Modeling the Global Electromagnetic Resonance Field Produced by Lightning Discharges with a Continuing Current

In lightning research, there is a growing interest in measuring the extremely low frequency (ELF, 3 Hz–3 kHz) electromagnetic (EM) radiation of lightning, as this frequency band can be used to infer various characteristics of lightning discharges that are currently not available from state-of-the-art lightning detection networks. One of these characteristics is the presence of a continuing current (CC), which can last for hundreds of milliseconds and therefore poses an increased risk of physical lightning damage. In this paper, we investigate the modeling capability of the global EM resonance field excited by lightning with a CC using a modified version of a well-known analytical model describing Schumann resonances (SRs) and a full FDTD model. Since analytical models are much faster and require significantly less memory than full numerical models, they are widely used to interpret ELF data. On the other hand, the flexibility of a full numerical model allows the simulation of model configurations that cannot be described by analytical models. To use the two models confidently, it is important to check their consistency for similar configurations. Here, we demonstrate that, for a uniform Earth-ionosphere cavity, the theoretical ELF spectra provided by the analytical and full numerical models show good agreement ∼7(±5) % for both the impulse-like (describing SRs) and exponentially decaying (describing the presence of a CC) current sources. Our results confirm that the analytical model is well suited to interpret ELF measurements for the purpose of studying global lightning activity or individual lightning discharges.