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The moon Ganymede has a strong internally generated magnetic field that separates the surface into two regions, with the polar surface magnetically connected to the Jovian environment. Consequently, the weathering of Ganymede's surface by plasma and energetic charged particles trapped in the Jovian magnetosphere is not uniform. At the same time, optical data suggest differences in the surface ice between the poles and the equator, with further refinements in the equatorial surface that have a longitudinal dependence. Here, we use Juno spacecraft data to characterize the charged particle environment along Ganymede's orbit (from about 50 eV to 1 MeV for electrons and 10 eV to 6 MeV for protons). These results put us into a better position to test the hypothesis that space weathering by electrons causes the brighter poles of Ganymede, given that electron fluxes are likely to be more clearly separated between the polar and equatorial regions. Electron weathering of ice may be the reason Ganymede's poles are brighter. These particles are well separated by the two magnetic topologies of Ganymede, whereas ions are not as well separated. We suggest ions may be converted to energetic neutral atoms near Ganymede and reach the surface despite the magnetic barrier. We quantify the environmental inputs in this paper. Juno orbital evolution has allowed better characterization of the region near Jupiter's magnetic equator and Ganymede's orbital distanceElectron and proton energy spectra from that region derived from Jovian Auroral Distributions Experiment (JADE) and Jupiter Energetic‐particle Detector Instrument (JEDI) data are presentedWe propose that polar versus equatorial albedo differences on Ganymede are due to changes resulting from electron precipitation Juno orbital evolution has allowed better characterization of the region near Jupiter's magnetic equator and Ganymede's orbital distance Electron and proton energy spectra from that region derived from Jovian Auroral Distributions Experiment (JADE) and Jupiter Energetic‐particle Detector Instrument (JEDI) data are presented We propose that polar versus equatorial albedo differences on Ganymede are due to changes resulting from electron precipitation

Jupiter is surrounded by intense and energetic radiation belts, yet most of the available in‐situ data, in volume and quality, were taken outward of Europa's orbit, where radiation conditions are not as extreme. Here, we study measurements of ions of tens of keV to tens of MeV at <10 Jupiter radii (RJ) distance to Jupiter, therefore inward of the orbit of Europa. Ion intensities drop around 6 RJ, near Io's orbit. Previous missions reported on radiation belts of tens and hundreds of MeV ions located between 2 and 4 RJ. Measurements of lower energies were not conclusive because high energy particles often contaminate the measurement of lower energy particles. Here, we show for the first time that ions in the hundreds of keV range are present and suggest that ions may extend even into the GeV range. The observation of charged particles yields information on the entire field line, not just the local field. We find that there is a region close to Jupiter where no magnetic trapping is possible. Jupiter's innermost radiation belt is located at <2 RJ, inward of the main ring. Previous work suggested that this belt is sourced by re‐ionized energetic neutral atoms coming steadily inward from distant regions. Here, we perform a phase space density analysis that shows consistency with such a local source. However, an alternative explanation is that the radiation belt is populated by occasional strong radial transport and then decays on the timescale of years. Planets with a magnetic field, like Earth and Jupiter, are surrounded by belts of natural charged particle radiation. These regions are called “radiation belts,” and they pose challenges to space exploration because of their severe effects on spacecraft and humans. Understanding the fundamental nature of radiation belts, for example, formation, structure, and dynamics has also been a scientific pursuit for decades, but there is still much to learn. Some of the most extreme radiation conditions are found at Jupiter, which makes that planet an ideal laboratory to study how radiation develops in space. Even though raw measurements from satellites in orbit of Jupiter exists, they often cannot be used as‐is. This is because strong radiation can interfere with radiation instruments in the same way that direct sunlight interferes with a thermometer. Here, we present results of a careful processing of data from the Juno mission to get around the instrument limitations. Our analysis not only extends the observed energy ranges of ions of Jupiter radiation belts but also forms the basis for testing new ideas. For example, our results suggest that the belts may form by ions that originate from a different region around Jupiter. Ion belts at a distance of 2–4 Jovian radii have significant intensities from hundreds of keV to GeVPhase space densities of the innermost ion belt suggest non‐steady state conditions or a local sourceA region without stable magnetic trapping exists close to Jupiter Ion belts at a distance of 2–4 Jovian radii have significant intensities from hundreds of keV to GeV Phase space densities of the innermost ion belt suggest non‐steady state conditions or a local source A region without stable magnetic trapping exists close to Jupiter

On July 13, 2016, the Hubble Space Telescope observed the onset of a dawn storm in Jupiter's northern ultraviolet aurora, while the NASA Juno spacecraft simultaneously traversed the dawnside outer magnetosphere. This represents the first concurrent auroral and in situ magnetospheric observations of the onset of a dawn storm at Jupiter. Mapping the auroral emission to the magnetosphere reveals the dawn storm corresponds to a source region at ∼60 Jupiter radii, and the eastward edge propagates toward local noon at speeds exceeding corotation. Particle observations from Jovian Auroral Distributions Experiment (JADE) and Jupiter Energetic particle Detector Instrument (JEDI) reveal the presence of enhanced hot plasma density in the outer magnetosphere during this interval, and pitch angle distributions measured with JEDI reveal pronounced field‐aligned proton and heavy ion motion. Juno magnetometer (MAG) signatures reveal a reversal in the azimuthal magnetic field at the time of storm onset, suggesting acceleration of the hot plasma population above typical sub‐corotational speeds. JEDI also detects a region of energetic particles which persists throughout the day following the storm, a feature which is not observed during subsequent perijoves. We interpret this dawn storm as the result of reconnection at earlier local times, possibly associated with a disruption of the azimuthal magnetodisk current. HST images captured a dawn storm while Juno was present in Jupiter's dawn outer magnetosphereJuno in situ measurements suggest a region of hot, accelerated plasma in the outer magnetosphere following storm onsetJEDI particle distributions reveal an extended region of energetic particles near the magnetopause following the storm HST images captured a dawn storm while Juno was present in Jupiter's dawn outer magnetosphere Juno in situ measurements suggest a region of hot, accelerated plasma in the outer magnetosphere following storm onset JEDI particle distributions reveal an extended region of energetic particles near the magnetopause following the storm

