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On days 2023‐364 and 2024‐034, the Juno spacecraft made close passages of Jupiter's moon Io, at altitudes of about 1,500 km. Data obtained from the first flyby, when the spacecraft was on magnetic field lines connected to both Jupiter and Io, revealed deep flux decreases. In addition, Juno's energetic particle detectors observed tens to hundreds of keV electron and proton beams. Such beams could be generated near Jupiter on field lines associated with Io. The second encounter occurred in the plasma wake and a more modest flux decrease was observed. Furthermore, data from both encounters suggest a spatially extensive decrease in >1 MeV electrons that includes regions inward of Io's orbit. In the immediate vicinity of Io, signatures of absorption likely dominate the data whereas diffusion and wave‐particle interactions are expected to be needed to understand MeV electron data in the wider spatial region around Io. Jupiter's magnetospheric plasma overtakes Io in its orbit. This causes a cavity to be formed over the hemisphere of Io that leads its orbital motion. Within this cavity subtle details of the plasma and energetic charged particle environment can be extracted from the signal, usually dominated by radiation at these distances. Examples include beams traveling in the direction from Jupiter to the moon, with little evidence of reflection at Io. Another major finding of this work is that there is a large region both along Io's orbit and even radially inward of it with a lower level of MeV electron flux. Guided by previous modeling, the most likely candidates for shaping the data are absorption by the moon, wave‐particle interactions that can scatter particles into Jupiter's atmosphere, and diffusion. Deep flux decreases over Io and in its immediate wake caused by losses to the moon's surface were observed in Juno JEDI dataJuno JEDI detected narrow field‐aligned beams of electrons and protons directed toward Io's north poleMoon absorption and wave‐particle interactions may explain the loss of >1 MeV electrons that extend both inward and outward of Io's orbit Deep flux decreases over Io and in its immediate wake caused by losses to the moon's surface were observed in Juno JEDI data Juno JEDI detected narrow field‐aligned beams of electrons and protons directed toward Io's north pole Moon absorption and wave‐particle interactions may explain the loss of >1 MeV electrons that extend both inward and outward of Io's orbit

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 Mshell ∼11 near the magnetic equator during perijove‐31. The electron injections are related to chorus wave bursts at 0.05–0.5 fcefrequencies, where fceis 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. 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 Chorus wave generation due to electron injections is demonstrated by their correlative occurrence and wave instability analysis Whistler‐mode waves scatter electrons into the loss cone and cause diffuse auroral precipitation with intensities of 60–160 erg/cm2/s Chorus waves cause local acceleration of MeV electrons in several days, further aided by the seed electron population from injections

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 orbitWe study the variability of electron fluxes inside and outside the Jovian magnetodisk as well as within Ganymede's magnetosphereGalileo‐based models underestimate the electron fluxes observed by Juno in particular at high energies 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

Juno passed by Europa at an altitude of 355 km on 29 September, day 272, 2022. As one of Juno's in situ science instruments, the Waves instrument obtained observations of plasma waves that are essential contributors to Europa's interaction with its environment. Juno observed chorus, a band at the upper hybrid frequency providing the local plasma density, and electrostatic solitary structures in the wake. In addition, impulses due to micron‐sized dust impacts on Juno were recorded with a local maximum very close to Europa. The peak electron density near Europa was ∼330 cm−3while the surrounding magnetospheric density was in the range of 50–150 cm−3. There was a significant separation between the Europa flyby and Juno's crossing of Jupiter's magnetic equator, enabling a unique identification of effects associated with the moon as opposed to magnetospheric phenomena normally occurring at the magnetic equator near 10 Jovian radii. Plasma waves are electromagnetic fields occurring in a plasma due to motions of the charged particles comprising the plasma. These waves can arise at various locations and at a range of frequencies depending on many factors, such as the number density of charged particles and the strength of the magnetic field. Here we discuss plasma waves observed by Juno during its 355‐km flyby of Europa on 29 September 2022. Some waves, called upper hybrid resonance emissions can provide information on the plasma density. Other waves, called electrostatic solitary waves are indicative of electron beams in the plasma. And yet other waves, called whistler‐mode chorus, are important in the interchange of energy between electrons and the waves, resulting in the acceleration of the electrons. Each of these types of waves were observed near Europa by the Juno plasma wave instrument and they are diagnostic of Europa's interaction with the Jovian magnetosphere. The Waves instrument also detects electrical impulses due to the collision of the spacecraft with dust grains moving at over 23 km/s that allow a determination of the concentration of dust near Europa. Two chorus bands, electrostatic solitary waves and upper hybrid emissions are observed at EuropaPlasma densities near Europa derived from the upper hybrid resonance frequency peak near the wake axis at about 330 cm−3Micron‐sized dust impacts peak near closest approach to Europa Two chorus bands, electrostatic solitary waves and upper hybrid emissions are observed at Europa Plasma densities near Europa derived from the upper hybrid resonance frequency peak near the wake axis at about 330 cm−3 Micron‐sized dust impacts peak near closest approach to Europa

Interchange instability is known to drive fast radial transport of particles in Jupiter's inner magnetosphere. Magnetic flux tubes associated with the interchange instability often coincide with changes in particle distributions and plasma waves, but further investigations are required to understand their detailed characteristics. We analyze representative interchange events observed by Juno, which exhibit intriguing features of particle distributions and plasma waves, including Z‐mode and whistler‐mode waves. These events occurred at an equatorial radial distance of ∼9 Jovian radii on the nightside, with Z‐mode waves observed at mid‐latitude and whistler‐mode waves near the equator. We calculate the linear growth rate of whistler‐mode and Z‐mode waves based on the observed plasma parameters and electron distributions and find that both waves can be locally generated within the interchanged flux tube. Our findings are important for understanding particle transport and generation of plasma waves in the magnetospheres of Jupiter and other planetary systems. The centrifugal interchange instability, which has been observed in rapidly rotating planets, like Saturn and Jupiter, moves cold plasmas inside of the magnetosphere further away, and transports hotter, less dense plasmas toward the inner magnetosphere. These moving flux tubes have been observed at Jupiter together with plasma waves, but their detailed characteristics are not fully understood. In the present study, we use observations from the Juno spacecraft to report multiple representative interchange events and evaluate the properties of energetic particles and plasma waves. Furthermore, we use linear theory to calculate the growth rates of Z‐mode and whistler‐mode waves during these events. Our findings reveal the typical features of plasma waves and particles during interchange events, which provide important insights into particle transport and generation of plasma waves at Jupiter and possibly other magnetized planets in our solar system and beyond. Several plasma transport events associated with interchange instability are identified alongside plasma waves using Juno observationsLinear growth rate analyses indicate that waves can be locally generated during interchange events due to anisotropic electron distributionsOur findings provide insights into electron transport and plasma wave dynamics during interchange events in planetary magnetospheres Several plasma transport events associated with interchange instability are identified alongside plasma waves using Juno observations Linear growth rate analyses indicate that waves can be locally generated during interchange events due to anisotropic electron distributions Our findings provide insights into electron transport and plasma wave dynamics during interchange events in planetary magnetospheres

We present simultaneous observations of aurorae at Jupiter from the Hubble Space Telescope and Hisaki, in combination with the in situ measurements of magnetic field, particles, and radio waves from the Juno Spacecraft in the outer magnetosphere, from ~ 80RJto 60RJduring 17 to 22 March 2017. Two cycles of accumulation and release of magnetic flux, named magnetic loading/unloading, were identified during this period, which correlate well with electron energization and auroral intensifications. Magnetic reconnection events are identified during both the loading and unloading periods, indicating that reconnection and unloading are independent processes. These results show that the dynamics in the middle magnetosphere are coupled with auroral variability. Accumulation and release of magnetic flux in the middle Jovian magnetosphere modulate auroral intensificationsMagnetic reconnection process occurs independently of Jupiter's global loading and unloading of magnetic fluxWe provide direct evidence that unloading of magnetic flux causes enhancements of auroral kilometric emissions

We present comparisons of precipitating electron flux and auroral brightness measurements made during several Juno transits over Jupiter's auroral regions in both hemispheres. We extract from the ultraviolet spectrograph (UVS) spectral imager H2emission intensities at locations magnetically conjugate to the spacecraft using the JRM09 model. We use UVS images as close in time as possible to the electron measurements by the Jupiter Energetic Particle Detector Instrument (JEDI) instrument. The upward electron flux generally exceeds the downward component and shows a broadband energy distribution. Auroral intensity is related to total precipitated electron flux and compared with the energy‐integrated JEDI flux inside the loss cone. The far ultraviolet color ratio along the spacecraft footprint maps variations of the mean energy of the auroral electron precipitation. A wide diversity of situations has been observed. The intensity of the diffuse emission equatorward of the main oval is generally in fair agreement with the JEDI downward energy flux. The intensity of the ME matches exceeds or remains below the value expected from the JEDI electron energy flux. The polar emission may be more than an order of magnitude brighter than associated with the JEDI electron flux in association with high values of the color ratio. We tentatively explain these observations by the location of the electron energization region relative to Juno's orbit as it transits the auroral region. Current models predict that the extent and the altitude of electron acceleration along the magnetic field lines are consistent with this assumption. We compare precipitated electron flux measured with JEDI to the auroral intensity and FUV color ratio observed with UVS at Juno's footprintThe different UV auroral features generally map well with the corresponding structures measured in the precipitated electron energy fluxComparison of energy flux at the two levels reveal a diversity of situations probably related to the location of the acceleration region

