Terrestrial Gamma-ray Flashes: The Invisible Storm Within Thunderstorms

Terrestrial gamma-ray flashes
Ksenija Brakočević
Ksenija Brakočević
5 min

Terrestrial gamma-ray flashes (TGF) are a relatively recent phenomenon likely caused by thunderstorms and lightning. These gamma rays, also known as dark lightning, are characterized by durations of 0.2-3.5 microseconds and energies up to 35 MeV. TGFs were first detected accidentally by Gerald Fishman from NASA in 1994 while he was observing extraterrestrial gamma rays from the Compton Gamma Ray Observatory. It took some time to realize their relation to lightning. Professor Umran Inan of Stanford University proved that a particular TGF was caused by a lightning strike that occurred 1.5 milliseconds before the detected TGF, thus confirming the atmospheric origin of these gamma rays.

One major difficulty in observing and understanding TGFs is that devices designed for detecting extraterrestrial gamma rays were not adequate for observing TGFs due to their very short duration. For example, the Burst and Transient Source Experiment (BATSE) managed to detect only 76 TGFs over nine years because it was designed to detect longer-lasting gamma rays from outer space. However, technological advancements have improved detection rates. The Ramaty High Energy Solar Spectroscopic Imager (RHESSI), established in the 2000s, observed TGFs at a much higher rate, about 50 occurrences each day. NASA's Fermi Gamma-ray Space Telescope, developed later, detects around 500 TGFs each day, and this number could be even higher. In 2018, the Atmosphere-Space Interactions Monitor (ASIM) was launched on the International Space Station to observe TGFs.

The mechanism behind TGF creation is still uncertain, but it is theorized that they are caused by electrons traveling at speeds close to the speed of light. These electrons collide with the nuclei of air atoms, releasing energy in the form of gamma rays. Lightning provides populations of these very fast electrons, creating a relativistic runaway electron avalanche. Other theories on TGF mechanisms are borrowed from other upper-atmospheric phenomena.

TGFs are closely related to upper-atmospheric lightning phenomena, also known as mega lightning, which includes sprites, elves, and blue jets. The discovery of TGFs and the presence of antimatter in lightning challenge established theories on lightning causes and mechanisms. In one notable instance in 2009, the Fermi Gamma-ray Telescope captured data of a storm where a lightning flash and gamma-ray flash released 100 trillion positrons.

It is important to note that the beginnings of life on Earth are often attributed to the violent nature of atmospheric occurrences, including lightning. Newly discovered TGFs provide further evidence of lightning's influence on the evolution of life on Earth. The curious phenomenon of TGFs highlights the fascinating nature of our planet's immediate surroundings. The complexity of the physics behind TGFs and their implications for both evolutionary and atmospheric science guarantee continued research. On a more philosophical level, TGFs remind us of the numerous undiscovered mysteries of our little blue planet.

References

  • Smith, D. M. (2005). Terrestrial Gamma-Ray Flashes Observed up to 20 MeV. Science, 307(5712), 1085–1088. doi:10.1126/science.1107466
  • Briggs, M. S., Xiong, S., Connaughton, V., Tierney, D., Fitzpatrick, G., Foley, S., … Hutchins, M. L. (2013). Terrestrial gamma-ray flashes in the Fermi era: Improved observations and analysis methods. Journal of Geophysical Research: Space Physics, 118(6), 3805–3830. doi:10.1002/jgra.50205
  • Dwyer, J. R., Smith, D. M., & Cummer, S. A. (2012). High-Energy Atmospheric Physics: Terrestrial Gamma-Ray Flashes and Related Phenomena. Space Science Reviews, 173(1-4), 133–196. doi:10.1007/s11214-012-9894-0
  • Dwyer, J. R. (2008). Source mechanisms of terrestrial gamma-ray flashes. Journal of Geophysical Research, 113(D10). doi:10.1029/2007jd009248
  • Marisaldi, M., Fuschino, F., Labanti, C., Tavani, M., Argan, A., Del Monte, E., Trifoglio, M. (2013). Terrestrial gamma-ray flashes. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 720, 83–87. doi:10.1016/j.nima.2012.12.029