Have been observed by ground-based detectors, providing new insights into mechanisms for accelerating electrons to high energies, as high as 10 MeV, in the atmosphere. Ground observations of thundercloud gammas has been made before as part of monitoring regular nuclear plant operations. The new measurements, however, represent the first time that such gamma studies were made with detailed scientific objectives in mind, including determinations of particle species, arrival direction, and energy spectrum.
On the night of 6 January 2007 two powerful low-pressure air masses collided over the Sea of Japan. A nearby array of gamma detectors provided information on the energy and the timing of the gammas, which are the highest-category of electromagnetic radiation. The array is operated by the University of Tokyo and the Cosmic Radiation Laboratory of RIKEN in Japan. The gamma production, the researchers believe, works like this: an energetic seed electron, perhaps liberated from an atom by an intruding cosmic ray, ionizes many air molecules, which in turn are accelerated by the high electric fields present in the thunderclouds.
This flock of fast electrons can then emit gamma radiation (bremsstrahlung, or "braking radiation") as they are slowed by surrounding air. The gamma production actually occurs before the eventual lightning strike, says Teruaki Enoto of the University of Tokyo (email@example.com, 81-3-5841-4173), and the reason for this is not entirely known.
Previous thundercloud-related gammas were studied by satellite and only measured very brief bursts, with durations of msec. By contrast, the Tokyo-RIKEN work indicates bursting behavior that could last for minutes, testifying to the quasi-static nature of the acceleration mechanism at work in the clouds. The electrostatic potential in the clouds might be as high as 10 million volts. (Tsuchiya et al., Physical Review Letters, upcoming article)