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Particle acceleration is a commonly observed phenomenon at dipolarization fronts. Many studies have attempted to determine the acceleration mechanism, with betatron acceleration being a major candidate. In previous work, we attempted to match the observed change in electron energy to the change predicted by betatron acceleration, but found that although this worked in some cases, overall betatron acceleration alone could not describe the observed energy spectrum changes. In this work, we attempted to study whether ion acceleration showed similar behavior and whether a quasi-adiabatic correction would be more accurate. On average the betatron acceleration equation overestimated the observed acceleration and the quasi-adiabatic correction did not account for the difference, although there are limitations to this study due to data fidelity. We then turned to study whether our assumptions about the source population having the same phase space density as the cold pre-existing background population in the plasma sheet are valid. We indirectly studied this by comparing the relative abundances of O+ and He++ as proxies for ionospheric and solar wind populations respectively. We found the betatron acceleration equation method performs slightly better when there is a stronger ionospheric component. This suggests that when more plasma containing O+ is present in the dipolarization front, it indicates that the source population is more local and therefore this method of using betatron acceleration is more valid.

Abstract Jupiter's poleward (Zone II) main aurora exhibits bi‐directional electron acceleration; upward acceleration dominates but downward acceleration generates strong aurora. During Juno's first perijove (PJ1), the upward acceleration manifested as narrow electron angular beams (within ∼5° of the magnetic field) over the 30–1,200 keV energy range of Juno's Jupiter Energetic Particle Detector Investigation (JEDI). These beams can be simply connected (non‐uniquely) to >10 to perhaps 100's of MeV electrons that penetrated the radiation shielding of the camera head of the Magnetometer Investigation's Advanced Stellar Compass (ASC). The most intense of those multiple MeV populations are shown to have been highly directional and propagating upwards. How auroral processes generate such beams is unknown. With azimuthal symmetry assumed (not demonstrated here), these beams provided >1026 s−1 of >30 keV electrons to Jupiter's vast magnetosphere, a possibly critical and dominating source of energetic electrons to that region and ultimately to Jupiter's radiation belts.

Abstract Interchange instability is known to drive fast radial transport of electrons and ions in Jupiter's inner and middle magnetosphere. In this study, we conduct a statistical survey to evaluate the properties of energetic particles and plasma waves during interchange events using Juno data from 2016 to 2023. We present representative examples of interchange events followed by a statistical analysis of the spatial distribution, duration and spatial extent. Our survey indicates that interchange instability is predominant at M‐shells from 6 to 26, peaking near 17 with an average duration of minutes and a corresponding M‐shell width of

Abstract 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.

Abstract 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.

Abstract 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−3 while 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.

Betatron acceleration is commonly cited as a primary accelerator of energetic electrons at dipolarization fronts, and many case studies compare observed energetic electrons measurements to a betatron model. In this work, we extend this to a statistical study. We identified 168 dipolarizations with an enhanced flux of energetic electrons at Magnetospheric Multiscale (MMS). We compared the observed flux of energetic electrons above 1 keV to a betatron acceleration model assuming a source population similar to the population in the quiet plasma sheet and found that, on average, the model slightly overestimated the observation, but there was a wide spread of errors. We then tested characteristics such as position, change in and strength of magnetic field, and wave power to determine if any of these characteristics affected the accuracy of the model; the only clear correlations were that the model was less accurate when the initial total magnetic field was smaller and when there was a higher Ey during the dipolarization. Since the betatron model did not explain our observations very well, we repeated with a full adiabatic model that included a Fermi acceleration component as well. We found that the adiabatic model slightly underestimated the observations, but with a smaller error than the betatron model under the same assumptions. Testing the same parameters, we found that the adiabatic model also did not strongly rely on any of the parameters except the initial magnetic field, and the anti-correlation with Ey was no longer present. The fact that neither model was generally applicable means that either adiabatic processes alone are not enough to explain electron acceleration at dipolarization fronts in general, or the common assumption we used, that the source population has the same phase space density as the cold pre-existing population, is not valid.

Abstract 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.

Abstract Chorus waves play a key role in outer Van Allen electron belt dynamics through cyclotron resonance. Here, we use Van Allen Probes data to reveal a new and distinct population of intense chorus waves excited in the heart of the radiation belt during the main phase of geomagnetic storms. The power of the waves is typically ~ 2–3 orders of magnitude greater than pre-storm levels, and are generated when fluxes of ~ 10–100 keV electrons approach or exceed the Kennel–Petschek limit. These intense chorus waves rapidly scatter electrons into the loss cone, capping the electron flux to a value close to the limit predicted by Kennel and Petschek over 50 years ago. Our results are crucial for understanding the limits to radiation belt fluxes, with accurate models likely requiring the inclusion of this chorus wave-driven flux-limiting process, that is independent of the acceleration mechanism or source responsible for enhancing the flux.

