Fermi's observations of terrestrial gamma rays have deepened the mystery. At least one of the flashes contain an unmistakable pattern of positrons -- the antimatter counterpart of electrons.
"It was a surprise, and now we have to explain it," said Fishman, who is working on a paper about the discovery with colleague Michael Briggs at the University of Alabama in Huntsville.
Scientists believe strong electric fields can trigger an avalanche of high-energy electrons that interact with the atmosphere, a sort of naturally occurring particle accelerator.
The electron avalanche may be what triggers lightning, as well as produce gamma radiation, Fishman said.
Lightning can generate a huge amount of power in a very short time, added University of Washington physicist Robert Holzworth, who oversees the lightning tracking project used by the Fermi scientists.
"Not many natural phenomena are as powerful as lightning," Holzworth said. "It's very easy for me to imagine enough energy to come up with an electron-positron pair."
