Science begins to unravel a mystery about lightning

Lightning strikes a beach in Lhokseumawe, North Aceh, Indonesia, on May 25. Scientists are now beginning to elucidate a complex physical mystery surrounding electrical discharges in the sky: the production of gamma rays | Azwar Ebank/AFP/Mitsul Meteorology

Thunderstorms produce powerful flashes of gamma rays, and scientists are now beginning to better understand the dynamics of using cameras at different wavelengths. Gamma rays are usually seen coming from objects in space, such as neutron stars or pulsars, but they can also be produced by thunderstorms on Earth.

So-called terrestrial gamma-ray flashes (TGFs) can damage nearby electronic equipment such as aircraft, but it is not yet known exactly why they occur, as a very small portion of the ray events appear to be produced by them.

Gamma rays are the most energetic light waves in the universe. Very active, in fact, they are seen in some radioactive decay, in many cosmic explosions, as well as in lightning. Lightning in particular has been a little tricky, as no one is quite sure how it produces gamma rays.

Terrestrial gamma ray flashes (TGFs) are produced by “bremsstrahlung,” a German word that translates to “braking radiation.” Electrically charged particles that are either accelerated or slowed down (hence the term braking) will emit electromagnetic radiation. In the case of TGFs, the cause is the acceleration of electrons.

In a new scientific paper, researchers report the recent observation of an optical analog of TGF. This is important because the two main hypotheses for gamma ray production by rays produce different signals in visible light.

The first is the Relativistic Charge Return (RFD) mechanism, which proposes that photons – particles of light – and positrons – the antimatter equivalent of an electron – produce reactions that generate a breakdown of electrons. If this model is correct, there should be very little or no visible light and some modest ultraviolet light. This is known as dark lightning.

An alternative explanation is the thermally runaway electron production mechanism, in which a highly electric local field generates the electron chain. Thermal escape will produce an optical signal at the same time as the gamma rays. During a storm on September 11, 2021, researchers were using the Telescope Array Surface Detector (TASD), which detected gamma rays and light signals from a thunderstorm, consistent with a thermal escape production mechanism.