The Jovian plasma sheet is a key region of the Jovian magnetosphere populated by a mix of warm and hot plasma. It is the main channel for radial transport of mass and energy in the Jovian magnetosphere and provides a favorable environment for magnetic reconnection and wave–particle interactions although the understanding of its plasma properties is incomplete. This study combines observations from the Jovian Auroral Distributions Experiment and Juno Energetic Particle Detector Instrument on board the Juno spacecraft during its first 31 orbits to analyze the plasma properties of the Jovian plasma sheet from 20 R _J to 100 R _J . Our results indicate that the plasma number density decreases from 1 cm ^−3 at 20 R _J to 0.005 cm ^−3 at 100 R _J , while the plasma pressure decreases from 2 nPa at 20 R _J to 0.02 nPa at 100 R _J . The plasma pressure inside the plasma sheet is comparable to the magnetic pressure in the lobe region, suggesting a rough balance between the two. In the plasma sheet with r > 70 R _J , the H ^+ density and pressure remain relatively constant, likely due to other plasma sources such as the solar wind. Additionally, we find that the pressure (density) ratios for heavy ions between the center and the edge of the plasma sheet are generally an order of magnitude, while that for H ^+ decreases with radial distance. These findings contribute to a more comprehensive understanding of the plasma properties of the Jovian plasma sheet.

In this study, we present a survey of energetic proton observations associated with Io’s footprint tail (FPT) and compare their signatures with in situ measurements of the plasma waves and lower-energy electron environments. We find further supporting evidence that proton acceleration in Io’s FPT is likely a consequence of wave–particle interactions via electromagnetic ion cyclotron waves that are generated by precipitating electrons into Jupiter’s ionosphere. This idea was originally proposed by Clark et al. (2020) and Sulaiman et al. (2020) based on NASA’s Juno mission likely transiting Io’s Main Alfvén Wing (MAW) during its twelfth orbit (i.e., PJ12). Additionally, the analysis of > 50 keV protons presented here highlights important observational details about the Io–Jupiter interaction as follows: 1) proton acceleration in Io’s FPT is a persistent feature and the energy flux carried by the protons is highest at smaller Io-Alfvén tail distances; 2) energetic protons exhibit positive correlations with both plasma waves and

Abstract 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.

In this study, we examine particle energization and injections that show energetic electron enhancements at both MMS in the magnetotail and Van Allen Probes in the inner magnetosphere. Observing injections along with a corresponding flow burst allows us to better understand injections overall. Searching for suitable events, we found that only a small number of events at MMS had corresponding injections that penetrated far enough into the inner magnetosphere to observe with Van Allen Probes. With the four suitable events we did find, we compared the energy spectra at the two spacecraft and mapped the boundary of where the injection entered the inner magnetosphere. We found that, among these injections in the inner magnetosphere, the electron flux did not increase above ∼400 keV, similar to previous results, but the corresponding signatures in the tail observed increased fluxes at 600 keV or higher. There does not appear to be a comparable flux increase at Van Allen Probes and MMS for a given event. None of our injections included ion enhancements at Van Allen Probes, but one included an ion injection at geosynchronous orbit in the GOES spacecraft. All of our injections were dispersed at Van Allen Probes, and we were therefore able to map an estimate of the injection boundary. All of the injections occurred in the premidnight sector. Although we found some events where particle energizations in the tail are accompanied by inner magnetospheric injections, we do not find a statistical link between the two.

We took a survey of boundary layer (or low-latitude boundary layer) crossings by the Magnetospheric Multiscale (MMS) mission. Out of 250 total crossings, about half showed enhancements of high-energy (>30 keV) electrons in the FEEPS sensor and a little less than half of those energetic electron events had whistler-mode waves present. Energetic electron enhancements were more likely to be present at magnetic local times closer to noon and at distances of less than about 20 Earth radii, but there was seemingly no correlation with magnetic latitude. For almost all of these events, the pitch angles of the FEEPS electrons were peaked at 90° or isotropic, not field-aligned. Most of the events for which we had data to make a determination showed either direct or indirect evidence of reconnection. Overall, energetic electron enhancements are a fairly common occurrence and there appears to be some connection between whistler waves, energetic electron enhancements, and reconnection, whether it is a direct link or some other process affecting all of them.

Abstract Dawn storms are among the brightest events in the Jovian aurorae. Up to now, they had only been observed from Earth‐based observatories, only showing the Sun‐facing side of the planet. Here, we show for the first time global views of the phenomenon, from its initiation to its end and from the nightside of the aurora onto the dayside. Based on Juno's first 20 orbits, some patterns now emerge. Small short‐lived spots are often seen a couple of hours before the main emission starts to brighten and evolve from a straight arc to a more irregular one in the midnight sector. As the whole feature rotates dawn‐ward, the arc then separates into two arcs with a central initially void region that is progressively filled with emissions. A gap in longitude then often forms before the whole feature dims. Finally, it transforms into an equatorward‐moving patch of auroral emissions associated with plasma injection signatures. Some dawn storms remain weak and never fully develop. We also found cases of successive dawn storms within a few hours. Dawn storms thus share many fundamental features with the auroral signatures of the substorms at Earth, despite the substantial differences between the dynamics of the magnetosphere at the two planets.