Terrestrial Gamma-Ray Flashes (TGFs) are among the most energetic natural phenomena on Earth, representing a form of atmospheric discharge invisible to the human eye. Colloquially termed “dark lightning,” this phenomenon occurs high above the ground, typically within active thunderstorms. Given the immense power involved, questions naturally arise about the potential danger they pose, particularly to people in aircraft. This article explores the nature of dark lightning and analyzes the calculated radiation exposure risk.
What is Dark Lightning?
Dark lightning is the popular name for a Terrestrial Gamma-Ray Flash, or TGF, a brief but intense burst of high-energy radiation originating in the atmosphere. Unlike traditional lightning, which is a massive electrical discharge resulting in a bright, visible plasma channel, dark lightning emits no light in the visible spectrum. The term “dark” refers to its invisibility, as the energy is released primarily in the form of gamma rays and high-energy X-rays. These bursts are incredibly short-lived, typically lasting only a few milliseconds, which is why they remained undiscovered until the 1990s by instruments on orbiting satellites. TGFs are considered to be the most powerful naturally occurring particle accelerators on our planet, linked to the intense electrical activity within large thunderclouds.
The Physics Behind Dark Lightning Generation
The creation of a Terrestrial Gamma-Ray Flash begins with the extremely strong electric fields that build up inside large thunderstorms. These fields accelerate free electrons within the cloud to speeds approaching the speed of light, creating “runaway electrons.” As these runaway electrons collide with air atoms, they knock loose other electrons, creating a cascade known as a Relativistic Runaway Electron Avalanche (RREA). When these near-light-speed electrons are suddenly decelerated by air molecules, they emit high-energy photons—gamma rays and X-rays—a process known as Bremsstrahlung radiation. This mechanism transforms the storm’s electrical energy into a brief, directed beam of high-energy radiation.
Quantifying the Radiation Exposure Risk
The primary concern regarding dark lightning centers on the direct exposure of aircraft passengers and crew to this high-energy radiation. A TGF is a tightly focused beam, and a direct strike on an aircraft at cruising altitude would deliver a significant radiation dose. Modeling suggests that a person inside a plane that flies directly through the core of an intense TGF could receive a dose equivalent to approximately ten full chest X-rays or even a full-body computed tomography (CT) scan.
This level of exposure, while high for a single event, is not immediately life-threatening but would exceed the annual radiation dose limit set for the general public. However, the vast majority of TGFs are directed upward, escaping into space and posing no threat to the ground or to most air traffic. For the average passenger, the radiation risk from a TGF is negligible because commercial aircraft actively avoid the severe thunderstorm cells that produce them.
The probability of an aircraft being directly hit by a TGF is extremely remote, with one study estimating that a specific flight route might experience a hit only once every 1,800 years. The radiation dose received from a TGF is often compared to the long-term exposure from cosmic rays, which all passengers absorb during high-altitude flight.
Frequency and Altitude of Occurrence
Dark lightning is a far more common occurrence than initially believed. Satellites and ground-based detectors estimate that globally, between 500 and 1,100 TGFs occur every day. Although often associated with conventional lightning strikes, they are not a one-to-one match with visible lightning.
These events are generated high in the atmosphere, predominantly within the upper regions of powerful thunderclouds. The source altitude for most TGFs is typically between 10 and 15 kilometers above the Earth’s surface. This range corresponds closely to the cruising altitude of most commercial aircraft, making the radiation risk primarily a concern for air travel. The concentration of TGFs in the high troposphere means the energy is largely absorbed by the atmosphere before it can reach the surface, shielding ground-level populations.