Abstract In Jupiter's magnetosphere, a key question is how plasma is transported outward from its source near Io to the distant magnetotail. Since 2016, the plasma disk has been extensively observed by the Juno mission. Using plasma data from the JADE instrument, we perform an analysis revealing significant temporal variation. For different Juno orbits, plasma disk observations are categorized as either enhanced or depleted based on plasma density, with extreme cases showing variations exceeding an order of magnitude. For plasma disk crossings by Juno, density enhancements and fluctuations in plasma density and magnetic field profiles are correlated, suggesting that flux tube interchange is triggered by increase in the plasma source from Io. Juno's in situ measurements show a correlation with remotely sensed Io torus ribbon brightness from the IoIO telescope, suggesting that the average plasma transport time from Io to the plasma disk between 10 and 30 RJ is around 40 days.

Ion energization and magnetic field energy dissipation within kinetic-scale plasma structures is an important problem of space plasma physics. A representative category of such structures consists of magnetic field discontinuities (or thin current sheets), which are widely observed in the solar wind and planetary magnetotails. As these discontinuities move through the ambient plasma, they generate convection electric fields that accelerate ions interacting with them. This acceleration process can be further modified by polarization electric fields that arise due to ion and electron decoupling around strong magnetic field gradients. We investigate ion interactions with moving polarized discontinuities in four distinct space plasma environments: the magnetotails of Earth and Mars, magnetodisk of Jupiter, and the near-Earth solar wind. Using multi-mission data sets, we characterize the convection and polarization electric fields by the magnitude of plasma flows along the discontinuity, , and the electron-to-ion temperature ratio . Discontinuities in Earth's magnetotail and Jovian magnetodisk are typically slow ( smaller than the ion thermal velocity) and feature hot ions , whereas solar wind discontinuities are fast ( exceeding the ion thermal velocity) and dominated by hot electrons . Martian magnetotail discontinuities can exhibit intermediate properties between these two regimes. We develop a theoretical model describing ion interactions with moving polarized discontinuities and quantify the respective roles of convection and polarization electric fields in ion acceleration. Main model predictions are compared with spacecraft observations and discussed in the context of heavy ion energization across the four examined space plasma systems.

Ions and neutral particles passing through thin carbon foils experience angular scattering, energy loss and straggling, and cause secondary electron emission from the foils. The secondary electrons are typically used for timing and coincidence detection. The angular scattering (and energy straggling) degrades the performance of a carbon-foil-based instrument. Thus, it is important to understand its effects for instrument design optimization and for enhancing data analysis using a better knowledge of the instrument response. The angular scattering of ions through a foil depends on the foil thickness. Therefore, measurements of the angular scattering of ions can be used to estimate the foil thickness as was shown in previous studies. In this study, we establish the necessary empirical relationships to reconstruct the angular scattering distributions of H ions by carbon foils, and we provide scaling factors to determine the same for heavier species. A single measurement of the scattering distribution of H for a given foil is sufficient to determine the scattering distributions for many low-Z ions at keV energies, which saves time consuming measurements. We also show how to derive a foil thickness estimate and the energy straggling of H for that foil.

Trihydrogen cations ( $${{\rm{H}}}_{3}^{+}$$ ), a key diagnostic of atmospheric energy balance, are produced through ionization by solar radiation and particle impacts in hydrogen-rich planetary atmospheres. At Jupiter, $${{\rm{H}}}_{3}^{+}$$ is produced most efficiently in the auroral region through electron precipitation and has a leading role in magnetosphere–ionosphere coupling by regulating ionospheric conductance. Previously, the properties of $${{\rm{H}}}_{3}^{+}$$ have been determined from remote sensing, with plasma parameters retrieved from its infrared emissions, although these measurements are limited by line-of-sight integration and restricted altitude resolution. Here we report the unambiguous, direct in situ detection of $${{\rm{H}}}_{3}^{+}$$ plasma in the auroral region. Furthermore, intermittent $${{\rm{H}}}_{3}^{+}$$ outflows are measured high above the ionosphere, with upwards velocities exceeding Jupiter’s escape speed, confirming atmospheric escape of $${{\rm{H}}}_{3}^{+}$$ . We propose that $${{\rm{H}}}_{3}^{+}$$ outflow originates in the auroral upwards electric current region, initially triggered by plasma-wave interactions and subsequently accelerated by the electric potential structure above the ionosphere. This study reveals a $${{\rm{H}}}_{3}^{+}$$ -mediated pathway for mass and energy transfer between the ionosphere and magnetosphere of Jupiter, constituting an atmospheric escape with a loss rate of order 1026 s−1. Similar mechanisms may operate at other planets with infrared aurorae and strong magnetic fields. In Jupiter’s polar region, $${{\rm{H}}}_{3}^{+}$$ has a key role in atmospheric heating and ionospheric conductance. Juno measurements show in situ detection of $${{\rm{H}}}_{3}^{+}$$ and provide direct evidence of its escape, revealing a pathway for energy transfer between the magnetosphere and the ionosphere.

