We Can't Explain What's Causing Jupiter's Mysterious Aurora

Jupiter's southern aurora, as seen by Juno. G. Randy Gladstone

Scientists have discovered a mysterious new type of aurora at Jupiter, which doesn’t seem to fit any of our existing models. It hints that something unknown is taking place on the planet.

That revelation is laid out in a rather interesting – but equally technical – paper today in Nature. Led by Barry Mauk from the Johns Hopkins University Applied Physics Laboratory in Maryland, the authors used data from NASA’s Juno spacecraft, currently in orbit around Jupiter, to study the gas giant’s aurorae.

What they found is confusing. Jupiter’s brightest aurora appears to be powered by a process that we’ve never seen before. This aurora can have a power of about 1,000 milliwatts per meter squared, compared to just 10 or 20 milliwatts per meters squared for Earth’s aurora.

“There is definitely something going on we don’t understand,” Mauk told IFLScience. “We’re seeing something brand new here.”

So what’s going on? Let’s explain. On Earth, we have three types of aurora. The brightest – responsible for the iconic undulating auroral patterns – are called discrete. Then there’s a much dimmer type called broadband or Alfvénic, which you can’t see with the naked eye, and an even dimmer one still called diffuse.

A reconstructed view of Jupiter's northern lights as seen by Juno. NASA/JPL-Caltech/Bertrand Bonfond

The brightest, the discrete aurorae, are caused by a downward flow of electrons in our magnetic field resulting from the solar wind. This causes electric potentials on the order of several thousand volts in our atmosphere. Diffuse and broadband, meanwhile, are much weaker and result from the turbulent scattering of magnetically trapped electrons.

At Jupiter, things are different. The electric potentials at Jupiter’s poles are 10 to 30 times larger than seen on Earth, up to 400,000 volts. But its brightest aurora cannot be explained by the discrete process, responsible for our own bright aurora.

“Instead, on Jupiter the brightest aurora are caused by some sort of turbulent acceleration process that we don’t understand very well,” said Mauk.

This brighter aurora is seen in addition to the other three we see on Earth, but those are in a different order. On Jupiter, the diffuse is the dimmest, followed by discrete, and then broadband aurora.

A close-up (false color on the left) of Jupiter's aurora, taken by Juno. G. Randy Gladstone/Bertrand Bonfond

We mentioned earlier that at Earth, the power source of our aurorae is the solar wind, which blows over our magnetic field and acts like a giant generator. At Jupiter, however, the power source is the spin of the planet itself. All of the power comes from its rotation.

The aurorae at Jupiter’s poles are basically a signature of its attempt to spin its magnetosphere. The planet is trying to shed infinitesimal amounts of its angular momentum, powering the aurorae. This was thought to be how the planet generated its own northern and southern lights.

But the discovery of this new aurora kind of throws our models out the window. The rotation of Jupiter, acting like a generator, cannot explain how it forms. Something else must be going on. What that is, we don’t know yet.

This could have implications for studying stars and planets outside the Solar System. If there’s some completely new type of process taking place, it may suggest some intricate workings in planetary or stellar magnetic fields that we simply haven’t encountered before.

Most of the data in this paper came from Juno’s first orbits of Jupiter. The other day the spacecraft completed its eighth orbit, so scientists will be using more data to try and get to the bottom of the mysterious aurora. We already knew Jupiter was pretty weird. Now, it just got weirder.

A Hubble image of Jupiter's aurora from last year. NASA/ESA/J. Nichols (University of Leicester)
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