Gamma rays from thunderclouds

During thunderstorms, atmospheric electric fields can be strong enough to accelerate charged particles to tens of MeV and produce bursts of radio, x-ray, and gamma-ray emissions. The bursts are separated from lightning in time and often in location as well. Although the x rays can last for a minute or more, the radio and gamma-ray burst durations typically lie in the micro- or millisecond range. Some unusual, longer-duration gamma events last tens of seconds or even minutes. A collaboration of researchers from four institutions in Japan has now described one such event that took place during a winter thunderstorm over the Sea of Japan on 6 January 2007. Specialized detectors atop a nuclear power station recorded about 40 seconds of gamma rays, followed some 70 seconds later by a lightning stroke. The photons had energies that reached and exceeded 10 MeV and were unambiguously determined to have come from the sky. The figure shows the spectra from two counters in one of the detectors. Knowing that winter thunderclouds in that location have a three-layer vertical charge structure of positive-negative-positive, the scientists suggest the following scenario: Cosmic rays generated energetic electrons that were further accelerated, both groundward and skyward, in the ambient electric fields. An avalanche of runaway electrons ensued that produced relativistically beamed bremsstrahlung radiation. The events are rarely seen because the narrow radiation cones almost never intercept an appropriate detector. To learn more about the mysteries of lightning and the role of cosmic rays and runaway electrons, see Physics Today, May 2005, page 37 . (H. Tsuchiya et al. , Phys. Rev. Lett. , in press.)