The Jovian polar regions produce X‐rays that are characteristic of very energetic oxygen and sulfur that become highly charged on precipitating into Jupiter's upper atmosphere. Juno has traversed the polar regions above where these energetic ions are expected to be precipitating revealing a complex composition and energy structure. Energetic ions are likely to drive the characteristic X‐rays observed at Jupiter (Haggerty et al., 2017, https://doi.org/10.1002/2017GL072866; Houston et al., 2018, https://doi.org/10.1002/2017JA024872; Kharchenko et al., 2006, https://doi.org/10.1029/2006GL026039). Motivated by the science of X‐ray generation, we describe here Juno Jupiter Energetic Particle Detector Instrument (JEDI) measurements of ions above 1 MeV and demonstrate the capability of measuring oxygen and sulfur ions with energies up to 100 MeV. We detail the process of retrieving ion fluxes from pulse width data on instruments like JEDI (called “puck's”; Clark, Cohen, et al., 2016, https://doi.org/10.1002/2017GL074366; Clark, Mauk, et al., 2016, https://doi.org/10.1002/2015JA022257; Mauk et al., 2013, https://doi.org/10.1007/s11214‐013‐0025‐3) as well as details on retrieving very energetic particles (>20 MeV) above which the pulse width also saturates. The Juno mission has observed high‐energy, heavy ions of the sort that are thought to be responsible for X‐Rays from Jupiter's poles. These heavy ions originate from Jupiter's volcanic moon Io and eventually precipitate into and interact with Jupiter's atmosphere resulting in X‐Ray emission. The Juno JEDI instrument is shown to have the unplanned capability to measure heavy ions to energies as high as 100 MeVAs such, the JEDI instrument has the capability to measure those ions needed to generate polar X‐rays at Jupiter (greater than tens of megaelectron volts O and/or S)Although not yet directly correlated with polar X‐rays, we show that heavy ions up to 100 MeV are indeed observed over Jupiter's polar regions

The polar orbit of Juno at Jupiter provides a unique opportunity to observe high‐latitude energetic particle injections. We measure energy‐dispersed impulsive injections of protons and electrons. Ion injection signatures are just as prevalent as electron signatures, contrary to previous equatorial observations. Included are previously unreported observations of high‐energy banded structures believed to be remnants of much earlier injections, where the particles have had time to disperse around Jupiter. A model fit of the injections used to estimate timing fits the shape of the proton signatures better than it does the electron shapes, suggesting that electrons and protons are different in their abilities to escape the injection region. We present ultaviolet observations of Jupiter's aurora and discuss the relationship between auroral injection features and in situ injection events. We find, unexpectedly, that the presence of in situ particle injections does not necessarily result in auroral injection signatures. High‐latitude observations at Jupiter reveal features of injections (L < 15 RJ) and associated auroral signatures not previously reportedIncluded are a near equality of electron and ion injections, puzzling differences between their signatures and signatures of old injectionsThere is no one‐to‐one correspondence between the in situ particle and auroral signatures of injections, contrary to literature expectations

We present a continuing investigation of mass‐/charge‐dependent interactions between energetic ions (greater than tens of kiloelectron volts) and planetary magnetopauses and of the escape of the ions across the boundary. Previous studies at Earth using Magnetospheric Multiscale mission data are refined and advanced showing profound behavior differences between light (H, He) and singly charged heavy ions (O+). We highlight a distinctive feature of oxygen ions: an angular distribution bifurcation providing clear indication of entrainment along the magnetopause in Speiser‐like orbits during relatively stable magnetic conditions. This signature, interpreted using a simple kinetic model, suggests that these ions tend to be carried substantial distances along the boundary (even with boundary‐normal magnetic fields) in a fashion that impedes their full dayside escape. While large fluctuations and waves can likely sometimes disrupt the observed ordering, the following picture emerges. Energetic particles with gyroradii much smaller than the magnetopause thickness (e.g., electrons and absent boundary‐normal magnetic fields) and ions with gyroradii much larger than the thickness (e.g., O+) are impeded from fully escaping across the boundary. However, energetic ions with intermediate‐sized gyroradii commensurate with the thickness (e.g., H+, He++, and O6+) can be effectively scattered within the boundary causing them to escape much more readily, with and without boundary‐normal fields. This picture is supported by observations from the Juno spacecraft at the near‐dawn meridian side of Jupiter's magnetopause. There it is observed that energetic electrons and heavy ions are more strongly contained by the magnetopause than are the energetic protons and helium ions. Strongly magnetized planets, like Earth, Jupiter, Saturn, Uranus, and Neptune, have space environments, called magnetospheres, that energize charge particles, like electrons and ions (charged atoms), to very high energies (thousands to millions of electron volts). Some of these energetic charge particles can leak across the “magnetopause” boundary that separates the magnetosphere from the interplanetary environment and become a part of that vast region of the sun‐generated solar wind. Charge particles in a magnetic field gyrate around in circular “gyro‐orbits.” The heavier the charged particle, the larger is its gyro‐orbit. We find in this paper that the ability of the ions to leak across the magnetopause boundary depends on the sizes of the gyro‐orbits in a counterintuitive fashion. Traditionally, it has been thought that the larger the gyroradii, the easier it is for the particle to escape. Here we find that ions (and electrons) with the smaller gyroradii and ions with the larger gyroradii (heavy ions like those of oxygen) are both impeded in their abilities to leak across the magnetopause boundary. It is the ions with intermediate‐sized gyroradii (like protons), with gyroradii sizes close the thickness of the magnetopause that find it easiest to escape across the boundary. Earth's >100‐keV‐injected magnetospheric O+ions encountering the magnetopause tend to stay with it and are impeded from fully escapingEnergetic protons, with gyroradii similar to magnetopause thicknesses, are more likely to scatter within the boundary and escapeObservations at Jupiter's dawn magnetopause also show that large gyroradii ions appear less likely to escape than intermediate ones

A shock in the solar wind was observed shortly after 05:00 by heliospheric spacecraft stationed at L1. Behind the shock, protons and electrons in the tens to hundreds of keV energy range were found to be enhanced several‐fold. The shock reached the Earth precisely at 06:00 and was detected by particle sensors aboard GOES and ARTEMIS, and by ground‐based magnetometers. In addition, the arrival of the shock was seen promptly by the magnetometer investigation aboard MMS, which at the time was located 16 REdown the geomagnetic tail. However, the arrival of the energetic particles at MMS was delayed for ~23 min, and first arrived moving earthward up the geomagnetic tail. Timing considerations between the solar wind speed and the energetic particles indicated that the solar particles first gained access to field lines connected to MMS around 115 REdownstream. Once the shock and its entrained solar particles passed the access region, the solar particles were no longer seen. A shock arrived at the Earth at 06:00 on 16 July 2017 behind which was a region of increased solar energetic particles as observed by several near‐Earth spacecraftTwenty‐three minutes later, MMS observed Earth‐directed particles in the magnetotail with the same composition, spectra, and time profilesThe sum of the observations indicate that there must have been a reconnection point located around 100 RE downstream of the Earth giving access to the magnetotail

We compare electron and UV observations mapping to the same location in Jupiter's northern polar region, poleward of the main aurora, during Juno perijove 5. Simultaneous peaks in UV brightness and electron energy flux are identified when observations map to the same location at the same time. The downward energy flux during these simultaneous observations was not sufficient to generate the observed UV brightness; the upward energy flux was. We propose that the primary acceleration region is below Juno's altitude, from which the more intense upward electrons originate. For the complete interval, the UV brightness peaked at ~240 kilorayleigh (kR); the downward and upward energy fluxes peaked at 60 and 700 mW/m2, respectively. Increased downward energy fluxes are associated with increased contributions from tens of keV electrons. These observations provide evidence that bidirectional electron beams with broad energy distributions can produce tens to hundreds of kilorayleigh polar UV emissions. Jupiter's ultraviolet (UV) aurora is produced by electrons that precipitate into the planet's atmosphere and interact with hydrogen molecules. A number of different UV auroral emission regions have been identified such as the main aurora, the aurora associated with Jupiter's satellites, and the polar aurora located poleward of the main aurora. We examine electron and UV observations from Juno in Jupiter's northern polar region to investigate the processes responsible for producing Jupiter's polar aurora. We show electrons and UV emissions having simultaneous enhancements during a time when they map to the same location of Jupiter's upper atmosphere at the same time. We present evidence that electrons with energies between 0.1 and 100 kilo electron volts (keV) are capable of producing the polar UV emissions studied here and that further acceleration of these electrons may be occurring at altitudes below the spacecraft. Simultaneous peaks are observed in electron energy flux and UV brightness when they map to same location in Jupiter's polar region at the same timeUpward greater than downward electron energy fluxes are observed, suggesting that primary acceleration region may be below ~1.5 jovian radiiDownward energy fluxes able to produce tens to hundreds of kilorayleigh polar UV emissions are identified; increases in energy flux due to tens of keV electrons

