The European Space Agency's (ESA) JUICE mission (JUpiter ICy moons Explorer) is en route to the Jovian system to characterize Ganymede's subsurface ocean. Determining the ocean's conductivity and depth requires precise measurements of its induced magnetic field at the position of JUICE. Electron reflectometry provides additional constraints for the surface magnetic field, but this would require knowledge of Ganymede's surface electric potential. Here, we model the global electrostatic surface potential distribution on Ganymede using a semi-analytical charging formulation, by feeding it with surface plasma environment parameters derived from different magnetospheric interaction simulations. We further contrast these estimates against self-consistent charging simulations using the Spacecraft Plasma Interaction Software (SPIS). Our results indicate that surface potentials should be mostly negative, ranging down to about -100 V. All model predictions exhibit a consistent morphological trend: closed field line regions are more negatively charged than open field line ones. Depending on the input used, the driver behind this trend can either be the spatial variations of plasma density alone or combined density and electron temperature patterns, especially in the ionosphere; the electron current is the dominant driver of surface potential, with the relative influence of secondary electron, ion, and photoelectron currents varying depending on the location at Ganymede. While estimates of absolute surface potential are consistent with aspects of Juno flyby data, they are heavily dependent on how simulated plasma parameters are extrapolated to Ganymede's surface. Consequently, advanced modeling is required to achieve more reliable constraints of Ganymede's near-surface environment description.

In Saturn's magnetosphere, the inward transport of magnetic flux is largely carried by localized injection flux tubes filled with warm, tenuous plasma, although their inflow speeds and spatio-temporal properties remain poorly constrained. Here, we propose that these flux tubes can modify electron microsignatures, the small-scale, absorption-induced flux depletions downstream of Saturn's moons, by twisting them into zigzag-shaped structures in the energy-time spectrograms of energetic electrons. Using observations from the Cassini spacecraft, we identify zigzag-shaped microsignatures and reproduce their morphology through test-particle simulations based on this scenario. The inferred radial velocities at low -shells (L similar to 5) are on the order of 1 km/s, suggesting significant braking of injection flux tubes and consistent with the observed decline in their occurrence inside L similar to 7. The ability to resolve such low inflow velocities remotely from microsignature distortions offers a new diagnostic for constraining mass and flux circulation in the magnetospheres of giant planets.

Coronal mass ejections (CMEs) are enormous bursts of plasma and magnetic field from the Sun that propagate explosively through the Solar System. CMEs are the primary drivers of the most severe space-weather events, posing major radiation hazards for human exploration beyond Earth’s magnetosphere, including missions to the Moon and Mars. During Europa Clipper’s cruise between Earth and Mars, its Plasma Instrument for Magnetic Sounding (PIMS) recorded unusual solar-wind conditions later identified as the wake of a complex CME. Data from an unprecedented network of 17 spacecraft across the inner Solar System revealed a hidden Earth-directed component that propagated asymmetrically and could not have been predicted without off-Sun-Earth-line observations. This component also affected Europa Clipper and Mars, demonstrating how a missed forecast of this kind could endanger a crewed mission. These findings highlight the need for multipoint observations, including planetary missions in cruise, to improve space-weather forecasting, mission planning, and operations.

We present a new theoretical framework to describe the rapid and spatially localized loss of energetic particles in planetary radiation belts, focusing on interactions between gas giant magnetospheres and their moons. Observations show that flux depletions--known as microsignatures--often refill on timescales comparable to a single drift period, which conflicts with traditional quasi-linear radial diffusion models that assume slow, gradual transport and predict refilling only over many drift periods. To resolve this inconsistency, we develop a drift-kinetic model that explicitly captures localized losses occurring on timescales similar to the azimuthal drift period. We demonstrate that such localized loss regions can synchronize the azimuthal Fourier modes of the particle distribution function, producing apparent refilling through phase-space synchronization rather than diffusion. The resulting governing equations are mathematically equivalent to a generalized Kuramoto model, widely used to describe synchronization phenomena. This framework provides a first-principles, non-diffusive explanation for the evolution of microsignatures near moons, highlighting synchronization as a fundamental yet overlooked mechanism in magnetized plasma environments.