We present Magnetospheric Multiscale (MMS) mission observations of substorm‐related ion injections at magnetotail positions between ∼7.5 REand 8.5 RE. Energization of supra‐thermal ions (50 keV–1,000 keV) is strongly species dependent, with oxygen reaching significantly higher peak energies than protons. Using a previously established correlation technique, we confirm that the highest energy (>400 keV) oxygen ions are multiply charged of solar wind origin, enabling them to reach high energies in rough proportion to their charge states. Significantly, we conclude that oxygen ions between 130 keV and 330 keV are singly charged, and that they sometimes achieve higher energizations relative to protons, that is, higher than expected based on their charge states. We conclude that nonadiabatic processes can boost the energies of the oxygen ions. The technique does not depend on “before injection” and “after injection” spectral comparisons, and therefore likely represents the most definitive test yet of nonadiabaticity. Earth's space environment, its magnetosphere, forms an invisible comet‐like shape, with a “magnetotail” extending away from the Sun. Satellite observations there often show “injections”; sudden enhancements of ions from 10 to 100s of kilo‐electron volts. The ions of hydrogen (H), helium (He), and oxygen (O) come from both the solar wind and from the Earth's ionosphere. Oxygen from the solar wind is more strongly ionized (charge state often +6) than that from the ionosphere (charge state +1). In the magnetosphere, ions gyrate around in spirals in Earth's magnetic field. Because singly charged oxygen (O+) gyrates more slowly and with larger gyrating orbits than other ions, we expect that such ions can be more strongly accelerated compared to lighter ions during injections. But, that expectation is difficult to prove from a single spacecraft. Here, we present a method that more definitively demonstrates that O+is sometimes preferentially energized compared to H+during injections. We revisit correlations of 50 keV –1,000 keV ion dynamics to infer charge‐ and mass‐dependent acceleration during magnetotail injectionsSingly charged oxygen ions (≤300 keV) are sometimes preferentially energized compared to protons, likely by nonadiabatic processesThe technique does not depend on time‐separated spectral comparisons and so is likely the most definitive test yet of nonadiabaticity We revisit correlations of 50 keV –1,000 keV ion dynamics to infer charge‐ and mass‐dependent acceleration during magnetotail injections Singly charged oxygen ions (≤300 keV) are sometimes preferentially energized compared to protons, likely by nonadiabatic processes The technique does not depend on time‐separated spectral comparisons and so is likely the most definitive test yet of nonadiabaticity

Field‐aligned proton beams are a systematic and identifiable feature associated with Jupiter's auroral emissions, transporting 3 ± 2 kg s−1away from Jupiter's ionosphere. This mass loss occurs at all longitudes sampled by Juno around the southern auroral oval, while the northern hemisphere exhibits upward proton beams predominantly on one portion in System III, near the auroral kink region. These beams are associated with upward inverted‐V structures indicative of quasi‐static magnetic field‐aligned parallel potentials. A lack of bidirectionality indicates these proton populations are pitch‐angle and/or energy scattered and incorporated into the magnetospheric charged particle environment. This mechanism is a significant, and potentially dominant, source of protons in Jupiter's middle and outer magnetosphere. If Jupiter's ionosphere is the primary source for protons in the inner magnetosphere, they are likely sourced equatorward of the main emissions and at energies <100 eV. We find fluxes of narrow proton beams streaming away from Jupiter, where 3 ± 2 kg s−1flows away from Jupiter's atmosphere into its local magnetic system. The mass loss occurs consistently around Jupiter's southern auroral emissions, but is not uniform in the northern hemisphere. These beams are found to be a significant source of protons for much of Jupiter's magnetic environment. Field‐aligned H+beams are a systematic feature near Jupiter's main auroral emissionsPotential structures >100 V transport 3 ± 2 kg s−1of ionospheric H+from Jupiter to the middle and outer magnetosphereThis phenomenon is a significant source of H+in Jupiter's magnetosphere Field‐aligned H+beams are a systematic feature near Jupiter's main auroral emissions Potential structures >100 V transport 3 ± 2 kg s−1of ionospheric H+from Jupiter to the middle and outer magnetosphere This phenomenon is a significant source of H+in Jupiter's magnetosphere

