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Juno's highly inclined orbits provide opportunities to sample high‐latitude magnetic field lines connected to the orbit of Io, among the other Galilean satellites. Its payload offers both remote‐sensing and in‐situ measurements of the Io‐Jupiter interaction. These are at discrete points along Io's footprint tail and at least one event (12th perijove) was confirmed to be on a flux tube Alfvénically connected to Io, allowing for an investigation of how the interaction evolves down‐tail. Here we present Alfvén Poynting fluxes and field‐aligned current densities along field lines connected to Io and its orbit. We explore their dependence as a function of down‐tail distance and show the expected decay as seen in UV brightness and electron energy fluxes. We show that the Alfvén Poynting and electron energy fluxes are highly correlated and related by an efficiency that is fully consistent with acceleration from Alfvén wave filamentation via a turbulent cascade process. Plain Language Summary Io and Jupiter are electrodynamically coupled resulting in the Io footprint tail. This is one of the most persistent, stable, and recognizable features of Jupiter's aurora. The Juno spacecraft routinely samples magnetic field lines connected to Io's orbit, allowing for an investigation of this powerful coupling. We use data recorded by Juno to estimate a proxy for the strength of this interaction, that is, electromagnetic energy, and show its dependence downstream of Io and how the interaction decays. We further show that the available electromagnetic energy and electron energy are intimately linked, suggesting a transfer of energy between wave and particles. This is the basis upon which electrons end up precipitating into Jupiter's upper atmosphere and generate some of the brightest auroras. Key Points Alfvénic Poynting fluxes and electron energy fluxes are highly correlated on magnetic field lines connected to Io's orbit The efficiency in the Main Alfvén Wing is ∼10%, fully consistent with Alfvén wave filamentation via a turbulent cascade process Field‐aligned current densities are quantified and exhibit a decay in magnitude down‐tail of Io

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. Plain Language Summary A recent Juno spacecraft close flyby of Jupiter's moon Europa revealed the presence of powerful electrons beams. Based on previous observations and modeling of electron beams at the moon Io, such beams were not expected to be observed so close to Europa. Overall, the proximity of the beams to Europa indicates that the acceleration of these electrons takes place much closer to Europa than anticipated and that these beams, therefore, stem from a new and previously unknown acceleration mechanism. The beams are predicted to have an outsized influence on the ionization of the constituents of Europa's tenuous atmosphere and are accompanied with large magnetic field perturbations. Hence, these electron beams are an important ionization source that modify the moon's ionosphere, the electric current systems, and the magnetic field environment. In particular, the presence of electron beams will affect plasma conditions that are used to infer the extent of a subsurface ocean via the magnetic induction signal. These beams significantly impact the space plasma environment around Europa which needs to be accounted for by future missions such as ESA's (European Space Agency) JUICE (Jupiter Icy Moons Explorer) and NASA's (National Aeronautics and Space Administration) Europa Clipper mission. Key Points Powerful electron beams that significantly shape Europa's space environment are discovered during a Juno flyby The beams enhance electron‐impact‐excited emissions in Europa's atmosphere and are associated with large jumps of the magnetic fields The beams' proximity to Europa and their pitch angle distribution constrain the source acceleration to be near or within the plasma disk

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 Alfvén 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. Plain Language Summary The passage of the Galilean moons‐ Io, Europa, Ganymede, and Callisto, perturbs the plasma flow in Jupiter's magnetosphere, creating waves that travel from the moon and reflect off Jupiter's ionosphere. These waves have been proposed to accelerate charged particles, and such accelerated particles had been observed by the Juno spacecraft during its passage through magnetic field lines connected to the satellite wakes. In this work, we find instances when this acceleration occurs selectively at specific energies that have constant separation in speed. We propose that this selective acceleration is due to resonance between particle bounce motion and the waves arising from the satellite wake perturbation. Bounce‐resonant acceleration is a promising fundamental process which can accelerate particles in Jupiter's inner magnetosphere and other plasma systems with similar geometries. Key Points Proton and electron flux enhancements in satellite and wake flux tubes often occur at discrete energies linearly separated in speed Broadband proton flux enhancements at <46 keV energies were also observed within satellite flux tube crossings Particles can be accelerated via resonance between bounce motion and standing Alfvén waves generated by moon‐magnetosphere interactions

