Jupiter has clouds, jet streams, and gigantic hurricanes, so it's not surprising that it has also lightning. But when Voyager 1 visited the gas giant in 1979, astronomers discovered something peculiar. The Jovian lightning strikes emitted radio waves in a way no Earthly counterpart does. But now, thanks to observations from the Juno mission, we understand what’s going on. The findings are reported in Nature.
Thanks to Juno’s incredible suite of instruments, it became clear that there was nothing weird going on. Instead, the previous observations were just quite limited. Using data from the first eight flybys, the team recorded 377 lighting discharges with radio waves both in the megahertz and gigahertz ranges, as expected.
"No matter what planet you're on, lightning bolts act like radio transmitters – sending out radio waves when they flash across a sky," lead author Shannon Brown of NASA's Jet Propulsion Laboratory said in a statement. "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."
But not all is similar between our planet and the gas giant. The distribution of lightning on Jupiter was different from what’s observed on Earth, leading the researchers to wonder what’s going on in the Jovian atmosphere to make it happen.
"Jupiter lightning distribution is inside out relative to Earth," added Brown. "There is a lot of activity near Jupiter's poles but none near the equator. You can ask anybody who lives in the tropics – this doesn't hold true for our planet."
The key to this is heat. Earth receives most of its heat from the Sun, so between the tropics, there are more convection movements (hot air rising) generating the right conditions for lightning. It’s not surprising that the place on Earth that experiences 1.2 million lightning strikes per year is in this region.
Jupiter is five times further away from the Sun than Earth so our Star actually has an opposite effect. Jupiter’s interior is quite hot and the sunrays on the equator warm the top clouds enough to disrupt convection. No such thing happens towards the poles, and that’s why lightning strikes are more common at higher latitudes.
What's more, another paper on Jupiter’s lightning was published in Nature Astronomy earlier this year. In this study, researchers presented the largest database of lightning-generated low-frequency radio emissions comprising 1,600 detections, with a peak rate of four lightning strikes per second similar to the peak rate in thunderstorms on Earth.