Cloudy, with a Chance of Gamma Rays

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Science  28 Jan 2011:
Vol. 331, Issue 6016, pp. 378
DOI: 10.1126/science.331.6016.378-d

The tremendous rumble of thunderclouds that sends children under their bedcovers and provides the electrifying atmosphere indicative of an impending storm also holds fascination for scientists who wish to understand the mechanism of lightning and the associated buildup and release of energy. Experiments have become more sophisticated since Benjamin Franklin's kite-flying forays to probe lightning directly. Of recent interest are terrestrial gamma-ray flashes (TGFs)—flashes of high-energy radiation that occur predominantly at the upper regions of clouds in the tropics and appear to originate deep in the atmosphere at an altitude of 10 to 20 km. Since space-based satellites first observed these TGFs in the 1990s, the formation mechanisms have tended to be described in terms of runaway electron acceleration models resulting from the large electric fields thunderclouds discharge. Tavani et al. report on a systematic study of observations across a wide energy range (350 keV to 100 MeV) by the Italian Space Agency's AGILE observatory that raise questions about those models. They argue that the emission spectrum at the very highest energies cannot be reconciled with existing models and that other mechanism(s) must be at play. Although no definitive model is provided, the detailed data set should prove useful for those trying to understand the energy dynamics of our atmosphere.

Phys. Rev. Lett. 106, 18501 (2011).

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