Terrestrial Gamma-Ray Flashes
Terrestrial gamma-ray flashes (TGFs), also known as dark lightning, are bursts of gamma rays that occur in Earth's atmosphere. These intense flashes of gamma radiation are remarkably short-lived, typically lasting between 0.2 and 3.5 milliseconds. The energies associated with TGFs can be exceptionally high, reaching up to 20 million electronvolts.
Discovery and Observation
TGFs were first identified in 1994 by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory, a NASA spacecraft. This discovery was groundbreaking in the study of atmospheric phenomena and gamma rays, as it unveiled a previously unknown source of terrestrial gamma radiation.
In 1996, researchers at Stanford University established a connection between TGFs and individual lightning strikes, showing that a TGF could occur within mere milliseconds of a lightning event. This correlation hinted at a deeper relationship between lightning and TGFs.
Mechanism and Hypotheses
The underlying mechanism of TGF generation is closely related to the intense electric fields produced above or within thunderstorms. The Atmosphere-Space Interactions Monitor (ASIM) has provided significant insights into this process. ASIM's simultaneous measurement of optical signals from lightning and gamma-ray signals from TGFs showed that these gamma-ray flashes are usually associated with optical flashes. This observation strongly implies that relativistic electrons, accelerated by the strong electric fields near lightning channels, are the precursors of TGFs.
One of the key processes believed to be responsible for TGFs is the relativistic runaway electron avalanche (RREA). This phenomenon involves the rapid acceleration of electrons to near-light speeds due to strong electric fields, which then produce gamma rays through interactions and collisions, a process known as Bremsstrahlung.
Connection with Other Atmospheric Phenomena
TGFs are part of a broader category of atmospheric events that include sprite lightning and other transient luminous events (TLEs). These phenomena are interconnected through the complex dynamics of thunderstorms and involve various types of high-energy particles and electromagnetic emissions.
Additionally, TGFs have been associated with the production of antimatter, specifically positrons, which are produced when gamma rays interact with the atmosphere. This interaction underscores the unique and powerful nature of TGFs as a natural source of high-energy radiation and particle acceleration.