Lightning at Jupiter is just like Earth’s. Except where it’s not

An artist’s impression of lightning bolts in the atmosphere if Jupiter. NASA’s Juno spacecraft has found jovian lightning produces the same sort of radio emissions as those on Earth, but they only seem to occur at high latitudes. Image: NASA/JPL-Caltech/SwRI/JunoCam

Lightning bolts in Jupiter’s turbulent atmosphere have long puzzled scientists because they did not appear to generate the same high-energy radio emissions that accompany discharges in Earth’s atmosphere. But the Juno spacecraft’s orbit around the giant planet carries it much closer than earlier probes and its Microwave Radiometer Instrument has, in fact, found megahertz and gigahertz emissions like those seen at Earth.

The Juno data also has shed light on why lightning tends to occur only at high latitudes on Jupiter while they are common in the equatorial tropics on Earth. The research was published 6 June in the journal Nature.

“No matter what planet you’re on, lightning bolts act like radio transmitters, sending out radio waves when they flash across a sky,” said Shannon Brown, a Juno researcher at NASA’s Jet Propulsion Laboratory in Pasadena, California.

“But until Juno, all the lightning signals recorded by spacecraft [Voyagers 1 and 2, Galileo, Cassini] were limited to either visual detections or from the kilohertz range of the radio spectrum, despite a search for signals in the megahertz range. Many theories were offered up to explain it, but no one theory could ever get traction as the answer.”

Juno braked into orbit around Jupiter in July 2016. During its first eight trips around the planet, its Microwave Radiometer Instrument detected 377 lightning blasts.

“They were recorded in the megahertz as well as gigahertz range, which is what you can find with terrestrial lightning emissions,” said Brown. “We think the reason we are the only ones who can see it is because Juno is flying closer to the lighting than ever before, and we are searching at a radio frequency that passes easily through Jupiter’s ionosphere.”

As for why lightning only seems to occur at high latitudes in Jupiter’s atmosphere, Brown said it all comes down to the planet’s average distance from the Sun, 779 million kilometres (484 million miles). At that distance, Jupiter receives 25 times less sunlight than Earth.

Even so, that level of heating still warms the equatorial atmosphere more than the poles, just enough to stabilise the upper altitudes and prevent the rise of warm air that otherwise would trigger convection and storm development. At the poles, that upper level warmth is not present, allowing warm air from the interior to rise, driving the convection that generates lightning.

In a separate paper published in Nature, a team of researchers led by Ivana Kolmašová of the Czech Academy of Sciences in Prague presented the largest database of low-frequency lightning-generated emissions collected to date. Juno’s Waves instrument detected more than 1,600 such “whistlers” with peak rates of four lightning strikes per second, similar to the rates seen on Earth.

“These discoveries could only happen with Juno,” said Scott Bolton, Juno principal investigator at the Southwest Research Institute, San Antonio, Texas. “Our unique orbit allows our spacecraft to fly closer to Jupiter than any other spacecraft in history, so the signal strength of what the planet is radiating out is a thousand times stronger. Also, our microwave and plasma wave instruments are state-of-the-art, allowing us to pick out even weak lightning signals from the cacophony of radio emissions from Jupiter. ”

But mysteries remain. Even though lightning has been detected near both of Jupiter’s poles, scientists do not yet know why discharges are more frequent at northern latitudes.