Observations of energetic charged particles associated with Io's footprint (IFP) tail, and likely within or very near the Main Alfvén Wing, during Juno's 12th perijove (PJ) crossing show evidence of intense proton acceleration by wave‐particle heating. Measurements made by Juno/JEDI reveal proton characteristics that include pitch angle distributions concentrated along the upward loss cone, broad energy distributions that span ~50 keV to 1 MeV, highly structured temporal/spatial variations in the particle intensities, and energy fluxes as high as ~100 mW/m2. Simultaneous measurements of the plasma waves and magnetic field suggest the presence of ion cyclotron waves and transverse Alfvénic fluctuations. We interpret the proton observations as upgoing conics likely accelerated via resonant interactions with ion cyclotron waves. These observations represent the first measurements of ion conics associated with moon‐magnetosphere interactions, suggesting energetic ion acceleration plays a more important role in the IFP tail region than previously considered. NASA's Juno spacecraft orbits Jupiter's polar region and makes direct measurements of the fields and particles that are responsible for creating Jupiter's powerful auroras. In this article, we present new observations that show intense proton acceleration occurring at altitudes near the auroral emissions created by the interaction between Jupiter's moon Io and the surrounding plasma and magnetic field environment. These unique observations provide clues on how particles are being accelerated and will help constrain particle acceleration theories. Juno's likely crossing of Io's Main Alfvén Wing (MAW) during PJ12 reveals evidence of transverse ion accelerationObservations suggest wave‐particle interactions with ion cyclotron waves as the favored acceleration mechanism; however, Alfvén acceleration was not ruled outIon conics generated in Io's footprint tail or near the MAW are more intense and energetic than observed in other auroral regions

Energetic Neutral Atom (ENA) cameras on orbiting spacecraft at Earth and Saturn helped greatly to diagnose these complex magnetospheres. Within this decade, the European Space Agency's Jupiter Icy Moons Explorer will make ENA imaging a major thrust in understanding Jupiter's complex magnetosphere. The present polar‐orbiting Juno mission at Jupiter carries no ENA camera. But, the Jupiter Energetic‐particle Detector Instrument is sensitive to >50 keV ENAs, provided there are no local charged particles to mask their presence. Juno offers great service to interpreting past serendipitous and future dedicated ENA imaging with its orbit providing unique viewing perspectives. Here we report Juno observations of ENAs from Jupiter's polar regions. These ENAs likely arise from energetic ions that nearly precipitate in the main auroral regions and mirror magnetically within, and charge exchange with, Jupiter's upper atmosphere. Jupiter proves itself different from Saturn, as ENAs generated from precipitating ions were not identified there. In the space environments (called magnetospheres) of magnetized planets, singly charged energetic particles, trapped by the planet's magnetic field, can steal electrons from cold gas atoms and become neutralized. These now Energetic Neutral Atoms (ENAs), no longer confined by the magnetic field, can travel out of the system similar to photons leaving a hot oven. ENA cameras on orbiting spacecraft at Earth and Saturn have helped greatly to diagnose these complex magnetospheres. The present polar‐orbiting Juno mission at Jupiter carries no ENA camera. But, the Jupiter Energetic‐particle Detector Instrument is sensitive to ENAs with energies >50 keV, provided there are no charged particles in the environment to mask their presence. Here we report on Juno observations of ENAs coming from Jupiter's polar regions. These ENAs likely arise from energetic ions that nearly precipitate, reaching the atmospheric regions of Jupiter's main aurora and mirroring magnetically within Jupiter's upper atmosphere. Energetic Neutral Atoms (ENAs) with energies >50 keV are observed to be emitted from Jupiter's north polar regionsThese ENAs appear to be from precipitating energetic ions that magnetically mirror within the upper atmosphere of Jupiter's main auroraFindings support previous proposals that precipitating ions contribute greatly to Jupiter ENA emissions, contrary to findings at Saturn Energetic Neutral Atoms (ENAs) with energies >50 keV are observed to be emitted from Jupiter's north polar regions These ENAs appear to be from precipitating energetic ions that magnetically mirror within the upper atmosphere of Jupiter's main aurora Findings support previous proposals that precipitating ions contribute greatly to Jupiter ENA emissions, contrary to findings at Saturn

Understanding the energization processes and constituent composition of the plasma and energetic particles injected into the near‐Earth region from the tail is an important component of understanding magnetospheric dynamics. In this study, we present multiple case studies of the high‐energy (≳40 keV) suprathermal ion populations during energetic particle enhancement events observed by the Energetic Ion Spectrometer (EIS) on NASA's Magnetospheric Multiscale (MMS) mission in the magnetotail. We present results from correlation analysis of the flux response between different energy channels of different ion species (hydrogen, helium, and oxygen) for multiple cases. We demonstrate that this technique can be used to infer the dominant charge state of the heavy ions, despite the fact that charge is not directly measured by EIS. Using this technique, we find that the energization and dispersion of suprathermal ions during energetic particle enhancements concurrent with (or near) fast plasma flows are ordered by energy per charge state (E/q) throughout the magnetotail regions examined (~7 to 25 Earth radii). The ions with the highest energies (≳300 keV) are helium and oxygen of solar wind origin, which obtain their greater energization due to their higher charge states. Additionally, the case studies show that during these injections the flux ratio of enhancement is also well ordered by E/q. These results expand on previous results which showed that high‐energy total ion measurements in the magnetosphere are dominated by high‐charge‐state heavy ions and that protons are often not the dominant species above ~300 keV. In the magnetotail during injections, the charge states of suprathermal He and O ions can be inferred with a correlation analysisEnergization of ionospheric and solar wind ions during injections in the magnetotail is remarkably coherent and ordered by charge stateThe highest energy ions (≳300 keV) observed are heavies of solar wind origin and reach higher energies due to their higher charge states

