George Randall Gladstone

Auroral emissions play a critical role in coupling the magnetosphere and atmosphere at magnetized planets. At Jupiter, isolated auroral patches equatorward of the main auroral oval have been associated with magnetospheric injections, yet the key conditions driving these emissions remain unclear. Here we combine in situ particle and wave measurements and remote-sensing auroral observations from Juno to uncover the origins of Jupiter’s patchy aurora. By examining both low-altitude and equatorial crossings, our results reveal that not all injections lead to auroral enhancements. Instead, auroral patches arise directly from electron precipitation driven by wave-particle interactions, while injections play a supportive role in facilitating wave growth and electron precipitation. These findings highlight the central role of plasma waves in auroral generation and in coupling Jupiter’s magnetosphere and ionosphere, providing broader implications for wave-driven auroral processes across planetary magnetospheres.

Context. Ultraviolet (UV) auroral emissions on Jupiter offer a unique window into the coupling between the planet’s magnetosphere and upper atmosphere. Color ratios derived from molecular hydrogen emissions provide valuable diagnostics for the energy of precipitating electrons and the structure of the auroral atmosphere. Aims. We aim to characterize the horizontal and vertical variability of hydrocarbon absorption in Jupiter’s auroral atmosphere using UV data from the Juno-UVS spectrograph and to investigate potential departures from the expected structure. Methods. We constructed color ratio maps sensitive to CH ₄ and C ₂ H ₂ absorptions for two of Juno’s close approaches to Jupiter, perijoves (PJs) 6 and 10, by integrating auroral H ₂ emission over hydrocarbon-sensitive spectral intervals. For CH ₄ , we redefined the absorbed spectral band, replacing the traditionally used 125-130 nm interval with 135-140 nm to mitigate higher order calibration issues. In regions of intense auroral brightness, we developed a correction method to account for spectral distortion due to detector nonlinearities at high fluxes. Results. The CH ₄ and C ₂ H ₂ absorptions generally follow the expected vertical distribution, with the CH ₄ density extending to higher altitudes than C ₂ H ₂ . However, several localized regions show unexpected spatial distribution of the absorption. In PJ6, such anomalies are attributed to instrumental nonlinearities. Following the correction, the CR distributions become consistent with standard hydrocarbon vertical distributions. In PJ10, however, some anomalous patterns persist despite the correction. Spectral modeling studies indicate that these can be explained by modifying the relative abundances of CH ₄ and C ₂ H ₂ , suggesting horizontal compositional variability and possible deviations in the homopause altitude between species. Conclusions. Our results confirm that hydrocarbon absorption in Jupiter’s aurora is vertically stratified, but that the altitude of this stratification itself varies horizontally with latitude and longitude. These findings underscore the importance of accounting for local atmospheric composition when interpreting UV auroral spectra and retrieving electron energy distributions. Juno-UVS thus provides a powerful diagnostic for probing both the auroral precipitation and upper atmospheric structure. The framework developed here enables more accurate retrievals of electron energies and provides new constraints on the spatial variability of Jupiter’s upper atmosphere. A systematic application to the full Juno dataset will offer deeper insights into the temporal and spatial dynamics of the Jovian aurora.

We present New Horizons Alice spectral observations of interstellar hydrogen Ly α emissions at a vantage point of 57 au from the Sun. The observations were conducted as a pair of orthogonal scans to produce high spatial angular resolution measurements of a dark patch of sky located at high Galactic latitudes. We find that the brightness of the UV background remains relatively constant over the selected sky region, with the exception of areas near a few bright stars and galaxies. Most stars do not affect the brightness of Ly α in this region; however, we detect a 25 Rayleigh Ly α enhancement near the location of hot subdwarf CD-38 222. This enhancement is consistent with nebular emission produced by ionizing photons from a bow shock as the object interacts with a high-latitude interstellar gas cloud. We do not find correlations of Ly α or background UV brightness with dust structures seen by the Planck space observatory. Our analysis at higher angular resolution and with the full Alice bandpass of 520–1870 Å confirms the results of the global Ly α maps produced by G. R. Gladstone et al. at lower spatial resolution. The near-constant Ly α is consistent with their interpretation that scattering of H within our Local Bubble produces a nearly isotropic Ly α distribution. Future Alice global scans in photometer mode can be used to assess changes in the Ly α sky as a function of solar distance. High-resolution FUV Ly α scans with the full spatial and spectral information should be repeated over the entire sky to resolve additional small-scale structures.

