Ever since NASA’s Voyager 1 flew past Jupiter in 1979, planetary researchers have wondered about the origin of Jovian lightning. That encounter confirmed the existence of the lightning, which had been theorized for years. But when the venerable explorer hurtled by, the data showed that the lightning-associated radio signals didn’t match the details of the radio signals produced by Earth’s lightning. In two papers published in the journals Nature and Nature Astronomy, researchers from NASA’s Juno mission describe the ways in which Jupiter’s lightning is actually analogous to Earth’s lightning.
This artist’s concept of lightning distribution in Jupiter’s northern hemisphere incorporates a JunoCam image with artistic embellishments. Data from NASA’s Juno mission indicates that most of the lightning activity on Jupiter is near its poles. Image credit: NASA / JPL-Caltech / SwRI / JunoCam.
“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, Juno scientist at NASA’s Jet Propulsion Laboratory and lead author of the Nature paper.
“But until Juno, all the lightning signals recorded by several NASA spacecraft 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.”
Enter NASA’s Juno orbiter. Among its suite of instruments is the Microwave Radiometer Instrument (MWR), which records emissions from Jupiter across a wide spectrum of frequencies.
“In the data from our first eight flybys, Juno’s MWR detected 377 lightning discharges,” Brown said.
“They were recorded in the megahertz as well as gigahertz range, which is what you can find with terrestrial lightning emissions. 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.”
“While the revelation showed how Jupiter lightning is similar to Earth’s, the paper also notes that where these lightning bolts flash on each planet is actually quite different.
“Jupiter lightning distribution is inside out relative to Earth. 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.”
Why do lightning bolts congregate near the equator on Earth and near the poles on Jupiter? Follow the heat.
Earth’s derives the vast majority of its heat externally from solar radiation. Because our equator bears the brunt of this sunshine, warm moist air rises (through convection) more freely there, which fuels towering thunderstorms that produce lightning.
Jupiter’s orbit is five times farther from the Sun than Earth’s orbit, which means that the giant planet receives 25 times less sunlight than Earth. But even though the gas giant’s atmosphere derives the majority of its heat from within the planet itself, this doesn’t render the Sun’s rays irrelevant.
They do provide some warmth, heating up Jupiter’s equator more than the poles — just as they heat up Earth.
Scientists believe that this heating at Jupiter’s equator is just enough to create stability in the upper atmosphere, inhibiting the rise of warm air from within. The poles, which do not have this upper-level warmth and therefore no atmospheric stability, allow warm gases from Jupiter’s interior to rise, driving convection and therefore creating the ingredients for lightning.
“These findings could help to improve our understanding of the composition, circulation and energy flows on Jupiter. But another question looms. Even though we see lightning near both poles, why is it mostly recorded at Jupiter’s north pole?” Brown said.
In the Nature Astronomy paper, a research team led by Dr. Ivana Kolmašová of the Czech Academy of Sciences presents the largest database of lightning-generated ‘whistlers’ (low-frequency radio emissions) around Jupiter to date.
The data set of more than 1,600 signals, collected by Juno’s Waves instrument, is almost 10 times the number recorded by Voyager 1.
Juno detected peak rates of four lightning strikes per second (similar to the rates observed in thunderstorms on Earth) which is six times higher than the peak values detected by Voyager 1.
“These discoveries could only happen with Juno,” said Juno principal investigator Dr. Scott Bolton, a researcher at the Southwest Research Institute and co-author of both papers.
“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.”
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Shannon Brown et al. 2018. Prevalent lightning sferics at 600 megahertz near Jupiter’s poles. Nature 558: 87-90; doi: 10.1038/s41586-018-0156-5
Ivana Kolmašová et al. Discovery of rapid whistlers close to Jupiter implying lightning rates similar to those on Earth. Nature Astronomy, published online June 6, 2018; doi: 10.1038/s41550-018-0442-z
This article is based on text provided by the National Aeronautics and Space Administration.
