Gamma-ray cloudbursts shed light on lightning

A team including researchers at RIKEN’s Discovery Research Institute in Wako and the University of Tokyo has observed a burst of high-energy gamma radiation emerging from a thundercloud over the Sea of Japan (1). The discovery could help to reveal the complex electrical processes that cause lightning.

“Free electrons, originally produced by cosmic rays, can be accelerated by the strong electric fields in thunderclouds,” explains project scientist Harufumi Tsuchiya. “If they reach relativistic energies, they can knock other electrons out of their atoms, causing a ‘runaway electron avalanche’.”

When one of the high-energy electrons is deflected by the nucleus of an atom, it loses energy in the form of gamma rays called Bremsstrahlung—literally ‘braking radiation’. Bursts of these gamma rays have been detected by near-Earth satellites above thunderclouds, and very short bursts are often recorded near the ground. Longer bursts lasting up to a few minutes appear to be very rare events, and physicists are unsure where they come from or what they consist of.

To answer these questions, the researchers built new radiation detectors based on devices on board the Suzaku cosmic x-ray satellite. The detectors were installed on the roof of the Kashiwazaki–Kariwa nuclear power plant in Niigata. On 6 January 2007, during a violent winter thunderstorm, they recorded a large radiation spike lasting over a minute, which could not be attributed to background radiation or electrical noise.

The spectrum of radiation included high-energy gamma rays that could not have been produced by thermal processes—which would require temperatures of billions of degrees Celsius. Therefore the burst must have been caused by Bremsstrahlung processes.

The burst was recorded approximately 70 seconds before a large flash of lightning, leading the researchers to speculate on whether the two events are related. In theory the runaway electrons could produce a large number of slower electrons, leading to electrical imbalance and lightning. “If thunderclouds frequently generate gamma ray bursts prior to lightning discharges, detailed observations of such rays would allow us to predict when lightning will occur,” claims Tsuchiya.

However, more observations are needed to prove such a link. “We believe the burst behaves like a searchlight beam, illuminating only a limited area on the ground,” says Tsuchiya, “so we were probably fortunate that the beam happened to pass over our detector.” To test this hypothesis, the researchers plan to spread several detectors over a large area, so that they might trace the movement of a gamma ray burst.

1. Tsuchiya, H., Enoto, T., Yamada, T., Yuasa, T., Kawaharada, M., Kitaguchi, T., Kokubun, M., Kato, H., Okano, M., Nakamura, S. & Makishima, K. Detection of high-energy gamma rays from winter thunderclouds. Physical Review Letters 99, 165002 (2007).