The successful search for infrared aurorae at Uranus

Thirty years of studying Uranus has given us hints of the infrared aurorae, but nothing concrete until now. Keck II telescope observations of Uranus at equinox have documented portions of the northern aurora, which are seen simultaneously with an average 88% in ion density at these locations.
Published in Astronomy
The successful search for infrared aurorae at Uranus
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I challenge you to find a single person that hasn’t seen or heard of the Aurora Borealis or Aurora Australis (Northern and Southern Lights respectively). These lights paint a colourful display that dance across the sky, but have you ever thought about the science behind these pretty performances? What about off world aurorae, what do they look like? And why even look at aurorae, what information do they provide us with?

Aurorae are a gateway of knowledge, highlighting the intricate relationship between a planet’s magnetic field, it’s upper atmosphere (known as the ionosphere) and the solar wind (a continuous flow of highly charged particles from the sun). By studying the shape, brightness and changes of these aurorae we can predict periods of intense solar activity, and hence prepare for space weather, such as solar flares, which can interrupt satellite systems and cause power outages (see the Carrington Event). These aurorae are not only observed at Earth, but throughout our solar system, with some of the biggest displays seen at the Gas Giant and Ice Giant planets. Of these planets, we know very little about the aurorae at Uranus and Neptune, as they both have had but a brief visit by the Voyager II spacecraft in 1986 and 1989.

Uranus presents a peculiar question with aurora, what happens when a planet rotates on its side and has a magnetic field almost perpendicular to that rotation? This is still a mystery, with ongoing observations by the Hubble Space Telescope tracking the ultraviolet (UV) aurora’s behaviour after Voyager II (Lamy, et al., 2012 2017 and 2020). What these investigations found was truly bizarre, one of the auroras stretched all the way down past the equator from a latitude of 50°N. If this occurred at Earth, you’d be lucky to see the aurora at the poles and instead would need to travel down to the equator! To add to this baffling find, this aurora was broken up into distinct patches, a feature not seen at either Jupiter or Saturn.

During this time, another field of auroral detections was emerging, looking instead at the light in the infrared (IR) wavelength spectrum. This spectrum of light (shown in Figure 1) appears similar to a barcode, in which the brightness of each line determines the temperature and the number of particles in a planet’s atmosphere. Hence, we have a very long ranged thermometer! These observing techniques first began in 1989, showing successful detection at both Jupiter (Drossart, et al., 1989) and Saturn (Stallard, et al., 1999) and so began a simple question at Uranus. Is there an IR aurora at Uranus?

Figure 1. An averaged spectra obtained by KECK II NIRSPEC between 3.4 to 4.0μm, including annotations for Q(1,0-), Q(2,0-), Q(3,0-), Q(3,1-) and Q(3,2-) emission lines, which were found at 3.9530, 3.9708, 3.9860, 3.9865 and 3.9946 µm respectively. These lines are no affected by doppler shift as the slit is aligned with the planet's rotational axis.

This question would prove more difficult than the successes at Jupiter and Saturn. IR light had been confirmed at Uranus in 1992 (Trafton, et al., 1993), but the authors could not confirm if there were aurorae and so the search continued. Years later in 2007, with the mystery still not resolved, Uranus would experience an autumnal equinox, so the chances of capturing both the planet’s northern and southern lights were extremely high, the next chance being in 42 years! Thus scientists were keen to document the planet in great detail! A group of these observers, which included coauthors Steve Miller and Tom Stallard, applied to the Keck II telescope, which was two and a half times the diameter and held a spectral resolution almost ten times that of UK Infrared Telescope (UKIRT) which had taken the initial Uranus investigations in 1992.

On September 4th and 5th, 2006 the Keck II telescope focused on Uranus’s upper atmosphere and though poor weather and technical issues plagued the first night, the team were triumphant in recording almost half of the planet’s atmosphere (Figure 2 shows a similar observing set up at IRTF focused on Uranus). In Thomas, et al. (2023) we report and discuss the key results of this observing run and what it means for the IR aurora search.

Figure 2. Observing view of Uranus with Kyle on the Infrared Telescope Facility (IRTF) on the 9th October 2021. The planet, its rings, moons and a background star are labelled. The original video was captured in black and white with an artificial colourmap (Python's colormap Viridis) being used to brighten the rings.

Our findings (the first shown in Figure 3) suggest dense patches of aurora found in both the northern and southern hemisphere, which matches up similarly to the planet’s northern aurorae as recorded by Voyager II. Excitingly, we have also located intriguing structures within the auroral signal, which we hope to further examine in future work. Combining this with the higher-than-expected numbers of ionised particles at these locations, this leads us to our conclusion – that we have seen the infrared aurora for the first time! Our work compares these results with past observations of the UV northern aurora as well as with the prior magnetic model, where we identify key differences and similarities.

Figure 3. Measured H3+ Q(1,0-) intensity mapped across the upper atmosphere of Uranus against Uranian latitude and arbitrary longitude all five Q-branch emission lines. The latitude is planetocentric whereas the longitude is arbitrary due to the loss of the Uranian Longitude System (ULS). The solid black lines mark out the boundaries of E1 (on the left) and E2 (on the right). Within the boundaries, the Enhanced regions are unshaded, the Dim regions are shaded with dots, and the Intermediate regions are shaded with diagonal lines.

This result has been the culmination of 30 years of auroral study at Uranus, and we are excited to see the future of aurora investigations at their weird and wonderful world. With continued research we hope to solve mysteries in our solar system, such as the “Energy Crisis” of the giant planets (Yelle and Miller, 2004) - where the planets are hundreds of Celsius above previous expectations. Current theories suggest that the aurorae heat up the planet, which has been confirmed at Jupiter. We eagerly await further investigations into Uranus’s own aurora to see if the trend continues.

We also know that a category of exoplanets known as sub-Neptune are similar to Neptune and Uranus in size. Potentially this can mean they also have similar magnetic fields and atmospheres. This analysis of Uranus’s aurora combined with longer term research, will guide our understanding of aurora at these worlds. By observing a world’s aurora, we can determine if these worlds have aligned, misaligned or no stable magnetic fields, which without will have a direct effect on protecting life from highly charged particles of space weather.

At the time of writing this post, we are currently awaiting JWST images of Uranus with baited breath. Since 2006, the light from Uranus in the infrared spectrum has continued to dim making this work more and more challenging. Hence our next goal is to use these images to not only identify the southern aurora (which we believe was missed during this investigation) but to analyse how the infrared light emitted from Uranus varies with time, to identify key auroral structures.

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