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Jupiter's poleward (Zone II) main aurora exhibits bi‐directional electron acceleration; upward acceleration dominates but downward acceleration generates strong aurora. During Juno's first perijove (PJ1), the upward acceleration manifested as narrow electron angular beams (within ∼5° of the magnetic field) over the 30–1,200 keV energy range of Juno's Jupiter Energetic Particle Detector Investigation (JEDI). These beams can be simply connected (non‐uniquely) to >10 to perhaps 100's of MeV electrons that penetrated the radiation shielding of the camera head of the Magnetometer Investigation's Advanced Stellar Compass (ASC). The most intense of those multiple MeV populations are shown to have been highly directional and propagating upwards. How auroral processes generate such beams is unknown. With azimuthal symmetry assumed (not demonstrated here), these beams provided >1026 s−1 of >30 keV electrons to Jupiter's vast magnetosphere, a possibly critical and dominating source of energetic electrons to that region and ultimately to Jupiter's radiation belts. Plain Language Summary: Aurora at Earth and Jupiter are generated by electron acceleration processes occurring at relatively low altitude above the polar atmospheric regions emitting the auroral lights. While downward electron acceleration causes the auroral emissions, upward electron acceleration can occur in a fashion that populates the distant planetary space environments (or "magnetospheres"). We find with NASA's Juno spacecraft that upward auroral electron acceleration at Jupiter has unique features. It is often more energetic and powerful than is the downward acceleration. The resulting electron beams are unexpectedly collimated along the magnetic field lines connecting the auroral regions to the spacecraft. The beams can extend in energy to unexpectedly high values, greater than 20 million electron volts (MeV). Finally, the quantity of energetic electrons coming from Jupiter's auroral regions is possibly a major, if not dominate, source of energetic electrons for Jupiter's vast magnetosphere. Key Points: Intense, narrow (±∼5°), 30–1200 keV, upward electron beams were observed over Zone II of Jupiter's main aurora during Juno perijove‐1They can be connected simply (non‐uniquely) to previously reported >10–100 s of MeV electron populations observed over the same regionAssuming symmetry, they provided >1026/s of >30 keV electrons to Jupiter's vast magnetosphere, a possibly critical and dominant source [ABSTRACT FROM AUTHOR]

An abundant multi‐MeV electron population beyond the orbit of Io is required to explain the intense inner radiation belt (electrons >50 ${ >} 50$ MeV) at Jupiter and its synchrotron radiation. In order to better understand the synergistic effect of radial transport and local wave‐particle interactions driven by whistler‐mode waves on the formation of Jupiter's radiation belt, we perform 3‐D Fokker‐Planck simulations for Jovian energetic electrons with the Versatile Electron Radiation Belt code. An empirical model of Jovian whistler‐mode waves updated with measurements from the Juno extended mission is used to quantify the local acceleration and pitch angle scattering. Resonant cyclotron acceleration by whistler‐mode waves leads to significant enhancement in the intensity of electrons above 1 MeV in the middle magnetosphere. Radial diffusion is capable of transporting MeV electrons accelerated by outer‐belt whistler‐mode waves into the M<10 $M< 10$ region, where they are further accelerated adiabatically to energies of about 10 MeV. Plain Language Summary: To sustain the most intense electron radiation belt in our solar system, Jupiter's magnetosphere needs to pre‐accelerate electrons to the energy level of multi‐MeV outside the orbit of Io. How such "seed electrons" of the synchrotron radiation belt are effectively accelerated is a challenging question. Gyro‐resonance between electrons and whistler‐mode waves is believed to be a potential candidate for such pre‐acceleration process. In this study, the efficiency of particle acceleration driven by Jovian whistler‐mode waves is quantitatively evaluated. With the quasilinear scattering rate calculated with the updated Jovian whistler‐mode wave model derived from Juno measurements, 3‐D diffusion simulations show that whistler‐mode waves between 5 and 25 RJ ${R}_{J}$ are capable of accelerating electrons upto about 10 MeV in tens of days. The recently discovered outer whistler‐mode belt is found to contribute to the acceleration of multi‐MeV electrons. Key Points: Scattering rates of electrons are quantified with the latest Jovian whistler‐mode wave model from Juno PJ01‐45 dataWhistler‐mode waves are able to accelerate MeV electrons in the middle magnetosphere of Jupiter in tens of daysElectrons accelerated in the outer whistler‐mode belt are further transported inward, indicating a two‐step acceleration mechanism [ABSTRACT FROM AUTHOR]

