A numerical model of lightning-generated EM waves and remote sensing applications

Lightning discharges radiate electromagnetic waves over a wide frequency bandwidth but the most energy is radiated at very low frequency (VLF, 3-30 kHz) and lower frequencies. These short pulses of VLF energy, known as atmospherics or sferics, propagate in the Earth-ionosphere waveguide with low attenuation. Because sferics are influenced by the ionosphere during the propagation in the waveguide and by the lightning source, they contain a great amount of information about both of these. This makes it possible to remotely probe the ionosphere and lightning parameters based on received sferics thousands of kilometers away from the lightning itself. Doing this requires an accurate model of electromagnetic wave propagation in the Earth-ionosphere waveguide. The electromagnetic fields produced by intense lightning affect the atmosphere, ionosphere and magnetosphere through different processes. To quantitatively evaluate these processes and the atmospheric phenomena related to these processes, a model for the simulations of the interactions between the ionosphere and the lightning-generated electromagnetic fields at various altitudes is necessary.; The 2-D cylindrical full-wave FDTD model developed in this work satisfies the above mentioned requirements. Unlike most of the previous models of the same problem, ours treats the ionosphere as true magnetized plasma by taking accounts for all the charged particles in the ionosphere. Furthermore, this model incorporates some of the nonlinear processes, namely heating and ionization, which might contribute to the generation of high altitude spectacular optical emissions above thunderstorms that have only been recently discovered. This model is verified by comparing with other mathematical formulations and the sferics collected by the extremely low frequency (ELF, 3-3000 Hz) and VLF remote sensing systems running at Duke University. This numerical model is shown to have some valuable advantages over other methods. This model is able to simulate the lightning-generated electromagnetic fields at high altitudes up to ∼200 km; the ionosphere profiles, Earth's magnetic field and the current source can be arbitrary; and the Earth curvature correction is included in the model. Although this model was developed for lightning-related applications, it is applicable to any VLF and ELF electromagnetic wave propagation applications in the ionosphere.; A recently reported PML formulation (NPML) was adopted to develop this model for ease of implementation without losing any performance. The NPML was proved mathematically to be equivalent to the standard PML under Cartesian coordinates.; The good agreement between the simulation results and the experimental data suggests the possibility of individual lightning discharge current waveform extraction from the received sferics. In this work, this FDTD model was applied on the lightning discharge current moment waveform remote sensing to find the relationship between the sprite initiation and lightning charge moment changes statistically by analyzing several hundreds of sprite events. The empirical sprite initiation probability criterion derived through this method should make it possible to determine the sprite occurrence rate globally by using limited number of ELF/VLF remote sensing systems. In addition, by including the nonlinear processes, heating and ionization, this FDTD model was used to rigorously test the QE sprite initiation theory on a detailed, event-level by incorporating the sferic data recorded by the ELF/ULF remote sensing system and the optical observation results of sprites.