We report on 0.032-32 keV electron observations during two Juno flybys of Io on 30 December 2023 and 3 February 2024. The first explored Io's northern Alfv & eacute;n wing, the second covering its downstream region, south of the plasma wake. Both had closest approach altitudes of similar to 1,500 km. Lower fluxes of >32 eV electrons in the Io torus transitioned to higher fluxes of energized, field-aligned electrons within these regions. The electron fluxes were spatially variable within the Alfv & eacute;n wing, highest at the boundaries, the distributions evolving from bi-directional to mono-directional as Juno traversed this region. Electron fluxes in the downstream region were also field-aligned, energized, and comparable to those during the northern flyby, supporting the interpretation of a glancing encounter with the southern Alfv & eacute;n wing. The electron energy flux in these regions ranged from 1-15 and 2-22 mW m(-2), respectively, which are enhanced compared to estimates from Galileo.

Abstract Banded energy distributions of H+, O++, S+++, and O+ or S++ ions between 100 eV and ∼20 keV are consistently observed in Jupiter's magnetosphere mapping to M‐shells between M = 10–20. The bands correspond to flux enhancements at similar speeds for different ion species, providing the first evidence of simultaneous bounce‐resonant acceleration of multiple ion species in Jupiter's magnetosphere. Ion enhancements occur for energies at which the bounce frequencies of the trapped ions matched integer harmonics of the System‐III corotation frequency. The observations highlight a previously unknown interaction between corotation and bounce motion of <10 keV energy ions that is a fundamental and persistent process occurring in Jupiter's magnetosphere.

The velocity distribution functions (VDFs) of He ^+ pickup ions (PUIs) through two interplanetary coronal mass ejection (ICME) events associated with two interplanetary shocks are examined. Their specific properties in pitch-angle space were used as tracers of the local physical processes. These VDFs were studied using observations from the PLasma And SupraThermal Ion Composition instrument on board the Solar Terrestrial Relations Observatory with 10 minutes cadence and full energy and angular resolutions. The analysis focuses on pitch angle distributions and their average/deviation to evaluate heating/cooling, acceleration/deceleration, and pitch angle diffusion. For an ICME accompanied by the quasi-perpendicular shock, (a) reflected and energized particles were found in the upstream, and pitch angle scattering and suprathermal particle production were conspicuous in the downstream; (b) strong heating was observed in the sheath region; (c) suprathermal particles were found in the sheath region and attributed to the velocity fluctuation within the solar wind; and (d) universal pitch angle scattering was found throughout the ICME. For an ICME accompanied by a quasi-parallel shock, (a) localized heating and acceleration were found around the shock location; (b) power-law suprathermal tail distributions were found near the shock, suggesting diffusive shock acceleration processes; (c) weak or no sheath heating was observed; and (d) pitch angle scattering was correlated with magnetic field power spectral density. In the magnetic cloud for this event, pitch angle diffusion consistent with the scattering by counterpropagating Alfvén waves was found. This type of analysis can be applied to study shock injection processes and PUI transport/diffusion processes under active conditions.

In Jupiter's magnetosphere, plasma originating from Io's escaping atmosphere is radially transported outward via two modes of centrifugally driven dynamics. In flux tube interchange events from the Rayleigh-Taylor instability, the cold and dense flux tube moves outward, while hot and depleted flux tubes are injected inwards. In the ballooning mode of the firehose instability, the flux tube breaks off with bursts of plasma blobs. Both modes suggest outward transport of cold, dense plasma blobs. In this study, we survey the cold blobs, analyzing 147 events based on thermal plasma measurements from the Juno/JADE-I instrument. The cold blobs are identified by searching time-of-flight spectra for very narrow signatures in the energy distributions. Plasma parameters determined by forward modeling reveal two types of cold blob events. The first type of event, occurring outside 25 RJ, exhibits high density and fast radial velocity accompanied by magnetic nulls and mainly occurs on the dawnside, suggesting an origin from the ballooning mode. The second type of event, occurring within 35 RJ, shows simultaneous outward moving cold, dense plasma with inward moving hot, tenuous plasma with no dependence on local time, indicating flux tube interchange. Based on statistics between 15 RJ and 30 RJ, the mass transport rate of the cold blobs is estimated to be about 100 kg/s, which is insufficient to account for the plasma production rate at Io.

