High-energy gamma rays have long been known to emanate from nuclear processes, ranging from terrestrial nuclear power plants to violent explosions in space. Researchers have now detected gamma rays that seem to emanate from thunderstorms.
The findings, they say, offer fascinating clues to how lightning is generated. Ironically, even though millions of lightning bolts strike the Earth each year, physicists still do not understand the mechanism underlying this basic natural phenomenon.
Duke engineer Steven Cummer analyzed the relationship between lightning events and gamma-ray emissions discovered by other scientists using satellite data. His team studied radio waves emanating from thunderstorms in the Caribbean to understand the correlation with gamma rays from thunderstorms there. Counterintuitively, the strong gamma outbursts also seem to precede associated lightning discharges by a split second, they found.
"All of this comes as a huge surprise," says Cummer, whose findings were published in the journal Geophysical Research Letters. "These are higher-energy gamma rays than come from the sun. And yet here they are coming from the kind of terrestrial thunderstorm that we see here all the time."
Their study will help scientists explore one popular theory that lightning is triggered by a process called "runaway breakdown," which occurs when extraterrestrial cosmic rays collide with atoms in the atmosphere, generating a few very high-energy electrons. A sufficiently strong electric field can further accelerate these electrons, causing additional collisions, producing more high- energy electrons until "the whole process avalanches," says Cummer.
Such an electron avalanche in the electrical field immediately after a strong lightning discharge could create a high-energy electron beam at altitudes of thirty to fifty kilometers, according to the hypothesis. That beam would then produce gamma rays as it interacted with the atmosphere.