Abstract. On 19–20 August 2024, the European Space Agency (ESA) Jupiter Icy Moons Explorer (JUICE) mission performed the first ever Lunar–Earth Gravity Assist (LEGA) manoeuvre. The mission was launched on April 14, 2023, and is currently on an 8-year interplanetary cruise to the Jovian system. It is equipped with the RADiation-hard Electron Monitor (RADEM), a facility instrument designed to measure the most energetic particle populations (electrons, protons and ions) in the Jovian environment where JUICE will operate. During LEGA, JUICE crossed the Van Allen belts, providing a unique opportunity to evaluate the in-flight response of RADEM and to optimize its configuration for the Jupiter phase. In this paper, we report RADEM observations of the Van Allen belts, showing clear sensitivity to trapped electrons and protons. We also discuss how the Earth-flyby geometry, including pitch-angle effects, influenced the measurements and the implications for future operations. The observations also demonstrate that while RADEM is a facility instrument, it has the potential to enhance the scientific return of the JUICE mission by monitoring a key energy range in Jupiter's radiation belts that no other instrument on JUICE is covering.

Abstract. The Jovian Energetic Neutrals and Ions (JENI), part of the Particle Environment Package (PEP), on the Jupiter Icy Moons Explorer (JUICE) spacecraft is a state-of-the-art detector, capable of analyzing the energy and direction of incident Energetic Neutral Atoms (ENAs) from planetary magnetospheres. Our analysis focuses on 2.86–85.5 keV Hydrogen ENA measurements between 21 to 24 August 2024 when JUICE performed the Lunar and Earth Gravity Assist (LEGA). During the LEGA, JENI had a unique opportunity to obtain continuous, 7.5-minute accumulation time global images of the terrestrial magnetosphere as the spacecraft moved outward from ~17 to 150 Earth Radii. Although there were no indications of substantial geomagnetic storms developing during that time, substorm activity was observed based on eight distinct dips of the SuperMAG AL (SML) index down to ~-250 nT for the four last ones and even down to ~-400 nT for the four first ones. Our results indicate increased ENA emissions associated with the onsets of these events, suggesting direct responses to the increased energetic ion activity during those substorm events. Unlike the periods of intense geomagnetic storms, during which proton intensities build up the ring current evolving from an asymmetric distribution during the main phase to a roughly symmetric distribution during the recovery phase, the images from JENI during this interval show non-uniform ENA emissions in the plasma sheet, suggesting transient processes, such as a series of repeated nightside injections. The nightside ENA intensities are generally brighter than those from the dayside, possibly reflecting magnetotail processes associated with energetic ions being injected.

To prepare for the arrival of the JUICE (JUpiter ICy moons Explorer) mission, developing a comprehensive and physically grounded magnetic field model is essential to understand the interactions with charged particles and the complicated plasma dynamics. Existing models range from simplified representations, such as the model only include dipole field and induced ocean, to computationally intensive magnetohydrodynamic (MHD) or hybrid simulation codes, yet a critical gap remains for a model balancing physical fidelity with computational efficiency. We aim to present the development of a semi-empirical magnetic field model of Ganymede, based on the frameworks used for Mercury and Earth. Our superposition model integrates observational constraints with theoretical insights to accurately represent the Jovian background field and Alfvén wing, Ganymede’s internal field, the ocean-induced field, and the field generated from different currents in Ganymede’s magnetosphere. The model enables rapid, scalable simulations of particle access across the magnetosphere. It will support mission planning, enhance the interpretation of JUICE data, and provide key predictions for particle-driven surface processes on magnetized icy moons. Ultimately, this work enhances our understanding of moon-magnetosphere interactions and the dynamic interplay between internal fields, induced currents, and external plasma environments.

Galileo flew-by Ganymede six times between 1996 and 2000. The Energetic Particles Detector (EPD) performed energy resolved, directional electron measurements, and together with the magnetometer data, pitch angle distributions (PADs) could be derived. PADs are a very useful diagnostic to investigate particle dynamics and the field geometry in the vicinity of the moon. Here we present PADs from five Ganymede flybys and highlight aspects of their topology and energy dependence in more detail compared to past studies and the Juno flyby in 2021. We find that at about 300 keV, a transition between a single to a double loss cone is observed on open field lines mapping to Ganymede's north pole (flyby G2), indicative of the plasma slowdown within Ganymede's magnetosphere. Butterfly PADs with a deep 90 degrees pitch angle minimum, seen also with Juno, are resolved in all Galileo flybys outside Ganymede's magnetopause, but their energy extent, exact PAD shape, and topology change drastically between different flybys and are only partly reproduced by numerical models. Finally, while previous studies reported field-aligned electron beams within Ganymede's magnetosphere extending up to about 500 keV, trapped distributions in Ganymede's closed field line regions reach well above 1 MeV. This indicates that even if field aligned beams are significant electron source for the closed field line region, additional energization or particle sources are needed to explain the electron energies within Ganymede's radiation belts.

Abstract. 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.