The Jupiter and Icy Moons Explorer (JUICE) mission of the European Space Agency (ESA) will investigate the Jovian system with multiple instruments over several years, beginning in early 2031. This paper describes the historical context and state of knowledge, as well as JUICE’s scientific goals and measurement techniques of the satellites that will not be encountered in close flybys. These include the large volcanically active moon Io, the four small inner moons Metis, Adrastea, Amalthea, and Thebe, and the numerous small Irregular (outer) moons. JUICE will provide multiple opportunities to observe Io from relatively remote distances of hundreds of thousands of kilometers. These observations will enable monitoring of Io’s surface for changes, and for the study of its neutral clouds and plasma torus. Io observations will be performed with the four optical remote sensing instruments and with the Particle Environment Package. For the small inner moons it is planned to obtain complete geographic longitude (scales up to 8 km/px), solar-phase and multi-color coverage, oblique polar views, and UV to near-IR spectra. Astrometric measurements will also be performed. The Irregular moons will mostly appear unresolved to the JUICE instruments. Nonetheless, long-duration disk-integrated lightcurves will be acquired to derive rotation periods, object dimensions, pole-axis orientations, and colors for most objects for the first time. From these data, convex-shape models will be generated and phase curves determined. Furthermore, the precision of the orbital elements will be improved via accurate astrometry. UV and near-IR measurements will be attempted for the largest of these objects.

Aims. We analyzed ultraviolet observations of Ganymede recorded on June 7, 2021, by the Juno spacecraft, which observed the aurora at high spatial resolution in order to detect the presence of auroral sub-structures. The emission mainly comes from two oxygen lines at 130.4 and 135.6 nm, which are excited under electron precipitation. Methods. We produced longitude–latitude projections of the oxygen emission lines from Ganymede’s atmosphere to investigate the presence of small-scale auroral features. We adopted a magnetic field model of Ganymede’s magnetosphere to map between the moon surface and the surrounding space, in order to determine the regions magnetically connected to the aurora. Results. We find auroral patches on the leading, downstream side of Ganymede. Their typical size is ∼50 km, and they have a brightness of up to ∼200 Rayleigh. These features map approximately to the downstream reconnection region and seemingly resemble the terrestrial and Jovian mesoscale auroras associated with sub-storms and dawn storms, respectively. This could indicate that magnetospheres of celestial bodies host similar physical process(es) despite their different conditions and dynamics.

Context . The interaction between Io and the Jovian magnetosphere produces the auroral “Io footprint”, which can be observed in the ultraviolet thanks to the deexcitation of atmospheric H and H ₂ under electron precipitation. Since 2016, Jupiter has been explored with Juno’s Ultraviolet Spectrograph, which monitors the emission from 68 to 210 nm. Aims . We used Juno observations of the Io footprint near the planetary limb to determine its vertical ultraviolet emission profile in the northern hemisphere. We simulated emission as a function of the altitude and the mean energy of the precipitating electrons, and we used the results to determine the energy spectrum associated with the footprint profile. Methods . We estimated the source location of the Io footprint UV emission, and we extracted its vertical profile. We analyzed the variability of the emission altitude, and we derived the corresponding energy spectrum of precipitating electrons using TransPlanet. The results were compared with the in situ measurements from Juno’s particle detectors. Results . The main spot emission peaks around ~500 km, with a variability of ~200 km correlated with longitude. The emission of the footprint tail within 20° from the main spot peaks around ~500±300 km, and it is moderately correlated with the magnitude of the magnetic field near Jupiter. The trans-hemispheric electron beam spot is located at ~300 km, and the associated precipitation appears depleted at low energy. The retrieved energy spectrum of the precipitation shows remarkable agreement with the particle measurements. Conclusions . The longitudinal modulation of the Io footprint altitude suggests that the morphology of the torus affects the transmission of the Alfvén waves responsible for the acceleration of auroral electrons. The dependency of the footprint tail altitude on the magnetic field strength indicates that the magnetic field also plays a role in the acceleration mechanism.

