We investigate the detectability of heliospheric helium ions at energies up to 100 keV by the New Horizons (NH) spacecraft during its flyby through Pluto's induced magnetosphere. The Pluto Energetic Particle Spectrometer Science Investigation energetic ion detector observed a reduction in their flux by an order of magnitude as the spacecraft passed through the non-uniform electromagnetic fields near the dwarf planet. This is despite these ions gyrating on scales up to several hundred Pluto radii. To contextualize these observations, we incorporate the electromagnetic fields from a hybrid model into a novel tracing tool for energetic H and generate synthetic time series of their flux into the detector along the NH trajectory and several hypothetical, idealized flyby geometries. Our major results are: (a) The detectability of perturbations to H fluxes highly depends on PEPSSI's look direction. Even along the same flyby trajectory, different viewing geometries may reveal changes in helium flux by up to a factor of five or no perturbations whatsoever. (b) The substantial reductions in H flux seen by NH may largely stem from the detector's finite field-of-view, filtering the incoming particles in velocity space. Without this effect, the reduction in flux by the draped fields would not exceed a factor of two. (c) The modeled flux perturbations gradually decrease with energy and become indiscernible above 20 keV. This behavior is qualitatively consistent with energetic ion dynamics at other small solar system bodies. (d) Across the range of plausible interplanetary field orientations, the model persistently suggests weaker reductions in H flux than observed.

We analyze ∼40–200 keV energetic particle observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) on board the New Horizons (NH) spacecraft from 2007 to 2024 and compare them with similar measurements from the Low Energy Charged Particle experiments on the Voyager 1 and Voyager 2 spacecraft when they were at comparable heliocentric distances (5–60 au). In both Voyager data sets, particle intensities generally decrease with increasing radial distance from the Sun, reaching a minimum in the outer heliosphere before rising again prior to their respective encounters with the heliospheric termination shock (TS). This radial behavior in the intensity-time profiles is described as a heliospheric valley. The NH/PEPSSI time series from 5 au (2007) to 60 au (2024) exhibits a similar decrease in particle fluxes with distance. Analysis of the radial dependence of energetic particle intensities from all three spacecraft normalized by observations at 1 au to account for the solar cycle effects reveals an approximate piece-wise composite power-law relationship, with a slope break (steeper decline) beyond ∼33 au. This break may reflect differences in the dominant transport and acceleration mechanisms operating in the two regions demarcated by this radial break. In addition, a radial scaling method is applied to Voyager observations to best match the NH data. This comparison provides an estimate for the NH TS crossing between 2027 (68 au) and 2034 (83 au).

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.

In the coming years, New Horizons (NH) is expected to exit the heliosphere by crossing the solar wind termination shock (TS) and make the first measurements of pick-up ions (PUIs) across the TS boundary. To date, the only working spacecraft to have crossed the TS are Voyager 1 and 2, with Voyager 1 encountering the TS on day of year (DOY) 351, 2004 at ~94 AU, and Voyager 2 undergoing multiple crossings between DOY 243 and 344, 2007 at ~83.6 AU. Although NH is approximately aligned in heliolongitude with Voyager 2, its trajectory lies near the heliographic equator, in contrast to the higher northern and southern heliolatitudes of Voyager 1 and 2, respectively.In this work, we analyze energetic particle observations (∼40–200 keV) from the Voyager Low Energy Charged Particle (LECP) instruments and the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) onboard NH to characterize radial intensity variations in the outer heliosphere. Voyager 1 and 2 observations show a systematic decrease in energetic particle intensities with increasing heliocentric distance, followed by a recovery prior to their respective TS crossings, forming a heliospheric energetic particle “valley.” NH/PEPSSI observations from 5 to 60 AU exhibit a comparable radial decline but have yet to show the expected increase on the march toward the TS crossing.To mitigate temporal variability associated with solar cycle effects, all observations are normalized using near-Earth energetic particle measurements from IMP-8/EPE and ACE/EPAM. The combined radial profiles from Voyager and NH are well described by a double power-law with a break at~33 AU. The combined radial profiles from Voyager and NH are well described by a power-law dependence with a distinct break beyond ~33 AU. This break likely reflects a transition in the dominant transport and/or acceleration mechanisms operating in the inner and outer regions separated by this radial distance. The presence of this break across multiple heliolatitudes suggests a global heliospheric feature, potentially reflecting changes in particle transport, acceleration, or local plasma conditions in the outer heliosphere. By scaling the Voyager observations to the NH measurements, we estimate a NH TS crossing between 2027 (~68 AU) and 2034 (~83 AU).

