Gamma-rays are the most energetic light wave produced in the universe. So energetic, in fact, that they are seen in certain radioactive decays, in many cosmic explosions, and also in lightning. Lightning in particular has been a bit of a head-scratcher, as no one knows how it produces gamma-rays.
The terrestrial Gamma-ray flashes (TGFs) are produced by bremsstrahlung, a German word that translates to "braking radiation". An electrically charged particle being accelerated or decelerated (hence the term braking) will emit electromagnetic radiation. In the case of the TGFs, the cause is electrons being accelerated.
In a new paper, currently available on the preprint repository ArXiv, researchers report the recent observation of the optical counterpart of a TGF. This is important because the two main hypotheses for the production of gamma-rays by lightning produce different signatures in visible light.
The first is the Relativistic FeeDback (RFD) mechanism, which suggests that photons – the particles of light – and positrons – the antimatter equivalent of the electron – produce feedback that generates electron avalanches. Antimatter in lightning clouds happens so this scenario had to be taken seriously. If this model is correct, there should be very little or no signature in visible light and some modest ultraviolet light. This is known as dark lightning.
The alternative explanation is the thermal runaway electron’s production mechanism, where a localized high-electric field would generate the electron cascade. These fields would be ten times higher than the conventional breakdown field.
“The thermal runaway electron’s production mechanism assumes that TGF events are produced in the vicinity of the tips of streamers related to lightning leaders. The free electron population is then accelerated further in the potential drops in front of lightning leader tips to runaway electrons via the thermal runaway mechanism,” the authors write in the paper.
The thermal runaway would produce an optical signal simultaneously with the gamma rays. And during a thunderstorm on September 11, 2021, researchers were using the Telescope Array Surface Detector (TASD), which detected gamma-rays and optical signals from a lightning storm, consistent with the thermal runaway production mechanism.
“Our data indicate a substantial increase in the visible light in correlation with the TGF production. From this result it seems reasonable to conclude that our result supports the thermal runaway mechanism model and does not support the idea that TGFs are dark events as proposed by Relativistic FeedBack mechanism,” the authors write in the study’s conclusion.
This is not the end of the story when it comes to gamma-rays from lightning, far from it. Scientists are only just scratching the surface of the complexities of thunderstorms and lightning.
[H/T: New Scientist]