Optical remote sensing observations have suggested that the top layer of Europa's icy surface is heavily affected by external weathering agents. To model and understand these effects, it is necessary to characterize the environment as fully as possible. In this paper, we focus on one agent in the environment (energetic electrons). We show Juno electron data from its 2022 Europa flyby and other time periods. While the Juno sensor used here (Jupiter Energetic Particle Detector Instrument) was not designed to obtain high quality electron data in an intense radiation environment, it is possible to extract information such as how Europa blocks energetic particles from accessing some of the surrounding space. The decrease in charged particle flux in Europa's wake provides an upper limit on the precipitation fluxes of the same particles. We also report that electron pitch angle distributions near Europa for the single energy channel considered here are time variable and not isotropic. We present energetic electron data obtained when the Juno spacecraft crossed the plasma wake of the moon Europa in 2022. A major goal of this work is to estimate the amount of blockage of MeV electrons near the moon to inform radiation estimates for future Europa orbiters and landers. While the Jupiter Energetic Particle Detector Instrument does not directly detect particles between 1 and 20 MeV, using knowledge gained since the instrument began obtaining data we were able to estimate that about 80% of the particles in question are blocked by Europa in its wake at low altitude. Research on the weathering of Europa's surface by electrons is also discussed. Jupiter Energetic Particle Detector Instrument electron data relevant to Europa are presentedWe estimate Europa's blockage causes an ∼80% reduction of ∼1–20 MeV electrons in the immediate plasma wakePitch angle distributions are computed from 70 keV electron data and show time variability Jupiter Energetic Particle Detector Instrument electron data relevant to Europa are presented We estimate Europa's blockage causes an ∼80% reduction of ∼1–20 MeV electrons in the immediate plasma wake Pitch angle distributions are computed from 70 keV electron data and show time variability

The radiation belts and magnetospheres of Jupiter and Saturn show significant intensities of relativistic electrons with energies up to tens of megaelectronvolts (MeV). To date, the question on how the electrons reach such high energies is not fully answered. This is largely due to the lack of high‐quality electron spectra in the MeV energy range that models could be fit to. We reprocess data throughout the Galileo orbiter mission in order to derive Jupiter's electron spectra up to tens of MeV. In the case of Saturn, the spectra from the Cassini orbiter are readily available and we provide a systematic analysis aiming to study their acceleration mechanisms. Our analysis focuses on the magnetospheres of these planets, at distances of L> 20 and L> 4 for Jupiter and Saturn, respectively, where electron intensities are not yet at radiation belt levels. We find no support that MeV electrons are dominantly accelerated by wave‐particle interactions in the magnetospheres of both planets at these distances. Instead, electron acceleration is consistent with adiabatic transport. While this is a common assumption, confirmation of this fact is important since many studies on sources, losses, and transport of energetic particles rely on it. Adiabatic heating can be driven through various radial transport mechanisms, for example, injections driven by the interchange instability or radial diffusion. We cannot distinguish these processes at Saturn with our technique. For Jupiter, we suggest that the dominating acceleration process is radial diffusion because injections are never observed at MeV energies. Space is filled with a radiation of protons and electrons moving with almost light speed. While in free space this is cosmicradiation of extreme energies, magnetized planets trap radiation particles of lower energies, typically in the megaelectronvolt range. These are the so called radiation belts that are found, for example, around Jupiter, Saturn, and Earth. All radiation particles initially start out from being almost at rest and are over time accelerated to almost light speed. The physical mechanisms responsible for this are the subject of ongoing research. Here we focus on the processes accelerating electrons around Jupiter and Saturn based on data from the Galileo and Cassini orbiters. During their life, electrons change their orbitsaround a planet, get closer to the planetary surface, and are exposed to stronger magnetic fields that these planets are producing. It is known that such changes in magnetic field exposure are in principle able to accelerate particles. Here we find that this exposure is indeed the main reason for acceleration of electrons at relatively large distances from Jupiter and Saturn, not other candidate processes that could in principle also have been responsible. Electrons in Saturn's and Jupiter's magnetosphere, outside the most intense radiation belts, are accelerated through adiabatic transport

Large amplitude electromagnetic plasma waves are observed simultaneously with intense fluxes of electrons precipitating on auroral field lines at Jupiter. Here we present plasma wave observations from the Juno Waves instrument obtained during an instance of very intense broadband electron precipitation observed by the Jupiter Energetic Particle Detector Instrument connecting to Jupiter's main auroral oval. The wave spectrum extends from 50 Hz to ~10 kHz with peak‐to‐peak amplitudes of ~10 nT in the magnetic channel and of ~1 V/m in the electric channel, representing some of the most intense plasma waves observed by Juno. The Eand Bfields of these electromagnetic waves are correlated and have apparent polarization perpendicular to Jupiter's magnetic field with a downward Poynting flux. We conclude the plasma waves are whistler mode emissions with a possible admixture of ion‐cyclotron or Alfvén waves and may be important in the broadband electron acceleration. Large amplitude whistler mode waves are found coincidently with intense fluxes of precipitating electrons across a broad energy range connecting to Jupiter's main auroral oval. The whistler mode waves are propagating downward, in the same direction as the precipitating electrons. The tight correspondence in time between the waves and the electrons strongly suggests an important interaction between the waves and electrons. Intense plasma waves are observed simultaneously with broad energy spectrum of precipitating electronsThe waves are propagating in the whistler mode in the same direction as the electronsThe intense whistler mode waves are not observed during nearby inverted V events

Earth and its magnetosphere are immersed in the supersonic flow of the solar-wind plasma that fills interplanetary space. As the solar wind slows and deflects to flow around Earth, or any other obstacle, a ‘bow shock’ forms within the flow. Under almost all solar-wind conditions, planetary bow shocks such as Earth’s are collisionless, supercritical shocks, meaning that they reflect and accelerate a fraction of the incident solar-wind ions as an energy dissipation mechanism1,2, which results in the formation of a region called the ion foreshock3. In the foreshock, large-scale, transient phenomena can develop, such as ‘hot flow anomalies’4–9, which are concentrations of shock-reflected, suprathermal ions that are channelled and accumulated along certain structures in the upstream magnetic field. Hot flow anomalies evolve explosively, often resulting in the formation of new shocks along their upstream edges5,10, and potentially contribute to particle acceleration11–13, but there have hitherto been no observations to constrain this acceleration or to confirm the underlying mechanism. Here we report observations of a hot flow anomaly accelerating solar-wind ions from roughly 1–10 kiloelectronvolts up to almost 1,000 kiloelectronvolts. The acceleration mechanism depends on the mass and charge state of the ions and is consistent with first-order Fermi acceleration14,15. The acceleration that we observe results from only the interaction of Earth’s bow shock with the solar wind, but produces a much, much larger number of energetic particles compared to what would typically be produced in the foreshock from acceleration at the bow shock. Such autogenous and efficient acceleration at quasi-parallel bow shocks (the normal direction of which are within about 45 degrees of the interplanetary magnetic field direction) provides a potential solution to Fermi’s ‘injection problem’, which requires an as-yet-unexplained seed population of energetic particles, and implies that foreshock transients may be important in the generation of cosmic rays at astrophysical shocks throughout the cosmos.

The relationship between electron energy flux and the characteristic energy of electron distributions in the main auroral loss cone bridges the gap between predictions made by theory and measurements just recently available from Juno. For decades such relationships have been inferred from remote sensing observations of the Jovian aurora, primarily from the Hubble Space Telescope, and also more recently from Hisaki. However, to infer these quantities, remote sensing techniques had to assume properties of the Jovian atmospheric structure—leading to uncertainties in their profile. Juno's arrival and subsequent auroral passes have allowed us to obtain these relationships unambiguously for the first time, when the spacecraft passes through the auroral acceleration region. Using Juno/Jupiter Energetic particle Detector Instrument (JEDI), an energetic particle instrument, we present these relationships for the 30‐keV to 1‐MeV electron population. Observations presented here show that the electron energy flux in the loss cone is a nonlinear function of the characteristic or mean electron energy and supports both the predictions from Knight (1973, https://doi.org/10.1016/0032‐0633(73)90093‐7) and magnetohydrodynamic turbulence acceleration theories (e.g., Saur et al., 2003, https://doi.org/10.1029/2002GL015761). Finally, we compare the in situ analyses of Juno with remote Hisaki observations and use them to help constrain Jupiter's atmospheric profile. We find a possible solution that provides the best agreement between these data sets is an atmospheric profile that more efficiently transports the hydrocarbons to higher altitudes. If this is correct, it supports the previously published idea (e.g., Parkinson et al., 2006, https://doi.org/10.1029/2005JE002539) that precipitating electrons increase the hydrocarbon eddy diffusion coefficients in the auroral regions. Measured profiles of the energetic electron energy flux and characteristic energies in Jupiter's main auroral regionData help constrain Jovian auroral acceleration theories ‐ various aspects in agreement with both Knight and MHD turbulence predictionsComparisons to Hisaki remote sensing data reveal that the hydrocarbon eddy diffusion coefficient in the auroral region may be enhanced

