Just saw this news today. Guess that’s what you call an atmospheric disturbance.
from New Scientist:
Thunderstorms emit gamma rays, known as terrestrial gamma ray flashes (TGFs), although what causes them is still a mystery. While observing these flashes, Fermi also detected a separate set of gamma rays with an energy of 511 kiloelectronvolts. These rays were produced when a barrage of positrons struck the spacecraft’s detectors and were annihilated by making contact with electrons there.