Infrared Aurora Detected on Uranus

While the ultraviolet aurora of Uranus has been observed since 1986, no confirmation of the infrared aurora had been observed until now.

The ice giants Uranus and Neptune are unusual planets in our solar system as their magnetic fields are misaligned with the axes in which they spin. While planetary scientists have yet to find an explanation for this, clues may lie in Uranus’ aurora.

Aurorae are caused by highly energetic charged particles, which are funneled down and collide with a planet’s atmosphere via the magnetic field lines.

On Earth, the most famous result of this process are the spectacles of the northern and southern lights.

At planets such as Uranus, where the atmosphere is predominately a mix of hydrogen and helium, this aurora will emit light outside of the visible spectrum and in wavelengths such as the infrared.

In the new research, University of Leicester Ph.D. student Emma Thomas and colleagues analyzed the near-infrared spectra of Uranus obtained with the NIRSPEC (Near-infrared Spectrograph) instrument on the Keck II telescope.

“From this, we can analyze the light — known as emission lines — from these planets, similar to a barcode,” they explained.

“In the infrared spectrum, the lines emitted by a charged particle known as H₃⁺ will vary in brightness depending on how hot or cold the particle is and how dense this layer of the atmosphere is. Hence, the lines act like a thermometer into the planet.”

The Keck II observations revealed distinct increases in H₃⁺ density in Uranus’ atmosphere with little change in temperature, consistent with ionization caused by the presence of an infrared aurora.

“The temperatures of all the gas giant planets, including Uranus, are hundreds of degrees Kelvin/Celsius above what models predict if only warmed by the Sun, leaving us with the big question of how these planets are so much hotter than expected?” Thomas said.

“One theory suggests the energetic aurora is the cause of this, which generates and pushes heat from the aurora down towards the magnetic equator.”

“A majority of exoplanets discovered so far fall in the sub-Neptune category, and hence are physically similar to Neptune and Uranus in size.”

“This may also mean similar magnetic and atmospheric characteristics too.”

“By analyzing Uranus’ aurora which directly connects to both the planet’s magnetic field and atmosphere, we can make predictions about the atmospheres and magnetic fields of these worlds and hence their suitability for life.”

“This paper is the culmination of 30 years of auroral study at Uranus, which has finally revealed the infrared aurora and begun a new age of aurora investigations at the planet.”

“Our results will go on to broaden our knowledge of ice giant auroras and strengthen our understanding of planetary magnetic fields in our Solar System, at exoplanets and even our own planet.”

The results may also give scientists an insight into a rare phenomenon on Earth, in which the north and south pole switch hemisphere locations known as geomagnetic reversal.

“We don’t have many studies on this phenomenon and hence do not know what effects this will have on systems that rely on Earth’s magnetic field such as satellites, communications and navigation,” Emma said.

“However, this process occurs every day at Uranus due to the unique misalignment of the rotational and magnetic axes.”

“Continued study of Uranus’ aurora will provide data on what we can expect when Earth exhibits a future pole reversal and what that will mean for its magnetic field.”

The findings were published in the journal Nature Astronomy.

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E.M. Thomas et al. Detection of the infrared aurora at Uranus with Keck-NIRSPEC. Nat Astron, published online October 23, 2023; doi: 10.1038/s41550-023-02096-5