The spatial distribution of whistler‐mode wave emissions in the Jovian magnetosphere measured during the first 45 perijove orbits of Juno is investigated. A double‐belt structure in whistler‐mode wave intensity is revealed. Between the two whistler‐mode belts, there exists a region devoid of 100 s keV electrons near the magnetic equator at 90.1 fceq) intensity is revealedNear‐equatorial energetic electron distributions measured by JEDI show similar radial and azimuthal profile as whistler‐mode wavesThe dawn‐dusk asymmetry in the outer whistler‐mode belt intensity corresponds to either the abundance of source electrons or auroral hiss [ABSTRACT FROM AUTHOR]

Magnetic reconnection has been commonly reported between the solar wind IMF and the magnetic field of Earth and other planets. A similar phenomenon is expected between Jupiter's magnetosphere and Ganymede's mini magnetosphere inside the Jovian magnetosphere. This article is the first report of a reconnection event in the tail region of Ganymede. We present compelling evidence that Juno flew in close proximity to an X‐line, that was not within the tail current sheet, but rather in the turbulent wake area of Ganymede. We report the observation of distinctive electron Bernstein mode waves with unique characteristics particular to a separatrix region of the reconnection site. We detect a clear reversal of a magnetic field component. Electron densities and pitch angle distributions also indicate that Juno possibly traversed the inflow, and outflow region surrounding the separatrix region. Finally, from the time sequence of the observations by the different instruments on Juno, we reconstruct a likely trajectory of Juno around the reconnection site. Key Points: Juno passed near a magnetic reconnection X‐line in Ganymede's downstream region on 7 June 2021Juno Waves observations provide a positive indication of being in the reconnection separatrix siteJuno JADE electron measurements show the flux increase and pitch angle distributions expected in a separatrix region [ABSTRACT FROM AUTHOR]

Juno's highly elliptical polar orbits provide unprecedented in‐situ observations of the electrodynamic interaction between Jupiter and its volcanic moon Io. These observations occur in regions never sampled before both near Io's orbit and near Jupiter's ionosphere and at distances between the two. Magnetic field data obtained during multiple traversals of magnetic field lines mapping to Io's orbit reveal remarkably rich and complex magnetic signatures near flux tubes connected to Io's orbital position. Here we present a methodology to model the distribution of currents along Io's flux tube (IFT) and Alfvén wings in such a way as to match the magnetic field signature observed during Juno's traversals of the IFT and Alfvén wings downstream of Io. We obtain the location, size and morphology of the current‐carrying region as well as the distribution of currents within the IFT and Alfvén wings. The observed field‐aligned currents exhibit strong filamentation, with upward and downward currents splitting into secondary cells rather than forming uniform structures. Additionally, there is a strong correlation between total field‐aligned current intensity, particle energy flux, and Poynting flux, indicating efficient energy transfer and coupling in the Jupiter‐Io system. Using all of Juno's traversals up to perijove (PJ) pass 42, we estimate the strength of the interaction with regards to distance along Io's extended tail, Io's position in the plasma torus and the magnetic field intensity at the footprint in Jupiter's ionosphere, illuminating the interaction of Jovian magnetospheric plasma with Io and setting important constraints in the Io‐Jupiter interaction. Plain Language Summary: This study explores the electrodynamic interaction between Jupiter and its moon Io using data from NASA's Juno spacecraft. Juno's unique polar orbits allow it to gather unprecedented in‐situ observations near Io's orbit and Jupiter's ionosphere. By analyzing magnetic field data collected during Juno's encounters of magnetic field lines connected to Io's orbit, intricate magnetic signatures near Alfvén wings linked to Io are revealed. We develop a method to model the distribution of currents along Io's Alfvén wings and determine the location, size, and morphology of the currents within Io's Alfvén wings. The results presented in this paper provide insights into the strength of the interaction between Io and Jupiter's magnetosphere, shedding light on the dynamics of plasma interactions and providing valuable constraints for understanding the Io‐Jupiter system. Key Points: A new approach in modeling the current distribution in Io's flux tube and Alfvén wings, offers deeper insights into Io‐Jupiter interactionThe strength of the Io‐Jupiter interaction is adequately characterized by a generalized model based on three independent physical parametersAssessing the strength of the interaction across various physical parameters highlights the complexity of the Io‐Jupiter electrical coupling [ABSTRACT FROM AUTHOR]