Plasma circulation and magnetic reconnection are crucial for understanding the dynamics of Jupiter's magnetosphere. Previous studies identified hundreds of reconnection sites at distances of 50-100 RJ ${\mathrm{R}}_{J}$ from magnetic field signatures. Recent analysis of thermal plasma data from the Juno/JADE instrument shows super-corotating flows exist in the pre-dawn sector, suggesting that the internally driven reconnection could be occurring much closer to Jupiter. In this study, we present cases of super-corotating flows exceeding 120% $\%$ corotation observed at the nightside equator. In Case 1, a super-corotating outflow event is observed at 26.9 RJ ${\mathrm{R}}_{J}$ and is accompanied by flux rope plasmoids. In Case 2, a super-corotating inflow event is observed at 24.7 RJ ${\mathrm{R}}_{J}$ and is accompanied by plasma heating and density depletion, with a strong enhancement in B theta ${B}_{\theta }$ indicating the presence of a dipolarization front. These observations provide evidence of super-corotating flows accelerated by reconnection events.

3D velocity distribution functions (VDFs) of He+ pickup ions (PUIs) were used to trace local physical processes around interplanetary shocks. He+ PUIs are measured by PLasma And SupraThermal Ion Compostion (PLASTIC) instrument onboard the Solar Terrestrial Relations Observatory (STEREO) in an unprecedented cadence and angular/velocity resolutions with mass and charge state unambiguously determined. We focused on the VDFs in terms of the interplanetary magnetic field orientation in the solar wind frame and indipendently evaluated the acceleration, heating, and pitch-angle scattering for both perpendicular and parallel shocks with notable differences. For the perpendicular shock: (a) Reflected and energized particles were found in the upstream, and the PUI properties were isolated between upstream and downstream, (b) Strong heating is observed in the sheath region, (c) Suprathermal particles are found in the sheath region and attributed to the compression within the solar wind, and (d) Universal pitch-angle scattering were found through out the ICME. For the parallel shock: (a) Locaized heating and acceleration were found around the shock location, (b) Harder suprathermal particle distributions were found near the shock, suggesting diffusive shock processes, (c) Weak or no sheath heating was observed, and (d) Pitch angle scattering were correlated with magnetic field power spectral density at the Helium cyclotron.

A key open question in astrophysics is how plasma is transported within strongly magnetized, rapidly rotating systems. Magnetic reconnection and flux tube interchange are possible mechanisms, with Jupiter serving as the best local analog for distant systems. However, magnetic reconnection at Jupiter remains poorly understood. A key indicator of active magnetic reconnection is the ion diffusion region, but its detection at Jupiter had not been confirmed previously. Here, we report a magnetic reconnection event in Jupiter's inner magnetosphere that presents the detection of an ion diffusion region. We provide evidence that this event involves localized flux tube interchange motion driven by centrifugal forces, which occurs inside a thin current sheet formed by the collision and twisting of two distinct flux tubes. This study provides insights into Io-genic plasma transport at Jupiter and the unique role of magnetic reconnection in rapidly rotating systems, two key unresolved questions.

Current sheets are quasi‐1D layers of strong current density, which play a crucial role in storing magnetic field energy and subsequently releasing it through charged particle acceleration and plasma heating. They are observed in planetary magnetospheres and solar wind flows, where they are also known as solar wind discontinuities. Despite significant variations in plasma parameters across different magnetospheres and the solar wind, current sheet configurations can remain fundamentally similar. In this study, we analyze current sheets observed in various regions, including the near‐Earth (within 30 Earth radii) and distant (50–200 Earth radii) magnetotail, Earth's dayside and nightside magnetosheath, the near‐Earth solar wind, and Martian and Jovian magnetotails. We examine three key plasma parameters: the plasma beta (ratio of plasma to magnetic pressure), the Alfvénic Mach number (ratio of plasma bulk flow speed to Alfvén speed in the current sheet reference frame), and the ion to electron temperature ratio. Additionally, we investigate the kinetic, thermal, and magnetic field energy densities. Our cross‐system analysis demonstrates that the same current sheet configuration can exist across a very wide parametric space spanning multiple orders of magnitude. We also highlight the distinct plasma environments of the Martian and Jovian magnetotails, characterized by large populations of heavy ions, emphasizing their significance in comparative magnetospheric studies.

