importance of in-flight calibrations for optimizing the scientific return. Planetary flybys give access to multiple low-energy particle populations besides the mono-energetic and highly directional solar wind.

Solar wind protons precipitating onto the lunar regolith result in backscattering and sputtering of surface atoms. Emitted particles are a source for the lunar exosphere. The interaction between precipitating solar wind and the surface does not conserve the charge state, thus positive [1] and negative [2] ions as well as energetic neutral atoms are emitted[3]. We combine data from recent measurements on the lunar surface by the NILS instrument[4] on Chang’e-6 and the ASAN instrument[5] on Chang’e-4, with data measured from orbit by Artemis[6] and Chandrayaan-1 [7] to investigate the full charge state distribution of emitted hydrogen. We discuss the fate of the different observed particle populations and their effect on in the near lunar environment.[1] Saito, Y., et al., (2008). Solar wind proton reflection at the lunar surface: Low energy ion measurement by MAP-PACE onboard SELENE (KAGUYA). Geophys. Res. Lett., doi:10.1029/2008GL036077.[2] Wieser, M., et. al, (2025). Direct observations of negative ions on the Lunar surface by Chang’E-6. Communications Earth & Environment, doi: 10.1038/s43247-025-02399-7.[3] Wieser, M., et al.,(2009). Extremely high reflection of solar wind protons as neutral hydro- gen atoms from regolith in space. Planetary and Space Science. doi: 10.1016/j.pss.2009.09.012.[4] R. Canu-Blot, et al. (2025), The Negative Ions at the Lunar Surface (NILS) Instrument on the Chang’E-6 Mission. Space Science Reviews, doi:10.1007/s11214-025-01162-w[5] M. Wieser, et al. (2020), The Advanced Small Analyzer for Neutrals (ASAN) on the Chang’E-4 Rover Yutu-2. Space Science Reviews, doi:10.1007/s11214-020-00691-w[6] C. Lue, et al.,(2018), Artemis observations of solar wind proton scattering off the lunar surface. Journal of Geophysical Research: Space Physics, doi:10.1029/2018JA025486[7] Barabash, S., et al., (2009). Investigation of the solar wind–Moon interaction onboard Chandrayaan-1 mission with the SARA experiment. Current Science, 96(4):526–532.

The Juice flyby of Earth in August 2024 gave us the first chance to evaluate the performance of the Jovian Plasma Dynamics and Composition analyzer (JDC) in environments similar to those expected at Jupiter. JDC is one of the sensors belonging to the Particle Environment Package (PEP) on the Juice spacecraft. It measures positive and negative ions as well as electrons in the energy range 1 eV/q to 35 keV/q. One of the most challenging observations at the final destination is those of the low energy ion populations in the tenuous ionospheres of Jupiter's icy moons. During the Juice flyby of Earth we discovered that the energies of the positive ions observed by JDC were not easy to interpret due to a problem with the energy sweep. Using measurements made on ground, we were able to reconstruct the observed energies and construct a new sweeping scheme that solves the problem and that will greatly improve future observations. We also used a simulation to explain the effects of the spacecraft velocity and spacecraft potential on the recorded positive ion fluxes when Juice passed through the Earth's plasmasphere. The study highlights the importance of in-flight calibrations for optimizing the scientific return. Planetary flybys give access to multiple low-energy particle populations besides the mono-energetic and highly directional solar wind.

Context . Airless planetary bodies are directly exposed to solar wind ions, which can scatter or become implanted upon impact with the regolith-covered surface, while also sputtering surface atoms. Aims . We constructed a semi-analytical model for the scattering of ions of hundreds of electron volts (eV) and the sputtering of surface atoms, both resulting in the emission of negative ions from the lunar surface. Our model contains a novel description of the scattering process that is physics-based and constrained by observations. Methods . We used data from the Negative Ions at the Lunar Surface (NILS) instrument on the Chang’e-6 lander to update prior knowledge of ion scattering and sputtering from lunar regolith through Bayesian inference. Results . Our model shows a good agreement with the NILS data. We find that a precipitating solar wind proton has a roughly 22 _−6.1 ^+4.9 % chance of scattering from the lunar surface in any charge state and 8.1 _−3.9 ^+7.9 % chance of sputtering a surface hydrogen atom. The resulting ratio of scattered to sputtered hydrogen flux is η ^sc /η ^sp = 1.5 _−1.1 ^+1.5 for a proton speed of 300 km/s. We found a high probability (7-20%) that a hydrogen atom leaves the surface negatively charged. The angular emission distributions at near-grazing angles for both scattered and sputtered fluxes are controlled by surface roughness. Our model also indicates significant inelastic energy losses for hydrogen interacting with the regolith, suggesting a longer effective path length than previously assumed. Finally, we estimated a surface binding energy of 5.5 eV, consistent with the observations. Conclusions . Our model describes the scattering and sputtering of particles of any charge state from any homogeneous, multi-species surface. Using NILS data, we successfully applied the model to update our understanding of solar wind interacting with lunar regolith and the emission of negative hydrogen ions.