We present statistical distributions of whistler‐mode chorus and hiss waves at frequencies ranging from the local proton gyrofrequency to the equatorial electron gyrofrequency (fce,eq) in Jupiter's magnetosphere based on Juno measurements. The chorus wave power spectral densities usually follow the fce,eq variation with major wave power concentrated in the 0.05fce,eq–fce,eq frequency range. The hiss wave frequencies are less dependent on fce,eq variation than chorus with major power concentrated below 0.05fce,eq, showing a separation from chorus at M < 10. Our survey indicates that chorus waves are mainly observed at 5.5 < M < 13 from the magnetic equator to 20° latitude, consistent with local wave generation near the equator and damping effects. The hiss wave powers extend to 50° latitude, suggesting longer wave propagation paths without attenuation. Our survey also includes the whistler‐mode waves at high latitudes which may originate from the Io footprint, auroral hiss, or propagating hiss waves reflected to high M shells. Plain Language Summary: Whistler‐mode chorus and hiss waves in Jupiter's magnetosphere are major plasma wave modes, characterized by perturbations in electric and magnetic fields at frequencies from the proton gyrofrequency to the electron gyrofrequency. Chorus waves are typically observed at 0.05fce,eq–fce,eq frequencies (fce,eq is the electron gyrofrequency at the equator) with coherent wave structures. Chorus waves, generated by hot electrons, could cause electron precipitation into the atmosphere and acceleration in the radiation belt. In contrast, hiss waves are usually incoherent with wave frequencies less dependent on fce,eq than chorus. Hiss waves have mixed sources and mainly drive energetic electron loss. Using Juno satellite measurements, we analyze the statistical distribution of chorus and hiss waves in Jupiter's magnetosphere. Our survey reveals different latitudinal coverages and statistical properties of chorus and hiss waves, suggesting their different sources and damping effects. Additionally, our survey includes whistler‐mode waves at high latitudes, potentially originating from various sources such as the Io footprint at the ionosphere, auroral hiss, or reflection of hiss waves at high M shells. The whistler‐mode wave distributions from our study provide valuable insights for future modeling of whistler‐mode wave sources and energetic electron dynamics in Jupiter's magnetosphere. Key Points: Intense chorus waves at 0.05–1 equatorial electron gyrofrequencies (fce,eq) are observed at 5.5 < M < 13 within 20° magnetic latitudesHiss waves from 50 Hz to 0.05 fce,eq have extended latitudinal coverage up to 50° and exhibit propagation effectsHigh latitude (>50°) whistler‐mode waves at 0.05–1 fce,eq are observed in two groups due to different sources [ABSTRACT FROM AUTHOR]

Interchange instability is known to drive fast radial transport of electrons and ions in Jupiter's inner and middle magnetosphere. In this study, we conduct a statistical survey to evaluate the properties of energetic particles and plasma waves during interchange events using Juno data from 2016 to 2023. We present representative examples of interchange events followed by a statistical analysis of the spatial distribution, duration and spatial extent. Our survey indicates that interchange instability is predominant at M‐shells from 6 to 26, peaking near 17 with an average duration of minutes and a corresponding M‐shell width of <∼0.05. During interchange events, the associated plasma waves, such as whistler‐mode, Z‐mode, and electron cyclotron harmonic waves exhibit a distinct preferential location. These findings provide valuable insights into particle transport and the source region of plasma waves in the Jovian magnetosphere, as well as in other magnetized planets within and beyond our solar system. Plain Language Summary: The radial transport of plasma around a magnetized planet is crucial for understanding the underlying magnetospheric dynamics. Jupiter's magnetospheric dynamics are primarily dominated by the rapid rotation and plasma source from Io. This rapid rotation drives the interchange instability, where hot, low‐density plasma is moved toward the inner magnetosphere. During this process, the inward moving flux tube builds up magnetic pressure, potentially leading to the trapping of particles alongside plasma waves. In this study, we present several typical examples of interchange events, and conduct a statistical analysis to explore their spatial distribution, duration and spatial extent, as well as the typical features of the associated plasma waves. This survey provides insights into mass transport, the source of these plasma waves in Jupiter's magnetosphere, with potential implications for other magnetized planets within and beyond our solar system. Key Points: Our statistical survey indicates that interchange events occur over L (or M)‐shells ∼6–26 at Jupiter with a peak occurrence rate at M ∼ 17During interchange events, various types of plasma waves are intensified, each exhibiting a distinct preferential locationThe duration and the corresponding spatial extent of interchange events are analyzed for multiple events [ABSTRACT FROM AUTHOR]

