The Fermi Gamma-ray Burst Monitor (GBM) has detected gamma-rays with energies of 511,000 electron volts, a signal indicating an electron has met its antimatter counterpart, a positron. When antimatter striking Fermi collides with a particle of normal matter, both particles are immediately annihilated and transformed into gamma-rays.
The GBM has detected over 4000 of these so-called Terrestrial Gamma-ray Flashes (TGFs) since Fermi's launch in 2008. The observatory was located immediately above a thunderstorm for most (but not all) of the observed TGFs. TGFs produce a beam of high-speed electrons and positrons, which then ride up Earth's magnetic field to strike Fermi. This beam can continue past Fermi, reach a location known as a mirror point, where its motion is reversed, and then hit Fermi a second time just milliseconds later. Each time, positrons in the beam collide with electrons in the spacecraft. The particles annihilate each other, emitting gamma-rays detected by the GBM.
The detection of positrons shows that many high-energy particles are being ejected from the atmosphere. In fact, scientists now think that all TGFs emit electron/positron beams.