Results have indicated that between 10^14 and 10^17 runaway electrons are required to produce a TGF, assuming a production altitude of 15 km and that the ratio of bremsstrahlung photons to electrons is roughly 1. According to Carlson et al. [2008], we can assume that cosmic rays produce a maximum seed population of 10^6 energetic electrons. Furthermore, we can expect an electric potential of 100 MV to be available in a large thundercloud, which would roughly correspond to 100 MV/7.3 MV = 13.9 avalanche lengths or a maximum multiplication e^13.9 ≈ 10^6 runaway electrons per seed electron. Combining this we get a multiplication of 10^12, which is 5 orders of magnitude lower than the required number of electrons from an average observed TGF produced at 15 km altitude. In addition, Dwyer [2008] made calculations on the initiation of RREAs from extensive air showers (EAS) and steady state background radiation, both mainly a product of cosmic rays. He found that neither of them is very likely to explain TGFs by its own. Thus, the high number of electrons required to produce a TGF cannot be explained by RREA multiplication alone. In response to this, two leading theories have been presented. Dwyer [2003] suggested that the feedback mechanism could provide further multiplication and thus explain the production of TGFs. Another possible solution has been presented by Celestin and Pasko [2011]. They show that seed electrons with energies on the order of 60 keV can be produced in the vicinity of the tips of lightning leaders by streamers and be further accelerated in the potential drops in front of lightning leader tips. They found that this process was capable of producing 10^17 energetic electrons.

• GEANT4 simulation results match with previous MC simulations, but only when using the GEANT4 LBE physics list (Livermore model, seems better at low energy losses)

• Confirms the results presented by Dwyer and constrain the conditions under which the feedback mechanism may play a role (needs 48.5MV @2500kV/m and 300MV @390kV/m)

• The ratio of bremsstrahlung photons to runaway electrons where 400kV/m < E < 2000kV/m was found to be between 1 and 0.1