Gamma Ray Flashes due to Plasma Effects in the Stratosphere: Role of Whister Waves

The gamma ray flashes observed by satellites are correlated with thunderstorms and can be due to the large electric fields associated with the latter. The proposed mechanism involves a number of physical processes. It starts with the creation of a beam of runaway electrons by the static electric field in the upper part of a thunder cloud. As the beam propagates upward it interacts with whistler waves generated during cloud-to-ground lightning discharges. This is followed by self-focusing of the whistler wave, producing ducts which guide the relativistic electron beam to higher altitudes. Such beams could reach altitudes in excess of 30 km so that the gamma rays generated by bremsstrahlung emission can escape the atmosphere. In this model for gamma ray generation the whistler waves play an essentila role. The whistler waves become unstable when the number density of the relativistic electron beam is high enough to overcome the damping by the population of cold electrons. The threshold conditions are studied for different ranges of the relevant atmospheric parameters. For whistler waves with large amplitudes due to the instability, the conditions for self-focusing and channel formation, and consequently the beam propagation are investigated.