Paola Ines Tiranti

Magnetosphere‐Ionosphere‐Thermosphere (MIT) coupling is widely considered the dominant energy source responsible for maintaining the elevated thermospheric temperatures observed at the giant planets. At Uranus, the strongly tilted and asymmetric magnetic field produces a highly asymmetric magnetospheric configuration that may fundamentally alter the spatial distribution of energy deposition in the upper atmosphere. In this study, we investigate how Uranus' magnetic geometry influences thermospheric heating and circulation using a ‐D thermospheric circulation model updated to represent Uranus‐like atmospheric conditions. The simulations show that magnetospheric forcing at the two magnetic poles produces markedly different thermospheric responses. At the magnetic south pole, where the magnetic field strength is larger and the pole lies close to the rotational pole, Joule heating produces a confined region of enhanced temperature resembling the polar hot spots observed at Jupiter and Saturn. In contrast, the magnetic north pole lies near the rotational equator and is associated with weaker magnetic field strengths and, therefore, higher ionospheric conductivities. Under these conditions, the imposed electric field drives heating across both hemispheres simultaneously, producing a much broader region of thermospheric heating that actually decreases near the rotational poles. These results demonstrate that Uranus' unusual magnetic geometry leads to a strongly asymmetric pattern of MIT‐driven heating that differs significantly from Jupiter and Saturn. Comparing these results to recent JWST observations, which show relatively uniform thermospheric temperatures, we hypothesize that highly efficient meridional transport from the magnetic north pole may redistribute this energy globally, potentially explaining these observations.

Saturn’s rings actively supply material to the planet through charged and neutral inflows. The Cassini mission revealed this coupling and the James Webb Space Telescope now sharpens the picture, yet the variability, chemistry and future of this erosion remain unresolved.

Past measurements of Saturn's upper atmosphere have allowed only a broad scale view of the temperature and ion density structures within the auroral region. However, Saturn's auroral currents include a planetary period current component that is produced by neutral atmospheric flows. In order to understand the ultimate source for Saturn's planetary period aurora, we need vastly improved measurements of Saturn's auroral region. Here, using the first James Webb Space Telescope (JWST)/NIRSpec observations of Saturn, we reveal incredibly high spatial resolution (<500 km/pixel) views of Saturn's auroral region for the first time. This provides at least an order‐of‐magnitude improvement in our measurement of the temperature and column density structure within Saturn's auroral region. It reveals an auroral region at the time of observation that is highly controlled by the planetary period currents, resulting in a series of upwards and downward currents from the pole to below the main auroral region. These are paired with reflected and inverted regions of heating and cooling across the auroral region. These patterns match surprisingly well to past model predictions, suggesting both the changes in column density and temperature are the direct result of localized heating positioned over the region of strongest particle precipitation. This strongly indicates that the planetary period currents are produced by a feedback loop, powered by the energy of magnetospherically generated aurora, preferentially heating one side of the auroral region.

We present the main Alfvén wing (MAW) spots of Io and Europa as observed by the Near‐Infrared Spectrograph onboard the James Webb Space Telescope. These auroral footprint features have been measured previously, but only in emission. Here, the derived ionospheric emission, temperature and column density are reported, as well as spectral radiance. At the Io footprint, temperatures are 670–900 K, excluding a spatially confined cold structure (538 17 K), localized to the MAW spot, with high densities (0.98 0.43 × ). There are suggestions of a similar, less extreme population associated with the Europa footprint. However, temperatures at Io's MAW spot show significant variability within different observational exposures, indicating precipitation energy changes, sampling various regions of the ionosphere's altitudinal temperature profile. This work provides a new window into understanding the auroral processes driven by moon‐magnetosphere interactions in the Jovian system.

On 19 January 2025, the James Webb Space Telescope (JWST) program 5073 observed Uranus for almost a full rotation with NIRSpec. We present the first ever vertical ionospheric profiles of temperature, volumetric density and total emission. Temperature profiles peak between 3,000–4,000 km at all longitudes. Densities peak near 1,000 km with values up to (4.45 0.12) × , lower than predicted by 1‐D models. We identify two bright emission regions at 50–110W and 220–290W, near the auroral regions. A distinct emission and density depletion occurs between 190–240W, possibly reflecting magnetic topology influences, analogous to ionospheric dark regions at Jupiter. These results offer new insights into Uranus's upper atmosphere, confirm its long‐term cooling trend (426 2 K), and provide key constraints for ice giant ionospheric and magnetospheric modeling.

