A Monte Carlo Model of Relativistic Runaway Breakdown in Realistic Thunderstorm Situations

Recent observations and theoretical advances have suggested that runaway breakdown may play an important role in the initiation of lightning. However, exactly how runaway breakdown manages to produce a lightning leader is still not clear. Monte Carlo simulations have proven to be useful tools to study runaway breakdown because they allow one to simulate the propagation and production of energetic electrons under realistic thunderstorm conditions based on detailed physical interactions. In this presentation, we discuss some of the different approaches we use to model physical aspects of electron production and propagation in our Monte Carlo simulation. We discuss approaches that offer reasonable accuracy without being too expensive computationally and discuss the errors introduced by the different interactions included in the model: propagation of electrons and positrons through fields and matter, Coulomb scattering, Moller scattering, bremsstrahlung radiation events and photon propagation, including photoelectric absorption, Compton scattering and pair production. While thunderstorm measurements of electric fields have shown electric fields with magnitudes larger than that needed for a runaway avalanche to occur, it is not clear if conditions exist inside thunderstorms to create large enough runaway avalanches to initiate lightning. Thus, we will also present results from running our model in realistic storm conditions. We will explore the effects of different electrical structures on the positron and x-ray feedback mechanisms and discuss the thunderstorm conditions necessary to produce enough runaway breakdown to create substantial fluxes of x-rays and/or significantly alter the electric field inside the storm.