# The Ionosphere of Uranus as Revealed by JWST

Henrik Melin, L. Fletcher, H. Hammel, S. Milam, L. Moore, T. Stallard, J.

O'donoghue, P. Tiranti, E. Thomas, K. Knowles, et al.

► To cite this version:

Henrik Melin, L. Fletcher, H. Hammel, S. Milam, L. Moore, et al. The Ionosphere of Uranus as Revealed by JWST. Geophysical Research Letters, 2025, 52 (22), 10.1029/2025GL118301. hal-05486948

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# Geophysical Research Letters®

## RESEARCH LETTER

10.1029/2025GL118301

**Key Points:**

* First James Webb Space Telescope Near-Infrared Spectrograph Integral Field Unit observations of the ionosphere of Uranus, providing a high spatial resolution view in the near infrared

* Auroral emissions detected with high ion densities, but without elevated temperatures, indicating limited heating or efficient transport

* First detection of dark non-auroral features, located along the magnetic dip equator, analogous to the equatorial ribbon seen at Jupiter

**Correspondence to:**

H. Melin,
henrik.melin@northumbria.ac.uk

**Citation:**

Melin, H., Fletcher, L. N., Hammel, H. B., Milam, S. N., Moore, L., Stallard, T. S., et al. (2025). The ionosphere of Uranus as revealed by JWST. *Geophysical Research Letters*, *52*, e2025GL118301. https://doi.org/10.1029/2025GL118301

Received 18 JUL 2025

Accepted 4 NOV 2025

**Author Contributions:**

**Conceptualization:** Henrik Melin,
L. N. Fletcher, H. B. Hammel, S. N. Milam

**Data curation:** L. N. Fletcher,
H. B. Hammel, M. Roman

**Formal analysis:** Henrik Melin, L. Moore,
T. S. Stallard, J. O'Donoghue, P. I. Tiranti,
E. M. Thomas, K. L. Knowles,
O. R. T. King, J. Harkett

**Methodology:** Henrik Melin

**Resources:** H. B. Hammel, L. Lamy

**Software:** Henrik Melin

**Validation:** M. Roman

**Visualization:** Henrik Melin

**Writing – original draft:** L. N. Fletcher

**Writing – review & editing:**
L. N. Fletcher, H. B. Hammel,
S. N. Milam, L. Moore, T. S. Stallard,
J. O'Donoghue, P. I. Tiranti,
E. M. Thomas, K. L. Knowles, M. Roman,
L. Lamy, O. R. T. King, J. Harkett

© 2025. The Author(s).
This is an open access article under the
terms of the Creative Commons
Attribution License, which permits use,
distribution and reproduction in any
medium, provided the original work is
properly cited.

<figure><img src="image_2.png" /></figure>

## The Ionosphere of Uranus as Revealed by JWST

Henrik Melin¹ ID, L. N. Fletcher² ID, H. B. Hammel³ ID, S. N. Milam⁴ ID, L. Moore⁵,⁶ ID, T. S. Stallard¹ ID, J. O'Donoghue⁷ ID, P. I. Tiranti¹ ID, E. M. Thomas¹ ID, K. L. Knowles¹ ID, M. Roman²,⁸ L. Lamy⁹,¹⁰ ID, O. R. T. King², and J. Harkett² ID

¹Department of Mathematics, Physics, and Electrical Engineering, Northumbria University, Newcastle upon Tyne, UK,

²School of Physics & Astronomy, University of Leicester, Leicester, UK, ³Association of Universities for Research in Astronomy, Washington, DC, USA, ⁴NASA Goddard Space Flight Center, Greenbelt, MD, USA, ⁵Department of Astronomy, Boston University, Boston, MA, USA, ⁶Center for Space Physics, Boston University, Boston, MA, USA,

⁷Department of Meteorology, University of Reading, Reading, UK, ⁸Universidad Adolfo Ibáñez, Santiago, Chile, ⁹LIRA, Observatoire de Paris, Université PSL, Sorbonne Université, Université Paris Cité, Cergy Paris Université, CNRS, Meudon, France, ¹⁰LAM, Aix Marseille Université, CNRS, CNES, Marseille, France

**Abstract** 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 $H_3^+$ spectra. We found that both the northern and southern aurora were present; confirmed by localized $H_3^+$ 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.