We report the first in situ observations of electron measurements at a Europa footprint tail (FPT) crossing in the auroral region. During its 12th science perijove pass, Juno crossed magnetic field lines connected to Europa's FPT. We find that electrons in the range ~0.4 to ~25 keV, with a characteristic energy of 3.6 ± 0.5 keV, precipitate into Jupiter's atmosphere to create the footprint aurora. The energy flux peaks at ~36 mW/m2, while the peak ultraviolet (UV) brightness is estimated at 37 kR. We estimate the peak electron density and temperature to be 17.3 cm−3and 1.8 ± 0.1 keV, respectively. Using magnetic flux shell mapping, we estimate that the radial width of the interaction at Europa's orbit spans roughly 3.6 ± 1.0 Europa radii. In contrast to typical Io FPT crossings, the instrument background caused by penetrating energetic radiation (> ~5–10 MeV electrons) increased during the Europa FPT crossing. Jupiter's moons interact with Jupiter's space environment, or magnetosphere, and create auroral spots and tails in Jupiter's ionosphere. Io's aurora footprint on Jupiter is the strongest and most persistent of all moons, but Ganymede, Callisto, and Europa's auroral footprints are also routinely observed by remote platforms. NASA's Juno mission and its instrument suite occasionally fly through regions that are connected to the moon‐magnetosphere interactions. During these crossings, Juno samples the electrons and ions that create the aurora. This paper is the first report of electron measurements taken during a Juno crossing of Europa's tail. These measurements confirm previous results based on remote observations. Most importantly, they provide a sample of the conditions in the regions associated with Europa's footprint aurora in Jupiter's magnetosphere. This is the first report of in situ electron measurements of a Europa footprint tail crossingPrecipitating electron energies range from ~0.4 to ~25 keV with a characteristic energy of 3.6 keV, consistent with a low color ratio of the auroral emissionsThe instrument background caused by > ~5–10 MeV penetrating electrons increased during the crossing, opposite to what is observed at Io

In this paper, we exploit the charge‐dependent nature of auroral phenomena in Jupiter's polar cap region to infer the charge states of energetic oxygen and sulfur. To date, there are very limited and sparse measurements of the >50 keV oxygen and sulfur charge states, yet many studies have demonstrated their importance in understanding the details of various physical processes, such as X‐ray aurora, ion‐neutral interactions in Jupiter's neutral cloud, and particle acceleration theories. In this contribution, we develop a technique to determine the most abundant charge states associated with heavy ions in Jupiter's polar magnetosphere. We find that O+and S++are the most abundant and therefore iogenic in origin. The results are important because they provide (1) strong evidence that soft X‐ray sources are likely due to charge stripping of magnetospheric ions and (2) a more complete spatial map of the oxygen and sulfur charge states, which is important for understanding how the charge‐ and mass‐dependent physical processes sculpt the energetic particles throughout the Jovian magnetosphere. Quasi‐static electric potentials in Jupiter's polar cap region are used to determine the energetic (>hundreds of keV) ion charge statesThe most abundant charge states associated with these precipitating ions are O+and S++and therefore iogenic in originThese observations are important for X‐ray auroral and ion‐neutral interaction physics

Jupiter displays many distinct auroral structures, among which auroral dawn storms and auroral injections are often observed contemporaneously. However, it is unclear if the contemporaneous nature of the observations is a coincidence or part of an underlying physical connection. We show six clear examples from a recent Hubble Space Telescope campaign (GO‐14634) that each display both auroral dawn storms and auroral injection signatures. We found that these conjugate phenomena could exist during intervals of either relatively low or high auroral activity, as evidenced by the varied levels of total auroral power. In situ observations of the magnetosphere by Juno show a strong magnetic reconnection event inside of 45 Jupiter radii (RJ) on the predawn sector, followed by two dipolarization events within the following 2 hr, coincident with the auroral dawn storm and auroral injection event. We therefore suggest that the auroral dawn storm is the manifestation of magnetic reconnection in the dawnside magnetosphere. The dipolarization region is mapped to the auroral injection, strongly suggesting that this was associated with the auroral injection. Since magnetic reconnection and dipolarization are physically connected, we therefore suggest that the often‐conjugate auroral dawn storm and auroral injection events are also physically connected consequences. Jupiter's auroral dawn storm and auroral injection events are conjugate and physically connected phenomenaWe report the recurrent nature of magnetic dipolarization at JupiterThese observations suggest that reconnection manifests auroral dawn storms and subsequent dipolarization produces auroral injection events

The Juno spacecraft has been orbiting Jupiter for the past 3 years. During this interval, Juno has collected large datasets of plasma, magnetic field, and wave measurements in Jupiter's magnetosphere. In this study, we conduct statistics on the plasma and wave characteristics in Jupiter's magnetodisk (nightside magnetosphere beyond 20 Jupiter radii, RJ). Distributions of electron fluxes and waves in the magnetodisk exhibit different characteristics in two regions: the plasma sheet with dense plasma and weak wave intensity and the plasma sheet boundary layer (PSBL) occupied by strong waves and rarefied plasma. We discuss that these waves can be generated by field‐aligned high‐energy electrons in the PSBL. We further compare plasma and wave properties in the PSBL of Jupiter's magnetodisk with the PSBL in Earth's magnetotail. This comparison suggests that the PSBL in Jupiter's magnetodisk may be formed by transient magnetic reconnection in the Jovian magnetosphere. The dawn‐dusk asymmetry of the PSBL properties further supports this scenario: the PSBL is more pronounced in the dawn flank of Jupiter's magnetodisk, where magnetic reconnection is predicted to occur by models of the rotation‐dominated magnetosphere. We further discuss properties of the Jovian PSBL and implications on magnetic reconnection in Jupiter's magnetosphere. Our results reveal the statistical properties of plasma sheet boundary layer in Jupiter's magnetodiskStrong electromagnetic waves with E/cB∼1 are observed at the plasma sheet boundary layerObserved dawn‐dusk asymmetry of the plasma sheet boundary layer is consistent with expected asymmetry of transient reconnection