ESA’s JUpiter ICy moons Explorer (JUICE) mission performed the world’s first Lunar-Earth flyby on the 19-20 of August 2024, successfully rerouting the spacecraft toward Venus for another gravity assist. In October and November 2025 the JUICE payload also attempted observations of the interstellar comet 3i/ATLAS. In this presentation, we focus on observations obtained from the Jupiter Energetic Neutrals and Ions (JENI) camera and the Jovian Energetic Electron (JoEE) magnetic spectrometer, which are a part of the comprehensive JUICE Particle Environment Package (PEP). The Lunar-Earth flyby brought JUICE to within ~750 km of the Moon’s surface and ~6,840 km over Earth. JUICE flew through Earth’s magnetotail visiting the plasma sheet, ring current, and radiation belt regions, before exiting the magnetosphere along the flank bringing the spacecraft back into the solar wind. JENI and JoEE made direct measurements of the energetic ions (~1 keV to several MeV) and electrons (~30 keV to 2 MeV) in those magnetospheric regions. During its outbound leg of the trajectory, JENI captures high-resolution images of Earth’s dynamical ring current. Several substorm injections of hot plasma were observed in Earth’s nightside.In the period 8-19 November was allowed to be on attempting ENA imaging of 3i/ATLAS and data will be downlinked by February 2026. In this presentation, we report on these exciting observations captured by JUICE discuss the instrument performance of JENI and JoEE.

Jupiter hosts the most energetic, intense and ion-rich radiation belts in the solar system. Understanding the composition and distribution of their heavy ions is essential for constraining source, acceleration, and loss processes across an ion mass and energy range not typically accessible in other planetary magnetospheres. Energetic heavy ions may alter Jovian moon surfaces and rings, generate X-ray emissions in the Jovian system, and pose risks to spacecraft via single event effects. Despite their importance, relatively little is known about their global distribution and dynamics within the Jovian magnetosphere, particularly above several MeV nucleon ^–1 or for species different than sulfur or oxygen. Here we survey the full Heavy Ion Counter dataset from the Galileo mission to characterize the composition of Jupiter’s >5 MeV nucleon ^−1 ions ( Z > 6), with an extra focus on lesser-studied species. We unambiguously resolve 10 different ion species and provide estimates of their energy and distance dependent relative abundances. Several new species are resolved (N, Ne, Si, K, and/or Ca and possibly Fe), each likely associated with different magnetospheric, weathering, and/or heliospheric processes. A key finding is that abundances of species like carbon, neon and silicon are reminiscent of those in solar energetic particles and comparable to or larger than magnetospheric sulfur, indicating a considerable solar input into Jupiter’s heavy ion radiation belts. A considerable abundance enhancement of N between Io and Ganymede, hints that a N source may exist in Jupiter’s magnetosphere.

Context. Io, the innermost Galilean moon of Jupiter, is the main source of plasma in the Jovian magnetosphere. The neutral gas coming from the moon will get ionized through ultra-violet radiation and electron impacts. The newly created ions then get picked up by the Jovian magnetic field and start gyrating, thereby creating a ring-beam distribution in velocity space. This type of distribution is unstable with respect to the generation of ion cyclotron waves. Aims. The aim of this study is to characterize the escaping gas from Io's atmosphere into the Jovian magnetosphere. Methods. The Galileo magnetometer data have been investigated for the five Io flybys that have magnetometer data available. The ion cyclotron waves can be measured with magnetometers and through spectral analysis the specific pick-up ions can be determined. Assuming that the energy of the ions in the ring-beam distribution is fully transferred to the cyclotron waves, the pick-up ion densities can be estimated for all these species. Results. We found evidence of sulfur-bearing ions SO3+, SO2+, SO+, and S+, as well as either H2S+ or S-34(+) (which have the same mass-to-charge ratio and cannot be discerned), and for non-sulfur-bearing ions: Cl-35(+), Cl-37(+), K+, and Si+. We also present a first plausible detection of Io-genic phosphorous through the detection of P+ cyclotron waves. Conclusions. The main pick-up densities are related to SO2+ and SO+, varying with distance from Io between similar to 10(8) and similar to 10(6) m(-3), with the other ions exhibiting a similar variation, but their pick-up densities are lower by an order of magnitude.

