FEM — the finite element method (3D models)

FEM (the finite element method) solves the fields in the frequency domain and makes it possible to construct a three-dimensional inhomogeneous Earth-ionosphere cavity. The space of the cavity is divided into a mesh of elements, on which the solution of the wave equation is sought for each frequency separately.

The key advantage of 3D-FEM is that it can reproduce line splitting of modes — the phenomenon in which a resonance peak splits as a result of the asymmetry of the cavity. The model is moreover independent of the source configuration: it computes the natural properties of the cavity itself, not the response to a specific lightning strike, so the results are not tied to an assumption about the position of the sources.

A 3D-FEM simulation model of the Earth-ionosphere cavity was described by Goncharov et al. (2019) Goncharov 2019. Because it works in the frequency domain, it is suitable for the systematic investigation of the resonance properties of the cavity and of fine spectral effects. Like FDTD, it is computationally demanding, in exchange for high fidelity in capturing inhomogeneities and line splitting of modes.