This work presents the first detailed analysis of foreshock bubbles (FBs) using high‐resolution Magnetospheric Multiscale (MMS) data. Between October 2017 and January 2019, MMS captured 10 foreshock transient events with burst resolution data that we show are consistent with FBs. One “textbook” event is examined and described in detail. Employing the multipoint nature of MMS, we demonstrate how the size and orientation, expansion speed, and distance since formation can be estimated. From all 10 events, FB sizes ranged from 1.1 to 9.9 RE(average of 4.4 RE), and expansion speeds ranged from 139 to 377 km/s (average of 257 km/s). FBs formed under a usual range of solar wind conditions between 3 and 20 REupstream of Earth's bow shock. We also report on new features of FBs: deep and localized magnetic “holes” within the cores of FBs, where the total field strength drops to <1 nT. Foreshock bubbles (FBs) are large (up to 10 RE), explosive (expansion speeds of >100 km/s) events upstream of the bow shockFBs form under a usual range of solar wind conditions between 3 and 20 REupstream of Earth's bow shockFB cores often include deep, localized magnetic holes where the B field drops to <1 nT

Upward‐moving energetic electrons with energies of 1 MeV and above were observed over the entire Jovian polar region. The electrons were found to be associated with intense broadband whistler mode waves, similar to terrestrial whistler mode auroral hiss. Upward‐propagating whistler mode hiss at Earth is known to be generated by upward‐moving, magnetic field‐aligned electron beams (from electric field‐aligned potentials), by a beam‐plasma instability at the Landau resonance. Assuming this process at Jupiter, we present a linear stability analysis, showing the electron distribution functions (based on inverted‐V observations made by the Juno Jovian Auroral Distributions Experiment, JADE‐E, instrument) are unstable. The polarization of the modeled waves is consistent with whistler mode hiss (right‐hand circularly polarized). From the results of the linear stability analysis, we find that the calculated growth rates are sufficient to produce the observed whistler mode waves. Upgoing electron beams can generate upward‐propagating whistler mode waves over the Jovian polar cap regionNumerical simulations show the electron beams are unstable and capable of producing the observed whistler mode wavesLarge growth rates are found for Landau (n= 0) resonance

In planetary magnetospheres, singly charged energetic particles, trapped by the planet's magnetic field, can steal electrons from cold gas atoms and become neutralized. These now energetic neutral atoms (ENAs), no longer confined by the magnetic field, can travel out of the system similar to photons leaving a hot oven. ENAs have been used to image magnetospheric processes at Earth, Jupiter, and Saturn. At Jupiter, the opportunities to image the magnetosphere have been limited and always from the perspective of the near‐equatorial plane at distance >139 RJ. The polar‐orbiting Juno mission carries the Jupiter Energetic particle Detector Instrument that is serendipitously sensitive to ENAs with energies >50 keV, provided that there are no charged particles in the environment to mask their presence. Here we report on the first ENA observations of Jupiter's magnetosphere from a nonequatorial perspective. In this brief report we concentrate on emissions seen during Perijove 22 (PJ22) during very active conditions and compare them with emissions during the inactive Perijove 23 (PJ23). We observe, and discriminate between, distinct ENA signatures from the neutral gases occupying the orbit of Io (away from Io itself), the orbit of Europa (away from Europa), and from Jupiter itself. Strong ENA emissions from Io's orbit during PJ22 are associated with energetic particle injections observed near Io's orbit several hours earlier. Some injections occurred planetward of Io's L‐shell (magnetic position), somewhat of a surprise given that injections are thought to be driven by outward transport of plasmas generated by Io. In the space environments of magnetized planets (magnetospheres), magnetic fields trap and confine energetic charged particles like protons and singly charged heavier ions. These ions can neutralize themselves by stealing electrons from cold gas atoms within the same environment. They become energetic neutral atoms (ENAs), and no longer confined by the magnetic field, can travel out of the system in a fashion similar to light leaving a hot oven. ENAs have been used to image magnetospheric processes at Earth, Jupiter, and Saturn. At Jupiter, the opportunities to image the magnetosphere have been limited and always from the perspective of the near‐equatorial plane at large distances (>139 RJ). The polar‐orbiting Juno mission carries the Jupiter Energetic particle Detector Instrument that is serendipitously sensitive to ENAs with energies >50 keV, provided that there are no charged particles in the environment to mask their presence. Here we report on the first ENA observations of Jupiter's magnetosphere from a nonequatorial perspective. That perspective allows us to observe distinct ENA signatures from the neutral gases occupying the orbit of the moon Io (away from Io itself), the gases in the orbit of the moon Europa (away from Europa), and from Jupiter itself. The first Jovian off‐equator energetic neutral atom (ENA) viewings reveal distinct emissions from Jupiter and the orbits of Io and EuropaStrong ENA emissions from Io's orbit are associated with energetic particle injections near Io's orbit observed several hours earlierEnergetic particle injections occur inside Io's orbit, a surprise given expectations that outward transport from Io drives injections

Transient magnetic reconnection and associated fast plasma flows led by dipolarization fronts play a crucial role in energetic particle acceleration in planetary magnetospheres. Despite large statistical observations on this phenomenon in the Earth's magnetotail, many important characteristics (e.g., mass or charge dependence of acceleration efficiency and acceleration scaling with the spatial scale of the system) of transient reconnection cannot be fully investigated with the limited parameter range of the Earth's magnetotail. The much larger Jovian magnetodisk, filled by a mixture of various heavy ions and protons, provides a unique opportunity for such investigations. In this study, we use recent Juno observations in Jupiter's magnetosphere to examine the properties of reconnection associated dipolarization fronts and charged particle acceleration. High‐energy fluxes of sulfur, oxygen, and hydrogen ions show clear mass‐dependent acceleration with energy ∼m1/3. We compare Juno observations with similar observations in the Earth's magnetotail and discuss possible mechanism for the observed ion acceleration. Heavy ion and proton acceleration by transient dipolarizations in the Jupiter magnetodiskIon acceleration results in flux increases in the energy range of [0.5,3] MeVObservations show that ion acceleration scales with mass as ∼m1/3

