Besides lightning, thunderstorms can produce strange lights, whose unexplained nature led meteorologists to give them names such as Sprites and Elves. Satellites have revealed even higher-powered events at wavelengths far too short for the human eye to see, but it has been debated if the two are connected. For the first time, these bursts of energy have been studied with detectors covering a broad expanse of the electromagnetic spectrum to answer that question.
Astronomers expend great effort to uncover the sources of gamma-ray bursts halfway across the universe, but we have some much closer to home. Known as terrestrial gamma-ray flashes (TGFs), these bursts of exceptionally powerful (that is short-wavelength) photons last less than a few milliseconds and have been detected by satellites passing over thunderstorms.
Atmospheric scientists have agreed for some time that TGFs result from electrons being accelerated in thunderstorms' electric fields; fields so powerful the charges travel close enough to light speed for the strange consequences of special relativity to take hold. Whether the electrons are released by the heat at the tip of a lightning bolt or produced by cosmic rays hitting the atmosphere has remained controversial.
TGFs are hard to predict, and therefore to study, but on October 10, 2018 one was seen above East Sulawesi, Indonesia, and scientists were ready. The International Space Station was directly overhead carrying X-ray and gamma-ray detectors trained downwards, along with three ultraviolet and optical photometers. No TGF has ever been observed across such a spectrum.
Along with the TGF, all this equipment captured the fast-building ultraviolet radiation and expanding ring of red known as an Elve.
A paper in Science analyzing the observations attributes the red glow to emissions from atomic oxygen, caused by the current in a lightning leader channel, where ionized gas precedes the actual lightning stroke. The lightning strike itself produced light at a wavelength just too short for human vision, but easily visible to many animals. Although Elves are fairly common, studying something so short-lived that can only be seen from space or high in the atmosphere has been a challenge.
The paper attributes both sets of observations to the leader of a lightning bolt jumping within the cloud, rather than coming to ground. The paper argues only a particularly swift and fast-changing, but very powerful, lightning source current could produce the specific forms of radiation observed here.
Although this work establishes that some TGFs and Elves are connected through a common origin, many more observations will be required to show if this is always the case, or if the phenomena sometimes occur independently.