Actually, the "Compton" telescope in the depths of space should look for mysterious gamma-ray pulses. These events, which often take place far beyond our Milky Way, release unimaginable amounts of energy within seconds - more than our Sun produces billions of years. But the Nasa Observatory was also found in the vicinity in 1994: In thunderstorms in the earth's atmosphere, so could prove his instruments at that time, short flashes of radiation with energies of 30 to 40 megaelectronvolts.
Terrestrial gamma-ray bursts thus belong to the most energetic natural phenomena of the earth. But the exact procedures have not been clarified with certainty. Among other things, it was unclear whether the visible light of lightning or the gamma radiation would appear first. At the annual American Geophysical Union (AGU) meeting in San Francisco, researchers have now come up with new findings. They are based on measurements from an instrument of the European Space Agency (ESA) attached to the exterior of the International Space Station.
The device called Atmosphere-Space Interactions Monitor, or Asim, has been investigating thunderstorms worldwide since last summer. "The special thing about it is that we can look for visible light and gamma radiation for the first time, which opens up completely new possibilities for us," says Nikolai Østgaard from the University of Bergen, Norway. He leads the Asim science team. In an article in the journal "Journal of Geophysical Research: Atmospheres" he now reports together with colleagues on the measurements of the first ten months.
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During this time, the device analyzed 217 Terrestrial Gammablitze. Some of them could also be detected with the space telescope "Fermi". The events lasted about one ten-thousandth of a second each and played out in 10 to 15 kilometers altitude. The radiation is created according to the findings of the researchers in lightning, which run from bottom to top within a cloud.
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When Østgaard describes how lightning moves in the cloud, he clenches two fists and holds them at a slight distance above each other: One must imagine that like the poles a battery, below is the negative pole, above the positive pole. If the voltage difference becomes too strong, a conductive channel is formed in the air. When this channel reaches the other pole, the voltage can flow. A flash, as we know it, arises.
What he and his colleagues now know thanks to Asim: When a gamma pulse develops, it is immediately ahead of the visible flash. It arises when the electrons hit air molecules and stimulate them so strongly that they have to give off radiation to calm down again.
Elves dance over the thunderstorm
The question of the order is thus clarified for the first time: first Gammapuls, then visible lightning. And the researchers can announce yet another discovery: it has to do with so-called elves. These are extremely weak luminous phenomena of visible and ultraviolet light, which can occur above thunderstorms about 90 to 100 kilometers high in the earth's atmosphere. These have already been observed with a camera from the International Space Station.
A team around Torsten Neubert from the Technical University of Denmark in Kongens Lyngby reports in the journal "Science" now based on Asim data that Terrestrial Gammablitze can also ensure the appearance of such elves. The electromagnetic pulse of some flashes is therefore responsible for the fact that particles of the ionosphere are excited. The result is a glow that then spreads like waves on the surface of a lake into which a stone has been thrown.