Jupiter's ultraviolet (UV) aurorae, the most powerful and intense in the solar system, are caused by energetic electrons precipitating from the magnetosphere into the atmosphere where they excite the molecular hydrogen. Previous studies focused on case analyses and/or greater than 30‐keV energy electrons. Here for the first time we provide a comprehensive evaluation of Jovian auroral electron characteristics over the entire relevant range of energies (~100 eV to ~1 MeV). The focus is on the first eight perijoves providing a coarse but complete System III view of the northern and southern auroral regions with corresponding UV observations. The latest magnetic field model JRM09 with a current sheet model is used to map Juno's magnetic foot point onto the UV images and relate the electron measurements to the UV features. We find a recurring pattern where the 3‐ to 30‐keV electron energy flux peaks in a region just equatorward of the main emission. The region corresponds to a minimum of the electron characteristic energy (<10 keV). Its polarward edge corresponds to the equatorward edge of the main oval, which is mapped at Mshells of ~51. A refined current sheet model will likely bring this boundary closer to the expected 20–30 RJ. Outside that region, the >100‐keV electrons contribute to most (>~70–80%) of the total downward energy flux and the characteristic energy is usually around 100 keV or higher. We examine the UV brightness per incident energy flux as a function of characteristic energy and compare it to expectations from a model. Aurorae, also commonly called Northern or Southern Lights, are among the most spectacular displays of nature. They are observed not only at Earth but at other planets too, such as Mars, Jupiter, and Saturn. In fact, Jupiter has the brightest aurora in the solar system. The aurora is created when electrons and/or ions in space precipitate into the atmosphere and excite the ambient gas. At Jupiter, they mostly shine in the ultraviolet which is invisible to our eyes but can be seen with suitable instrumentation. The faster the electrons, the deeper they go into the atmosphere, but also the more energy they carry, which eventually can be converted to create more light. This study is about characterizing the electrons that create Jupiter's aurora using many instruments from the National Aeronautics and Space Administration's Juno Mission. We find that different ultraviolet emissions correspond to different electron characteristics. Knowing the differences will help us to understand the bigger picture to explain the processes that create the aurora. We present a survey of Jovian auroral electrons characteristics from 50 eV to 1000 keV by JunoWe present a metric to identify main oval crossings in electron data using 3‐30 keV electrons energy fluxWe estimate the UV brightness per incident electron energy flux as a function of characteristic energy

The four Magnetospheric Multiscale (MMS) spacecraft observed energetic ( E>50keV) ion bursts exhibiting an inverse dispersion in the magnetosheath on 28 December 2015. We consider the possibility that these ions originate from the magnetosphere. The ion composition ratios, flux levels, and the spectral slopes of the energetic ion energy spectra observed in the foreshock and in the magnetosheath resemble those in the outer magnetosphere but differ significantly from those seen further upstream from the bow shock at ACE. The particle gyrocenters lie earthward from the spacecraft, indicating that the maximum ion fluxes come from close to the magnetosphere. We provide important evidence that argues against an explanation of the particle source in terms of hot flow anomaly acceleration. A three‐dimensional global hybrid simulation shows that escaping magnetospheric ions can be scattered and transported across the magnetosheath. Ground magnetometer observations suggest that a solar wind pressure increase accelerates escaping magnetospheric ions via betatron acceleration, resulting in an inverse energy dispersion in the magnetosheath. However, there are no pressure changes detected on the MMS and ACE spacecraft and the ground magnetic field strength does not appear to be large enough to be consistent with the large magnetospheric compression needed to account for a betatron acceleration. Therefore, we suggest that the inverse energy dispersion event can be explained by a magnetic field rotation that connects MMS to the subsolar magnetosphere, enabling high‐energy particles from deep within the inner magnetosphere gain access to the magnetopause and magnetosheath. The four MMS spacecraft observed an energetic ion burst exhibiting an inverse dispersion in the magnetosheathThe energetic ion energy spectra observed in the foreshock and in the magnetosheath resemble those in the outer magnetosphereA 3‐D global hybrid simulation shows that escaping magnetospheric ions can be scattered and transported across the magnetosheath