**Plain Language Summary** Uranus is a peculiar planet: it rotates on its side and it has a significantly titled magnetic field. This creates a dynamic and complex interaction with the surrounding space environment, that is, poorly understood. Using observations from the Near-Infrared Spectrograph instrument on the James Webb Space Telescope (JWST) we were able to map emissions from Uranus' charged-particle ionosphere to determine temperatures and densities across the disk. We saw auroral emissions in the northern and southern hemispheres. Because of the strong tilt of the magnetic field, the Uranian auroral emissions occur at the equator and mid-latitudes, unlike the Earth, Jupiter, and Saturn, where they are seen at the poles. At Jupiter and Saturn, the auroral process injects large amounts of energy into the upper atmosphere, however at Uranus, the observed temperatures are not significantly elevated across the auroral regions. We also see dark bands across the pole of the planet that appear fixed in longitude. We suggest that these are linked to the structure of the magnetic field close to the planet, and by studying these we can constrain models of the magnetic field using observations from JWST.

## 1. Introduction

The magnetic configuration of Uranus is unique within our Solar System, with a rotational axis tilted 98° from the normal to the ecliptic, and a magnetic field that is offset 59° from the rotational axis (Connerney et al., 1987). This creates a highly dynamic magnetosphere with a twisted magnetotail (Cao & Paty, 2017; Cowley, 2013; Griton et al., 2018; Pantellini, 2020; Tóth et al., 2004). During its 84 year orbit, the orientation of the magnetic field relative to the solar wind changes dramatically, and so the coupling between the space environment, magnetosphere and atmosphere may be strongly seasonally dependent (Lamy et al., 2017; Masters, 2014; Turner et al., 2024; Zomerdijk-Russell et al., 2025).

The morphology of the aurora of Uranus was first observed by Voyager 2 in the ultraviolet (UV; Herbert, 2009), presenting a patchy aurora but without the clear oval or arc-like structures seen at Jupiter and Saturn (Clarke et al., 2009). The UV $H_2$ bands are relatively weak, with a peak brightness of 9 kR. Subsequent observations by the Hubble Space Telescope (HST), obtained after equinox, have shown similar but variable morphologies, with brightness reaching 24 kR (Lamy et al., 2012, 2017, 2025). The presence of variable aurora suggests varying electric currents that enter, and flow through, the upper atmosphere. At Jupiter and Saturn, Pedersen currents
produce significant amounts of resistive Joule heating, which causes elevated thermospheric temperatures at the magnetic poles (Badman et al., 2015).

Both solar photoionization and particle impact ionization produce $H_3^+$ (via $H_2^+$). The former process generates $H_3^+$ across the entire dayside, and the latter produces localized density enhancements in the auroral regions. By fitting the observed $H_3^+$ spectrum in the near-infrared, the temperature of the upper atmosphere and the column integrated $H_3^+$ density can be determined (Melin, 2025c), revealing the physical properties of the charged particle ionosphere and the neutral thermosphere (Miller et al., 2020).

$H_3^+$ was first discovered at Uranus in 1992 (Trafton et al., 1993), with a globally averaged temperature of 740 ± 25 K, in good agreement with the exospheric temperature measurements of Voyager 2 (Broadfoot et al., 1986). This temperature is much hotter than can be produced by heating from sunlight, a discrepancy that is seen at all four giant planets, colloquially termed the *giant planet energy crisis* (Yelle & Miller, 2004). Since the discovery, disk averaged observations have revealed that the ionosphere has cooled at a rate of ~8 K per year (Melin et al., 2011, 2013, 2019), a behavior perhaps also seen at Neptune (Melin, Moore, et al., 2025). There is currently no clear consensus on the cause of this cooling, but several mechanisms have been proposed, for example, changes in the homopause height as a function of latitude (Melin et al., 2019), and simple geometric considerations (Melin et al., 2011). A long-term decrease in the solar wind pressure which could modulate auroral heating has also been proposed (Masters et al., 2024), however recent analysis shows that the solar wind pressure has been gradually increasing since 2008 (Jasinski & Velli, 2025), which puts this hypothesis into doubt. There is also a significant amount of day-to-day variability, which is suggested to be linked to changes in auroral intensity and heating (Lam et al., 1997; Melin et al., 2013, 2019).