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.

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.

Abstract. This report presents an overview of the observations and performance of the Jovian Energetic Neutrals and Ions (JENI) and the Jovian Energetic Electrons (JoEE) belonging to the Particle Environment Package (PEP) onboard the Jupiter Icy Moon Explorer (JUICE) during its Lunar-Earth Gravity Assist (LEGA) 19–23 August 2024. Observations from the Lunar and Earth flybys are presented and discussed. While the environment around the Moon was very quiet, the Earth flyby presented a characteristic, nearly symmetric recovery phase ring current with adiabatically increasing energies with decreasing distance to Earth, and butterfly pitch-angle distributions (PADs) at high L-shells transitioning to distributions rounded around 90° pitch angle at lower L-shells. The electron radiational belt showed a double-structured outer zone, possibly from a fresh intensification, and the characteristic slot region created by loss through interactions with plasmaspheric hiss. On the outbound, JENI was operated as an Energetic Neutral Atom (ENA) camera and observed the energetic ions of the inner magnetosphere from 17.4 RE to 150 RE. During this period multiple intensifications were observed in the night side magnetosphere consistent with substorm injections. The LEGA observations provided useful data for further calibrating the sensors that overall showed a nominal performance.

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 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 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.

Understanding the balance between charged particle acceleration and loss is central to radiation belt research. Jupiter's Galilean moons orbit within its intense radiation environment and can act both as sources and sinks of energetic particles. Using observations from the Juno spacecraft, we identify large-scale depletions of energetic electrons along Europa's orbit. These depletions are too deep to result from direct absorption by the moon alone. Here we show that rapid electron losses, occurring within a timescale shorter than Jupiter's rotation, are driven by pitch angle scattering via whistler-mode waves co-located with Europa's orbit. This suggests that Europa maintains a plasma environment capable of sustaining a slot-like region, similar to the one seen in Earth's Van Allen belts. However, this Jovian slot only partially extends along Europa's path, implying that additional, unidentified acceleration mechanisms may act to refill the region and maintain high radiation levels close to Jupiter.

The Jupiter Icy Moons Explorer (JUICE) is a European Space Agency mission to explore Jupiter and its three icy Galilean moons: Europa, Ganymede, and Callisto. Numerous JUICE investigations concern the magnetised space environments containing low-density populations of charged particles that surround each of these bodies. In the case of both Jupiter and Ganymede, the magnetic field generated internally produces a surrounding volume of space known as a magnetosphere. All these regions are natural laboratories where we can test and further our understanding of how such systems work, and improved knowledge of the environments around the moons of interest is important for probing sub-surface oceans that may be habitable. Here we review the magnetosphere and plasma science that will be enabled by JUICE from arrival at Jupiter in July 2031. We focus on the specific topics where the mission will push forward the boundaries of our understanding through a combination of the spacecraft trajectory through the system and the measurements that will be made by its suite of scientific instruments. Advances during the initial orbits around Jupiter will include construction of a comprehensive picture of the poorly understood region of Jupiter’s magnetosphere where rigid plasma rotation with the planet breaks down, and new perspectives on how Jupiter’s magnetosphere interacts with both Europa and Callisto. The later orbits around Ganymede will dramatically improve knowledge of this moon’s smaller magnetosphere embedded within the larger magnetosphere of Jupiter. We conclude by outlining the high-level operational strategy that will support this broad science return.

A common problem in space physics is how the energetic particles we observe in space are accelerated to high energies. In the magnetospheres and radiation belts of magnetized planets like the Earth and Saturn, we find electrons with up to MeV energies. There are two fundamental acceleration processes. Electrons can gain energy when they are transported closer to the planet (radial acceleration), where the magnetic field is stronger. The alternative is that the electrons are accelerated locally, through fluctuating electric or magnetic fields and wave-particle interaction. In this work, we use a modified version of the Versatile Electron Radiation Belts code to perform the simulations of the radiation belts at Saturn. Using convection terms of a modified Fokker-Planck equation, the zebra stripes and banana orbit signature is reproduced in the convection-diffusion code. Using the flexibility of our simulation framework, we explore the effects of radial diffusion, coulomb scattering and the local diffusion. Throughout the series of simulations, we aim to understand the role of the controlling processes of the radial transport acceleration (e.g., due to variable electric field) and the role of local acceleration, as well as any other processes needed to reproduce the observations.