SEATTLE — Forget about gamma rays from the hearts of distant galaxies. Scientists now believe gamma rays, as well as beams of energetic particles of antimatter, are common components of lightning storms right here on Earth.
In 2009, researchers announced that NASA’s Fermi Gamma-ray Space Telescope had, for the first time, detected gamma rays produced by antimatter generated in terrestrial lightning storms (SN: 12/5/09, p. 9).
Now, after analyzing additional gamma-ray signals produced by terrestrial positrons — the antimatter counterpart to electrons — Michael S. Briggs of the University of Alabama in Huntsville and his colleagues think that the antimatter beams do not require special conditions to be generated. Briggs presented the latest findings during a news briefing January 10 at the winter meeting of the American Astronomical Society. Details will also appear in an upcoming Geophysical Research Letters.
“The idea that any planet has thunderstorms that not only produce antimatter but then launch it into space seems like something straight out of science fiction,” commented Steven Cummer of Duke University in Durham, N.C., who was not part of the study. “That our own planet does this, and has probably done it for hundreds of millions of years, and that we’ve only just learned it, is amazing to me.”
According to Briggs, positrons and electrons form in terrestrial gamma-ray flashes, short bursts of gamma rays produced inside thunderstorms. (First observed in the 1990s, gamma-ray flashes are still not well understood.) When the positrons meet up with the electrons, they annihilate each other, producing gamma rays of a specific energy: 511,000 electron volts.
The Fermi observatory has usually been located directly above thunderstorms when it has detected positron-generated gamma rays, but in four cases the lightning storms were thousands of kilometers away from the region on Earth that the telescope was observing.
In one striking event on December 14, 2009, Fermi was orbiting over Egypt when the only active storm was in Zambia, some 4,500 kilometers to the south. Because the storm was not in Fermi’s line of sight, the craft could not have detected gamma rays that came directly from the terrestrial disturbance. Yet Fermi did record gamma rays characteristic of annihilation between electrons and positrons that lasted for 30 milliseconds, the longest it has ever recorded such terrestrial signals.
Briggs’ team suggests that electrons and positrons produced in the Zambia storm surfed along Earth’s magnetic field to strike the Fermi craft. Many of the positrons met up with electrons in the spacecraft and the two annihilated each other immediately, producing the telltale gamma rays. But some of the positrons continued on past Fermi and were magnetically reflected back toward the craft 23 milliseconds later, only then pairing off with electrons in the craft to produce the gamma rays.
The newly discovered antimatter beams “gives us some very important information that we can use to piece together what is going on when lightning initiates and propagates,” Cummer said.