Jupiter's plasma sheet has been understood to be primarily composed of Io‐genic sulfur and oxygen, along with protons at lower mass density. These ions move radially away from Jupiter, filling its magnetosphere. The material in the plasma sheet interacts with Europa, which is also a source of magnetospheric pickup ions, primarily hydrogen and oxygen. Juno's thermal plasma instrument JADE, the Jovian Auroral Distributions Experiment, has provided comprehensive in situ observations of the composition of Jupiter's plasma sheet ions with its Time‐of‐Flight mass‐spectrometry capabilities. Here, we present observations of the magnetospheric composition in the Europa‐Ganymede region of Jupiter's magnetosphere. We find material from Europa is intermittently present at comparable densities to Io‐genic plasma. The intermittency of Europa‐genic signatures suggests Europa's neutral oxygen toroidal cloud is more localized to Europa's vicinity than its hydrogen cloud. These observations reveal a more complex and compositionally diverse magnetosphere than previously thought. Plain Language Summary Jupiter's charged particle environment is overwhelmingly driven by material lost from Io. This material interacts with the icy moon Europa, which can also inject charged particles into the environment. We find that Europa appreciably contributes to and modifies its local charged particle environment, revealing a more complex and compositionally diverse magnetosphere than previously thought. Key Points Three distinct heavy ion populations observed in Jupiter's plasma sheet: Io‐genic plasma, Europa‐genic plasma, and Io‐genic energetic particles The mixture of Io‐genic and Europa‐genic plasma varies greatly throughout the Europa‐Ganymede region We find evidence Europa's oxygen neutral toroidal clouds are more localized than its hydrogen cloud

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. Plain Language Summary Trapped plasma particles in Jupiter's magnetosphere experience a “bounce” motion—traveling from the northern to the southern hemisphere and back along magnetic field lines. In addition, plasma as a whole is driven to rotate with the planet, a process referred to as corotation. These are two distinct periodic processes—the former dictates how individual particles travel along a field line, whereas the latter is a global scale process However, for low energy plasma particles, both processes have similar timescales (>100 min). In this work, we show evidence from plasma measurements of resonance between these two processes, which could also be important for plasma acceleration. Key Points Banded energy distributions (0.01–45 keV/q) occur simultaneously for different ion species mapping to M = 10–20 Bounce frequencies of banded ions match harmonics of the corotation/System‐III frequency Corotation‐modulated bounce resonance accelerates low‐energy plasma ions in Jupiter's magnetosphere

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 Alfvén wing, the second covering its downstream region, south of the plasma wake. Both had closest approach altitudes of ∼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 Alfvén 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 Alfvén 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. Plain Language Summary The interaction of charged particles in Jupiter magnetosphere with the obstacle posed by its satellite Io leads to a structure connecting the moon to Jupiter's ionosphere called an Alfvén wing. Strong electric currents, plasma waves, and slowed and deflected flows which increase along the flanks are key features of this region. The Galileo mission discovered the presence of intense electron beams within the Alfvén wing and other regions near Io, electrons that travel along the local magnetic field and can interact with Io's atmosphere. New results from Juno shown here indicate that the electron beam properties within Io's northern Alfvén wing are not uniform, the flux of electrons in these beams being largest at the wing boundaries and weaker inside. This has implications for where the interaction of electrons with Io's atmosphere is most concentrated. The electron beams measured downstream of the satellite show similar features over a narrow region, supporting the idea that Juno briefly encountered Io's Alfvén wing in the southern hemisphere. Key Points Sharp enhancements in electron flux and energy were observed when Juno entered the northern Alfvén wing and downstream region near Io The electron fluxes were spatially variable in the northern Alfvén wing, being largest at the wing boundaries and smaller inside Electron fluxes downstream of Io were comparable to the northern flyby, suggesting a glancing encounter with the southern Alfvén wing

