The spacecraft was located immediately above a thunderstorm for most of the observed TGFs (terrestrial gamma-ray flash), but in four cases, storms were far from Fermi. In addition, lightning-generated radio signals detected by a global monitoring network indicated the only lightning at the time was hundreds or more miles away. During one TGF, which occurred on Dec. 14, 2009, Fermi was located over Egypt. But the active storm was in Zambia, some 2,800 miles to the south. The distant storm was below Fermi's horizon, so any gamma rays it produced could not have been detected.
"Even
though Fermi couldn't see the storm, the spacecraft nevertheless was
magnetically connected to it," said Joseph Dwyer at the Florida
Institute of Technology in Melbourne, Fla. "The TGF produced high-speed
electrons and positrons, which then rode up Earth's magnetic field to
strike the spacecraft."
Source: http://science.nasa.gov/science-news/science-at-nasa/2011/11jan_antimatter