Abstract
The terrestrial gamma-ray flashes (TGF) were discovered by BATSE detector on Compton Gamma Ray Observatory satellite, which recorded a total of ~76 of them. At present time, they are observed by RHESSI satellite at a rate of 10-20 per month. The TGF are observed in regions with high lightning activity and many of them have been associated with lightning discharges. We investigate the hypothesis of TGF production by relativistic runaway electrons accelerated by post-discharge electric field above thunderclouds. The runaway electron avalanche process is studied using Monte Carlo simulations. The results include electron distributions and avalanche growth rates and depend on the values of reduced electric and magnetic fields and the angle between them. The calculated avalanche growth rate and electron drift velocities are applied to a fluid model of the avalanche above a thunderstorm, seeded by cosmic rays. We present results of two-dimensional cylindrical and cartesian models, and compare levels and spectra of calculated gamma emissions with observations. The RHESSI observations suggest existenceof runaway electron avalanche and TGF production at the geomagnetic equator. We discuss the problems with the avalanche being strongly impeded by the geomagnetic field at high altitudes.