Gamma-Ray Glows: A Common Signature of Thunderstorms ?

Predicted by Wilson in the 1920s, thunderstorms act as natural particle accelerators. Charged particles, mainly electrons, can be energized by the strong electric fields inside thunderclouds, becoming runaway electrons that reach relativistic energies. During this acceleration, these relativistic electrons produce secondary electrons through atmospheric ionization, leading to a Relativistic Runaway Electron Avalanche (RREA) while emitting X-rays through bremsstrahlung. This mechanism underlies the high-energy atmospheric phenomena generated by thunderstorms, such as terrestrial gamma-ray flashes (TGFs), flickering gamma-ray flashes (FGFs), and gamma-ray glows (GRGs).

GRGs are long-lasting X-ray emissions produced by sustained RREAs, typically lasting from seconds to tens of minutes. They are usually observed close to their sources either by aircraft, high-altitude sites located on mountain, or from the western coast of Japan where thunderclouds frequently develop near sea level.

Since 2023, we are conducting a ground-based observational campaign by equipping several strategic sites to detect these high-energy events and study their occurrence and characteristics. Three sites were instrumented with scintillators: Chofu (Tokyo, Japan), Pic du Midi de Bigorre (French Pyrenees), and Normandy (France).

In this presentation, we introduce a new statistical method designed to detect GRGs and potentially TGFs and FGFs. The method combines Gaussian filtering, continuous wavelet transforms, and Bayesian inference. It enabled the detection of more than ten GRGs at Pic du Midi between April and November 2025, as well as two GRGs at sea level in Chofu and Normandy demonstrating the method’s efficiency and showing that GRGs are common and associated with all thunderstorms.