Thunderclouds fire beams of antimatter upward

A terrestrial gamma-ray flash in a storm makes pairs of electrons and positrons. NASA's Fermi telescope has caught the positrons travelling out along Earth's magnetic field, including from storms over the horizon.

Inside a thundercloud, electrons are accelerated hard enough to throw gamma rays. Some of those gamma rays pass near a nucleus and become an electron and a positron, a particle and its antiparticle. The positron is antimatter. Most of this happens and ends inside the storm. In a few cases the charged particles are guided along Earth's magnetic field, out of the atmosphere, and along the field line to wherever a detector happens to be. Fermi's gamma-ray burst monitor recorded the signature: a flash, and then, milliseconds later, the positrons arriving. Four of the events came from storms that were not under the spacecraft. The beam had travelled along the field.

This is not a cloud that stores antimatter, and it is not a threat to aircraft in any ordinary sense. The beam is brief, thin, and aimed by the magnetic field, not by the storm's anger. About five hundred such gamma-ray flashes are estimated to happen on Earth each day. Fermi sees the ones its orbit happens to thread. The surprise that survives the scale is the identity of the particles. The same positrons a PET scanner uses on purpose, a weather system makes by accident, and throws into space.