red sprites april 4 2018
© Martin Popek
Red sprites over western Czeckia on April 4, 2018
Two red sprites, transient optical phenomena appearing as luminous reddish-orange flashes, have been recorded over the Czech Republic on April 4, 2018, marking the early beginning of 2018 sprite season in the northern hemisphere.

Lucky photographer this season was again well known Martin Popek who captured them above storms over western Czeckia on April 4 below Pleiades star cluster:

Because sprites are associated with thunderstorms, they tend to occur in late spring and summer. However, western and central Europe recorded more than 100 000 lightning strikes over the past 48 hours. It was enough to produce sprites and allow us to photograph them.

"Sprites are a true space weather phenomenon," lightning scientist Oscar van der Velde of the Technical University of Catalonia in Spain explained. "They develop in mid-air around 80 km [50 miles] altitude, growing in both directions, first down, then up. This happens when a fierce lightning bolt draws lots of charge from a cloud near Earth's surface. Electric fields [shoot] to the top of Earth's atmosphere - and the result is a sprite. The entire process takes about 20 milliseconds."

Sprites have been observed since the 1800s, though rarely visible from the ground. Aircraft pilots began reporting sightings of sprites in the 1950s and '60s, but they were not formally identified until 1989 when the Space Shuttle (STS-34) recorded the flashes as it passed over a thunderstorm in northern Australia.

Sprites and other phenomena, including elves - that bring a millisecond flash of light that fills the entire night sky within a 100 kilometer (62 miles) radius of the associated lightning strike - are generating much interest because of their strong electric fields and electromagnetic pulses that may interact with the Earth's ionosphere and magnetosphere.

Some studies has shown that some lightning strikes might be associated with terrestrial gamma-ray flashes (TGFs), intense explosions of gamma rays lasting only about one millisecond that are emitted into space from the upper atmosphere. Scientists believe electrons traveling at nearly the speed of light scatter off atoms and decelerate just above thunderclouds, emitting TGFs.
A group of scientists, along with help from Japan's NHK television, sought sprites regularly for two weeks in the summer of 2011 and captured it on film.

Video credit: NHK.

Sprite lightning in slow motion

Video credit: H. H. C. Stenbaek-Nielsen (U. Alaska, Fairbanks), DARPA, NSF.