The Moons Of Jupiter Are Doing Something Really Weird To Its Aurora

A view of Jupiter's aurora, taken by Hubble in 1998. NASA et al

Scientists have discovered that the aurorae of Jupiter are shaped by some of its moons in highly unusual ways – and this may tell us more about the moons themselves.

Published in the journal Science, researchers led by Alessandro Mura from the Institute for Space Astrophysics and Planetology in Italy used images from NASA’s Juno spacecraft to study the gas giant. Juno has been busy flying close to the planet over its poles for the past two years.

We’d previously known that the moons Io and Ganymede could create a “footprint” on the planet’s aurora, where they interact with the particles that create it. But looking closely at these footprints (regions where the aurora suddenly brightens) with Juno's Jovian InfraRed Auroral Mapper (JIRAM) instrument, things get wild and wonderful.

“When we got very close with Juno, what we found is that have an intrinsic structure,” Mura told IFLScience. “It’s like an aurora inside an aurora.”

This is the split double tail footprint of Io as it passes over Jupiter's aurora. Mura et al

As the moons orbit the gas giant, they are constantly interacting with its magnetic field. As we know from Earth, this magnetic field drives the aurora, channeling high-energy electrons to the poles, where they collide with atoms in the atmosphere and produce those pretty auroral patterns and lights.

But on Earth, our Moon plays little to no role in what our aurora looks like. Jupiter’s moons both directly affect the aurora, Io because of its numerous volcanic eruptions, and Ganymede because of its magnetic field – the only moon in the Solar System known to have one.

The researchers found that the moons cause the aurora of Jupiter to brighten in places. Io, in particular, causes a pattern similar to a von Kármán vortex street. This is a system of swirling vortices, with the particles seeming to clump together in places.

Extending away from these bright spots for thousands of kilometers are long tails, described as a sort of “shadow” of the moons. The exact processes behind this aren’t clear, but this study has shown for the first time that two tails sometimes appear.

A representation of Io's motion through the aurora seen by Juno (inset) on a Hubble image (main). NASA/Mura/et al

Io’s magnetic field casts several “shadows” in the aurora of Jupiter, while Ganymede casts a double shadow. Interestingly the bright spots can appear pretty large, up to a few hundred kilometers across, which may give a clue to their formation, namely that the moons are playing a part.

“This is not a coincidence,” said Mura. “It’s probably related to the interaction itself.”

Another interesting consequence of this is that we can actually study the moons from afar, such as probing the strength of Ganymede’s magnetosphere. And there may be more moons taking part in this auroral effect too, such as Callisto, although that’s not known for sure yet.

Juno is scheduled to continue orbiting Jupiter until 2021 at the earliest, so the researchers will continue to monitor the aurora, and try and see exactly what’s going on.

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