Upward traveling electrons with energies from tens of kiloelectron volts to several megaelectron volts have been observed over the Jovian polar regions in association with intense upward propagating whistler mode waves. The electrons have a power law‐like energy distribution, indicative of a stochastic acceleration process. The energy flux of the upward propagating whistler mode waves is comparable to and strongly correlated with the energy flux of the upward traveling energetic electrons, suggesting that the whistler mode waves may be accelerating the electrons. We propose that a downward field‐aligned current over the polar cap generates strong downward parallel electric fields and associated upward electron beams in the low‐density regions of Jupiter's upper ionosphere, a mechanism similar to the formation of inverted‐Vs in Earth's auroral regions. At Jupiter, the upward‐traveling electron beams produce intense upward propagating whistler mode emissions over a broad frequency range, similar to upward propagating auroral hiss at Earth. As the whistler mode waves propagate upward out of the inverted‐V source region, the waves are absorbed by the plasma, thereby accelerating the electrons. We attribute the stochastic power law‐like energy spectrum of the accelerated electrons to the development of Hamiltonian chaos (velocity space diffusion), the signature of which is indicated by the occurrence of spiky soliton‐like variations in the whistler mode electric fields. Acceleration to high energies may be facilitated by the rapid increase in the phase velocity of the whistler mode waves with increasing altitude, which could accelerate some of the electrons trapped in the wavefield to very high relativistic energies. Upgoing energetic electrons with power law‐like spectrums and strong upgoing whistler mode waves are observed over the Jovian polar capThe upgoing broadband whistler mode waves are generated by an electron beam instability at low altitudes in the upper ionosphereThe upgoing electrons are stochastically accelerated by the broadband whistler mode waves via the development of Hamiltonian chaos

A large set of observations of Jupiter's ultraviolet aurora was collected with the Hubble Space Telescope concurrently with the NASA‐Juno mission, during an eight‐month period, from 30 November 2016 to 18 July 2017. These Hubble observations cover Juno orbits 3 to 7 during which Juno in situ and remote sensing instruments, as well as other observatories, obtained a wealth of unprecedented information on Jupiter's magnetosphere and the connection with its auroral ionosphere. Jupiter's ultraviolet aurora is known to vary rapidly, with timescales ranging from seconds to one Jovian rotation. The main objective of the present study is to provide a simplified description of the global ultraviolet auroral morphology that can be used for comparison with other quantities, such as those obtained with Juno. This represents an entirely new approach from which logical connections between different morphologies may be inferred. For that purpose, we define three auroral subregions in which we evaluate the auroral emitted power as a function of time. In parallel, we define six auroral morphology families that allow us to quantify the variations of the spatial distribution of the auroral emission. These variations are associated with changes in the state of the Jovian magnetosphere, possibly influenced by Io and the Io plasma torus and by the conditions prevailing in the upstream interplanetary medium. This study shows that the auroral morphology evolved differently during the five ~2 week periods bracketing the times of Juno perijove (PJ03 to PJ07), suggesting that during these periods, the Jovian magnetosphere adopted various states. The study presents the results of a large HST campaign observing Jupiter's UV aurora, concurrently with the in situ exploration of its magnetosphere with JunoThe study provides an original analysis method based on the definition of six morphological families tentatively associated with Jupiter's magnetospheric stateThis HST data set provides crucial synergistic measurements and additional opportunities to augment the Juno mission science return

Juno's Jupiter Energetic particle Detector Instrument often detects energetic electron beams over Jupiter's polar regions. In this paper, we document a subset of intense magnetic field‐aligned beams of energetic electrons moving away from Jupiter at high magnetic latitudes both north and south of the planet. The number fluxes of these beams are often dominated by electrons with energies above about 1 MeV. These very narrow beams can create broad angular responses in the Jupiter Energetic particle Detector Instrument with unique signatures in the detector count rates, probably because of >10 MeV electrons. We use these signatures to identify the most intense beams. These beams occur primarily above the swirl region of the polar cap aurora. This polar region is described as being of low brightness and high absorption and the most magnetically “open” at Jupiter. We find that there are very intense beams of energetic (probably dominated by >1 MeV) electrons moving upward over Jupiter's north and south poles that are likely to be magnetically connected to the swirl region in the aurora. We use data from Juno's Jupiter Energetic particle Detector Instrument to characterize the upward beams and also present Ultraviolet Spectrograph data showing the swirl region in a color ratio. We have created a table of times when these very intense beams are present from the first through the eighth perijove pass of the spacecraft. We survey persistent, intense upward MeV electron beams in Jupiter's polar regionsThese intense beams appear to be well correlated with the swirl region of the Jovian auroraWe consider Juno JEDI data from perijove 1 through perijove 8

Two new Juno‐observed particle features of Jupiter's main aurora demonstrate substantial diversity of processes generating Jupiter's mysterious auroral emissions. It was previously speculated that sometimes‐observed potential‐driven aurora (up to 400 kV) can turn into broadband stochastic acceleration (dominating at Jupiter) by means of instability. Here direct evidence for such a process is revealed with a “mono‐energetic” electron inverted‐V rising in energy to 200 keV, transforming into a region of broadband acceleration with downward energy fluxes tripling to 3,000 mW/m2, and then transforming back into a mono‐energetic structure ramping down from 200 keV. But a second feature of interest observed nearby is unlikely to have operated in the same way. Here a downward accelerated proton inverted‐V, with inferred potentials to 300–400 kV, occurred simultaneously with downward accelerated broadband electrons with downward energy fluxes as high as any observed (~3,000 mW/m2). This latter feature has no known precedent with Earth auroral observations. Two new particle features are identified within the intense auroral acceleration regions at Jupiter demonstrating great diversityOne feature supports a hypothesis that potential‐driven aurora can become unstable and convert over to broadband, stochastic accelerationThe other feature contradicts that hypothesis and has no qualitative precedent within Earths' auroral acceleration regions

The Juno spacecraft's Jupiter Energetic‐particle Detector Instrument has observed field‐aligned, unidirectional (upgoing) electron beams throughout most of Jupiter's entire polar cap region. The Waves instrument detected intense broadband whistler mode emissions occurring in the same region. In this paper, we investigate the pitch angle scattering of the upgoing electron beams due to interactions with the whistler mode waves. Profiles of intensity versus pitch angle for electron beams ranging from 2.53 to 7.22 Jovian radii show inconsistencies with the expected adiabatic invariant motion of the electrons. It is believed that the observed whistler mode waves perturb the electron motion and scatter them away from the magnetic field line. The diffusion equation has been solved by using diffusion coefficients which depend on the magnetic intensity of the whistler mode waves. Significant pitch angle scattering was observed on Perijove 1 in the north and Perijove 3 in the north and the south polar regionsModeled diffusion plots fit well for Perijove 3 in the south, but not enough to explain observations in the north for Perijove 1 and Perijove 3Interactions with whistler mode waves contributes to the overall pitch angle scattering of the electron beams

Mirror mode waves are ubiquitous in the Earth's magnetosheath, in particular behind the quasi‐perpendicular shock. Embedded in these nonlinear structures, intense lion roars are often observed. Lion roars are characterized by whistler wave packets at a frequency ∼100 Hz, which are thought to be generated in the magnetic field minima. In this study, we make use of the high time resolution instruments on board the Magnetospheric MultiScale mission to investigate these waves and the associated electron dynamics in the quasi‐perpendicular magnetosheath on 22 January 2016. We show that despite a core electron parallel anisotropy, lion roars can be generated locally in the range 0.05–0.2fceby the perpendicular anisotropy of electrons in a particular energy range. We also show that intense lion roars can be observed up to higher frequencies due to the sharp nonlinear peaks of the signal, which appear as sharp spikes in the dynamic spectra. As a result, a high sampling rate is needed to estimate correctly their amplitude, and the latter might have been underestimated in previous studies using lower time resolution instruments. We also present for the first‐time 3‐D high time resolution electron velocity distribution functions in mirror modes. We demonstrate that the dynamics of electrons trapped in the mirror mode structures are consistent with the Kivelson and Southwood (1996) model. However, these electrons can also interact with the embedded lion roars: first signatures of electron quasi‐linear pitch angle diffusion and possible signatures of nonlinear interaction with high‐amplitude wave packets are presented. These processes can lead to electron untrapping from mirror modes. Intense lion roars are observed in mirror modes by high time resolution instruments on board Magnetospheric MultiScale missionNonlinear lion roars are observed up to 0.4fcedue to their high amplitude, which may have been underestimated in previous studiesPossible signatures of linear and nonlinear resonant interaction between lion roars and electrons, which can be untrapped from mirror modes

