Scientists at Denmark’s National Space Institute have published a paper looking at strange blue flashes occurring above the surface of Earth.

The strange phenomenon has often been reported by pilots, but its existence has been debated. So in 2015, ESA astronaut Andreas Mogensen snapped footage of thunderstorms from the International Space Station (ISS) to try and look for it.

The paper, published in Geophysical Research Letters, confirms that about 18 kilometers (11 miles) above Earth, blue flashes stretching more than 1 kilometer (0.6 miles) appeared. One particular jet even reached a height of 40 kilometers (25 miles). Morgensen filmed the events over the Bay of Bengal in the Indian Ocean.

“It is not every day that you get to capture a new weather phenomenon on film, so I am very pleased with the result,” Mogensen said in a statement. “But even more so that researchers will be able to investigate these intriguing thunderstorms in more detail soon.”

According to GeekWire, Mogensen here is referring to a new experiment that will be installed on the Columbus module on the ISS this year, called the Atmosphere-Space Interactions Monitor (ASIM). This will look for these so-called "transient luminous events" (TLEs) in more detail.

How exactly these flashes form isn’t clear at the moment. They’re linked to a similar phenomenon called red sprites, caused by a thunderstorm discharging into space rather than down to the ground.

Most of our research on them so far though has come from satellites, which are rather limited in their view. Now that we know they can be studied from the ISS, it might open the door to understanding this strange atmospheric phenomenon in more detail in the future.