The team also photographed mysterious 'blue jets' of light, from research aeroplanes flying over the midwest of America. These appear to be closely associated with the vast red jellyfish- like flashes, first observed high above thunderstorms by the same team last year.
The red flashes are nicknamed 'sprites' because they appear and disappear so quickly. They survive for a few thousandths of a second, yet can extend to a height of 70 miles.
Gene Wescott, one of the Nasa researchers working at the University of Alaska, said the blue jet sightings were a complete surprise. 'When we first saw these things we just yelled 'wow' because we had never seen anything like it before.' His colleague, Davis Sentman, said: 'The flashes look like the fourth of July, like roman candles with fountains.'
The US space agency is concerned that such massive, apparently electrical events, could prove a hazard to military and research aircraft. The Nasa team may conduct a joint research effort with the US Air Force. Commercial aircraft usually fly too low to be affected.
Professor Wescott thinks the difference in colour between the jets and sprites may be due to the different atmospheric constituents at the heights at which they appear. The blue jets apparently originate from the centre of a storm, above the thunderclouds, and propagate upwards at about 300 times the speed of sound. Each flares upwards to a height of about 25 miles in a narrow cone.
The research aircraft flew at just over 40,000 feet, slightly above the cloud tops. 'We saw the first one on the evening of 1 July over Arkansas, then about 45 in a half hour period, and 6 sprites,' Professor Wescott said.
'One intense thunderstorm appeared to be generating all of these. Every night we flew we saw sprites, over every thunderstorm we looked at. But we only saw blue jets on that one evening.'
The team is more or less convinced that the sprites are caused by some form of electrical discharge, but the blue jets are more difficult to explain because they travel too slowly. The sprites travel at about half the speed of light, a thousand times faster than the blue jets. The jets also last longer, surviving for about one-tenth of a second.
At the moment explanations are guesswork, Professor Wescott admitted. 'We're working on it. They look for all the world like an injection of material, like a water fountain that someone is turning on and then off. One thought is that they could be hypersonic shock waves, but then they should spread out more.'
But the past year's work has provided some fresh clues on the nature of sprites. 'This year we had two aircraft instead of one, so could get a much better idea of the altitude of the sprites.'
Shuttle astronauts have reported seeing sprites in the past and Professor Wescott said they could be seen from the ground if observers knew what to look for. 'The pilots we have flown with are very skilled at spotting them, sometimes before us scientists.'