Previous Juno mission event studies revealed powerful electron and ion acceleration, to 100s of kiloelectron volts and higher, at low altitudes over Jupiter's main aurora and polar cap (PC; poleward of the main aurora). Here we examine 30–1200 keV JEDI‐instrument particle data from the first 16 Juno orbits to determine how common, persistent, repeatable, and ordered these processes are. For the PC regions, we find (1) upward electron angle beams, sometimes extending to megaelectron volt energies, are persistently present in essentially all portions of the polar cap but are generated by two distinct and spatially separable processes. (2) Particle evidence for megavolt downward electrostatic potentials are observable for 80% of the polar cap crossings and over substantial fractions of the PC area. For the main aurora, with the orbit favoring the duskside, we find that (1) three distinct zones are observed that are generally arranged from lower to higher latitudes but sometimes mixed. They are designated here as the diffuse aurora (DifA), Zone‐I (ZI(D)) showing primarily downward electron acceleration, and Zone‐II (ZII(B)) showing bidirectional acceleration with the upward intensities often greater than downward intensities. (2) ZI(D) and ZII(B) sometimes (but not always) contain, respectively, downward electron inverted Vs and downward proton inverted Vs, (potentials up to 400 kV) but, otherwise, have broadband distributions. (3) Surprisingly, both ZI(D) and ZII(B) can generate equally powerful auroral emissions. It is suggested but demonstrated for intense portions of only one auroral crossing, that ZI(D) and ZII(B) are associated, respectively, with upward and downward electric currents. The science objectives of the Juno mission, with its spacecraft now orbiting Jupiter in a polar orbit, include understanding the space environments of Jupiter's polar regions and generation of Jupiter's uniquely powerful aurora. In Jupiter's polar cap regions (poleward of the main auroral oval encircling the northern and southern poles), we find here that (1) beams of electrons aligned with the upward magnetic field direction are ever‐present with energies extended to the 100s to 1,000s of kilo electron volts and (2) downward magnetic field‐aligned electrostatic potentials reaching greater than a million volts occur over broad regions for 80% of the polar cap crossings. For the main auroral oval, we find three distinct zones: designated here as diffuse aurora (DifA), Zone‐I (ZI(D)) showing downward electron acceleration to 100s of kiloelectron volts, and Zone‐II (ZII(B)) showing bidirectional acceleration with the upward intensities often greater than downward intensities. ZI(D) sometimes shows upward electrostatic potentials reaching 100s of kilovolts and is associated with upward magnetic field‐aligned electric currents. ZII(B) sometimes shows downward electrostatic potentials reaching 100s of kilovolts and is associated with downward electric currents. Unexpectedly from Earth studies, ZI(D) and ZII(B) are just as likely to generate the most intense auroral emissions. Jupiter's polar caps have upward electron beams essentially everywhere (100s of kiloelectron volts) and often downward megavolt electric potentialsEnergetic particles reveal three main auroral acceleration zones: diffuse aurora (DifA), Zone‐I (downward), and Zone‐II (bidirectional)ZI(D) and ZII(B) sometimes (but not always) contain, respectively, downward electron inverted Vs and downward proton inverted Vs

The Jovian Auroral Distributions Experiment Ion sensor (JADE‐I) on Juno is a plasma instrument that measures the energy‐per‐charge (E/Q) distribution of 0.01 to 46.2 keV/qions over a mass‐per‐charge (M/Q) range of 1– 64 amu/q. However, distinguishing O+and S2+from JADE‐I's measurements is a challenging task due to similarities in their M/Q(∼16 amu/q). Because of this, O+and S2+have not been fully resolved in the in situ measurements made by plasma instruments at Jupiter (e.g., Voyager PLS and Galileo PLS) and their relative ratios has been studied using physical chemistry models and ultraviolet remote observations. To resolve this ambiguity, a ray tracing simulation combined with carbon foil effects is developed and used to obtain instrument response functions for H+, O+, O2+, O3+, Na+, S+, S2+, and S3+. The simulation results indicate that JADE‐I can resolve the M/Qambiguity between O+and S2+due to a significant difference in their charge state modification process and a presence of a large electric potential difference (∼8 kV) between its carbon foils and MCPs. A forward model based on instrument response functions and convected kappa distributions is then used to obtain ion properties at the equatorial plasma sheet (∼36 RJ) in the predawn sector of magnetosphere. The number density ratio between O+and S2+for the selected plasma sheet crossings ranges from 0.2 to 0.7 (0.37 ± 0.12) and the number density ratio between total oxygen ions to total sulfur ions ranges from 0.2 to 0.6 (0.41 ± 0.09). A technique to resolve mass‐per‐charge ambiguity of O+and S2+for ESA‐TOF instruments using carbon foil effectsAn analysis tool to extract ion distributions from in situ measurements of Juno JADE‐IIon bulk properties derived from the forward model with convected kappa distributions in the jovian plasma sheet

Many attempts have been made to model X‐ray emission from both bremsstrahlung and ion precipitation into Jupiter's polar caps. Electron bremsstrahlung modeling has fallen short of producing the total overall power output observed by Earth‐orbit‐based X‐ray observatories. Heavy ion precipitation was able to reproduce strong X‐ray fluxes, but the proposed incident ion energies were very high ( >1 MeV per nucleon). Now with the Juno spacecraft at Jupiter, there have been many measurements of heavy ion populations above the polar cap with energies up to 300–400 keV per nucleon (keV/u), well below the ion energies required by earlier models. Recent work has provided a new outlook on how ion‐neutral collisions in the Jovian atmosphere are occurring, providing us with an entirely new set of impact cross sections. The model presented here simulates oxygen and sulfur precipitation, taking into account the new cross sections, every collision process, the measured ion fluxes above Jupiter's polar aurora, and synthetic X‐ray spectra. We predict X‐ray fluxes, efficiencies, and spectra for various initial ion energies considering opacity effects from two different atmospheres. We demonstrate that an in situ measured heavy ion flux above Jupiter's polar cap is capable of producing over 1 GW of X‐ray emission when some assumptions are made. Comparison of our approximated synthetic X‐ray spectrum produced from in situ particle data with a simultaneous X‐ray spectrum observed by XMM‐Newton shows good agreement for the oxygen part of the spectrum but not for the sulfur part. Heavy ion precipitation into the Jovian atmosphere can produce the observed auroral X‐ray emissionUsing Juno measurements of ion fluxes over Jupiter's pole, we simulate X‐ray spectraWe compare our approximated synthetic X‐ray spectra produced by in situ data to observed emission

