Theoretical studies of terrestrial gamma flashes and relativistic
runaway electrons

Nikolai G. Lehtinen, Brant E. Carlson, Umran S. Inan and Timothy
F. Bell

STAR Laboratory, Stanford University, Stanford, CA 94305

The terrestrial gamma-ray flashes (TGF) were discovered by BATSE
detector on Compton Gamma Ray Observatory (CGRO) satellite, which
recorded a total of ~76 of them, at an average rate of once per
month. At present time, they are observed by Reuven Ramaty High Energy
Solar Spectroscopic Imager (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
electron avalanche (RREA). The initial seed for the avalanche is
provided by high-energy cosmic rays. The relativistic runaway
electrons are accelerated by a combined action of the electromagnetic
pulse (EMP) from a lightning return stroke and the post-discharge
quasi-electrostatic (QES) 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 investigate various parameters necessary for the production of a
detectable TGF, including the return stroke speed and maximum value,
the charge removed by a continuing current in the positive
cloud-to-ground discharge, and the dependence on the
pre-discharge atmosphere conductivity profile.

We compare the calculated gamma-ray emissions to the TGF observations,
and also investigate theoretically the possibility of optical and radio
emissions from the relativistic runaway electron avalanche.