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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

On 7 June 2021, NASA's Juno mission obtained unique measurements of Ganymede's magnetosphere during a close flyby that brought the spacecraft within ∼1,000 km of its surface. Here Jupiter Energetic particle Detector Instrument observations are presented and analyzed. The electron pitch angle distributions reveal distinct regions of Ganymede's magnetosphere that can be characterized as inbound and outbound magnetospheric boundaries, a magnetotail/wake region, and Ganymede's open field line region. Evidence for energy dependent electron pitch angle structuring is also documented both outside and within Ganymede's magnetosphere. Electron precipitation is observed and mapped to Ganymede's surface along Juno's magnetic footpoint. Not only is Ganymede the largest moon in our solar system, it is also the only moon known to generate its own internal magnetic field and mini‐magnetosphere. This makes Ganymede extremely fascinating to study. On 7 June 2021, NASA's Juno mission performed a historic flyby with a closest approach altitude of ∼1,000 km above its surface. The data collected by Juno is providing us with new insights into this unique mini‐magnetosphere. This letter focuses on the energetic ion and electron environment that surrounds Ganymede and the features revealed by Juno's Jupiter Energetic particle Detector Instrument. Energetic particle measurements reveal new insights into Ganymede's magnetosphere for the only close Juno flybyElectron pitch angle distributions reveal distinct regions of Ganymede's magnetosphereEvidence for energetic electron precipitation onto Ganymede's surface within the open field line region Energetic particle measurements reveal new insights into Ganymede's magnetosphere for the only close Juno flyby Electron pitch angle distributions reveal distinct regions of Ganymede's magnetosphere Evidence for energetic electron precipitation onto Ganymede's surface within the open field line region

We report on plasma observations from Juno/Jovian Auroral Distributions Experiment during the Ganymede flyby on 7 June 2021. Juno approached Ganymede from southern latitudes, passed through the wake region, then through its magnetosphere to closest approach (1,046 km from the surface) on the night side, and then back into Jupiter's plasma disk. We describe general plasma properties in the regions explored along the trajectory. We infer that Juno traversed a region of open field lines where one end intercepts Ganymede and the other Jupiter. The observations do not support Juno crossing into the closed field line region. The ion composition near Ganymede is very different than that of the nearby plasma environment. H2+and H3+ions were detected near Ganymede and in the wake region. Low energy (∼0.1–1 keV) electrons are enhanced just outside the magnetopause, in the wake (inbound trajectory) and in the magnetopause boundary layer (outbound trajectory). On 7 June 2021 the Juno mission came as close as 1,046 km from the surface of Ganymede, the largest moon in the solar system. Similar close encounters were previously made by the Galileo mission, from which we learned much of the interaction of the moon, with its own intrinsic magnetic field, and Jupiter's magnetosphere. In this paper, we present an overview of the plasma observations, that is, ions and electrons in the lower part of the energy spectrum, made by the Jovian Auroral Distributions Experiment. We find that the ion composition near Ganymede is very different than that from Jupiter's magnetosphere. Near Ganymede, the plasma composition is dominated by molecules and ions that originate from water in the atmosphere or the surface. One surprising observation is the presence of the molecular ion H3+inside Ganymede's magnetosphere and in a region just outside and downstream, that we call the wake. H3+was not included in various models of Ganymede's atmosphere. The plasma observations show that Juno crossed into the open field line region, but do not support crossing into a closed field line regionThe ion composition near Ganymede is very different from its local plasma environmentH2+and H3+ions were detected inside Ganymede's magnetopause and outside in the wake region The plasma observations show that Juno crossed into the open field line region, but do not support crossing into a closed field line region The ion composition near Ganymede is very different from its local plasma environment H2+and H3+ions were detected inside Ganymede's magnetopause and outside in the wake region

