Mjolnir is the mythical hammer of Thor, the Norse god of thunder. Mjolnir was a fearsome weapon said to be capable of leveling mountains and unleashing lightning. The Vikings wore amulets shaped like Mjolnir for protection, and more than a thousand have been found across Northern Europe. This constellation symbolizes Fermi's work investigating Terrestrial Gamma-ray Flashes (TGFs) associated with lightning from thunderstorms on Earth.

A thunderstorm is a powerful natural particle accelerator. It can generate streams of subatomic particles moving near the speed of light that result in a flash of gamma rays lasting about a thousandth of a second. TGFs were discovered in the 1990s by Fermi's predecessor, the Compton Gamma Ray Observatory, and Fermi has greatly expanded on its findings. Fermi's Gamma-ray Burst Monitor (GBM) detects about two TGFs every day as it orbits Earth.

When TGFs were first discovered, scientists thought the source must be high in the atmosphere, many kilometers above the cloud tops, because gamma rays are absorbed as they pass through air. Recent measurements show that TGFs originate deeper in the atmosphere, within the cores of thunderstorms, which means the source must be even more intense to produce a flash detectable from orbit.

In most thunderstorms, tiny ice crystals near the top of the storm carry a positive electrical charge and heavier snow pellets called "graupel" at lower altitudes carry negative charge. Lightning discharges are powered by the strong electric field between these layers. We don't yet understand the details, but electrons in the air become accelerated to near light speed by the electric field. Because electrons are negatively charged, they are attracted to the positive charge layer, which is usually at higher altitudes, so the beam of accelerated electrons is usually directed upward. As the electrons move upward through the air, they slow down, producing gamma-ray radiation called bremsstrahlung, a German term meaning "braking radiation," and a broad beam of gamma rays emerges from the top of the thunderstorm, heading toward space.

As the gamma rays travel through the atmosphere, some of them interact with the nuclei of nitrogen and oxygen atoms in the air and convert into a matter-antimatter pair, an electron and positron, moving at near the speed of light. If that conversion happens deep in the atmosphere, the electron and positron lose their energy in collisions with the air atoms and become absorbed. But if the conversion happens very high in the atmosphere, at altitudes of 25 miles (40 kilometers) or more, the electron and positron pair can escape into space.

These charged particles must spiral along lines of Earth's magnetic field. Occasionally, one of these particle beams strikes a spacecraft. The antimatter positrons annihilate when they strike normal matter, generating gamma rays of a specific energy. Fermi's GBM first detected those specific gamma rays in 2011 — and discovered that thunderstorms can launch antimatter beams into space.

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