Of all the weather phenomena our beautiful planet throws at us, lightning is one of the most spectacular – and the most mysterious. Although storms are frequent, we still make an effort to understand and describe their crackling electrical discharges generated in the air.
One kind of lightning is so strange and rare that we didn’t even have concrete evidence that it existed until 1990, when researchers identified its distinctive ‘rocket-like’ motion in video footage of NASA’s Space Shuttle from the previous year.
The streaks, later called ‘blue jets’, are now recognized as brilliant flashes of light lasting only a few hundred milliseconds as lightning shoots up from the clouds and into the stratosphere.
A blue jet photographed in Hawai’i. (Gemini Observatory / AURA / Wikimedia Commons)
We can’t easily see this phenomenon under a curtain of clouds – but that doesn’t mean scientists can’t observe it from above. About 400 kilometers (250 miles) above the planet orbits the International Space Station, and for some time, onboard instruments have been watching these mysterious flashes of upside-down lightning.
Now, after being installed in 2018, a European Space Station observatory equipped with optical sensors, photometers, and gamma and X-ray detectors has registered five blue flashes from the top of a storm cloud, one of which ended with a blue beam stripe. high in the stratosphere.
These rare glimpses provide valuable insight into the beginnings of the mysterious discharges, according to a team of researchers led by physicist Torsten Neubert of the Technical University of Denmark.
Blue jets are believed to start when a positively charged cloud cap encounters a layer of negative charge at the cloud boundary and the sky layer above it. This is believed to cause an electrical fault that forms a conductor – an invisible conductive channel of ionized air along which lightning travels.
However, our understanding of the blue fighter jet leader is quite limited. This is where the data analyzed by Neubert and his team fills in gaps.
On February 26, 2019, the Atmosphere-Space Interactions Monitor (ASIM) observatory recorded five blue flashes, each about 10 microseconds long, at the top of a storm cloud not far from the Pacific Island of Nauru.
One of these flashes produced a blue beam that reached as far as the stratopause – the interface between the stratosphere and the ionosphere, at an altitude of about 50 to 55 kilometers (about 30 to 34 miles).
In addition, the observatory recorded atmospheric phenomena called ELVES (short for Emission of Light and Very Low Frequency perturbations to Electromagnetic Pulse Sources). These are expanding rings of optical and ultraviolet radiation in the ionosphere that appear above thunderclouds and last only a millisecond, as illustrated in the animation below.
They are believed to be generated by an electromagnetic pulse at the bottom of the ionosphere caused by lightning strikes.
However, the leader’s red emission was weak and very limited. This, the research team said, suggests that the leader itself is very small and localized, compared to fully developed lightning conductors between the ground and the clouds.
This also suggests that the flashes and the blue beam are themselves some kind of discharge beams: branched, twisting sparks emanating from high voltage sources, such as Tesla coils, in a chain reaction of ionizing air particles.
“So we propose that the UV pulses are elves generated by the streamer’s flash currents, rather than lightning currents,” the researchers write in their paper.
The flashes, they think, are similar to narrow bipolar events. These are high power radio frequency discharges that occur in clouds during thunderstorms, and are known to cause lightning in the cloud. The blue flashes in the cloud tops, the team said, are likely the optical equivalent of this phenomenon and could develop into blue jets.
Since narrow bipolar events are quite common, this could mean that the blue flashes are also more common than we thought. Knowing more about how common they occur can help us gain a much better understanding of storms and lightning, not to mention our atmosphere and all the complex interactions in it.
The team’s research is published in Nature.