Due to the small angular size of Uranus in the sky (<3.8"), extracting $H_3^+$ spatial information on the disk from ground-based observations is challenging. However, spatially resolved observations using the Keck telescope (Thomas et al., 2023) have shown localized density enhancements on the disk, likely associated with the auroral process, whilst showing no significant auroral heating. In addition, Melin et al. (2019) observed a brightening on the dawn limb, confirmed to be driven by density (Thomas et al., 2025).

The unrivaled sensitivity and spatial resolution offered by the James Webb Space Telescope (JWST) provides a new platform with which to investigate the ionosphere of Uranus, and the first spectroscopic observations of Uranus acquired by the telescope are analyzed here. In the next Section we describe the observations and how they were analyzed, in Section 3 we will outline the results, followed by a discussion in Section 4 and finally a summary in Section 5.

## 2. Observations and Analysis

The JWST observations analyzed here are part of the Solar System Guaranteed Time Observations (GTO) awarded to H. B. Hammel (Principal Investigator L. N. Fletcher, programme #1248), detailed in Table 1. Uranus was observed on 8 January 2023 and 9 January 2023 with the Near-Infrared Spectrograph Integral Field Unit (NIRSpec IFU, Jakobsen et al., 2022). These were coordinated with HST and were timed to approximately coincide with a solar wind compression. Three observations, each separated by ~120° longitude, provided full spatial coverage of the observable latitudes. The IFU has 30 × 30 spaxels (spatial pixels), each subtending 0.1" by 0.1". For each observation, a four-point dither was used (exposures at slightly different telescope pointing), and the sub-JWST planetocentric latitude was 55.1°. The G395H/L290P filter/grating produces a spectrum between 2.9 and 5.3 µm at a spectral resolving power of $R = 2,700$. Each dither used 10 Groups and 4 Integrations, equivalent to an exposure time of 107s, with a spatial resolution per spaxel on the planet of ~1,400 km. Here, we consider the region between 3.30 and 3.98 µm, containing over 30 $H_3^+$ emission lines. By fitting the observed $H_3^+$ spectrum to a model, the $H_3^+$ temperature and column integrated density can be retrieved.