Juno conducted two close Io flybys on 30 December 2023 and 03 February 2024, both at a minimum altitude of 1,500 km. Filamentary structures in the electric and magnetic field spectra indicate Juno crossed the Alfvén wing, the magnetic structure connecting Io to Jupiter's polar ionosphere. We show that the first pass took Juno diametrically through the northern Alfvén wing, while the second pass had Juno graze the southern Alfvén wing boundary, enabling extended measurements of the transition region between Io's vicinity and the Jovian magnetosphere. Of note, evidence of local electron beams is inferred from whistler‐mode emissions. We demonstrate that their energies are sub‐keV, are sourced from Io's ionosphere or local torus, and are part of a distributed current system connecting Io to Jupiter. Finally, upper hybrid resonances indicate electron densities are significantly elevated in Io's polar region (∼28,000 cm−3) compared to the local Io torus (∼2,000 cm−3). Plain Language Summary: Juno conducted two close Io flybys. Signatures in the electromagnetic field indicate that Juno sampled the region dominated by the electrodynamic interaction between Io and Jupiter. Powerful electric currents flow within this region that deliver energetic electrons into Jupiter's polar atmosphere, resulting in the Io auroral footprint. Juno also spent an extended period along the boundary of this region, providing an in situ view of the complexity of this region's structure. Local density measurements indicate significantly elevated densities in Io's poles. Key Points: Juno plasma wave observations are consistent with Alfvén wing crossings during close Io flybysWhistler‐mode radiation indicate the presence of sub‐keV electron beams operating just outside the Alfvén wingElectron densities are significantly elevated in Io's polar region [ABSTRACT FROM AUTHOR]

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 polesThis mechanism is a universal process that can operate at any satellite‐planet system with adequate interaction conditionsPlasma flow within Io's Alfvén wing is slowed to ≤12% of its upstream value, allowing for filamentation [ABSTRACT FROM AUTHOR]

We survey crossings of Jupiter's dawn magnetopause during the Juno prime mission to identify and characterize Jupiter's magnetopause boundary layer. Using plasma and magnetic field observations from Jovian Auroral Distributions Experiment and Juno Magnetic Field investigation, we identify 53 boundary layer events from the 62 magnetopause crossings studied here. We find that the boundary layer generally exhibits mixed properties of magnetosheath and magnetosphere electron distributions, including lower characteristic electron energies and denser ion populations than in the magnetosphere, but higher characteristic electron energies and less dense ion populations than in the magnetosheath. Boundary layer proton speeds are on average slower than both the magnetosheath and magnetosphere. Other proton parameters in the boundary layer have intermediate values between the magnetosheath and magnetosphere. Through ion composition analysis in regions adjacent to the magnetopause, we find evidence of solar wind and magnetospheric plasma in the boundary layer that suggests plasma is transported across the magnetopause in both directions. This mass and energy transport may be the result of solar wind interactions such as magnetic reconnection and Kelvin‐Helmholtz instabilities. However, many boundary layer events do not exhibit local signatures of these solar wind interactions and plasma may be transported by a non‐local process or diffusively transported. Plain Language Summary: Jupiter has the strongest planetary magnetic field in the solar system and over one metric ton per second of sulfur and oxygen is ejected via volcanic activity on the moon, Io. These two factors inflate Jupiter's magnetosphere as material from Io slowly travels outward from Jupiter to near the magnetopause, the boundary between the magnetosphere and magnetosheath. Jupiter's magnetopause boundary layer is located between its outer magnetosphere and surrounding magnetosheath. The magnetosheath is a region of primarily solar wind plasma between the bow shock and the magnetopause and is draped over the magnetosphere. We find that the dawnside boundary layer that separates the magnetosphere and the magnetosheath is comprised of plasma from both regions and travels across this region due to a variety of physical processes. Additionally, we find solar wind plasma in the magnetosphere and plasma from Io in the magnetosheath. This result implies that plasma travels across the boundary layer in both directions. Lastly, our survey shows the plasma in the dawnside boundary layer moves in the opposite direction of Jupiter's rotation, contrary to the plasma located closer to Jupiter. Key Points: The primary characteristic of Jupiter's magnetopause boundary layer is a dual distribution of magnetosphere and magnetosheath electronsIon composition in the boundary layer and adjacent regions show that plasma is transported across the magnetopause in both directionsIon flow speeds in Jupiter's dawn flank magnetopause boundary layer are hundreds of km/s tailward, opposite to corotation on the dawnside [ABSTRACT FROM AUTHOR]

