Nikolai G. Lehtinen 1, Brant E. Carlson 2, Morris B. Cohen 1
1 Stanford University, Stanford, CA, U.S.A., 2 University of Bergen, Bergen, Norway
This talk is an overview of Stanford University research on RREA and TGF. The Relativistic Runaway Electron Avalanche (RREA) is the most plausible cause of the Terrestrial Gamma Flashes (TGF), energetic (up to >40 MeV) short (<1 ms) gamma ray bursts originating at the Earth's surface and observed by low Earth orbiting satellites. VLF observations of lightning signals (sferics) have shown the TGF occur in association with thunderstorms, or even with individual lightning discharges [Cohen et al, 2006]. The electric fields which accelerate the runaway electrons may be caused by several mechanisms: (1) quasi-electrostatic fields produced after lightning discharges; (2) the electromagnetic pulses radiated by rapid lightning return strokes; and (3) the direct production of radiation by the lightning channel. The theoretical studies of RREA and TGF include Monte Carlo modelling of collisional processes, modeling of ionization in the ionosphere under the combined action of a thundercloud electric field and relativistic electrons, and modeling of the lightning channel with high fields in the lightning leader tips. It was found that the extreme parameters of lightning discharges are required for the first two mechanisms to produce the observed levels of TGF emissions [Lehtinen et al, 1999; Inan and Lehtinen, 2005]. The time-domain method-of-moments modeling of the lightning channel showed that the electric fields at the tips of leader channels may produce gamma rays [Carlson et al, 2009]. Monte Carlo modelling was used to calculate: (1) the RREA multiplication rates, in the presence of the geomagnetic field [Lehtinen et al, 1999]; (2) gamma photon propagation was used to set constraints on TGF production parameters, in particular, the altitude of the gamma ray source [Carlson et al, 2007]; (2) RREA seeding efficiency by various kinds of energetic particles [Carlson et al, 2008]. It was also shown that the electrons produced in Compton scattering process of gamma photons may reach ionosphere and be observed by satellites in the form of Terrestrial Electron Flashes (TEF) [Carlson et al, 2009; Cohen et al, 2010]. Another result is the calculation of neutron flux from TGFs [Carlson et al, 2010].