Context . Ganymede is the only moon in the Solar System with an intrinsic magnetic field that actively interacts with the Jupiter magnetosphere. This precipitates energetic electrons that generate ultraviolet (UV) auroral emission. Aims . In sunlit auroral regions, the observed emission partly overlaps with the solar continuum reflected by the surface. An accurate modeling of the observed UV spectra therefore requires precise knowledge of the surface spectral reflectance. Methods . We analyzed Juno/UVS data acquired during the 34th perijove (PJ) flyby to constrain the Ganymede surface reflectance in the 140−205 nm range. We used the non-local thermal equilibrium radiative transfer model originally developed to simulate the auroral emission of Ganymede, which also accounts for the reflection of solar flux by the satellite surface, to fit the observed spectra in sunlit auroral regions. Results . Our results revealed that the reflectance varies strongly spatially and spectrally from 0.1% to 8% in the [140 nm; 205 nm] wavelength range. This indicates a significant surface heterogeneity. This variability likely reflects long-term interactions between the icy surface of Ganymede and precipitating energetic particles, which alter the ice structure and crystallinity and its chemical composition. In addition, the derived reflectance maps show no clear correlation with the visible surface features of Ganymede, suggesting that the UV reflectance is primarily shaped by irradiation-driven processes and not by the geological morphology. Conclusions . The resulting reflectance maps provide a critical input for future UV auroral emission modeling, particularly in preparation for observations by the Juice/UVS mission.

The highly elliptical polar orbit of the Juno mission provides a unique opportunity to simultaneously measure the compression state of Jupiter's magnetosphere and the total power emitted by the planet's ultraviolet aurora, using a single spacecraft. This allows us to study how Jupiter's aurora respond to a compression event. In this paper, we present a case study of an extreme compression event that occurred on December 6–7 2022 when Juno was a distance of 70 R_J from Jupiter. This extreme compression was accompanied by a very large increase in the ultraviolet auroral emissions to 12 TW, a factor of six higher than the baseline level. This event coincided with the predicted arrival of a powerful interplanetary shock, which was expected to cause the largest increase in the solar wind dynamic pressure seen thus far during the Juno mission. The simultaneous occurrence of the interplanetary shock, the extreme compression and the bright ultraviolet aurora suggests that in this case, the auroral brightening was caused by the solar wind shock compressing the magnetosphere.

During 2023 September the Alice ultraviolet spectrograph on the New Horizons (NH) spacecraft was used to map diffuse Lyα emission over most of the sky, at a range of ∼56.9 au from the Sun. At that distance, models predict that the interplanetary medium Lyα emissions result from comparable amounts of resonant backscattering of the solar Lyα line by interstellar hydrogen atoms (H i) passing through the solar system, in addition to an approximately isotropic background of ∼50 ± 20 R from the local interstellar medium (LISM). The NH observations show no strong correlations with nearby cloud structures of the LISM or with expected structures of the heliosphere, such as a hydrogen wall associated with the heliopause. To explain the relatively bright and uniform Lyα of the LISM, we propose that hot, young stars within the Local Hot Bubble shine on its interior walls, photoionizing H i atoms there. Recombination of these ions can account for the observed ∼50 R Lyα background, after amplification of the diffuse Lyα by resonant scattering, although sophisticated (i.e., 3D) radiative transfer models should be used to confirm this conjecture. Future observations of the diffuse Lyα, with instruments capable of resolving the line profile, could provide a new window on H i populations in the LISM and heliosphere. The NH Alice all-sky Lyα observations presented here may be repeated at some point in the future, if resources allow, and the two maps could be combined to provide a significant increase in angular resolution.