Planetary upper atmospheres couple the deep atmosphere to the space environment. The dynamics and energetics of this rarefied, partially ionized region govern atmospheric evolution. At Jupiter, decades of past plasma measurements have revealed a variable and enigmatic ionosphere inconsistent with photochemical predictions and unusual global structures imprinted by the planet’s powerful magnetic field. Upper-atmospheric temperatures have been measured sporadically and thus are unable to fully characterize the energy transfer mechanism responsible for its unexpectedly hot thermosphere. Observations to date have been too limited spatially, or too insensitive, to uncover the driving mechanisms behind the strong variability and magnetically organized features found in Jupiter’s upper atmosphere. Here, we present high spatial resolution global maps of ion densities and temperatures constructed from >175,000 Keck/NIRSPEC spectra collected over 4 yr. Irregular ionospheric emission features, first seen more than 25 yr ago, are shown to be persistent and due to local ion density modifications. Global temperatures decrease steadily from auroral to equatorial latitudes and are remarkably stable, with equatorial deviations of <10% night to night. Thus, despite appearing stochastic in previous observations, Jupiter’s upper atmosphere exhibits predominantly spatial rather than temporal variability, yielding a steady global structure generated by local plasma dynamics. These results illustrate how neutral winds and magnetic fields can create globally persistent plasma structures on hydrogen-dominated worlds and provide an explanation for decades of puzzling Jovian upper-atmospheric observations.

Observations of continuous vertical electron and ion distributions at Jupiter were obtained simultaneously for the first time on 2023 September 7 UTC. On that date, Juno performed a radio occultation experiment and, in parallel, JWST and Keck scanned Jupiter’s limb, focused on characterizing H 3 + . Here, using Keck infrared spectra, we derive constraints on H 3 + densities and temperatures. While on-disk column densities are roughly constant with latitude, with a mean value of (5.7 ± 2.1) × 10 ¹⁵ m ⁻² , temperatures decrease smoothly from 774 ± 50 K at high latitude to 683 ± 28 K at low latitude. Vertical H 3 + temperatures are comparable to their on-disk values and mostly isothermal. Keck spectra sampled Jupiter’s dawn limb, finding H 3 + volumetric number densities of (1–40) × 10 ⁸ m ⁻³ with peak altitudes near 1500 km. Comparison with the Juno electron altitude density profile confirms Jupiter’s ionosphere is proton dominated. Ionospheric model simulations that can broadly replicate observed plasma densities indicate that field-aligned vertical transport driven by neutral winds is primarily responsible for the observed high-altitude peak. We use model outputs, validated by comparison with plasma observations, to demonstrate that H 3 + is in photochemical equilibrium (PCE) below ∼1600 km altitude in Jupiter’s ionosphere. PCE analysis further reveals that the diurnal peak in N H 3 + can occur before noon, that peak electron densities should generally be 11 m ⁻³ , and that rapidly varying auroral H 3 + emissions can be explained by narrow ( 3 + just above the homopause produced by energetic particle precipitation.

The first spectroscopic observations of Uranus by the James Webb Space Telescope were obtained in January 2023. Using these observations, we explored the physical properties of the planet's ionosphere through the analysis of near‐infrared spectra. We found that both the northern and southern aurora were present; confirmed by localized column density enhancements. The median ionospheric temperature across the disk was 415 13 K, which is at least 300 K lower than measured by Voyager 2 in 1986, making these the lowest temperatures ever recorded. Surprisingly, auroral temperatures are not elevated in the south, and are only enhanced by tens of Kelvin in the north, indicative of limited heating or very efficient transport away from the auroral regions. Significant non‐auroral ionospheric structure is observed, including a dark band aligned with the magnetic dip equator, revealing the first imprints of Uranus's magnetic field on its ionosphere.

Past observations of Saturn's sub‐auroral ionosphere have provided no detailed longitudinal information, and variations in underlying stratospheric emission from methane fluorescence have never been resolved spatially. Here, we present the first near‐infrared observations of Saturn using JWST's NIRSpec‐IFU, revealing the 2.8–5.2 micron wavelength region in unprecedented detail. We observe the rotating auroral enhancement associated with planetary‐period currents, the phase of which is re‐discovered for the first time since Cassini. We discover a series of ionospheric dark beads located between 55 and 65N, that appear to be formed by local thermospheric dynamics. In the fluorescing methane, we reveal strange dark arms extending equatorward from a dark polar cap at 60N; four arms from a six‐pointed star, each extend below 45N with the final two arms missing. Neither set of features has any known analog at other planets.

We present near‐infrared observations of the northern aurora of Jupiter using the NIRSpec instrument on the James Webb Space Telescope, mapping emissions from and across the polar region. The data were acquired over a period of 4 hr, providing a temporally averaged view of these emissions. From the spectra we derive the temperature of the upper atmosphere and ion densities. Temperatures are elevated along the main auroral oval at both dawn and dusk, though the highest temperatures recorded were poleward of the main oval at dawn, approaching 1500 K. The highest ion densities were observed dusk‐ward of the main oval, closely correlating with the observed radiance. Using overlapping individual observations (or dithers) we investigate the temporal variability of the temperatures, which we found change too fast to represent wholesale changes to the vertical temperature structure of the upper atmosphere. Instead, these fast changes are likely connected to variable electron precipitation energies, which produce at different altitudes that sample different parts of the thermospheric temperature profile. The 3.3 m fundamental and hotband emissions are brightest at 210W close to the pole, which has been seen previously. However, we also see emission along the main oval, suggesting excitation of this non‐LTE emission by direct precipitation. Lastly, we suggest that the band ratios can be used to trace the penetration depth of the precipitating electrons, and therefore their energies.