Each dither was processed separately to Level 3 in the JWST pipeline (version 1.18.1, Bushouse et al., 2022), and the Calibration References Data System context was *jwst_1238.pmap*. Bad pixels were identified from the background observation and were interpolated over in the individual dithers. The contribution of spectral reflectance features from the lower atmosphere were subtracted using a scaled NEMESIS (Irwin et al., 2008) radiative transfer retrieval model based on the same data set (Roman et al., 2023), together with a linear background fit.
<table><caption>**Table 1**
Summary of the James Webb Space Telescope Observation Analysis in This Study, Shown in Figure 1</caption><thead><tr><th></th><th>Observation Name</th><th>Dither</th><th>Observation time (Universal time)</th><th>Longitude (IAU<sub>new</sub>)</th><th>Longitude (ULS)</th></tr></thead><tbody><tr><td>(a)</td><td>NIRSPEC-Lon1</td><td>1</td><td>2023-01-08 15:33:21</td><td>194.3°E</td><td>244.7°W</td></tr><tr><td>(b)</td><td>NIRSPEC-Lon1</td><td>2</td><td>2023-01-08 15:44:16</td><td>198.0°E</td><td>248.5°W</td></tr><tr><td>(c)</td><td>NIRSPEC-Lon1</td><td>3</td><td>2023-01-08 15:55:11</td><td>201.8°E</td><td>252.3°W</td></tr><tr><td>(d)</td><td>NIRSPEC-Lon1</td><td>4</td><td>2023-01-08 16:06:06</td><td>205.6°E</td><td>256.1°W</td></tr><tr><td>(e)</td><td>NIRSPEC-Lon2</td><td>1</td><td>2023-01-08 21:02:10</td><td>308.6°E</td><td>359.1°W</td></tr><tr><td>(f)</td><td>NIRSPEC-Lon2</td><td>2</td><td>2023-01-08 21:13:05</td><td>312.4°E</td><td>2.9°W</td></tr><tr><td>(g)</td><td>NIRSPEC-Lon2</td><td>3</td><td>2023-01-08 21:24:00</td><td>316.2°E</td><td>6.7°W</td></tr><tr><td>(h)</td><td>NIRSPEC-Lon2</td><td>4</td><td>2023-01-08 21:35:00</td><td>320.0°E</td><td>10.5°W</td></tr><tr><td>(i)</td><td>NIRSPEC-Lon3</td><td>1</td><td>2023-01-09 02:47:59</td><td>68.9°E</td><td>119.4°W</td></tr><tr><td>(j)</td><td>NIRSPEC-Lon3</td><td>2</td><td>2023-01-09 02:58:54</td><td>72.7°E</td><td>123.2°W</td></tr><tr><td>(k)</td><td>NIRSPEC-Lon3</td><td>3</td><td>2023-01-09 03:09:49</td><td>76.5°E</td><td>127.0°W</td></tr><tr><td>(l)</td><td>NIRSPEC-Lon3</td><td>4</td><td>2023-01-09 03:20:44</td><td>80.3°E</td><td>130.8°W</td></tr><tr><td>-</td><td>NIRSPEC Background</td><td>1</td><td>2023-01-09 05:14:05</td><td>Off disk</td><td>Off disk</td></tr></tbody></table>

Note. Three observations were acquired, each with four dithers, as well as a background observation. The sub-observer longitude is given in both the IAU<sub>new</sub> coordinate system (Lamy et al., 2025) and Uranus Longitude System (ULS) system (Ness et al., 1986).

There are two commonly used coordinate systems at Uranus, defining the northern pole in opposing ways: the Uranus Longitude System (ULS) defined the northern hemisphere as the sunlit one during the 1986 encounter, whereas the International Astronomical Union (IAU) defines it as the southern. Recently, Lamy et al. (2025) proposed a modified IAU<sub>new</sub>, with an updated rotation rate based on HST auroral observations. Since (a) the IAU<sub>new</sub> system is valid from 1986 to the current epoch and (b) the difference between the longitude position of the magnetic poles in the ULS and IAU<sub>new</sub> systems is 50.6°, we can now directly compare the JWST observations to models derived from the Voyager 2 magnetic field measurements (e.g., Connerney et al., 1987). In the text we use ULS longitudes and latitudes, and both IAU<sub>new</sub> (East) and ULS (West) longitudes are indicated in Table 1 and Figure 3. Note that these two systems use the same rotational period and are only offset in longitude.

Figure 2 shows two $H_3^+$ spectra and fits from individual spaxels in observation (a) (see Table 1). The weaker of the two spectra has a signal-to-noise of about five, resulting in significant uncertainties in derived parameters. To increase the signal-to-noise, and improve the fits, we project and combine all the Uranus observations in Table 1 to produce a cube with dimensions of longitude, latitude, and wavelength. This is done in the following manner: (a) An analytic line-of-sight correction was applied to each wavelength slice to correct for the increasing path-length of the $H_3^+$ ionosphere toward the limb; (b) Each wavelength slice of each dither was projected to a ULS longitude and latitude grid with a resolution of 2.5°. The bright dusk region of $H_3^+$ was excluded from this projection to remove the strongest instance of local-time effects, though others may be present, for example, an expected solar zenith angle dependence (Moore et al., 2019); (c) The median of these projections formed the wavelength slice of our data-combined projection; (d) This was subsequently performed for each wavelength slice to complete the cube; (e) The data-combined $H_3^+$ spectrum at each longitude and latitude point in our grid was fitted using h3ppy (Melin, 2025c) to produce maps of $H_3^+$ temperature (T), column integrated density (N), and total emission (E; Miller et al., 2013).