During the first Earth gravity assist maneuver of JUICE on 20th August 2024 the spacecraft passed through the Earth plasmasphere for about 2 hours. Before closest approach at a distance of 2.1 R_E the Jovian Electron and Ion spectrometer JEI of the PEP instrument suite was switched on for 40 min in an ion mode test configuration. The high plasma density of about 3000/cm³ (observed by the RPWI plasma wave instrument) led to a negative charging of the spacecraft which allowed a rare observation of the cold and dense plasmaspheric ion populations. Since the ions are only corotating with the Earth at a velocity of about 1 km/s at this distance the observed ion speed is dominated by the spacecraft velocity of about 8 km/s. For this reason ions with different mass appear at different energies in the energy spectrum observed by the JEI sensor. In addition the spacecraft potential leads to specific filtering of ion masses in the observed angular distribution. By calculating the sensor response function for these specific observing conditions it is possible to quantify densities of the different ion species. But this response calculation depends critically on the response of the JEI channel electron multipliers to the ion velocity and mass. Since the sensor was operated with a low post-acceleration further laboratory calibrations may be needed for this specific setup. Still we can already conclude from the observations that ions with mass >4 amu/q contribute at least 30 % to the observed total ion density. A flux peak observed in the energy spectrum at 15 eV can only be explained by the presence of heavy molecular ions with mass ~30 amu/q. Molecular ions have only been rarely detected in the outer Earth plasmasphere. The observations indicate that the JEI sensor can also be used to achieve ion composition measurements in the exospheres of the Jovian moons.

The Juice flyby of Earth in August 2024 gave us the first chance to evaluate the performance of the Jovian Plasma Dynamics and Composition analyzer (JDC) in environments similar to those expected at Jupiter. JDC is one of the sensors belonging to the Particle Environment Package (PEP) on the Juice spacecraft. It measures positive and negative ions as well as electrons in the energy range 1 eV/q to 35 keV/q. One of the most challenging observations at the final destination is those of the low energy ion populations in the tenuous ionospheres of Jupiter's icy moons. During the Juice flyby of Earth we discovered that the energies of the positive ions observed by JDC were not easy to interpret due to a problem with the energy sweep. Using measurements made on ground, we were able to reconstruct the observed energies and construct a new sweeping scheme that solves the problem and that will greatly improve future observations. We also used a simulation to explain the effects of the spacecraft velocity and spacecraft potential on the recorded positive ion fluxes when Juice passed through the Earth's plasmasphere. The study highlights the importance of in-flight calibrations for optimizing the scientific return. Planetary flybys give access to multiple low-energy particle populations besides the mono-energetic and highly directional solar wind.

The Juice flyby of Earth in August 2024 gave us the first chance to evaluate the performance of the Jovian Plasma Dynamics and Composition analyzer (JDC) in environments similar to those expected at Jupiter. JDC is one of the sensors belonging to the Particle Environment Package (PEP) on the Juice spacecraft. It measures positive and negative ions as well as electrons in the energy range 1 eV/q to 35 keV/q. One of the most challenging observations at the final destination is those of the low energy ion populations in the tenuous ionospheres of Jupiter's icy moons. During the Juice flyby of Earth we discovered that the energies of the positive ions observed by JDC were not easy to interpret due to a problem with the energy sweep. Using measurements made on ground, we were able to reconstruct the observed energies and construct a new sweeping scheme that solves the problem and that will greatly improve future observations. We also used a simulation to explain the effects of the spacecraft velocity and spacecraft potential on the recorded positive ion fluxes when Juice passed through the Earth's plasmasphere. The study highlights the importance of in-flight calibrations for optimizing the scientific return. Planetary flybys give access to multiple low-energy particle populations besides the mono-energetic and highly directional solar wind.

During the first Earth gravity assist maneuver of JUICE on 20th August 2024 the spacecraft passed through the Earth plasmasphere for about 2 hours. Before closest approach at a distance of 2.1 RE the Jovian Electron and Ion spectrometer JEI of the PEP instrument suite was switched on for 40 min in an ion mode test configuration. The high plasma density of about 3000/cm3 (observed by the RPWI plasma wave instrument) led to a negative charging of the spacecraft which allowed a rare observation of the cold and dense plasmaspheric ion populations. Since the ions are only corotating with the Earth at a velocity of about 1 km/s at this distance the observed ion speed is dominated by the spacecraft velocity of about 8 km/s. For this reason ions with different mass appear at different energies in the energy spectrum observed by the JEI sensor. In addition the spacecraft potential leads to specific filtering of ion masses in the observed angular distribution. By calculating the sensor response function for these specific observing conditions it is possible to quantify densities of the different ion species. But this response calculation depends critically on the response of the JEI channel electron multipliers to the ion velocity and mass. Since the sensor was operated with a low post-acceleration further laboratory calibrations may be needed for this specific setup. Still we can already conclude from the observations that ions with mass >4 amu/q contribute at least 30 % to the observed total ion density. A flux peak observed in the energy spectrum at 15 eV can only be explained by the presence of heavy molecular ions with mass ~30 amu/q. Molecular ions have only been rarely detected in the outer Earth plasmasphere. The observations indicate that the JEI sensor can also be used to achieve ion composition measurements in the exospheres of the Jovian moons.