In Jupiter's magnetosphere, the outward transport of Io-genic plasma and the planetward injection of energetic particles are facilitated by flux tube interchange motion driven by centrifugal instabilities. Flux tube content and entropy are two competing key quantities that determine the behavior of the system. In this study, we first review the centrifugal force driven dynamics in different regions. Benefiting from Juno's high-quality in situ data in the off-equatorial region, we then analyze the flux tube integrals and interchange instability criteria between M-shells 13 and 40 using numerical integration. On average, the entropy term dominates the system, resulting in a generally stable system for interchange motion. Finally, we investigate an injection event, in which predictions from the criteria align with the observed flow direction. This study illustrates the overall stability of the system with small-scale, intermittent dynamic phenomena.

The Juno spacecraft had previously observed intense high frequency wave emission, broadband electron and energetic proton energy distributions within magnetic flux tubes connected to Io, Europa, Ganymede, and their wakes. In this work, we report consistent enhancements in <46 keV energy proton fluxes during these satellite flux tube transit intervals. We find enhanced fluxes at discrete energies linearly separated in velocity for proton distributions within Io wake flux tubes, and both proton and electron distributions within Europa and Ganymede wake flux tubes. We propose these discrete enhancements to be a result of resonances between particles' bounce motion with standing Alfven waves generated by the satellite-magnetosphere interaction. We corroborate this hypothesis by comparing the bounce and field-line resonance periods expected at the satellites' orbits. Hence, we find bounce-resonant acceleration is a fundamental process that can accelerate particles in Jupiter's inner magnetosphere and other astrophysical plasmas.

Magnetic reconnection is a fundamental mechanism for the transport of mass and energy in planetary magnetospheres and astrospheres. While the process of reconnection is itself ubiquitous across a multitude of systems, the techniques used for its analysis can vary across scientific disciplines. Here we frame the latest understanding of reconnection theory by missions such as NASA’s Magnetospheric Multiscale (MMS) mission for use throughout the solar system and beyond. We discuss how reconnection can couple magnetized obstacles to both sub- and super-magnetosonic upstream flows. In addition, we address the need to model sheath plasmas and field-line draping around an obstacle to accurately parameterize the possibility for reconnection to occur. We conclude with a discussion of how reconnection energy conversion rates scale throughout the solar system. The results presented are not only applicable to within our solar system but also to astrospheres and exoplanets, such as the first recently detected exoplanet magnetosphere of HAT-11-1b.

Jupiter’s moon Europa has a predominantly water-ice surface that is modified by exposure to its space environment. Charged particles break molecular bonds in surface ice, thus dissociating the water to ultimately produce H 2 and O 2 , which provides a potential oxygenation mechanism for Europa’s subsurface ocean. These species are understood to form Europa’s primary atmospheric constituents. Although remote observations provide important global constraints on Europa’s atmosphere, the molecular O 2 abundance has been inferred from atomic O emissions. Europa’s atmospheric composition had never been directly sampled and model-derived oxygen production estimates ranged over several orders of magnitude. Here, we report direct observations of H 2 + and O 2 + pickup ions from the dissociation of Europa’s water-ice surface and confirm these species are primary atmospheric constituents. In contrast to expectations, we find the H 2 neutral atmosphere is dominated by a non-thermal, escaping population. We find 12 ± 6 kg s −1 (2.2 ± 1.2 × 10 26 s −1 ) O 2 are produced within Europa’s surface, less than previously thought, with a narrower range to support habitability in Europa’s ocean. This process is found to be Europa’s dominant exogenic surface erosion mechanism over meteoroid bombardment.

Plasmoids and magnetic field dipolarizations are reconnection-related phenomena often resulting in reconfiguration of the magnetic field and energetic particle acceleration in planetary magnetotail. Building on the work of Bl & ouml;cker et al. (2023) (), we selected seven specific events from their magnetic field dipolarization analysis, each exhibiting distinct ion dynamics during the time interval of the magnetic field dipolarizations. To gain further insights into the understanding why certain events were associated with ion intensity variations while others were not, we analyzed plasma moments, specifically ion flow velocity and density, for these selected events. Our findings revealed that certain magnetic field dipolarizations within our database exhibit sub-Alfv & eacute;nic flows and lack the properties typically associated with reconnection-related magnetic field dipolarizations. These magnetic field dipolarizations also do not accelerate ions. Furthermore, we present a survey of Jovian plasmoids and magnetic field dipolarizations during the first 47 orbits of Juno. Applying Juno magnetic field data, we identified 119 magnetic field dipolarizations and 94 plasmoids within a local time range of 18:00-06:00. The majority of plasmoids were detected in the predawn sector, whereas magnetic field dipolarizations were observed closer to Jupiter and were not limited to a specific local time. Combining the statistics of plasmoids and dipolarizations is useful for contextualizing them within the framework of reconnection. Certain magnetic field dipolarizations exhibit sub-Alfv & eacute;nic flows and deviate from typical reconnection-related properties Nonadiabatic ion acceleration is only observed in magnetic field dipolarizations with super-Alfv & eacute;nic ion flows Plasmoids are concentrated in the predawn sector, while dipolarizations are distributed from 20:00 to 5:00 local time closer to Jupiter

