Emma Miriam Thomas

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.

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.

Jupiter's auroral regions have previously been defined by broad‐scale auroral structures, but these are typically obscured by the wide array of temporal variability observed at timescales between minutes and days, making it difficult to understand the underlying magnetospheric biases driving these brightness differences. Here, we follow on from an initial study of Jupiter's aurora, again utilizing a data set of 13,000 images of Jupiter mapped into latitude, longitude and local time, smoothed over tens of hours of integration and many days of observing. Having removed correlations between brightness and both magnetic field and planetary local time identified in the first study, we examine morphological changes in emission with both planetary and magnetic local time. We reveal that the main auroral emission is enhanced by a factor of three in the region mapping into the dusk magnetosphere. An additional strong auroral darkening is observed near noon, aligned with previous ultraviolet observations of an auroral discontinuity in this region, though this rotates duskward slightly in magnetic local time, as the ionospheric source mapping to this region moves duskward. The polar aurora contrasts with this strongly, showing brightness enhancement when the auroral pole points toward the dawn and dusk limbs. It also shows that the Dark region is fixed in local time, close to the dawnward edge of the polar region, while the Swirl region appears to match well with predictions from recent MHD models when the magnetic pole points toward dawn, but changes significantly at other magnetic pole directions.

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.

We present the first pole‐to‐pole observations of Jupiter's ionosphere, capturing dawn and dusk vertical structure up to 5,000 km altitude using the James Webb Space Telescope (JWST), simultaneous to Juno's radio occultation experiments (ROX) in September 2023. We produce vertical temperature global maps showing largely constant temperatures with altitude. Mean temperatures range from 650 K at low latitudes to 800 K at high latitudes. Auroral emissions were observed at the north and south pole, with northern temperature profiles showing signatures of localized heating (up to 1900 K) at low altitudes. Temperature‐latitude gradients exhibit steeper slopes at dusk compared to dawn at all altitudes, particularly between 1,500 and 1,700 km. We also produce the first pole‐to‐pole vertical volumetric number density map, with auroral densities of up to peaking near 1,500 km altitude. We find a dawn‐dusk asymmetry at sub‐auroral and equatorial latitudes, with higher densities at dawn ( ) compared to dusk ( ) between 1,500 and 1,700 km. While photochemical equilibrium (PCE) theory predicts higher densities at dusk, the presence of enhanced dawn densities implies that additional processes—such as particle precipitation and plasma transport, particularly at high and mid latitudes—play a significant role in shaping the ionospheric structure.

Jupiter's auroral regions are marked by considerable spatial and temporal variability, with numerous sources of auroral enhancements, making it difficult to isolate individual causes of emission brightness. Here, we utilize a data set of 13,000 near‐infrared images of Jupiter mapped into latitude, longitude and planetary local time to separate out various sources of emission brightening, smoothed over hundreds of hours of integration and tens of days of observing. We show that equatorial emission is well correlated to planetary local time, indicating that equatorial ionospheric emission is dominated by solar Extreme Ultraviolet (EUV) ionization on the dayside of the planet. main auroral emission is strongly anti‐correlated with surface magnetic field strength (with a Pearson correlation of −0.90), and is also strongly correlated with planetary local time (changing in brightness in the same way as equatorial emission with a Pearson correlation of 0.93). This is the first time such strong correlations have been shown at Jupiter, and may be the first evidence of such direct correlations at any planet, including Earth. Both of these correlations are thought to ultimately result from changes in ionospheric electrical conductivity driven by changing magnetic field strength and solar EUV ionization. This suggests the aurora is significantly controlled by breakdown in co‐rotation currents flowing deep into the ionosphere, and that this deep layer contains an important component of solar ionization. However, polar auroral emission does not correlate well with planetary local time, suggesting much more complex processes drive the varying emission within this region.

For decades, the influence of Jupiter's higher order magnetic field on its non‐auroral ionosphere has remained enigmatic. NASA's Juno spacecraft has revealed great complexities in the Jovian magnetic field, with significant features in the sub‐auroral regions aligning with puzzling structures identified in near‐infrared emissions from the ionosphere. Here, we directly compare ground‐based measurements of Jupiter's ionosphere with the latest magnetic field models borne in the era of Juno and reveal aspects of global ionosphere‐magnetosphere coupling not previously explored. Ionospheric emissions exhibit either enhancements or reductions where the surface field is weakest and are found to correlate with both the strength and geometry of the magnetic field, where the latter is more dominant and with a unique control in each magnetic hemisphere. Therefore, we have illustrated that there may be distinct electrodynamic processes responsible for shaping Jupiter's non‐auroral ionosphere in these key regions.