Juno performed two close flybys of Io and found enhanced field‐aligned proton fluxes are absorbed by Io. These protons are absorbed at mass input rates comparable to previous estimates for hydrogen losses from Io, hence Jupiter is likely the source of hydrogen at Io. The conditions necessary for this to occur are: (a) formation of Alfvén waves at Io, (b) wave‐particle coupling to energize protons, (c) anti‐planetward transport of ions due to the magnetic mirror force and/or parallel acceleration, and (d) strong sub‐Alfvénic interaction slowing the flow connected to Io's fluxtube allowing for sufficient travel time for energized ions to transit to Io. The derived slowdown of ≤12% the upstream value is linked to filamentation within the Alfvén wing. This mechanism is likely operating at all strongly interacting satellites and provides an avenue to transfer material from a planetary body to its satellites, including exoplanets and brown dwarfs. Plain Language Summary Juno performed two close flybys of Jupiter's moon Io, where it observed highly directional charged hydrogen streaming along magnetic field lines. These protons hit Io and the overall influx of them is sufficient to account for previously observed proton losses from Io. Therefore, Io's protons are not from Io's interior and instead provided from Jupiter and its charged particle environment. Furthermore, the observations allow for an estimation of the speed at which plasma flows across Io, which is found to be reduced to ≤12% of the flow speed upstream from Io. These processes are likely operating at similarly interacting bodies throughout our solar system and other solar systems far from the Sun. Key Points Io's Alfvénic interaction provides a mechanism to transport ∼10's g s−1 of Jovian hydrogen ions to Io's poles This mechanism is a universal process that can operate at any satellite‐planet system with adequate interaction conditions Plasma flow within Io's Alfvén wing is slowed to ≤12% of its upstream value, allowing for filamentation

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. Water molecules in Europa’s icy surface are split into hydrogen and oxygen by charged particle bombardment. NASA’s Juno spacecraft flew near Europa and constrained the production of oxygen in Europa’s surface ice, thus providing only a narrow range to support habitability in its subsurface ocean.

We report on observations of negative carbon and oxygen pickup ions (PUIs) originating from dust orbiting Jupiter. The PUIs are observed at altitudes of a few thousand kilometers (∼4,800–10,200 km) above the 1‐bar level of Jupiter's atmosphere and up to ∼11,000–15,000 km from the equatorial plane, thus providing constraints on the location of the dust population and its composition. The Jovian Auroral Distributions Experiment–Electron sensors on Juno detect these PUIs because of the combination of a fast‐moving spacecraft and the large Keplerian orbital speed of the dust near Jupiter. We demonstrate that this scenario is consistent with the observations. We find a PUI C/O ratio of 10 ± 5 and a PUI energy release of ∼11 ± 9 eV. Electron stimulated desorption is a likely process for creating these PUIs. The dust is well inside the halo population and likely carbonaceous.

Like Earth, Jupiter has aurorae generated by energetic particles hitting its atmosphere. Those incoming particles can come from Jupiter's moons Io and Ganymede. Mura et al. used infrared observations from the Juno spacecraft to image the moon-generated aurorae. The pattern induced by Io showed an alternating series of spots, reminiscent of vortices, and sometimes split into two arcs. Aurorae related to Ganymede could also show a double structure. Although the cause of these unexpected features remains unknown, they may provide a way to examine how the moons produce energetic particles or how the particles propagate to Jupiter. Science , this issue p. 774 Auroral features induced on Jupiter by the moons Io and Ganymede have complex spatial structures. Jupiter’s aurorae are produced in its upper atmosphere when incoming high-energy electrons precipitate along the planet’s magnetic field lines. A northern and a southern main auroral oval are visible, surrounded by small emission features associated with the Galilean moons. We present infrared observations, obtained with the Juno spacecraft, showing that in the case of Io, this emission exhibits a swirling pattern that is similar in appearance to a von Kármán vortex street. Well downstream of the main auroral spots, the extended tail is split in two. Both of Ganymede’s footprints also appear as a pair of emission features, which may provide a remote measure of Ganymede’s magnetosphere. These features suggest that the magnetohydrodynamic interaction between Jupiter and its moon is more complex than previously anticipated.