Most gamma rays come from black holes and other galactic phenomenon, but there's a mysterious source that we are just figuring out: Earth-based thunderstorms.
Researchers think these radioactive waves are coming from a newly discovered phenomena called "dark lightning."
The Fermi space telescope was searching for space-based radiation when it was struck by some jettisoned from Earth:
Scientists think these gamma ray bursts are "dark lighting" that had been guided to the craft by the Earth's magnetic fields. Scientists think that dark lighting exists for the same reason that light lightning does — to equalize the negative and positive charges that build up in storm clouds and on the Earth.
Lightning does this by using slow electrons to ferry current between the ground and the clouds, or between differentially charged areas within a thundercloud. These particles light up because of the current they carry.
Dark lightning, on the other hand, uses higher energy charged particles. When those electrically charged particles in the thundercloud interact with molecules of air, they spit out gamma rays. These gamma rays make electrons and their anti-matter twins positrons. These particles keep hitting more molecules of air and spitting out more and more gamma rays, similar to how nuclear fission creates a nuclear explosion.
In the diagram below the charged particle is the little yellow and the molecule in the air is the blue ball. They come in contact, shooting out gamma rays — the pink thing:
The particles spread out and up after they dissipate the charge differential in the cloud in a quick burst:
These radiation bursts are similar to regular lighting, but because they are made of gamma waves instead of regular light waves, they are mostly invisible. They have been detected everywhere on Earth that thunderstorms happen:
Sometimes packets of the gamma rays are bent by the magnetic fields of the Earth, bundle-ing them together.
One of these bundles, traveling along a magnetic field line, is what hit the Fermi telescope. These bursts can also hit planes, possibly bombarding the passengers with radiation, but probably not to a harmful degree.