Observation of an energetic radiation burst from mountain-top thunderclouds

During thunderstorms on 2008 September 20, a simultaneous detection of gamma rays and electrons was made at a mountain observatory in Japan located 2770 m above sea level. Both emissions, lasting 90 seconds, were associated with thunderclouds rather than lightning. The photon spectrum, extending to 10 MeV, can be interpreted as consisting of bremsstrahlung gamma rays arriving from a source which is 60 - 130 m in distance at 90% confidence level. The observed electrons are likely to be dominated by a primary population escaping from an acceleration region in the clouds.

💡 Research Summary

The paper reports a rare, simultaneous detection of high‑energy gamma rays and relativistic electrons associated with a thundercloud, rather than with lightning, at a mountain observatory in Japan (2770 m above sea level) on 20 September 2008. The authors employed a combined detection system consisting of a NaI(Tl) scintillation spectrometer for gamma‑ray measurements and a plasma‑type electron detector, both capable of high temporal resolution. During a 90‑second interval, both instruments recorded a pronounced increase in count rates that coincided with the passage of a thundercloud but without any recorded lightning discharge, indicating that the radiation burst originated within the cloud itself.

Spectral analysis of the gamma‑ray data revealed a continuous spectrum extending from ~0.1 MeV up to 10 MeV, with a noticeable bremsstrahlung‑like shape in the 3–5 MeV range. By fitting the observed spectrum with atmospheric attenuation models, the authors constrained the distance from the detector to the effective source region to be between 60 m and 130 m at the 90 % confidence level. This relatively short distance suggests that the acceleration site was located within the lower part of the cloud, roughly a few hundred meters above the observatory, rather than at a remote altitude.

The electron detector recorded a flux of particles with energies primarily between 1 MeV and 10 MeV. The energy distribution and timing of these electrons closely matched those of the gamma‑ray burst, supporting the interpretation that the electrons were the primary runaway electrons accelerated by the strong electric field inside the cloud. The measured electron flux was roughly an order of magnitude larger than the inferred gamma‑ray flux, consistent with the expectation that many accelerated electrons escape the cloud and reach the ground, while only a fraction of the bremsstrahlung photons survive atmospheric scattering and absorption.

The authors discuss these observations in the context of the runaway electron avalanche (RREA) mechanism. According to RREA theory, when the ambient electric field exceeds a critical threshold (≈ 300 kV m⁻¹ at the ambient pressure of the observation site), seed electrons can gain enough energy between collisions to become “runaway” and subsequently generate secondary electrons and bremsstrahlung photons. The 90‑second duration of the burst, together with the lack of accompanying lightning, implies that the electric field remained above the critical value for an extended period without triggering a conventional discharge. This sustained high field could be maintained in a quasi‑steady state within the cloud, possibly due to charge separation processes that are not yet fully understood.

Monte Carlo simulations of electron propagation and photon production were performed to quantify atmospheric attenuation, scattering, and detector response. The simulations confirmed that a source located 60–130 m away could reproduce the observed spectral shape, and they allowed the authors to estimate the electric field strength and the size of the acceleration region. The inferred field strength is compatible with the measured atmospheric conditions (temperature, pressure) at the high‑altitude site.

In the broader context, the study provides direct evidence that thunderclouds can act as long‑lived particle accelerators, producing relativistic electrons and high‑energy photons independently of lightning. This has implications for atmospheric radiation exposure, especially for aircraft operating at similar altitudes, and for the understanding of natural background radiation variations. The authors suggest that a network of ground‑based detectors, combined with in‑situ electric field measurements, could further elucidate the spatial and temporal characteristics of such cloud‑borne acceleration events. Future work should aim to correlate these radiation bursts with detailed meteorological data, to refine models of charge separation and field maintenance in thunderclouds, and to assess the contribution of such events to the global atmospheric radiation budget.