Abstract. 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 Lunar-Earth Gravitational Assist (LEGA) of 19-20 August 2024 marked the first in-flight opportunity beyond functional checks to perform MAJIS (Moons and Jupiter Imaging Spectrometer) observations on-board the ESA's Jupiter Icy Moons Explorer (JUICE) spacecraft. This unique double flyby involved sequential close approaches to the Moon and Earth, offering an unprecedented configuration to evaluate MAJIS under high radiance, rapidly changing geometric, and operationally constrained conditions. A total of 24 hyperspectral image cubes were acquired (5 targeting the Moon and 19 the Earth) providing a dataset of approximately 7.5 Gbit. This work presents the primary goal of this observation campaign, which was to verify key aspects of MAJIS performance, including radiometric and spectral calibration, straylight behavior, geometric alignment, the use of onboard browse products, and interference tests with other JUICE instruments. This event also enabled assessment of thermal behavior and susceptibility to electromagnetic interference, and provided a first operational benchmark for MAJIS and a basis for refining future observation strategies and data analyses during JUICE's cruise and science phases. In addition, despite limited spatial and temporal coverage of the observations, the analyses presented here and in a series of companion papers of the special issue "The first-ever lunar-Earth flyby: a unique test environment for JUICE" demonstrated the instrument's ability to characterize mineralogical features on the Moon and atmospheric constituents on Earth. Observations include detection of mafic minerals (some associated to fresh excavated materials), thermal emission, and emissivity variations on the Moon at spatial scale of 100-200 m. Characterization of atmospheric absorption features, thermal brightness, icy cloud properties are captured for the Earth at km-scale and briefly discussed in the framework of the atmospheric biosignatures relevant to exoplanet habitability studies. Near-coincident acquisitions with other JUICE instruments and Earth-orbiting spectrometers provided valuable inter-calibration and cross-validation opportunities.

The Jovian Energetic Neutrals and Ions (JENI) Camera and the Jovian Energetic Electrons (JoEE) belong to the six-sensor suite Particle Environment Package (PEP) on board the JUICE mission. JENI is a combined ion and ENA camera with 90˚x120˚ Field-of-View and an energy range from a few keV to 110 keV for ENAs and 5 MeV for ions. Only one mission, Cassini, has captured ENA images of the Jovian system before during its distant flyby. Those images revealed emissions coming from the Europa neutral gas torus, but were too distant to resolve details on its spatial distribution and variability. The Juno mission has detected ENA emissions originating from both the Europa and also the Io torus, that indicate azimuthally asymmetric distributions. In ENA mode, JENI will image the Europa and Io tori, to investigate their spatial distribution and long-term variability providing global constraints to physical models of their sources. Although a predominant fraction of the ENAs from the tori originate from charge exchange between magnetospheric energetic ions and the neutral gas, a significant fraction may originate from charge exchange between the energetic ions and the ambient plasma in the tori. This opens up the intriguing possibility to also diagnose the plasma dynamics and distribution of the tori. JENI also targets the explosive recurrences of vast regions of heated plasma in the Jovian magnetotail (“injections”) that may be the engine behind the periodic radio emissions from rotating, magnetized planets, such as Saturn, Jupiter and perhaps even brown dwarfs. In ion mode, JENI will provide the detailed in-situ measurements of the energetic ion environment necessary to understand the physical heating and transport processes underlying the global context provided by the ENA images. JoEE is an electron spectrometer that near-simultaneously provide the energetic electron spectrum in multiple look directions over the energy range from 28 keV up to 2 MeV. JoEE’s prime objectives are to investigate the acceleration mechanisms of Jovian radiation belt electrons and their interaction with the Jovian moons. The Juno mission has recently made important electron measurements that provides useful guidance for deepening the JoEE objectives.In this presentation an overview is given of JENI and JoEE, with emphasis on the ENA observations and their expected science return. This includes imaging of the Europa and Io tori distribution and variability, quasi-periodic magnetospheric injections, and their relation to rotationally periodic radio emissions from planets and brown dwarfs.

ESA’s JUpiter ICy moons Explorer (JUICE) mission performed the world’s first Lunar-Earth flyby on the 19-20 of August 2024, successfully rerouting the spacecraft toward Venus for another gravity assist. The Lunar-Earth Gravity Assist (LEGA) was not only critical for JUICE’s route to Jupiter, but it provided an invaluable opportunity for the science payload to gather measurements of Earth’s space environment. In this presentation, we focus on observations obtained from the Jupiter Energetic Neutrals and Ions (JENI) camera and the Jovian Energetic Electron (JoEE) magnetic spectrometer, which are a part of the comprehensive JUICE Particle Environment Package (PEP). The Lunar-Earth flyby brought JUICE to within ~750 km of the Moon’s surface and ~6,840 km over Earth. JUICE flew through Earth’s magnetotail visiting the plasma sheet, ring current, and radiation belt regions, before exiting the magnetosphere along the flank bringing the spacecraft back into the solar wind. The JENI and JoEE instruments were on and collecting science data for most of the excursion, except for critical spacecraft maneuvers. Therefore, JENI and JoEE made direct measurements of the energetic ions (~1 keV to several MeV) and electrons (~30 keV to 2 MeV) in those magnetospheric regions. During its outbound leg of the trajectory, the spacecraft performed a roll that placed Earth within JENI’s FoV allowing the camera to capture high-resolution images of Earth’s dynamical ring current. Several injections of hot plasma were observed in Earth’s nightside magnetosphere and appear to be linked to substorms. In this presentation, we summarize these exciting observations captured by JUICE during its LEGA and discuss the instrument performance of JENI and JoEE.

