James Hunter Waite

The total power of the Jovian aurora is about 10 ¹³ –10 ¹⁴ W with ultraviolet emission observed in the Lyman and Werner bands of molecular hydrogen. X‐ray, radio, and infrared emissions have also been observed. The Jovian Auroral Distributions Experiment and the Jovian Energetic Particle Detector Instrument onboard the Juno spacecraft, orbiting Jupiter, measured large downward and upward fluxes of energetic electrons and ions in the polar regions. The main auroral oval is mainly due to energetic electron precipitation, but proton and heavy ion (sulfur and oxygen) precipitation contribute to the aurora poleward of the main oval. The current paper presents the results of a Monte Carlo model of energy deposition by proton precipitation into Jupiter's molecular hydrogen atmosphere for incident energies ranging from tens of keV up to 25 MeV. Cross sections for H ⁺ , H, and H ⁻ collisions with H ₂ were determined using a classical trajectory Monte Carlo method. The model generates ionization, dissociation, and excitation rates of H ₂ for a wide range of incident proton energy and altitude. The contribution of secondary electrons to the overall energy deposition is included. The model results indicate that 50 keV protons deposit their energy near 500 km, where the H ₂ density is about 3 × 10 ¹¹ cm ⁻³ , with 10 MeV protons depositing their energy near 200 km, where the H ₂ density is about 5 × 10 ¹⁵ cm ⁻³ , which is in the stratosphere.

The Cassini Radio & Plasma Wave Science (RPWS) and Ion and Neutral Mass Spectrometer (INMS) observations of Saturn’s ionosphere during the proximal orbits (Rev) 288 – 293 in the altitude range 1450 – 2500 km (above the 1-bar pressure level) are modelled with the inclusion of a dust component. Previous reports have revealed that large amounts of nm- to mm-sized dust grains precipitate from the D-ring into the atmosphere of Saturn’s equatorial region. We find that the processed charged dust (< 2 nm radius) has a profound effect on the ionospheric structure, enhancing the ion number density well above photochemical equilibrium levels, while the free electrons tend to become attached to the dust population as a result of a low photo-electron detachment rate by EUV light from the Sun at the distance of Saturn. The charged dust layers are influenced by the strong near horizontal magnetic field inhibiting vertical ambipolar diffusion as most charged components are magnetized above about 1800 km. Our dust-ionosphere model calculations reveal that layers of mostly singly negatively charged dust can explain much of the observed ionospheric densities. Modelling uncertainties include sticking coefficients, recombination rates, dust distributions and the presence of negative ions as well as variations thereof along the spacecraft trajectory. Our models generate overall acceptable fits to the observed RPWS electron and ion densities. The observations are therefore compatible with charged grains (and/or cluster ions) being present and playing a dominant role in the Kronian ionosphere structure and chemistry. Similar processes may be active at gas giants with associated ring systems.

Ganymede, the only satellite in the Solar System possessing an intrinsic magnetic field, is continuously immersed in Jupiter’s magnetosphere. The relative motion of this conducting body through the jovian magnetic field, together with the presence of closed magnetic field regions around Ganymede, leads to the precipitation of energetic electrons along these closed field lines, producing ultraviolet auroral emissions.In sunlit auroral regions, these emissions are superimposed on the solar radiation reflected by the surface, making an accurate knowledge of the surface spectral reflectance essential for a proper interpretation of the observed UV spectra.We used Juno/UVS observations obtained during the 34th perijove to derive the spectral reflectance of Ganymede’s surface in the 140-205 nm range. The analysis was performed with a non-local thermodynamic equilibrium radiative transfer model initially developed for auroral emission studies, which self-consistently includes the reflection of the incident solar flux by the surface. By fitting the UVS spectra in illuminated auroral regions, we retrieved spatially resolved reflectance values.The inferred reflectance exhibits strong spatial and spectral variability, ranging from about 0.1% to 8% over the [140-205] nm interval, revealing a highly heterogeneous surface. This variability is likely the signature of long-term irradiation by energetic particles, which modifies the physical structure, crystallinity, and chemical composition of surface ice. The resulting UV reflectance maps show no clear correlation with visible-wavelength surface morphology, indicating that irradiation-driven processes dominate over geological features in controlling the UV albedo.These new reflectance constraints constitute a key input for future modeling of Ganymede’s ultraviolet aurora and will be particularly valuable for the interpretation of upcoming observations by the JUICE/UVS instrument.