While planetary radiation belts are all embedded within the low energy solar wind, Ganymede is in the unique situation of being surrounded by the energetic particles of Jupiter's magnetosphere. Here we study Ganymede's environment based the recent flyby of Juno, as well as through reanalysis of past measurements from the Galileo spacecraft. We find that Ganymede is surrounded by a radiation cavity with intensities that are lower compared to Jupiter. Particles are lost in the cavity due to absorption by Ganymede that is likely enhanced by scattering. In the core of the cavity we find radiation belts. Their intensities are comparable to Jupiter for ions but lower for electrons. The radiation belts form peaks in phase space density, which indicates that they cannot be produced through steady inward influx and accumulation of Jupiter particles. Other processes are needed such as local acceleration, or time dependent transport and loss processes. Some planetary bodies such as Earth or Jupiter produce strong magnetic fields that trap charged particle radiation in space and form radiation belts. Other planetary bodies such as most moons of the giant planets are only weakly magnetized and mostly absorb the radiation in their environment. Jupiter's moon Ganymede is unique because it is the only known moon that produces its own magnetic field. Different to a planet, it is not embedded in the solar wind but in the magnetosphere of Jupiter. How the interaction between the respective magnetic fields works is poorly understood and the reason for the current study. Here we find that Ganymede is surrounded by a radiation cavity, were radiation intensities are smaller compared to the environment, likely because some of the particles are absorbed by the body of Ganymede itself. Embedded in the cavity we find radiation belts that resemble their planetary equivalents. We find that these belts cannot simply be populated by inflowing particles from Jupiter and discuss what additional physics may be needed. Ganymede's radiation cavity has lower intensities compared to JupiterGanymede has a radiation belt of at least tens and hundreds of keV ions and electronsGanymede's radiation belt cannot be produced through influx of Jupiter particles alone Ganymede's radiation cavity has lower intensities compared to Jupiter Ganymede has a radiation belt of at least tens and hundreds of keV ions and electrons Ganymede's radiation belt cannot be produced through influx of Jupiter particles alone

The JEDI instrument made measurements of energetic charged particles near Ganymede during a close encounter with that moon. Here we find ion flux levels are similar close to Ganymede itself but outside its magnetosphere and on near wake and open field lines. But energetic electron flux levels are more than a factor of 2 lower on polar and near‐wake field lines than on nearby Jovian field lines at all energies reported here. Flux levels are relevant to the weathering of the surface, particularly processes that affect the distribution of ice, since surface brightness has been linked to the open‐closed field line boundary. For this reason, we estimate the sputtering rates expected in the polar regions due to energetic heavy ions. Other rates, such as those related to radiolysis by plasma and particles that can reach the surface, need to be added to complete the picture of charged particle weathering. This paper uses Juno data to quantify the levels of energetic charged particle flux near Jupiter's moon Ganymede. We have computed energy spectra (energy vs. charged particle intensity) for electrons, protons, oxygen and sulfur ions, from a near encounter of Ganymede by the Juno spacecraft in June 2021. Particle fluxes may be important for weathering processes on Ganymede, such as the sputtering of water ice in the top layer. The distribution of water ice in the upper layer of Ganymede's surface does not appear to be consistent with sublimation alone, so questions remain about which processes are dominant in shaping the ice distribution. Quantification of the flux levels is also needed for planning, for example, for ESA's JUICE mission, expected to orbit Ganymede in the future. We present energy spectra from energetic charged particle data from Juno's close flyby of Ganymede in 2021We find a drop in electron fluxes on polar field lines compared to the surrounding region whereas ion fluxes are similar in both regionsWe compute sputtering rates to evaluate particle weathering, as a step toward understanding the distribution of Ganymede's surface ice We present energy spectra from energetic charged particle data from Juno's close flyby of Ganymede in 2021 We find a drop in electron fluxes on polar field lines compared to the surrounding region whereas ion fluxes are similar in both regions We compute sputtering rates to evaluate particle weathering, as a step toward understanding the distribution of Ganymede's surface ice

The Juno Waves instrument measured plasma waves associated with Ganymede's magnetosphere during its flyby on 7 June, day 158, 2021. Three distinct regions were identified including a wake, and nightside and dayside regions in the magnetosphere distinguished by their electron densities and associated variability. The magnetosphere includes electron cyclotron harmonic emissions including a band at the upper hybrid frequency, as well as whistler‐mode chorus and hiss. These waves likely interact with energetic electrons in Ganymede's magnetosphere by pitch angle scattering and/or accelerating the electrons. The wake is accentuated by low‐frequency turbulence and electrostatic solitary waves. Radio emissions observed before and after the flyby likely have their source in Ganymede's magnetosphere. Observations of plasma waves during the Juno flyby of Ganymede on 7 June 2021 provided evidence of a wake region behind the moon as well as two distinct regions within Ganymede's magnetosphere. Some narrowband wave features enabled the determination of the density of electrons in the magnetosphere. Other intense emissions known as chorus and hiss likely interact with energetic electrons and may be at least partially responsible for auroras seen on the moon. Radio waves detected near Ganymede are likely generated in its magnetosphere. Whistler‐modes, electron cyclotron harmonics, upper hybrid band and radio emissions are detected in and near Ganymede's magnetosphereThree distinct regions in and near the magnetosphere are observed, wake, nightside, and daysideThe dayside ionosphere exhibits higher and more variable electron densities than does the nightside Whistler‐modes, electron cyclotron harmonics, upper hybrid band and radio emissions are detected in and near Ganymede's magnetosphere Three distinct regions in and near the magnetosphere are observed, wake, nightside, and dayside The dayside ionosphere exhibits higher and more variable electron densities than does the nightside

