NASA's Juno spacecraft has detected brief, powerful flashes of ultraviolet light in the upper atmosphere of Jupiter. They are believed to be the Jovian counterparts of sprites and elves on Earth — electric phenomena in the atmosphere that can sometimes be seen high above powerful lightning discharges.
Sprites and Elves on Io
© NASA / JPL-Caltech / SwRI
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.
Scientists found the flashes in data from Juno's ultraviolet imaging spectrograph, Rohini Giles (Southwest Research Institute) told the virtual meeting of the Division of Planetary Sciences (DPS) on October 27th. Between August 2016 and July 2020, Juno saw a total of 11 UV flashes.

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).

Sprites
© Jesper Grønne / S&T Online Photo Gallery
The rare atmospheric phenomenon known as sprites appear red when captured above Earth.
Back in 2009, a team led by Yoav Yair (Open University of Israel) predicted what TLEs in the atmosphere of other planets would look like. According to Giles, the UV flashes observed by Juno meet all the expectations for Jovian sprites. Yair's coauthor Ute Ebert (Eindhoven University of Technology, the Netherlands) says it looks like "a beautiful confirmation" of the predictions. "We also want to study the role of planetary magnetic fields," she says. That might be easier if TLEs can be studied on different planets.

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."