Juno’s Extended Mission trajectory enables unprecedented high-resolution (~6-35 km scale) Stellar Reference Unit (SRU) limb imaging and aurora observations on Jupiter’s night side. The SRU is a low-light, broadband visible wavelength (450-1000 nm) star tracker, with a peak sensitivity from ~570-800 nm, that Juno utilizes as a multi-disciplinary science instrument. High altitude views of the atmosphere on Jupiter’s limb have been acquired in equatorial and high northern latitude regions, including within the auroral region. Our presentation will discuss Juno’s findings from this unique data set, including structural features observed from a few hundred to over a thousand km above the 1 bar level and their place within the interconnected inner Jovian system.

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.

Sprawling, turbulent cloud formations dominate the meteorology of Jupiter's mid‐to‐high latitudes, known as Folded Filamentary Regions (FFRs). A multi‐wavelength characterization by Juno reveals the spatial distribution, vertical structure, and energetics of the FFRs. The cloud tops display multiple lobes of stratiform aerosols, separated by darker, cloud‐free lanes, and embedded with smaller eddies and high‐altitude cumulus clouds. These cyclonic FFRs are microwave‐bright in shallow‐sounding wavelengths ( bars) and microwave‐dark in deep‐sounding wavelengths ( bars), with the transition potentially associated with the water condensation layer (6–7 bars). Associating microwave contrasts with temperature anomalies, this implies despinning of cyclonic eddies above/below their mid‐planes. Despite deep roots (being detectable in wavelengths sounding bars), they are “pancake vortices” with horizontal extents at least an order of magnitude larger than their depth. In the northern hemisphere, FFRs are most common in cyclonic belts poleward of N (all latitudes are planetocentric), particularly a North Polar Filamentary Belt (NPFB) near N that defines the transition from organized belts/zones to the chaotic polar domain. This distribution explains the high lightning rates from N, peaking in a belt poleward of N, which may trace the availability of water for moist convection. Many observed lightning flashes can be associated to specific FFRs containing bright storms, but some FFRs display no activity, suggesting quiescent periods during storm evolution. Analogies to Earth's oceanic eddies suggest that cyclones deform isentropic surfaces at their midplanes, raising deep water‐rich layers upwards to promote moist convection, release latent heat, and inject clouds into the upper troposphere.

Context . Ganymede’s UV aurorae, observed by HST and Juno/UVS, trace interactions between its atmosphere and Jupiter’s magneto-sphere. These emissions, dominated by O I lines at 130.4 and 135.6 nm, are driven by electron impact on species such as H ₂ O, O, and O ₂ , and yet the properties of the precipitating electrons remain poorly constrained. Aims . Our aim was to retrieve the energy and flux of precipitating electrons using UV observations from Juno/UVS during PJ34 and to assess the dominant atmospheric species producing the observed emissions. Methods . Using the TransPlanet electron transport model and a non-local thermodynamic equilibrium (non-LTE) radiative transfer module, we simulated O I emissions for 17 auroral subregions, testing both monoenergetic and kappa-type electron distributions. The I (135.6 nm)/ I (130.4 nm) line ratio was used as a diagnostic, with values varying by target species. Results . Monoenergetic distributions fit most regions better, with mean energies of 17–300 eV and fluxes up to 2 mW m ⁻² . Kappa and Maxwellian distributions yielded higher fluxes, but poorer spectral fits. Poor fits in some regions reflect low S/N or non-ideal electron populations. Conclusions . Our results suggest that Ganymede’s UV aurorae are mainly driven by low- to intermediate-energy electrons. Upcoming high-resolution observations and in situ data from Juice and Europa Clipper will be key to refining these diagnostics.