Juno's arrival at Jupiter in 2016 revealed unprecedented details about Jupiter's ultraviolet aurorae thanks to its unique suite of remote sensing and in situ instruments. Here we present results from in situ observations during Juno flybys above specific bright auroral spots in Jupiter's polar aurora. We compare data observed by Juno‐UVS, JEDI, JADE, Waves, and MAG instruments when Juno was magnetically connected to bright polar auroral spots (or their immediate vicinity) during perijove 3 (PJ3), PJ15, and PJ33. The highly energetic particles observed by JEDI show enhancements dominated by upward electrons, which suggests that the particle acceleration region takes place below the spacecraft. Moreover, brightness and upward particle flux were higher for the northern bright spot in PJ3 than the southern spots found in PJ15 and PJ33. In addition, we notice the intensification of whistler‐mode waves at the time of the particle enhancements, suggesting that wave‐particle interactions contribute to the acceleration of particles that cause the UV aurorae. The MAG data reveal magnetic perturbations during the PJ3 spot detection by Juno, which suggests the presence of significant field‐aligned electric currents. While the stable positions of the bright spots in System III suggest that the phenomenon is fixed to the planet's rotation, the presence of field‐aligned currents leaves the possibility of an origin rooted much farther in the magnetosphere. Jupiter's auroral bright spot emissions observed by Juno‐Ultraviolet Spectrograph were simultaneously measured with the JADE, JEDI, Waves, and MAG instrumentsFor each event, we observe characteristic changes in particle distributions, wave emissions, and magnetic field disturbancesWhistler waves and electric currents appear to play a role in generating bright auroral polar spots Jupiter's auroral bright spot emissions observed by Juno‐Ultraviolet Spectrograph were simultaneously measured with the JADE, JEDI, Waves, and MAG instruments For each event, we observe characteristic changes in particle distributions, wave emissions, and magnetic field disturbances Whistler waves and electric currents appear to play a role in generating bright auroral polar spots

Whistler mode chorus waves are a naturally occurring electromagnetic emission observed in Earth's magnetosphere. Here, for the first time, data from NASA's Magnetospheric Multiscale (MMS) mission were used to analyze chorus waves in detail, including the calculation of chorus wave normal vectors, k. A case study was examined from a period of substorm activity around the time of a conjunction between the MMS constellation and NASA's Van Allen Probes mission on 07 April 2016. Chorus wave activity was simultaneously observed by all six spacecraft over a broad range of Lshells (5.5 < L< 8.5), magnetic local time (06:00 < MLT < 09:00), and magnetic latitude (−32° < MLAT < −15°), implying a large chorus active region. Eight chorus elements and their substructure were analyzed in detail with MMS. These chorus elements were all lower band and rising tone emissions, right‐handed and nearly circularly polarized, and propagating away from the magnetic equator when they were observed at MMS (MLAT~−31°). Most of the elements had “hook”‐like signatures on their wave power spectra, characterized by enhanced wave power at flat or falling frequency following the peak, and all the elements exhibited complex and well‐organized substructure observed consistently at all four MMS spacecraft at separations up to 70 km (60 km perpendicular and 38 km parallel to the background magnetic field). The waveforms in field‐aligned coordinates also demonstrated that these waves were all phase coherent, allowing for the direct calculation of k. Error estimates on calculated krevealed that the plane wave approximation was valid for six of the eight elements and most of the subelements. The wave normal vectors were within 20–30° from the direction antiparallel to the background field for all elements and changed from subelement to subelement through at least two of the eight elements. The azimuthal angle of kin the perpendicular plane was oriented earthward and was oblique to that of the Poynting vector, which has implications for the validity of cold plasma theory. Cross‐correlation analysis was used to calculate the wave normal vector for eight rising tone lower band chorus elements and several subelementsSubstructure and wave phase were consistently coherent at all four MMS; plane wave approximation valid for six of eight elements up to 70 km scaleElements observed at L~8.2, MLT~06:53, MLAT~−31°; kwas consistently oriented earthward and significantly oblique to B0and S

We present observations of 0.1–100 keV electrons from Juno's Jovian Auroral Distributions Experiment Electron instrument over Jupiter's polar auroral region for periods around four Juno perijoves (PJ1, PJ3, PJ4, and PJ5). The observations reveal regions containing magnetic field aligned beams of bidirectional electrons having broad energy distributions interspersed between beams of upward electrons with narrow, peaked energy distributions, regions void of these electrons, and regions dominated by penetrating radiation. The electrons show evidence of acceleration via parallel electric fields (inverted‐V structures) and via stochastic processes (bidirectional distributions). The inverted‐V structures shown here were observed from ~1.4 to 2.9 RJand had spatial scales of hundreds to thousands of kilometers along Juno's trajectory. The upward electron energy flux was typically greater than the downward flux, the latter ranging between ~0.01 and 5 mW m−2for two cases shown here which we estimate could produce ~0.1–50 kR of ultraviolet emission. JADE observed 0.1–100 keV electrons, regions void of these electrons, and regions of penetrating radiation in Jupiter's polar aurora regionThe electrons consisted of bidirectional beams with broad energies interspersed between upward beams having narrow energy distributionsThe energy flux of downward electrons shown here ranged from ~0.01 to 5 mW m−2, the upward flux being up to ~100 times larger We report on observations of 0.1 ‐ 100 kilo‐electron volt electrons from the Jovian Auroral Distributions Experiment Electron instrument (JADE‐E) on Juno over the region where Jupiter's ultraviolet (UV) polar aurora is produced. The observations show electrons moving both towards and away from Jupiter. These electrons show both broad and narrow energy distributions, suggesting the presence of at least two different acceleration mechanisms. Regions void of these electrons and regions dominated by penetrating radiation were also identified. The energy flux of the electrons moving towards Jupiter was sufficient to produce the weaker UV polar auroral emissions observed at Jupiter but a different source of electrons, likely with higher energies, is required to account for the brighter emissions.

Recent results of the first ever orbit through Jupiter's auroral region by NASA's Juno spacecraft did not show evidence of coherent acceleration in the auroral or polar region. However, in this letter, we show energetic particle data from Juno's Jupiter Energetic‐particle Detector Instrument instrument during the third auroral pass that exhibits conclusive evidence of downward parallel electric fields in portions of Jupiter's polar region. The energetic particle distributions show inverted‐V ion and electron structures in a downward electric current region with accelerated peaked distributions in hundreds of keV to ~1 MeV range. The origin of these large electric potential structures is investigated and discussed within the current theoretical framework of current‐voltage relationships at both Earth and Jupiter. Parallel electric fields responsible for accelerating particles to maintain the aurora/magnetospheric circuit appear to be a common phenomenon among strongly magnetized planets with conducting ionospheres; however, their origin and generation mechanisms are subjects of ongoing research. Evidence of magnetic field‐aligned electric potentials above portions of Jupiter's polar cap regionsElectrons show upward peaked energy distributions of several hundreds of keVIons (H+, On+, and Sn+) show downward peaked energy distributions from hundreds of keV to a few MeV

The most intense auroral emissions from Earth’s polar regions, called discrete for their sharply defined spatial configurations, are generated by a process involving coherent acceleration of electrons by slowly evolving, powerful electric fields directed along the magnetic field lines that connect Earth’s space environment to its polar regions. In contrast, Earth’s less intense auroras are generally caused by wave scattering of magnetically trapped populations of hot electrons (in the case of diffuse aurora) or by the turbulent or stochastic downward acceleration of electrons along magnetic field lines by waves during transitory periods (in the case of broadband or Alfvénic aurora). Jupiter’s relatively steady main aurora has a power density that is so much larger than Earth’s that it has been taken for granted that it must be generated primarily by the discrete auroral process. However, preliminary in situ measurements of Jupiter’s auroral regions yielded no evidence of such a process. Here we report observations of distinct, high-energy, downward, discrete electron acceleration in Jupiter’s auroral polar regions. We also infer upward magnetic-field-aligned electric potentials of up to 400 kiloelectronvolts, an order of magnitude larger than the largest potentials observed at Earth. Despite the magnitude of these upward electric potentials and the expectations from observations at Earth, the downward energy flux from discrete acceleration is less at Jupiter than that caused by broadband or stochastic processes, with broadband and stochastic characteristics that are substantially different from those at Earth.

We study the spatiotemporal characteristics of energetic particle losses from the magnetosphere using test‐particle trajectories in electromagnetic fields from a global magnetosphere magnetohydrodynamic (MHD) simulation. We use a dynamically evolving distribution of high‐resolution electromagnetic fields from the Lyon‐Fedder‐Mobarry global MHD model and trace large ensembles of 100 keV hydrogen and oxygen ions as well as electrons from a near‐Earth plasma sheet location through their escape from the magnetosphere. In agreement with recent MMS observations, we demonstrate that both ions and electrons have access to and escape throughout the dayside magnetopause, including magnetically drift‐shadowed regions. Also, in agreement with MMS observations, the depth of penetration and persistence of particles in the magnetosheath has a clear mass dependence, heavier particles penetrating further and lingering longer. We demonstrate both magnetic local time and latitude dependence of particles losses as manifested by their crossings of the open‐closed boundary and relate them to the complex field topology. Finally, we establish a significant role of Kelvin‐Helmholtz instability in facilitating particle losses at the magnetopause flanks. Energetic particles have access to and escape from both sides of the dayside magnetosphereSalient features of losses for different species are in agreement with recent MMS observationsKelvin‐Helmholtz instability enhances losses, particularly for smaller gyroradius particles

