Rosie Eleanor Johnson

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.

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.

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.

A striking feature of Jupiter’s aurora are the emissions induced by the Galilean satellites. Their auroral footprints represent the ionospheric signature of the electromagnetic interaction between the Galilean moons and Jupiter's co-rotating magnetospheric plasma. We present a new high-resolution view of the auroral footprints of Io and Europa in the near-infrared, as observed by the Near-Infrared Spectrograph (NIRSpec) onboard the James Webb Space Telescope (JWST). We report measurements of ionospheric H3+ spectral radiance, total emission, column-averaged temperature and ion density, as well as the spectral radiance from lower altitude CH4.The H3+ spectral radiance for both footprints is found to be primarily driven by the ion densities, as opposed to the local thermospheric temperature, with significant variability on the timescales of ∼30 minutes observed in both the H3+ temperature (∼50%) and density (∼500%) at the core of the Io spot. A spatially confined cold structure was uniquely seen, localised to the centre of the Io spot, with extremely high H3+ densities and surrounded by a “halo” of relatively hot and less dense H3+ extending into the footprint tail. The variability in the ionization of H3+ at the Io spot is likely driven by changes in the precipitating electron flux and energy creating H3+ across different altitudes, therefore sampling various regions of the ionosphere’s altitudinal temperature profile, and/or NIRSpec’s changing observing geometry is revealing different vertical extents of the footprint. There are also suggestions of a similar, yet less extreme, cold and dense population of H3+ associated with the Europa footprint. Our findings highlight the unique insights gained from analysing the near-infrared emissions from the auroral footprints of the Galilean moons. Such observations can supply valuable context for the in-situ measurements acquired by Juno as it traversed within the moons’ orbits during its prime and extended missions, as well as to support future investigations made by JUICE and Europa Clipper.

Uranus stands as an enigma within the field of planetary science. What little we know of its ionosphere and aurorae have proven perplexing, whether this be ultraviolet (UV) or infrared (IR) emissions, considerable intensity and column density variability have been observed across the surface of Uranus’s ionosphere [Lamy et al., 2012, 2017, 2020, Lam et al., 1997, Melin et al., 2019 and Thomas et al., 2023 and 2025]. For IR emissions of the molecular ion H3+, a key tracer of energy in Uranus’s ionosphere, only two observations have published spatial intensity maps with a 10 year gap between investigations, this difference prevents a direct comparison as to the variability between emission intensities, temperatures and column densities. To address this, our investigation will study the shorter term variability of Uranus by mapping multiple half planet scans (using Keck-NIRSPEC) in October 2023 and January 2025, an example shown in Fig. 1, to identify ionospheric fluctuations in key auroral and non-auroral regions. Through comparison of these intensity scans with prior JWST observations in December 2021 our discussions will focus on short-term auroral and thermal drivers in Uranus’s ionosphere.Figure 1. Averaged global projections (~ 5hr) of the spectral radiance of the Q(1,0-) H3+ emission line from Uranus as observed by Keck-NIRSPEC on the 29th September 2023. The intensity has been normalised in this figure with the S representing the ULS southern pole of Uranus.

Jupiter's polar aurorae deliver significant heating at the poles, thought to spread across the planet through atmospheric winds. Additionally, ground‐based Keck observations have revealed a large‐scale high‐temperature region, spatially distinct from the aurorae. Here, we investigate the origins and characteristics of the feature using Keck data, in‐situ Juno spacecraft measurements, and solar wind modeling. Juno exited the magnetosphere on approach to Jupiter, coinciding with modeled high‐speed solar wind impact that compressed the magnetosphere. This hot feature may be dynamic, transported equatorward by winds following auroral activity enhancements from magnetospheric compression akin to a large‐scale traveling ionospheric disturbance on Earth, or driven by the inner magnetosphere particle precipitation. Exploring the dynamic case, we calculated equatorward velocities ranging from 0.46 to 2.02 km , similar to those seen at Earth. Our study underscores the importance of the solar wind at all planets, exemplified by its ability to alter Jupiter's upper‐atmospheric energy balance globally.

To date, no investigation has documented ionospheric flows at Uranus. Previous investigations of Jupiter and Saturn have demonstrated that mapping ion winds can be used to understand ionospheric currents and how these connect to magnetosphere‐ionosphere coupling. We present a study of Uranus's near infrared emissions (NIR) using data from the Keck II Telescope's Near InfraRed SPECtrograph (NIRSPEC) and the InfraRed Telescope Facility's iSHELL spectrograph. H ₃ ⁺ emission lines were used to derive dawn‐to‐dusk intensity, ionospheric temperatures and ion densities to identify auroral emissions, with their Doppler shifts used to measure ion velocities. We confirm the presence of the southern NIR aurora in 2016, driven by elevated H ₃ ⁺ column densities up to 6.0 × 10 ¹⁶ m ⁻² . While no auroral emissions were detected in 2014, we find a 14%–20% super rotation across the planet's disk in 2014 and a 7%–18% super rotation in 2016.