Scientists now know that thunderstorms, working as powerful natural terrestrial particle accelerators, produce intense flashes of ionizing radiation called "dark lightning." To further their understanding of this phenomenon, researchers at the U.S. Naval Research Laboratory's (NRL) Space Science Division are making precise observations of these flashes from space and running detailed simulations to understand the nature of dark lightning, and to quantify the radiation exposure it may present to the crew and passengers of aircraft.
Terrestrial Gamma-ray Flashes (TGFs) are extremely intense, sub-millisecond bursts of gamma rays and particle beams of matter and anti-matter. They are associated with strong thunderstorms and lightning, although scientists do not fully understand the details of the relationship to lightning. Unlike visible lightning, TGF beams are sufficiently broad — perhaps about half a mile wide at the top of the thunderstorm — that they do not create a hot plasma channel and optical flash; hence the name "dark lightning."
NRL researchers are studying the radiation environment in the vicinity of thunderstorms and dark lightning flashes. Using a calorimeter built by NRL on NASA's Fermi Gamma ray Space Telescope, they are measuring the energy content of dark lightning and, for the first time, using gamma rays to geolocate the flashes.
As a next step, the team is using NRL's SoftWare for the Optimization of Radiation Detectors (SWORD) to create the first-ever simulations of a dark lightning flash striking a Boeing 737. The radiation dosage to the passengers and crew can be calculated from these Monte Carlo simulations. Previous estimates have indicated it could be as high as the equivalent of hundreds of chest X-rays, depending on the intensity of the flash and the distance to the source. SWORD simulations allow researchers to study in detail the effects of variation in intensity, spectrum, and geometry of the flash.