## 3. Results

Figure 1 shows the individual dithers (columns) of each observation (rows) in sky-oriented coordinates (right ascension and declination), and the details of each is listed in Table 1. The L-shells of the $Q_3$ magnetic field model (Connerney et al., 1987) are shown in black. A magnetic L-shell is the furthest distance from a planet that a particular magnetic field line extends, so that field lines along the $L = 20 R_U$ contour extend to $20 R_U$, which is close to the magnetopause boundary (~22 $R_U$; Jasinski et al., 2025). Overall, they show that $H_3^+$ is bright across the entire disk of Uranus with an average radiance of $0.14 \mu Wm^{-2}sr^{-1}$ and a standard deviation of 0.04
<figure><img src="image_3.png" /><figcaption>**Figure 1.** The James Webb Space Telescope observations of $H_3^+$ emissions from Uranus obtained in January 2023. The columns shows individual dithers from the three observations (see Table 1 for details). The white latitude and longitude grid has a resolution of 20°, and the auroral L-shells are shown as black contours (in units of Uranus radii), with the northern auroral region being the more extended of the two. Dawn is at the bottom and dusk is at the top, and the errorbar in the colourbar indicates the uncertainty in the observations.</figcaption></figure>

$\mu$Wm$^{-2}$sr$^{-1}$. The uncertainty in the integrated radiances in Figure 1 is 0.015 $\mu$Wm$^{-2}$sr$^{-1}$, based on the scatter of the $H_3^+$ signal above the terminator of the planet. In these images, the dawn limb (bottom) appears darker than the rest of the disk and there is a bright region of $H_3^+$ at the dusk limb (top), which may be consistent with the buildup of $H_3^+$ across the dayside (see Melin et al., 2019).

<figure><img src="image_4.png" /><figcaption>**Figure 2.** The top panels show the Near-Infrared Spectrograph Integral Field Unit spectrum of Uranus and the $H_3^+$ spectral fit using *h3ppy* (Melin, 2025c) for two individual spaxels from Figure 1a, one at the southern aurora (a), and one at the dark feature adjacent to it (b). The gray regions are not included in the $H_3^+$ fitting. The bottom panels shows the data-model residual and the $3-\sigma$ uncertainty in the data. Whilst the temperatures are similar, the main driver of the difference in spectral radiance is the $H_3^+$ column density.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** The combined Near-Infrared Spectrograph Integral Field Unit data projected to Uranus Longitude System (ULS) planetocentric latitude and longitude showing (a) $H_3^+$ total emission with L-shells indicated as per Figure 1, (b) the L-shells for three different magnetic field models: Q$_3$: Connerney et al. (1987), UH$_5$: Holme and Bloxham (1996), and AH$_5$: Herbert (2009). The magnetic dip equator for these models is seen close to the pole, and the gray background is the Q$_3$ total magnetic field strength. The “×” signs indicate the base of the traced field lines from the northern aurora (black) to the southern polar region (colors, for each model), and the ★ signs indicate the same for the southern (black), which maps to a points on the northern hemisphere (colors). Both ULS (black) and IAU$_{new}$ (gray) longitudes are indicated. (c) $H_3^+$ column integrated density, with the uncertainty shown in panel (d). Panel (e) $H_3^+$ temperature, with the uncertainty shown in panel (f). The dotted thick white chevron indicates the morphology of the dark polar “>” feature in panel (a).</figcaption></figure>

Figures 1a–1d shows the four individual dithers for the first observation. Here, a bright feature consistent with the southern aurora is visible in the upper left, just inside $L = 20 R_U$. The $H_3^+$ radiance component of this feature is on average 64% (0.083 µWm$^{-2}$sr$^{-1}$) higher than the background ionospheric disk signal with a variability of 13%.
Emissions consistent with the southern aurora are also visible in Figure 1k (and partially visible in Figure 1i) and this feature is ~94% more radiant than the background ionosphere and peaks at 0.11 µWm⁻²sr⁻¹.