We present the first comparison of Jupiter's auroral morphology with an extended, continuous, and complete set of near‐Jupiter interplanetary data, revealing the response of Jupiter's auroras to the interplanetary conditions. We show that for ∼1–3 days following compression region onset, the planet's main emission brightened. A duskside poleward region also brightened during compressions, as well as during shallow rarefaction conditions at the start of the program. The power emitted from the noon active region did not exhibit dependence on any interplanetary parameter, though the morphology typically differed between rarefactions and compressions. The auroras equatorward of the main emission brightened over ∼10 days following an interval of increased volcanic activity on Io. These results show that the dependence of Jupiter's magnetosphere and auroras on the interplanetary conditions are more diverse than previously thought. Jupiter's auroras (northern lights) are the brightest in the solar system, over a hundred times brighter than the Earth's. Auroras on Earth are driven by the solar wind, a million mile‐per‐hour stream of charged particles flowing away from the Sun, hitting the Earth's magnetic field, and stirring it around, but it is not known whether the solar wind causes any significant auroras on Jupiter. The main reason for this uncertainty is a lack of observations of the planet's auroras obtained while spacecraft have been near Jupiter and able to supply a full and continuous set of measurements of the solar wind and its accompanying magnetic field. In early mid‐2016 Juno approached Jupiter, providing such an interplanetary data set, and we obtained over a month's worth of observations of Jupiter's auroras using the Hubble Space Telescope. We saw several solar wind storms, each causing auroral fireworks on Jupiter. We captured the most powerful auroras observed by Hubble to date, brightened main oval emissions, and flashing high‐latitude patches of auroras during the solar wind storms. These results indicate that Jupiter's auroral response to the solar wind is more diverse than we previously have thought. We present the first comparison of Jupiter's auroras with an extended and complete set of near‐Jupiter interplanetary dataDuring compressions, the well‐defined sector of Jupiter's emission and the dusk poleward region brightened, the latter pulsatingThe power emitted from the noon active region did not exhibit dependence on any interplanetary parameter, though the morphology changed

Juno ultraviolet spectrograph (UVS) observations of Jupiter's aurora obtained during approach are presented. Prior to the bow shock crossing on 24 June 2016, the Juno approach provided a rare opportunity to correlate local solar wind conditions with Jovian auroral emissions. Some of Jupiter's auroral emissions are expected to be controlled or modified by local solar wind conditions. Here we compare synoptic Juno‐UVS observations of Jupiter's auroral emissions, acquired during 3–29 June 2016, with in situ solar wind observations, and related Jupiter observations from Earth. Four large auroral brightening events are evident in the synoptic data, in which the total emitted auroral power increases by a factor of 3–4 for a few hours. Only one of these brightening events correlates well with large transient increases in solar wind ram pressure. The brightening events which are not associated with the solar wind generally have a risetime of ~2 h and a decay time of ~5 h. Juno ultraviolet spectrograph (UVS) observations of Jupiter's aurora obtained during approach are presented. Jupiter's auroras are thought to be controlled in part by the solar wind, a stream of charged particles flowing outward from the Sun. The Juno approach was a rare opportunity to compare solar wind conditions near Jupiter with the Jovian ultraviolet aurora, using observations made during June 2016. Although Jupiter's aurora is always present, four brightening events were seen in the data, in which the total auroral power increased to several times the typical level for a few hours. Only one of these brightening events appears well connected with solar wind activity. The brightening events which are not associated with the solar wind all increase in brightness in about 2 h and then dim back down again in about another 5 h. Jupiter's aurora and the solar wind have a complex relationshipThe single solar wind structure that correlated with an auroral brightening event was a CIRAuroral brightening events which are not related to solar wind conditions have a similar time evolution

Recent studies have utilized different charge states of oxygen ions as a tracer for the origins of plasma populations in the magnetosphere of Earth, using O+as an indicator of ionospheric‐originating plasma and O6+as an indicator of solar wind‐originating plasma. These studies have correlated enhancements in O6+to various solar wind and geomagnetic conditions to characterize the dominant solar wind injection mechanisms into the magnetosphere but did not include analysis of the temporal evolution of these ions. A sixth‐order Fourier expansion model based empirically on a superposed epoch analysis of geomagnetic storms observed by Polar is presented in this study to provide insight into the evolution of both ionospheric‐originating and solar wind‐originating plasma throughout geomagnetic storms. At high energies (~200 keV) the flux of O+and O6+are seen to become comparable in the outer magnetosphere. Moreover, while the density of O+is far higher than O6+, the two charge states have comparable pressures in the outer magnetosphere. The temperature of O6+is generally higher than that of O+, because the O6+is injected from preheated magnetosheath populations before undergoing further heating once in the magnetosphere. A comparison between the model results with O+observations from the Magnetospheric Multiscale mission and the Van Allen Probes provides a validation of the model. In general, this empirical model agrees qualitatively well with the trends seen in both data sets. Quantitatively, the modeled density, pressure, and temperature almost always agree within a factor of at most 10, 5, and 2, respectively. An empirical model was created for equatorial O+and O6+flux during geomagnetic storms and compared to O+observations from MMS and VAPWhile O6+pressure is sometimes comparable to O+, the O6+temperatures generally exceeds that of O+The main regions of O6+density enhancements are along the dayside and the flanks, consistent with reconnection and KHI‐related injection

Juno's first perijove science observations were carried out on 27 August 2016. The 90° orbit inclination and 4163 km periapsis altitude provide the first opportunity to explore Jupiter's polar magnetosphere. A radio and plasma wave instrument on Juno called Waves provided a new view of Jupiter's auroral radio emissions from near 10 kHz to ~30 MHz. This frequency range covers the classically named decametric, hectometric, and broadband kilometric radio emissions, and Juno observations showed much of this entire spectrum to consist of V‐shaped emissions in frequency‐time space with intensified vertices located very close to the electron cyclotron frequency. The proximity of the radio emissions to the cyclotron frequency along with loss cone features in the energetic electron distribution strongly suggests that Juno passed very close to, if not through, one or more of the cyclotron maser instability sources thought to be responsible for Jupiter's auroral radio emissions. First polar view of Jupiter's auroral radio emissionsEmissions composed of multiple V‐shaped spectral featuresJuno passed close to or through five or more source regions

After a successful orbit insertion, the Juno spacecraft completed its first 53.5 day orbit and entered a very low altitude perijove with the full scientific payload operational for the first time on 27 August 2016. The Jupiter Energetic particle Detector Instrument measured ions and electrons over the auroral regions and through closest approach, with ions measured from ~0.01 to >10 MeV, depending on species. This report focuses on the composition of the energetic ions observed during the first perijove of the Juno mission. Of particular interest are the ions that precipitate from the magnetosphere onto the polar atmosphere and ions that are accelerated locally by Jupiter's powerful auroral processes. We report preliminary findings on the spatial variations, species, including energy and pitch angle distributions throughout the prime science region during the first orbit of the Juno mission. The prime motivation for this work was to examine the heavy ions that are thought to be responsible for the observed polar X‐rays. Jupiter Energetic particle Detector Instrument (JEDI) did observe precipitating heavy ions with energies >10 MeV, but for this perijove the intensities were far below those needed to account for previously observed polar X‐ray emissions. During this survey we also found an unusual signal of ions between oxygen and sulfur. We include here a report on what appears to be a transitory observation of magnesium, or possibly sodium, at MeV energies through closest approach. JEDI observed heavy ions to energies >10 MeV precipitating over the northern and southern auroral regionsThe heavy ion intensity is too low to produce the polar X‐raysJEDI observed a strong transient signature of heavy ions with mass between those of sulfur and oxygen

Energetic charged particle measurements by the Jupiter Energetic Particle Detector Instrument (JEDI) on board Juno have revealed a radiation belt of hundreds of keV ions up to the atomic mass of sulfur, located between Jupiter's rings and atmosphere. Proton energy spectra display an unusual intensity increase above 300 keV. We suggest that this is because charge exchange in Jupiter's neutral environment does not efficiently remove ions at such high energies. Since this innermost belt includes heavy ions, it cannot be exclusively supplied by cosmic ray albedo neutron decay, which is an important source at Earth and Saturn but only supplies protons and electrons. We find indications that the stripping of energetic neutral atoms in Jupiter's high atmosphere might be the ion source. Since the stripped off electrons are of low energy, this hypothesis is consistent with observations of the ratio of energetic electrons to ions being much less than 1. Planets with their own a magnetic field, as Earth and Jupiter, are surrounded by “belts” of radiation. In case of Jupiter, most of this radiation extends to relatively large distances to Jupiter (several times the size of the planet) but is so strong that it is difficult to engineer satellites that can fly through them safely. Juno is the first spacecraft that repeatedly passes close to Jupiter and its Jupiter Energetic Particle Detector Instrument (JEDI) was now able to measure and quantify a small, separate radiation belt close to Jupiter. The measured population was unknown before and is located between Jupiter's atmosphere and its tenuous rings, inward to the main radiation belt. There is a radiation belt within Jupiter's ringsThis innermost belt consists of ion masses up to sulfur but very few electronsEnergetic neutral atoms may be the source of this innermost belt

