As outlined in a January 11, 2011 NASA press release, Michael Briggs of the University of Alabama at Huntsville announced, at the January 2011 meeting of the American Astronomical Society, that thunderstorms make antimatter, specifically positrons. This discovery was made with the NASA Fermi gamma-ray space telescope, which was formerly known as the GLAST mission.
Terrestrial Gamma Ray Flashes
Terrestrial Gamma Ray Flashes (TGFs) were discovered, quite unexpectedly, in the early 1990s by the Compton Gamma-Ray Observatory. Scientists noticed that TGFs were associated with lightning flashes and thunderstorms but did not understand what caused TGFs. Like the Compton observatory, the Fermi mission was primarily designed to study gamma-rays from celestial sources, but it can also observe TGFs.
Since NASA launched the Fermi mission in 2008 it has observed 130 TGFs. In all but four of these TGFs, the Fermi satellite was above a thunderstorm, which was the source of the gamma-rays. In the four cases where the terrestrial thunderstorms and lightning flashes were far from the Fermi telescope, evidence indicates that the observed gamma-rays originated from positrons striking the Fermi telescope.
What Are Positrons and Antimatter?
Positrons are the antimatter particles corresponding to electrons. Each type of matter particle has a corresponding antimatter particle with the same mass and opposite electric charge.
When the corresponding matter and antimatter particles come into contact, they annihilate each other to produce energy in the form of two gamma-rays. The reverse process, called pair production, can also occur. A gamma-ray having enough energy can, when near an atomic nucleus, produce a matter-antimatter pair of particles, such as a positron and an electron.
How Gamma-Rays Indicate Positrons
The Fermi gamma-ray telescope was designed to detect gamma-rays not positrons. It can however indirectly detect positrons. If a positron collides with the telescope and interacts with an electron in one of the telescope's atoms, the electron and positron will mutually annihilate. The Fermi telescope can detect the resulting gamma-rays. To detect a positron, the gamma-rays must have at least the amount of energy contained in the annihilated electron-positron pair as determined by Einstein's equation, E=mc^2.
How Thunderstorms Produce Positrons
Lightning flashes in thunderstorms are very strong electric currents consisting of a large number of speedy high energy electrons. In a process physicists call bremsstrahlung (German for "braking radiation"), the electrons emit gamma-rays when they pass near to and are slowed by atoms in the atmosphere. This process produces the TGFs observed in thunderstorms.
Some of the gamma-rays nearly collide with the nuclei of atoms. If these gamma-rays have enough energy they can make an electron-positron pair. The electrons and positrons thus produced stream outwards from the thunderstorm.
Fermi Telescope Detects Positrons
Gamma-rays, like light and other electromagnetic waves, usually travel in a straight line. Electrons, positrons, and other charged particles however travel in a spiral path around magnetic field lines. The magnetic force on moving charged particles causes this spiral path.
When the Fermi mission detected TGFs with no directly visible thunderstorms, the gamma-rays produced directly from the lightning flash could not reach the Fermi telescope because it was not in the line of sight from the storm. The positrons, however followed Earth's magnetic field lines until they reached the Fermi telescope. These TGFs with gamma-rays above the minimum energy produced by electron-positron annihilation constitute the evidence that terrestrial thunderstorms make antimatter, specifically positrons.
So far antiprotons have not been detected. Protons and antiprotons are about 2000 times more massive than electrons and positrons. Producing them therefore requires gamma-rays with 2000 times more energy.
The Fermi mission was designed to study gamma-rays from distant celestial objects, not Earth. This unexpected discovery of antimatter being produced in terrestrial thunderstorms provides a nice example of astronomical research increasing knowledge about our home planet as well as the distant universe.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