Emission consistent with northern aurora is seen in the bottom left in Figures 1e–1h, and it appears on the edge of the usable part of the IFU. Nonetheless, the mean auroral signal above the background ionosphere is 0.12 ± 0.02 µWm⁻²sr⁻¹ with a variability in the radiance of 37%. The northern aurora is also visible in Figures 1j and 1l, but appears very close to the limb, and is mixed with the enhancement of H₃⁺ at dusk, and therefore it is challenging to extract meaningful statistics.

Away from the auroral regions, there are significant structures, including dark spots and distinct bands. The darkest spot appears just poleward next to the southern auroral feature at 60°S, seen in Figures 1a–1d. The spectrum of both the bright southern auroral feature and the dark feature is shown in Figure 2. The H₃⁺ temperature retrieved from these regions is similar (425 ± 4 and 388 ± 16 K, respectively), but the column density is 1.9 times smaller in the dark feature. This suggests that a change in the column density is the main driver of changes in the integrated radiance. There are additional dark bands seen across the disk in all of the dithers in Figure 1.

Figure 3 shows the results of the H₃⁺ spectral fitting of the projected and combined data, looking down onto the southern ULS pole. The L-shells of the Q₃ model are shown in white. Figure 3b shows the L-shells derived from two additional models: UH₅ (Holme & Bloxham, 1996) and AH₅ (Herbert, 2009). The locations of the magnetic dip equator (where the magnetic field lines are parallel to the “surface”) for the three models are seen traversing the pole of the planet, and the total magnetic field strength of the Q₃ model is shown as the background. Using the three magnetic field models we trace the field lines (using JupiterMag, which also contains Uranus magnetic field models; Wilson et al., 2023) from the brightest emissions in the northern auroral region (black ×) to where they map in the southern magnetic pole (color-coded ×), as well as the same for the brightest emissions in the southern auroral region (⋆). The auroral emissions are not magnetically conjugate for any of the models, though it must be noted that the northern and southern aurora were not observed at the same time (Figure 1).

Figure 3a shows the H₃⁺ total emission, which reflects on what is seen in the integrated radiance of the individual dithers in Figure 1, but with higher signal-to-noise. The disk integrated total emission is 21.6 GW. Overall, the geographic polar region appears darker, and there is a dark ">" feature centered on it (the *polar chevron*). The features associated with the northern and southern aurora (contained within the white L-shell contours) are the brightest, but there are also bright extended region of H₃⁺ at around 270° W and 135° W. The latter features may be due to subtle local-time effects that have not been taken into account here.

The retrieved H₃⁺ column density and its uncertainty are shown in Figures 3c and 3d with a median value of 2.27 × 10¹⁵ m⁻² and a standard deviation of 0.63 × 10¹⁵ m⁻². The increase in column density at the bright features close to the magnetic poles confirm that these are indeed auroral in origin: they have localized enhancements of H₃⁺ which in turn must be associated with localized ionization of H₂ by particle precipitation. The H₃⁺ temperatures and their uncertainty are shown in Figures 3e and 3f with a median value of 415 K and a standard deviation of 13 K. Overall, the temperature decreases toward the southern geographic pole with a gradient of 0.40 ± 0.02 K per degree latitude southward of 25°S. Note that the bright features seen in the uncertainties (Figures 3d, 3f) are associated with the presence of bright tropospheric clouds, which are challenging to correct for.