The dynamics of the Jovian magnetosphere is controlled by the interplay of the planet's fast rotation, its solar‐wind interaction and its main plasma source at the Io torus, mediated by coupling processes involving its magnetosphere, ionosphere, and thermosphere. At the ionospheric level, these processes can be characterized by a set of parameters including conductances, field‐aligned currents, horizontal currents, electric fields, transport of charged particles along field lines including the fluxes of electrons precipitating into the upper atmosphere which trigger auroral emissions, and the particle and Joule heating power dissipation rates into the upper atmosphere. Determination of these key parameters makes it possible to estimate the net transfer of momentum and energy between Jovian upper atmosphere and equatorial magnetosphere. A method based on a combined use of Juno multi‐instrument data and three modeling tools was developed by Wang et al. (2021, https://doi.org/10.1029/2021ja029469) and applied to an analysis of the first nine orbits to retrieve these parameters along Juno's magnetic footprint. We extend this method to the first 30 Juno science orbits and to both hemispheres. Our results reveal a large variability of these parameters from orbit to orbit and between the two hemispheres. They also show dominant trends. Southern current systems are consistent with the generation of a region of sub‐corotating ionospheric plasma flows, while both super‐corotating and sub‐corotating plasma flows are found in the north. These results are discussed in light of the previous space and ground‐based observations and currently available models of plasma convection and current systems, and their implications are assessed. We analyze the first 30 orbits of Juno to retrieve the properties of current systems and plasma flows associated with Jovian main aurorasSouthern hemisphere results are consistent with ionospheric plasma sub‐corotationThe two opposite patterns, sub‐corotation and super‐corotation, are observed in the northern hemisphere We analyze the first 30 orbits of Juno to retrieve the properties of current systems and plasma flows associated with Jovian main auroras Southern hemisphere results are consistent with ionospheric plasma sub‐corotation The two opposite patterns, sub‐corotation and super‐corotation, are observed in the northern hemisphere

Jupiter displays the most powerful auroral emissions in our solar system, which result from strong energy dissipation in Jupiter's surrounding space environment. Although mass and energy in Jupiter's magnetosphere mostly come from the innermost Galilean moon Io's volcanic activity and Jupiter's rotation, solar wind perturbations can play crucial roles in releasing magnetospheric energy. The systematic response of the aurora to a solar wind compression remains poorly understood because of timing uncertainties. Here we report the analysis of a set of auroral images from the Hubble Space Telescope with contemporaneous in situ magnetopause detections from Juno, allowing for a more direct comparison. By analyzing the dawn side main auroral emission, we distinguish two non‐mutually exclusive types of auroral enhancements: auroral dawn storm (ADS) featured with latitudinal extension in limited longitudes, and a long‐lasting main auroral brightening with limited extension in latitudes while extending over a large longitude range. Only the latter systematically appears under a compressed magnetopause, while the dawn storms could occur whatever the state of the magnetopause. The results could provide important constraints to improve theoretical models and numerical simulations. During expanded magnetopause conditions, Jupiter's aurora displayed either quiet or dawn storm morphology. The result is consistent with recent discovery of the initiation of ADSs in midnight and post‐midnight, possibly driven by magnetic reconnection plasma instabilities in night magnetotail. Our results show that some typical auroral morphologies could be used as a diagnostic of solar wind conditions at Jupiter. Planetary aurorae are an image of the perturbations of energetic particles in the planetary magnetosphere. Jupiter has the largest magnetosphere in our solar system, and it produces the most powerful auroral emission. Unlike the terrestrial magnetosphere that is mainly driven by solar wind activities, the major plasma source in Jupiter's magnetosphere comes from the innermost Galilean moon Io's volcanic activity. The respective impact of the solar wind and internal plasma sources on Jupiter's magnetosphere and aurorae has been under debate for decades, mostly due to the lack of direct connection between solar wind conditions and auroral morphologies. Using contemporaneous measurements from the Hubble Space Telescope and Juno spacecraft, we can systematically determine the relation between auroral morphologies and magnetopause compression at Jupiter. The results could crucially constrain the physical interpretation of Jupiter's main aurora. We quantify two types of auroral intensifications at JupiterDuring solar wind compression, Jupiter's main aurora was systematically enhanced globallyDuring Quiet solar wind conditions, aurora could have either quiet or dawn storm morphologies We quantify two types of auroral intensifications at Jupiter During solar wind compression, Jupiter's main aurora was systematically enhanced globally During Quiet solar wind conditions, aurora could have either quiet or dawn storm morphologies