"Understanding the mechanisms behind sprites is becoming increasingly important because these sprites affect the upper layers of the atmosphere up to the near-Earth space environment," Füllekrug told environmentalresearchweb. "As more and more business goes into space it is important to find out how these electrical discharges and accelerated particles will affect equipment out in space."

So on the evening of 29 August 2012 at a site in the south of France, when Füllekrug and his colleagues observed a huge thunderstorm and a very large sprite they were quite excited. The sprite was so large that it was even picked up by a quasi-static current sensor in south-west England.

"We knew we had seen something important, but we also knew that we did not have enough evidence to work out the mechanism behind the sprite or to write it up into a good research paper," said Füllekrug. But as he started documenting his observations, he was contacted by researchers from all over the world who wanted to help piece together the evidence. "It was amazing," said Füllekrug. "I had contact with researchers in Hungary, the UK, the Czech Republic and France, who had all detected the sprite and wanted to help. I even had contact with some amateur astronomers in Italy who, while looking for meteor showers, had also seen it."

Using this new data, Füllekrug was able to conclude that two different mechanisms were involved in this sprite: the first positive lightning discharge initiates a sprite where electrons are accelerated during the exponential growth and branching of the sprite streamers. This preconditioned plasma above the thundercloud is then exposed to a second positive lightning discharge associated with a bouncing-wave discharge, which causes a re-brightening of the existing sprite streamers above the thundercloud and initiates a subsequent relativistic electron beam, as the team writes in Environmental Research Letters (ERL) as part of the ERL Focus on High Energy Particles and Atmospheric Processes.

"These observations do not fit any currently known lightning signatures," said Füllekrug. "We saw two effects in close succession, which would indicate that the two effects may be connected."

Füllekrug hopes that this work will help inform two space missions that are due to be launched in 2015 – the French TARANIS mission that aims to analyse radiation from lightning and sprites; and the European Space Agency's Atmosphere-Space Interactions Monitor project, which will use instruments on the International Space Station to measure high-altitude lightning discharged from thunderclouds.

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