The magnetospheric cusp connects the planetary magnetic field to interplanetary space, offering opportunities for charged particles to precipitate to or escape from the planet. Terrestrial cusps are typically found near noon local time, but the characteristics of the Jovian cusp are unknown. Here we show direct evidence of Jovian cusps using datasets from multiple instruments onboard Juno spacecraft. We find that the cusps of Jupiter are in the dusk sector, which is contradicting Earth-based predictions of a near-noon location. Nevertheless, the characteristics of charged particles in the Jovian cusps resemble terrestrial and Saturnian cusps, implying similar cusp microphysics exist across different planets. These results demonstrate that while the basic physical processes may operate similarly to those at Earth, Jupiter's rapid rotation and its location in the heliosphere can dramatically change the configuration of the cusp. This work provides useful insights into the fundamental consequences of star-planet interactions, highlighting how planetary environments and rotational dynamics influence magnetospheric structures.

Jupiter's moon Europa contains a subsurface ocean whose presence is inferred from magnetic field measurements, the interpretation of which depends on knowledge of Europa's local plasma environment. A recent Juno spacecraft flyby returned new observations of plasma electrons with unprecedented resolution. Specifically, powerful magnetic field‐aligned electron beams were discovered near Europa. These beams, with energies from ∼30 to ∼300 eV, locally enhance electron‐impact‐excited emissions and ionization in Europa's atmosphere by more than a factor three over the local space environment, and are associated with large jumps of the magnetic fields. The beams therefore play an essential role in shaping Europa's plasma and magnetic field environment and thus need to be accounted for electromagnetic sounding of Europa's ocean and plume detection by future missions such as JUICE and Europa Clipper.

The thermal and energetic electrons along Ganymede's orbit not only weather the surface of the icy moon, but also represent a major threat to spacecraft. In this article, we rely on Juno plasma measurements to characterize the temporal and spatial variability of the electron environment upstream of Ganymede. In particular, we find that electron spectra observed by Juno have fluxes larger by a factor of 2-9 at energies above 10 keV than what was measured two decades earlier by Galileo. This result will advance our understanding of the surface weathering and may be a concern for the radiation safety of the JUICE mission. Furthermore, the June 2021 close fly-by of Ganymede through the moon's wake reveals that the open field line regions of its magnetosphere attenuate electron fluxes at all energies by a factor of 1.6-5, thereby offering a natural shelter to visiting spacecraft crossing this region. Ganymede, the only magnetized moon in our Solar System, orbits deep inside the giant magnetosphere of Jupiter where it interacts with the temporally and spatially variable magnetized disk of plasma in corotation around the planet, its magnetodisk. The intensities of ions and electrons precipitating to the surface of Ganymede in particular depend on the location of the moon with respect to the Jovian magnetodisk. In this work, we provide a full quantification of electron properties along the orbit of Ganymede as observed by Juno. This is done by combining observations from two instruments in order to build composite electron energy spectra and derive their omnidirectional fluxes, densities, and pressures. We report that the average electron omnidirectional fluxes are significantly attenuated when measured above or below the magnetodisk, as well as strongly inside the magnetosphere of the moon where its intrinsic magnetic field provides additional shielding. We confirm that the electron total density is dominated by the thermal population, whereas the total pressure is dominated by the suprathermal one. When comparing our results with Galileo-based observations and models, we find that the latter the latter two underestimate fluxes in particular at high energies, and we put these observations in context for the future exploration of Ganymede by JUICE. We present composite electron energy spectra combining all Juno particle data from 07/2017 to 08/2022 at Ganymede's orbit We study the variability of electron fluxes inside and outside the Jovian magnetodisk as well as within Ganymede's magnetosphere Galileo-based models underestimate the electron fluxes observed by Juno in particular at high energies