From the projected and combined data we extract the spectrum from both the northern and southern aurora, as well as a non-auroral spectrum (i.e., *L* < 3 *R*<sub>U</sub>). The h3ppy fits to the median non-auroral spectrum is *T*=411 ± 1 K and *N* = 2.32 ± 0.05 m⁻², reflecting the H₃⁺ component generated by solar photoionization. The spectral fits for the northern aurora give *T* = 432 ± 1 and *N* = 3.06 ± 0.07 m⁻², whilst the southern aurora produces *T*=417 ± 2 and *N*=3.11 ± 0.10 m⁻². Again, the localized enhancements in the H₃⁺ column density confirms the detection of auroral emissions at both magnetic poles.

## 4. Discussion

The integrated radiance maps in Figures 1a–1d provide the first view of the short-term variability of the near-infrared H₃⁺ aurora. The southern aurora varies 13% over the course of 33 min (the time between the first and the last dither). If this is purely driven by changes in the H₃⁺ column density, as indicated above, this implies a rate
of change of $7 \times 10^{10} \text{ m}^{-2}\text{s}^{-1}$. This is very modest in comparison to the fast changes seen in the aurora of Jupiter, up to up to $10^{13} \text{ m}^{-2}\text{s}^{-1}$ (Melin, Stallard, et al., 2025). This could be driven by the long $H_3^+$ lifetimes at Uranus (1 hr; Moore et al., 2019) compared to the very short lifetimes seen in the aurora of Jupiter ($\le 150$ s; Nichols et al., 2025). The long lifetime is a product of the relatively low electron densities observed at Uranus (Lindal et al., 1987). A long $H_3^+$ lifetime would act to smooth the rate of change in density produced by rapid changes in the precipitation flux. In contrast, HST observations have observed auroral variability of directly excited $H_2$ on time-scales from minutes to tens of minutes (Lamy et al., 2017).

At Jupiter and Saturn, the auroral regions are observed to be hotter than non-auroral latitudes by several hundreds of K. (e.g., Brown et al., 2020; O'Donoghue et al., 2021). In contrast, the $H_3^+$ temperatures across the disk of Uranus are confined within a limited range: the maximum fitted temperature is $449 \pm 4$ K, and the minimum temperature is $379 \pm 3$ K. Thomas et al. (2023) also observed very little variability in temperature across the disk between 162°W and 287°W (Lamy et al., 2025), consistent with this picture. In addition, no significantly elevated temperatures are measured in the southern auroral zone ($417 \pm 2$ K), with only limited heating in the northern ($432 \pm 1$ K). This suggests that either (a) auroral heating is severely limited at Uranus or (b) the heating can very efficiently be redistributed away from this region, generating very little temperature contrast across the disk. The median temperature derived here is 415 K with a standard deviation of 13 K, consistent with a continued cooling of Uranus' thermosphere (Melin et al., 2019).

The dark chevron seen at the southern pole in Figure 3a has never been observed before, and is driven by a reduction in column density (Figure 3c). The lower part of this feature (centered on 0°W) lines up well with the locations of the magnetic dip equators (Figure 3b) of the Connerney et al. (1987) and Holme and Bloxham (1996) models, but agrees more poorly with the Herbert (2009) model. Stallard et al. (2018) observed a darkening of Jupiter's $H_3^+$ ionosphere along the magnetic dip equator. If we are seeing the same feature at Uranus, then this provides a way to constrain future magnetic field models. The upper part of the polar chevron feature (centered on 180°W) connects to the southern aurora and traverses across a broad range of dip angles and field strengths (Figure 3b). The origin of this feature is unclear, but similar features are also seen at Jupiter (Knowles et al., 2025).