The Jovian Infrared Auroral Mapper (JIRAM) is an imager/spectrometer on board NASA/Juno mission for the study of the Jovian aurorae. The first results of JIRAM's imager channel observations of the H3+infrared emission, collected around the first Juno perijove, provide excellent spatial and temporal distribution of the Jovian aurorae, and show the morphology of the main ovals, the polar regions, and the footprints of Io, Europa and Ganymede. The extended Io “tail” persists for ~3 h after the passage of the satellite flux tube. Multi‐arc structures of varied spatial extent appear in both main auroral ovals. Inside the main ovals, intense, localized emissions are observed. In the southern aurora, an evident circular region of strong depletion of H3+emissions is partially surrounded by an intense emission arc. The southern aurora is brighter than the north one in these observations. Similar, probably conjugate emission patterns are distinguishable in both polar regions. Detailed observation of a very long tail of the Io magnetic footprintIdentification of thin multiple arc structures in the northern and southern ovals, and of bright spots and depletion in the south poleFilamentation of alternatively upward and downward current in the 0°–90° SIII sector in the north

The Jupiter InfraRed Auroral Mapper (JIRAM) aboard Juno observed the Jovian South Pole aurora during the first orbit of the mission. H3+(trihydrogen cation) and CH4(methane) emissions have been identified and measured. The observations have been carried out in nadir and slant viewing both by a L‐filtered imager and a 2–5 μm spectrometer. Results from the spectral analysis of the all observations taken over the South Pole by the instrument are reported. The coverage of the southern aurora during these measurements has been partial, but sufficient to determine different regions of temperature and abundance of the H3+ion from its emission lines in the 3–4 μm wavelength range. Finally, the results from the southern aurora are also compared with those from the northern ones from the data taken during the same perijove pass and reported by Dinelli et al. (2017). H3+intensity, column density, and temperature maps of the Jupiter southern aurora are derived from Juno/JIRAM data collected on the first orbitEmissions from southern aurora are more intense than from the NorthDerived temperatures are in the range 600°K to 1400°K

Juno obtained unique low‐altitude space environment measurements over Jupiter's poles on 27 August 2016. Here Jupiter Energetic‐particle Detector Instrument observations are presented for electrons (25–800 keV) and protons (10–1500 keV). We analyze magnetic field‐aligned electron angular beams over expected auroral regions that were sometimes symmetric (bidirectional) but more often strongly asymmetric. Included are variable but surprisingly persistent upward, monodirectional electron angular beams emerging from what we term the “polar cap,” poleward of the nominal auroral ovals. The energy spectra of all beams were monotonic and hard (not structured in energy), showing power law‐like distributions often extending beyond ~800 keV. Given highly variable downward energy fluxes (below 1 RJaltitudes within the loss cone) as high as 280 mW/m2, we suggest that mechanisms generating these beams are among the primary processes generating Jupiter's uniquely intense auroral emissions, distinct from what is typically observed at Earth. Upward, energy‐monotonic energetic electron angular beams are unexpectedly persistent over Jupiter's polar capsJupiter's aurora appears not to be associated with monoenergetic electron beams but with other processesJupiter's aurora is powered by the downward portion of bidirectional, energy‐monotonic electron angular beams and diffuse precipitation

On 27 August 2016, the NASA Juno spacecraft performed its first close‐up observations of Jupiter during its perijove. Here we present the UV images and color ratio maps from the Juno‐UVS UV imaging spectrograph acquired at that time. Data were acquired during four sequences (three in the north, one in the south) from 5:00 UT to 13:00 UT. From these observations, we produced complete maps of the Jovian aurorae, including the nightside. The sequence shows the development of intense outer emission outside the main oval, first in a localized region (255°–295° System III longitude) and then all around the pole, followed by a large nightside protrusion of auroral emissions from the main emission into the polar region. Some localized features show signs of differential drift with energy, typical of plasma injections in the middle magnetosphere. Finally, the color‐ratio map in the north shows a well‐defined area in the polar region possibly linked to the polar cap. During Perijove 1, Juno‐UVS acquired the first spatially and spectrally resolved Far‐UV observations of Jupiter's nightside auroraeDuring the 10 h sequences, large and intense outer emissions progressively developed and a protrusion formed with the southern main ovalA well‐defined area identified in the northern polar region resembles the shape, location, and area of the predicted open field line region

In inner planetary magnetospheres, pitch angle distributions of charged particles are shaped by the global magnetic field topology when they bounce fast along magnetic field lines. Therefore, one can estimate the magnetic field intensity at the top of the atmosphere from the loss cone size, inferred from particle pitch angle distributions at higher altitudes. We propose such a technique to estimate the magnetic field intensity near the top of Jovian atmosphere based on Jupiter Energetic Particle Detector Instruments particle distributions captured by JUNO and verify the algorithm by using plasma measurements from Van Allen Probes in the Earth's inner magnetosphere. Our results demonstrate that the low‐altitude magnetic field can be significantly larger than the expected field from present models, which supports recent reports on the anomaly of Jovian internal magnetic field from in situ JUNO measurements. We also discuss possible modifications and applications of the proposed technique to future investigations of the global Jovian magnetic field configuration. A technique to estimate the planetary magnetic field from the loss cone distribution of energetic particles is developedThe algorithm is verified with plasma measurements from the Earth's inner magnetospherePlasma pitch angle distributions indicate the presence of a Jovian magnetic field anomaly

The Juno Waves instrument detected a new broadband plasma wave emission (~50 Hz to 40 kHz) on 27 August 2016 as the spacecraft passed over the low‐altitude polar regions of Jupiter. We investigated the characteristics of this emission and found similarities to whistler mode auroral hiss observed at Earth, including a funnel‐shaped frequency‐time feature. The electron cyclotron frequency is much higher than both the emission frequency and local plasma frequency, which is assumed to be ~20–40 kHz. The E/cBratio was about three near the start of the event and then decreased to one for the rest of the period. A correlation of the electric field spectral density with the flux of an upgoing 20 to 800 keV electron beam was found, with a correlation coefficient of 0.59. We conclude that the emission is propagating in the whistler mode and is driven by the energetic upgoing electron beam. A new broadband plasma wave emission between 50 Hz and 40 kHz was discovered from Juno Waves data in Jupiter's high‐latitude polar regionsThe plasma waves are assumed to be propagating in the whistler mode below 40 kHzSignificant correlation found between Waves data and an upgoing electron beam from JEDI, indicating that the beam is the source of the radiation

After its capture into Jupiter orbit early in the summer of 2016, the Juno spacecraft made three close flybys of the planet to date. The Jupiter Energetic Particle Detector Instrument (JEDI) made continuous measurements during perijoves in late August and early December. Here we describe the radiation (approximately hundreds of keV to more than 10 MeV charged particles) that was measured close to Jupiter. The purpose of this paper is to present some of the first direct energetic charged particle measurements ever obtained at high magnetic latitude very close to Jupiter and to interpret these data using techniques that rely on the instrument design. We generate an electron energy spectrum in an intense radiation region where the JEDI foreground is only about 40% of the rate due to >15 MeV electrons. Parameter that characterizes high fluxes of >15 MeV electronsQuantify Jovian radiation regions at high latitudesEnergy spectrum for MeV electrons in radiation belts

During the first orbit around Jupiter of the NASA/Juno mission, the Jovian Auroral Infrared Mapper (JIRAM) instrument observed the auroral regions with a large number of measurements. The measured spectra show both the emission of the H3+ion and of methane in the 3–4 μm spectral region. In this paper we describe the analysis method developed to retrieve temperature and column density (CD) of the H3+ion from JIRAM spectra in the northern auroral region. The high spatial resolution of JIRAM shows an asymmetric aurora, with CD and temperature ovals not superimposed and not exactly located where models and previous observations suggested. On the main oval averaged H3+CDs span between 1.8 × 1012cm−2and 2.8 × 1012cm−2, while the retrieved temperatures show values between 800 and 950 K. JIRAM indicates a complex relationship among H3+CDs and temperatures on the Jupiter northern aurora. First global maps of H3+intensity, column density, and temperature for the Jupiter northern aurora with high spatial resolutionOne side of the auroral oval shows higher H3+column density and lower temperatures in comparison with the other sideColumn densities main oval and temperature main oval do not superimpose

NASA's Juno spacecraft successfully completed its first science polar pass over Jupiter's northern and southern aurora, with all the instruments powered, on 27 August 2016. Observations of conical energetic proton distributions at low altitudes (<6 RJ) over the northern polar region are interpreted as resulting from transversely (to the local magnetic field lines) accelerated H+at a position planetward of the point of observation. The proton conics were observed within a broad region of upward beaming electrons and were accompanied by broadband low‐frequency wave emissions as well as low‐altitude trapped magnetospheric protons and heavy ions. The characteristic energies associated with these accelerated ion conics are ~100 times more energetic than similar distributions observed in the Earth's auroral region and similar in energy to those found at Saturn. In addition, the ion conics also exhibited pitch angle dispersion with time that is interpreted as a consequence of the structure of the source location. Mapping these distributions along magnetic field lines connected from the spacecraft to the ionosphere suggests that the source region exists at altitudes between ~3 and 5 RJ. These new and exciting observations of accelerated ions over the polar region of Jupiter open up new areas for comparative planetary auroral physics. Ion conic distributions observed over Jupiter's polar magnetospherePitch angle mapping suggests source region between 3 and 5 Jovian radiiApproximately 100 times more energetic than ion conics observed in Earth's auroral region, but similar to ion conics at Saturn

