
A sprite — a bright, brief, flash — is circled in yellow in this ultraviolet image of Jupiter's south pole from NASA's Juno spacecraft. The UV image also captures the auroral ring around the pole, as well as the magnetic "footprint" of the moon Io.
They can't be lightning bolts themselves, explains Giles. "Jupiter's atmosphere is very opaque to ultraviolet radiation, so the flashes must originate much higher" than the planet's ammonium-rich thunderclouds, she says — probably some 260 kilometers (160 miles) above the 1-bar pressure level in the atmosphere.
Each flash lasts for just a few milliseconds and looks like a point source to Juno, which means the source cannot be larger than 1,000 kilometers across or so. The flashes occur above turbulent regions in the atmosphere known to host thunderstorms.
In today's Journal of Geophysical Research: Planets, Giles and her colleagues argue that the flashes are probably similar to sprites and elves, labeling them transient luminous events (TLEs). These have incidentally been observed above terrestrial thunderclouds for decades but were only caught on camera for the first time in 1989. They are caused by electromagnetic disturbances, generated by energetic lightning displays. Unlike on Earth, though, Jovian TLEs are not easily visible at optical wavelengths, since they are produced by the excitation of molecular hydrogen (H2) instead of molecular nitrogen (N2).

The rare atmospheric phenomenon known as sprites appear red when captured above Earth.
However, one prediction hasn't yet been tested: Juno can't check whether the ultraviolet flashes occur simultaneously with optical lightning discharges, because its UV and visible-light instruments are mounted on different sides of the triangular, spinning spacecraft. This design results in a 10-second offset between the two types of measurements, says Giles, so the team needs a different approach.
"Our best hope is the Waves instrument, which studies radio and plasma waves," she says. "The very strongest discharges might possibly produce longer-lasting waves."