| A three-dimensional Monte Carlo model of the uniform relativistic runaway electron breakdown in air in the presence of static electric and magnetic fields is developed and used to calculate electron distribution functions, avalanche rates and the direction and velocity of avalanche propagation.; The Monte Carlo simulation results are used in a fluid model of a runaway electron beam in the middle atmosphere accelerated by quasi-electrostatic fields following a positive lightning stroke. We consider the case of lightning discharges which drain positive charge from remote regions of a laterally extensive (>100 km) thundercloud in a thunderstorm located at ∼45° geomagnetic latitude, using a translationally invariant two-dimensional model. We also consider a cylindrically symmetric model with a vertical axis of symmetry, constrained to a vertical geomagnetic field.; In both models, the optical emission intensities produced by the runaway electrons are found to be negligible compared to the emissions produced by thermal electrons heated in the conventional type of breakdown. The calculated γ-ray flux is of the same order as the terrestrial γ-ray flashes observed by the BATSE detector on the Compton Gamma Ray Observatory.; The energetic electrons leaving the atmosphere undergo intense interactions with the background magnetospheric plasma, leading to rapid growth of Langmuir waves with rate found based on the energy electron distribution and intense scattering of the electrons. In the nonlinear stage, beam electrons acquire an isotropic thermal distribution with a typical energy of ∼1 MeV within one interhemispheric traverse along the Earth's magnetic field lines. While the electrons within the loss cone precipitate out, most of the electrons get trapped and form detectable energetic electron curtains surrounding the Earth.; Electrons with pitch angles below the loss cone encounter the Earth's atmosphere at the conjugate point, are scattered and produce light, ionization and γ-ray emissions, much like a beam of precipitating auroral electrons. A Monte Carlo approach is used to model the interaction of the downcoming electrons with the conjugate atmosphere, including the backscattering of electrons as well as the production of optical emissions, enhanced secondary ionization and γ-ray emissions. Results indicate that these conjugate ionospheric effects are detectable and may be used to quantify the runaway electron mechanism. |
