How Jupiter’s ‘surfing’ ions make a spectacular light show

12 Jul 2021

Image: © Ser/Stock.adobe.com

After baffling researchers for decades, new readings from NASA and the ESA have revealed the cause of Jupiter’s chaotic auroras.

On the fifth planet from the sun, a dazzling light show has stumped researchers for decades. Jupiter’s auroras remind us of those on Earth but follow none of our rules.

A new research paper has finally cracked what is going on however, and may offer insight into Saturn, Uranus, Neptune and even planets outside of our solar system.

Auroras are streams of red and green lights that appear in the sky – most famously on Earth at a belt surrounding the magnetic poles between 65 and 80 degrees latitude.

Most commonly called the Northern Lights, there is also a southern counterpart that is just as spectacular (called aurora australis).

Jupiter boasts comparable light shows, but with a few key differences. Most noteworthy is their independence. On Earth, there is a link between the north and south auroras, while there is no such connection on Jupiter.

When the ESA examined Jupiter, they found the southern auroras to pulse every 11 minutes, while the northern pole flared chaotically and sporadically.

The x-ray ‘colours’ of these auroras show that they are triggered by electrically charged particles called ions crashing into Jupiter’s atmosphere. But astronomers had no idea how the ions were able to get to the atmosphere in the first place.

By combining data from the ESA’s XMM-Newton telescope and NASA’s Juno spacecraft, this latest research has cracked these puzzles and figured out the mechanics of Jupiter’s auroras.

A particular advantage came from Juno’s presence circling the planet and its ability to closely observe what is going on.

This is how Zhonghua Yao from the Institute of Geology and Geophysics at the Chinese Academy of Sciences, Beijing, and lead author of the new study, was able to see beyond previous models and account for Jupiter’s odd behaviour.

On Earth, auroras are visible only in that specific belt surrounding the magnetic poles. Beyond this belt, auroral emission disappear because the magnetic field lines here leave Earth and connect to the magnetic field in the solar wind.

These are called open field lines and, typically, Jupiter’s high-latitude polar regions are not expected to emit substantial auroras.

Jupiter’s x-ray auroras don’t follow this assumption, however. They exist poleward of the main auroral belt, pulsate regularly, and can sometimes be different at the north pole compared to the south pole.

These are typical features of a closed magnetic field, where the magnetic field line exits the planet at one pole and reconnects with the planet at the other.

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Using computer simulations, Yao and colleagues predicted that Jupiter’s pulsating x-ray auroras could be linked to closed magnetic fields that are generated inside the planet and then stretch out millions of kilometres into space before turning back.

On 16 and 17 July 2017, the Juno spacecraft was exactly where it needed to be according to Yao’s simulations. XMM-Newton observed Jupiter continuously for these 26 hours and saw x-ray auroras pulsating every 27 minutes.

And so the researchers trawled through the data on Juno at these moments to try and find magnetic readings that could give some insight into the ongoing processes.

And then, eureka! They found their info. The pulsating x-ray auroras were being caused by fluctuations of Jupiter’s magnetic field, clearly seen from the readings.

As the planet rotates, it drags around its magnetic field. The magnetic field is then struck directly by the particles of the solar wind and compressed.

These compressed heat particles are trapped in Jupiter’s magnetic field. This triggers a phenomenon called electromagnetic ion cyclotron (EMIC) waves, in which the ion particles are directed along the field lines.

These ion particles are guided by the field and ‘surf’ the EMIC wave across millions of kilometres of space, eventually slamming into the planet’s atmosphere and triggering the x-ray aurora.

“What we see in the Juno data is this beautiful chain of events,” said William Dunn from the Mullard Space Science Laboratory at University College London, who co-led the research.

“We see the compression happen, we see the EMIC wave triggered, we see the ions, and then we see a pulse of ions travelling along the field line. And then a few minutes later, XMM sees a burst of x-rays.”

Now that the process responsible for Jupiter’s x-ray auroras has been identified for the first time, researchers are aware of a fundamental process that could guide future research. This process could be studied on Saturn, Uranus, Neptune and possibly other planets outside our solar system.

For example, at Jupiter, the magnetic field is filled with sulphur and oxygen ions that are spewed out by the volcanoes on the moon Io. At Saturn, the moon Enceladus jets water into space, filling Saturn’s magnetic field with water ions.

While each planet is different, these similarities of process even grant insight into auroras on Earth.

In the case of Earth, the ion responsible is a proton, which comes from the hydrogen atom. While not energetic enough to create x-rays, the basic process is the same. Jupiter’s x-ray aurora is also fundamentally an ion aurora, just at much higher energy than the proton aurora here on Earth.

According to Dunn, these EMIC waves could possibly have a role to play in transferring energy across the cosmos.

Sam Cox is a journalist at Silicon Republic covering sci-tech news

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