CHICAGO — More than electricity can illuminate a thundercloud.

Brilliant bursts of gamma radiation, known as dark lightning or terrestrial gamma-ray flashes, also explode in lightning storms. And on rare occasions, those powerful blasts — the most energetic radiation to naturally arise on Earth — might even , researchers reported December 13 at the American Geophysical Union meeting. The zap could briefly expose passengers to unsafe levels of radiation. First , dark lightning is estimated to flash . But scientists have only a hazy understanding of how it initiates. They generally agree dark lightning is sparked by the electric fields generated by thunderstorms and lightning bolts. These fields can spur electrons to velocities approaching the speed of light, amassing breakneck electron avalanches. When the streaming particles smash into airborne atoms, gamma radiation is unleashed. Dark lightning often occurs around 10 to 15 kilometers high in the sky, altitudes frequented by airlines. The new analysis combines dark lightning observations and airline routes to suggest that dark lightning might strike near a plane around once every 1 to 4 years, atmospheric scientist Mélody Pallu said at the meeting. However, that’s probably “an upper limit of the real probability,” or even 10 times the actual rate, she said, largely because the calculations didn’t factor in pilots’ avoidance of thunderstorms. Previous computer simulations have revealed that passengers flying within 200 meters of a strong terrestrial gamma-ray flash’s initiation point may become exposed to radiation doses , said Pallu, now at the Astroparticle and Cosmology Laboratory in Paris. Such levels would surpass the occupational safety level of per year put forth by the International Commission on Radiological Protection. Though somewhat nebulous, the findings make one thing clear: Further investigations are needed to figure out how dark lightning impacts passengers flying through the sky.