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 flybyThe beams enhance electron‐impact‐excited emissions in Europa's atmosphere and are associated with large jumps of the magnetic fieldsThe beams' proximity to Europa and their pitch angle distribution constrain the source acceleration to be near or within the plasma disk [ABSTRACT FROM AUTHOR]

Energetic particle injections are commonly observed in Jupiter's magnetosphere and have important impacts on the radiation belts. We evaluate the roles of electron injections in the dynamics of whistler‐mode waves and relativistic electrons using Juno measurements and wave‐particle interaction modeling. The Juno spacecraft observed injected electron flux bursts at energies up to 300 keV at M shell ∼11 near the magnetic equator during perijove‐31. The electron injections are related to chorus wave bursts at 0.05–0.5 fce frequencies, where fce is the electron gyrofrequency. The electron pitch angle distributions are anisotropic, peaking near 90° pitch angle, and the fluxes are high during injections. We calculate the whistler‐mode wave growth rates using the observed electron distributions and linear theory. The frequency spectrum of the wave growth rate is consistent with that of the observed chorus magnetic intensity, suggesting that the observed electron injections provide free energy to generate whistler‐mode chorus waves. We further use quasilinear theory to model the impacts of chorus waves on 0.1–10 MeV electrons. Our modeling shows that the chorus waves could cause the pitch angle scattering loss of electrons at <1 MeV energies and accelerate relativistic electrons at multiple MeV energies in Jupiter's outer radiation belt. The electron injections also provide an important seed population at several hundred keV energies to support the acceleration to higher energies. Our wave‐particle interaction modeling demonstrates the energy flow from the electron injections to the relativistic electron population through the medium of whistler‐mode waves in Jupiter's outer radiation belt. Key Points: Chorus wave generation due to electron injections is demonstrated by their correlative occurrence and wave instability analysisWhistler‐mode waves scatter electrons into the loss cone and cause diffuse auroral precipitation with intensities of 60–160 erg/cm2/sChorus waves cause local acceleration of MeV electrons in several days, further aided by the seed electron population from injections [ABSTRACT FROM AUTHOR]

Author(s): Y. Kaspi (corresponding author) [1]; E. Galanti [1]; W. B. Hubbard [2]; D. J. Stevenson [3]; S. J. Bolton [4]; L. Iess [5]; T. Guillot [6]; J. Bloxham [7]; [...]

Author(s): T. Guillot (corresponding author) [1]; Y. Miguel [1, 2]; B. Militzer [3]; W. B. Hubbard [4]; Y. Kaspi [5]; E. Galanti [5]; H. Cao [6, 7]; R. Helled [8]; [...]

Author(s): A. Adriani (corresponding author) [1]; A. Mura [1]; G. Orton [2]; C. Hansen [3]; F. Altieri [1]; M. L. Moriconi [4]; J. Rogers [5]; G. Eichstdt [6]; T. Momary [...]

Author(s): B. H. Mauk (corresponding author) [1]; D. K. Haggerty [1]; C. Paranicas [1]; G. Clark [1]; P. Kollmann [1]; A. M. Rymer [1]; S. J. Bolton [2]; S. M. [...]