Several studies have validated the accuracy of the maximum magnetic shear model to predict the location of the reconnection site at the dayside magnetopause. These studies found agreement between model and observations for 74% to 88% of events examined. It should be noted that, of the anomalous events that failed the prediction of the model, 72% shared a very specific parameter range. These events occurred around equinox for an interplanetary magnetic field (IMF) clock angle of about 240°. This study investigates if this remarkable grouping of events is also present in data from the recently launched MMS. The MMS magnetopause encounter database from the first dayside phase of the mission includes about 4,500 full and partial magnetopause crossings and flux transfer events. We use the known reconnection line signature of switching accelerated ion beams in the magnetopause boundary layer to identify encounters with the reconnection region and identify 302 events during phase 1a when the spacecraft are at reconnection sites. These confirmed reconnection locations are compared with the predicted location from the maximum magnetic shear model and revealed an 80% agreement. The study also revealed the existence of anomalous cases as mentioned in an earlier study. The anomalies are concentrated for times around the equinoxes together with IMF clock angles around 140° and 240°. Another group of anomalies for the same clock angle ranges was found during December events. The maximum magnetic shear model predicts magnetic reconnection at the Earth's magnetopause 80% accurateAnomalies in the predicting ability known from earlier studies have been confirmedThe anomalies in predicting the reconnection location concentrate on Equinox and IMF clock angles of 120° and 240°

We analyze two ion scale dipolarization fronts associated with field‐aligned currents detected by the Magnetospheric Multiscale mission during a large substorm on 10 August 2016. The first event corresponds to a fast dawnward flow with an antiparallel current and could be generated by the wake of a previous fast earthward flow. It is associated with intense lower hybrid drift waves detected at the front and propagating dawnward with a perpendicular phase speed close to the electric drift and the ion thermal velocity. The second event corresponds to a flow reversal: from southwward/dawnward to northward/duskward associated with a parallel current consistent with a brief expansion of the plasma sheet before the front crossing and with a smaller lower hybrid drift wave activity. Electromagnetic electron phase‐space holes are detected near these low‐frequency drift waves during both events. The drift waves could accelerate electrons parallel to the magnetic field and produce the parallel electron drift needed to generate the electron holes. Yet we cannot rule out the possibility that the drift waves are produced by the antiparallel current associated with the fast flows, leaving the source for the electron holes unexplained. Dipolarization fronts associated with field‐aligned currents are observed at the plasma sheet edge with a few ion inertial length scaleIntense lower hybrid drift waves are detected at the front and can accelerate electrons parallel to BElectromagnetic electron phase‐space holes are detected near the lower hybrid drift waves and could be a latter by‐product of these

This study examines multipoint observations during a conjunction between Magnetospheric Multiscale (MMS) and Van Allen Probes on 7 April 2016 in which a series of energetic particle injections occurred. With complementary data from Time History of Events and Macroscale Interactions during Substorms, Geotail, and Los Alamos National Laboratory spacecraft in geosynchronous orbit (16 spacecraft in total), we develop new insights on the nature of energetic particle injections associated with substorm activity. Despite this case involving only weak substorm activity (maximum AE<300 nT) during quiet geomagnetic conditions in steady, below‐average solar wind, a complex series of at least six different electron injections was observed throughout the system. Intriguingly, only one corresponding ion injection was clearly observed. All ion and electron injections were observed at <600 keV only. MMS reveals detailed substructure within the largest electron injection. A relationship between injected electrons with energy <60 keV and enhanced whistler mode chorus wave activity is also established from Van Allen Probes and MMS. Drift mapping using a simplified magnetic field model provides estimates of the dispersionless injection boundary locations as a function of universal time, magnetic local time, and L shell. The analysis reveals that at least five electron injections, which were localized in magnetic local time, preceded a larger injection of both electrons and ions across nearly the entire nightside of the magnetosphere near geosynchronous orbit. The larger ion and electron injection did not penetrate to L < 6.6, but several of the smaller electron injections penetrated to L < 6.6. Due to the discrepancy between the number, penetration depth, and complexity of electron versus ion injections, this event presents challenges to the current conceptual models of energetic particle injections. Weak substorm activity during quiet geomagnetic conditions reveals the complex nature of energetic particle injectionsFifteen spacecraft observe a series of electron injections but only one clear ion injection, injection boundaries estimated at different LThere are two distinct injection types: localized in L and MLT with only electrons observed and broad range of L and MLT with ions and electrons

Juno observed the low‐altitude polar region during perijove 1 on 27 August 2016 for the first time. Auroral intensity and false‐color maps from the Ultraviolet Spectrograph (UVS) instrument show extensive diffuse aurora observed equatorward of the main auroral oval. Juno passed over the diffuse auroral region near the System III longitude of 120°–150° (90°–120°) in the northern (southern) hemisphere. In the region where these diffuse auroral emissions were observed, the Jupiter Energetic Particle Detector Instrument (JEDI) and Jovian Auroral Distributions Experiment (JADE) instruments measured nearly full loss cone distributions for the downward going electrons over energies of 0.1–700 keV but very few upward going electrons. The false‐color maps from UVS indicate more energetic electron precipitation at lower latitudes than less energetic electron precipitation, consistent with observations of precipitating electrons measured by JEDI and JADE. The comparison between particle and aurora measurements provides first direct evidence that these precipitating energetic electrons are mainly responsible for the diffuse auroral emissions at Jupiter. Diffuse auroral emissions were observed in a broad region located equatorward of the main oval by JunoIn the diffuse auroral region, precipitating electrons with nearly full loss cone distributions were observed over 100 s eV‐100 s keVThe pattern of characteristic energy of precipitating electrons is consistent with the auroral Juno‐UVS false‐color map