Throughout the first orbit of the NASA Juno mission around Jupiter, the Jupiter InfraRed Auroral Mapper (JIRAM) targeted the northern and southern polar regions several times. The analyses of the acquired images and spectra confirmed a significant presence of methane (CH4) near both poles through its 3.3 μm emission overlapping the H3+auroral feature at 3.31 μm. Neither acetylene (C2H2) nor ethane (C2H6) have been observed so far. The analysis method, developed for the retrieval of H3+temperature and abundances and applied to the JIRAM‐measured spectra, has enabled an estimate of the effective temperature for methane peak emission and the distribution of its spectral contribution in the polar regions. The enhanced methane inside the auroral oval regions in the two hemispheres at different longitude suggests an excitation mechanism driven by energized particle precipitation from the magnetosphere. Evidence of diffuse CH4emission inside the northern and southern Jupiter auroral ovalsDetailed maps of the distribution of the CH4emission are obtained for both polesEstimated rotational temperatures of the CH4emission are about 500 K for the north pole and 650 K for the south pole

We report observations of energetic electron butterfly distributions measured in a narrow range of Jupiter's magnetic latitudes by Juno during perijove 1. The electron butterfly distributions are characterized as clear electron flux peaks at 30–80° pitch angles, compared with the 90°‐peaked pitch angle distributions of the trapped electrons. Jupiter Energetic Particle Detector Instrument measurements during the close approach to Jupiter indicate a specific electron population with butterfly distributions formed between the main auroral oval and the radiation belt. The off‐90° flux peak is most clearly observed at tens of keV energies and gradually merges toward 90° pitch angle at higher energies. By projecting the observed electron pitch angle distributions along the magnetic field line, we found that the electron butterfly distributions are observed close to their source region. The particular electron distribution is possibly formed by parallel acceleration of electrons through Landau resonance with electrostatic waves. Juno observed electron butterfly‐shaped pitch angle distributions between the main auroral oval and the radiation beltThe electron butterfly distributions were locally produced through nonadiabatic processes or observed close to their source regionsLandau resonant interactions between the electrons and electrostatic waves may cause the energy‐dependent electron butterfly distributions

We report on plasma and magnetic field observations from Juno's Jovian Auroral Distributions Experiment and Magnetic Field Investigation at 18 magnetopause crossings when the spacecraft was located at ~6 h magnetic local time and 73–114 Jovian radii from Jupiter. Several crossings showed evidence of plasma energization, accelerated ion flows, and large magnetic shear angles, each representing a signature of magnetic reconnection. These signatures were observed for times when the magnetosphere was in both compressed and expanded states. We compared the flow change magnitudes to a simplified Walén relation and found ~60% of the events to be 110% or less of the predicted values. Close examination of two magnetopause encounters revealed characteristics of a rotational discontinuity and an open magnetopause. These observations provide compelling evidence that magnetic reconnection can occur at Jupiter's dawn magnetopause and should be incorporated into theories of solar wind coupling and outer magnetosphere dynamics at Jupiter. Observations at Jupiter's dawn magnetopause by Juno revealed accelerated ion flows and large magnetic shear angles at several crossingsCase studies of two magnetopause crossings showed evidence of rotational discontinuities and an open magnetopauseCompelling evidence for magnetic reconnection at the Jupiter's dawn magnetopause during Juno's approach to Jupiter and first capture orbit

The four Magnetospheric Multiscale (MMS) spacecraft observed a ∼1 min burst of energetic ions (50–1000 keV) in the region upstream from the subsolar quasi‐perpendicular bow shock on 6 December 2015. The composition, flux levels, and spectral indices of these energetic protons, helium, and oxygen ions greatly resemble those seen in the outer magnetosphere earlier while MMS crossed the magnetopause and differ significantly from those simultaneously observed far upstream by Advanced Composition Explorer (ACE). However, the event cannot be explained solely in terms of leakage from the magnetosphere. The strongly southward orientation of the interplanetary magnetic field (IMF) lines at the time of the event precludes any connection to the magnetosphere. This point is confirmed by the presence of energetic electrons, known to occur on magnetic field lines that graze the bow shock rather than connect to the magnetosphere. We suggest that the ions gradient drifted out of the nearby quasi‐parallel foreshock and into the quasi‐perpendicular bow shock. Each of the ion species exhibited an inverse energy dispersion. As predicted by models for shock drift acceleration, the energies of the ions increased as θBn, the angle between the IMF and the shock normal, increased. Finally, we note that a similar event was observed a few minutes later in the subsolar magnetosheath, indicating that such events can be swept downstream of the bow shock. The energetic ions observed in the upstream and downstream region of the bow shock originated in the magnetosphereMagnetospheric energetic ions gradient drifted out of the nearby quasi‐parallel foreshock and into the quasi‐perpendicular bow shock regionInverse dispersions can be caused by the IMF orientation changes, from quasi‐perpendicular to (nearly) perpendicular local bow shock

Using Juno‐measured >30 keV electrons, three regions with substantial ultraviolet emissions were identified previously for Jupiter's main aurora (excluding the polar cap): low‐latitude diffuse aurora, mid‐latitude Zone I of downward acceleration, and higher latitude Zone II of bi‐directional acceleration. Zone I, associated with upward magnetic field‐aligned currents, was represented as bimodal: sometimes supporting coherent downward electron electrostatic acceleration and sometimes downward electron broadband acceleration, with broadband acceleration usually delivering the most intense electron energy flux at Juno. Recent observations of up‐going ion beams within Zone I represent a challenge as to whether coherent electrostatic acceleration invariably accompanies broadband acceleration. Is this region strictly bi‐modal, or is there a continuum between these two modes? We address these questions by combining multiple ion and electron data sources to diagnose electrostatic potentials both above and below the spacecraft. We find: (a) During Zone I downward electron broadband events, there are examples where evidence of downward electron electrostatic acceleration completely disappears and examples where it endures at some level. (b) Most often, evidence of downward electron electrostatic acceleration is strongly suppressed with strong downward electron broadband acceleration. Residual potentials most often (not always) have values small (<10 kV) compared to the electron characteristic energies of 100–400 keV. (c) Care must be exercised in these studies because plasmasheet electron precipitation spectra can mimic broadband acceleration spectra. At least for weaker auroral broadband accelerations, there is likely to be a continuum of electrostatic and broadband participation. Why either process is favored at any one time is unknown. Zone I downward electron broadband acceleration sometimes has upward electrostatic potentials fully disappearing and other times enduringZone I potentials of 30–360 kV often disappear to <10 kV with strong broadband electrons having characteristic energies of 100–400 keVWhy one process (electrostatic or broadband) is favored over the other at any one time remains unknown Zone I downward electron broadband acceleration sometimes has upward electrostatic potentials fully disappearing and other times enduring Zone I potentials of 30–360 kV often disappear to <10 kV with strong broadband electrons having characteristic energies of 100–400 keV Why one process (electrostatic or broadband) is favored over the other at any one time remains unknown

The driver of energetic electron precipitation into Ganymede's atmosphere has been an outstanding open problem. During the Juno flyby of Ganymede on 7 June 2021, Juno observed significant downward‐going electron fluxes inside the bounce loss cone of Ganymede's polar magnetosphere. Concurrently, Juno detected intense whistler‐mode waves, both in the quasi‐parallel and highly oblique directions with respect to the magnetic field line. We use quasi‐linear model to quantify energetic electron precipitation driven by quasi‐parallel and very oblique whistler‐mode waves, respectively, in the vicinity of Ganymede. The data‐model comparison indicates that in Ganymede's lower‐latitude (higher‐latitude) polar region, quasi‐parallel whistler‐mode waves play a dominant role in precipitating higher‐energy electrons above ∼100s eV (∼1 keV), whereas highly oblique waves are important for precipitating lower‐energy electrons below 100s eV (∼1 keV). Our result provides new evidence of whistler‐mode waves as a potential primary driver of precipitating energetic electrons into Ganymede's atmosphere. During the Juno flyby of Ganymede on 7 June 2021, the Juno spacecraft detected energetic electrons precipitating into Ganymede's atmosphere. Simultaneously, Juno detected intense electromagnetic whistler‐mode waves in the vicinity of Ganymede. We use a physics‐based model to quantify the role of the observed whistler‐mode waves in energetic electron precipitation. The comparison between the Juno observation and modeling results reveals that whistler‐mode waves potentially play a dominant role in precipitating energetic electrons into Ganymede's atmosphere over a broad energy range from tens of eV to several hundred keV. Our findings are potentially important for understanding the loss process of energetic electrons in Ganymede's magnetosphere, as well as the generation of Ganymede's diffuse aurora. We provide new evidence of whistler‐mode waves as a potential primary driver of precipitating energetic electrons into Ganymede's atmosphereThis finding is potentially important for the generation of aurora and the loss of energetic electrons in the vicinity of GanymedeJuno observations and quasi‐linear modeling are used to quantify energetic electron precipitation driven by whistler‐mode waves We provide new evidence of whistler‐mode waves as a potential primary driver of precipitating energetic electrons into Ganymede's atmosphere This finding is potentially important for the generation of aurora and the loss of energetic electrons in the vicinity of Ganymede Juno observations and quasi‐linear modeling are used to quantify energetic electron precipitation driven by whistler‐mode waves