These JWST observations can be placed in the context of the baseline of past HST auroral observations at Uranus (Lamy et al., 2012, 2017, 2025). The auroral morphology observed here is similar to those observed by HST, spot-like and with limited spatial extent, albeit less variable. The HST observations show that both the northern and southern aurora appears on a range of L-shells, and that the brightness of the aurora has been increasing since 2011. Lamy et al. (2017) suggested that the variable southern aurora is produced by dayside magnetopause reconnection, possibly triggered by compression of the magnetosphere by the solar wind (also seen at Saturn, e.g., Jasinski et al., 2014). In this study, the northern and southern auroral emissions (Figure 3a) are not magnetically conjugate, as they do not sit on magnetic field-lines that are connected (Figure 3b). The northern aurora appears more extended, with the caveat that they are closer to the limb, and so the spaxels cover a larger latitude and longitude area. The magnetic field lines that connect the northern aurora to the southern magnetic pole are confined to the sunlit southern hemisphere of the planet, and so this may also be associated with reconnection on the dayside. Lastly, the locations of northern and southern auroral emissions observed here (Figure 3a) are consistent with the predicted locations of Lamy et al. (2025). Because this new longitude system is also valid during the Voyager 2 encounter, the JWST observations do not indicate any clear secular variation in the internal magnetic field of Uranus in the 37 years since 1986.

**5. Summary**

We have presented the first JWST NIRSpec IFU observations of Uranus that captured the morphology of the $H_3^+$ ionosphere, and from the analysis of the combined data we derive the physical properties of this region. We can summarize our findings as follows:

1. The ionosphere of Uranus shows significant emission across the whole observable disk, with distinct southern and northern auroral emissions, as confirmed by a non-solar localized increases in the $H_3^+$ density.

2. Separate dithers reveal that the southern $H_3^+$ aurora exhibits minimal temporal variation (13%), which may be related to the relatively long lifetime of $H_3^+$ at Uranus.
3. No elevated temperatures are seen at the southern aurora, whereas they are slightly elevated for the northern aurora. This suggests that the auroral currents produce very little Joule heating during the observing epoch, or that efficient transport processes are able to redistribute thermal energy from the auroral regions.

4. A dark non-auroral polar $H_3^+$ chevron is driven primarily by a reduction in the $H_3^+$ density. One part of the chevron aligns well with the magnetic dip equator, perhaps analogous to the darkening seen at Jupiter's magnetic dip equator.

5. The median $H_3^+$ temperature derived here is 415 ± 13 K, which is the coldest ever recorded at Uranus, demonstrating that the long-term cooling has continued until January 2023.

## Conflict of Interest

The authors declare no conflicts of interest relevant to this study.

## Data Availability Statement

The JWST Uranus GTO NIRSpec IFU data used here is available in Melin (2025a). The $h\alpha p\gamma H_3^+$ modeling and fitting code is available in Melin (2025c). The magnetic modeling was performed using *JupiterMag* (Wilson et al., 2023). The projected and combined spectra and the fitted parameters (Figure 3), along with relevant code is available in Melin (2025b). This archive also includes the NAIF kernels for IAU$_{new}$ and ULS longitudes based on Lamy et al. (2025).

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## Acknowledgments

HM was supported by the STFC James Webb Fellowship (ST/W001527/2) at Northumbria University, UK. LNF, MR and ORTK, were supported by STFC Consolidated Grant reference ST/W00089X/1. HBH and SNM acknowledge support from NASA JWST Interdisciplinary Scientist Grant 21-SMDSS21-0013. TSS and EMT was supported by an STFC Consolidated Grant (ST/Y005325/1) at the Northumbria University, UK. LM acknowledges support from Program number JWST-GO-05073.002-A provided through a Grant from the Space Telescope Science Institute under NASA contract NAS5-03127. JO'D. was supported by the STFC Ernest Rutherford Fellowship ST/X003426/1 at the University of Reading. PIT and JH was supported by a STFC studentship. KLK was supported by a Northumbria University Research Studentship. LL acknowledges funding from CNES spatial agency and CNRS/INSU national programmes of heliophysics (PNST, also funded by CEA) and planetology (PNP). For the purpose of open access, the author has applied a Creative Commons Attribution (CC BY) license to the Author Accepted Manuscript version arising from this submission. A portion of this work used the ALICE high performance computing facility at the University of Leicester.
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