The Jupiter bound JUICE mission (JUpiter ICy moons Explorer) was successfully launched on April 14, 2023 and is currently executing its 8-year interplanetary cruise phase. JUICE carries three comprehensive instrument suites to fully characterize particles, fields, and waves. The JUICE space plasma instrumentation constitutes the most comprehensive and capable heliophysics payload ever flown, or planned, in the important but not-well explored the solar system region between 1 and 5 au. Science objectives that can be addressed by the JUICE payload include solar wind evolution, collisionless shock interactions and propagation, and particle acceleration, solar energetic particles, pick-up ion (PUI) origin and evolution, turbulent interactions, energetic neutral atom (ENA) imaging, and interplanetary hydrogen observations. JUICE enables an expansion of inner heliospheric science, connecting to observations in the outer heliosphere and Very Local Interstellar Medium (VLISM) by NASA’s New Horizons, Voyager, and Interstellar Mapping and Acceleration Probe (IMAP) (to launch Feb 2025). New science opportunities are also enabled by the simultaneous observations from Europa Clipper in the same region. JUICE will spend the next six years between 0.7 au and 2.5 au, and in 2029-2031 it will explore the region out to Jupiter. JUICE has no thermal constraints past 1.34 au and the data volume from relevant sensors live well within the available data downlink through its weekly passes using the European Space Tracking (ESTRACK). In this presentation, we discuss the unique heliophysics observations the six-sensor suite Particle Environment Package (PEP) on board can do in conjunction with other space physics measurements on board JUICE and Europa Clipper. The PEP-suite measures species-resolved energy and angular distributions of electrons (~ 1 eV to ~1.5 MeV), ions (~1 eV to > 10 MeV; energy rage is species dependent); and ENA (~ 5eV to 300 keV). In addition to the PEP instrument suite, JUICE also carries a radiation monitor (RADEM) that can provide complimentary energy and species resolved measurements of very energetic electrons and ions in the solar wind.

The past decades of Cassini’s investigation around Saturn have revealed the frequent occurrence of ∼1 hr quasiperiodic (QP) signatures in the in situ magnetic field and plasma measurements and plasma wave observations. Saturn’s aurora, as the response to the planet’s magnetospheric dynamics, is also observed to exhibit transient features that can reoccur with a strikingly similar periodicity. Previous investigations have associated these features with energetic plasma injections, standing Alfvén waves, and dayside magnetodisk reconnections, which all require a closed-field configuration. However, both previous and our analyses of the Hubble Space Telescope (HST) images, as Saturn approached the equinox (when both poles are in the field of view), reveal that transient auroral emissions are strictly nonconjugate between hemispheres—they occur in either the north or south alone. This picture is matchable with the open-configured, north–south asymmetric magnetopause reconnection. We also present a detailed case analysis of Cassini’s measurements for a prolonged period before and after a sequence of transient auroral brightenings observed in HST’s visits G1–G4. We show that the northern lobe featured hourly QP pulsations in the magnetic field, electron flux, and whistler-mode hiss. In an interval outside HST exposures, we observed tens to hundreds of keV electrons in the antiparallel direction associated with upward field-aligned currents, which can in turn produce aurora. This is potentially suggestive of reconnection processes at the magnetopause, but are limited in the spatial scale.

The Galileo spacecraft performed close flybys of the moon Ganymede between 1996 and 2001. We reanalysed data of the energetic particles detector EPD onboard Galileo and derived the particle fluxes, energy spectra, and pitch angle distributions in the energy range of several keV to MeV during Ganymede flybys G2, G7, G8, G28, and G29. We find sharp dropouts in ion and electron fluxes as signatures of the loss cones inside the Ganymede magnetosphere as well as trapped electron distribution. Additionally, bi-directional field-aligned and butterfly distributions were found as well. We discuss these findings compared with simulation results in charactering Ganymede’s magnetosphere and in the context of future measurements with the Particle Environment Package PEP onboard the Juice mission which will be in orbit around Ganymede in 2032.