For the first time, scientists have observed the high-energy auroras pulsing on both of Jupiter's poles at the same time. They discovered that, unlike the northern and southern lights here on Earth, these two auroras on Jupiter behave nothing alike.
By studying rare observations of the gas giant’s polar lights, scientists helmed by William Dunn and Andrew Coates at University College London found that the northern and southern auroras brightened and faded completely independently from one another. The surprising finding, described today in the journal Nature Astronomy, is a step toward understanding what exactly is behind Jupiter’s auroras that shimmer with invisible X-rays at the poles.
Auroras are the signatures of a planet’s magnetic field, which are thought to be key for life because they protect a planet’s atmosphere from the scouring winds blowing off its nearby star. Magnetic fields show up on pulsars, exoplanets, and brown dwarfs, but Jupiter is close to home. So, by studying Jupiter’s auroras and magnetic fields, we can better understand what’s happening on far-away worlds.
“If we’re going to search other planets for other life, then we’re going to want to find places that have magnetic fields,” Dunn says. “Understanding in our Solar System what the signatures for northern lights are and what they mean is important, because hopefully at some point in the future, we’ll be looking at these signatures at extra-solar planets.”
Here on Earth, auroras are best known as the glowing bands of green or reddish light that appear when electrically charged particles ejected from the Sun rain down on our planet along the Earth’s magnetic field lines. The charged particles accelerate and smash into gas molecules in the atmosphere, producing light. But there are also auroras we can’t see made out of ultraviolet light, infrared light, or high-energy X-rays. X-ray auroras can occur weakly on Earth, Coates says. But they’re especially strong on Jupiter. “Jupiter is just a completely different beast,” Dunn says.
On Jupiter, charged particles are thought to blow in from the Sun, as well as from Jupiter’s tiny volcanic moon Io. Highly charged molecules of sulfur, oxygen, and carbon align along the planet’s magnetic field lines like iron filings around a magnet. Jupiter’s superfast rotation then drives the acceleration of these particles, which hit the atmosphere with tens of megavolts of energy. The particles strip away electrons already in the atmosphere, and release high-energy X-rays in the process. “Everything is happening in a supercharged way,” Coates says.
Jupiter’s orientation means that the X-ray auroras on its southern pole are difficult to see. But for about 12 hours each on May 24th, 2007 and June 1st, 2016, the Chandra X-ray Observatory and XMM-Newton space telescopes orbiting Earth were in precisely the right positions to observe both poles simultaneously.
The rare view revealed that auroras on both poles behave differently: one didn’t always brighten when the other did. That’s surprising, says Jonathan Nichols, an astrophysicist at the University of Leicester who was not involved in the study. Since magnetic field lines create a continuous arc between the poles, “You might imagine that what affects the auroras in the north would affect the auroras in the south,” Nichols says. At least, that’s generally what happens on Earth.
Even stranger, while the southern aurora pulsed rhythmically every nine to 12 minutes (it had previously been seen to pulse regularly every 40 to 45 minutes), the northern aurora was more erratic. Sometimes, it brightened every five to eight minutes, but other times it was more irregular. The brightness of the two auroras also differed, and varied from one pulse to the next.
It’s not exactly clear why the auroras are behaving this way, but Dunn has some ideas about what could cause the rhythmic pulsing of the southern X-ray aurora. The solar winds could be creating waves along Jupiter’s field lines, causing the charged particles surfing along those waves to reach the poles at intervals of, say, every 11 minutes, Dunn says.
NASA’s Juno spacecraft, which has been orbiting the gas giant since July 2016, could soon provide some answers. The probe is equipped with instruments to detect magnetic fields and charged particles, so it could tell us more about what’s going on at the poles. “It provides us with a juicy problem to try and solve,” Nichols says.
An important one at that: understanding what’s happening on Jupiter is key to figuring out what’s happening on planets beyond our Solar System, Nichols says. “Jupiter acts as an analogy for exoplanets, for brown dwarfs, for pulsars — a whole variety of astrophysical objects that we can’t get to,” he says. “So if we can understand Jupiter’s magnetic field then we can understand these whole different classes of objects.”