This work presents a two-dimensional (2D) subgrid technology for the geodesic finite different time-domain (FDTD) algorithm, which is applied to solve global extremely low frequency (ELF) electromagnetic EM wave propagation problems in the Earth-ionosphere system. The new technology provides arbitrarily locale resolution to study finer structure without disturb the global grid structure. Combined with the subgrid technique, the new geodesic FDTD algorithm can solve EM propagation problems in specific locale regions without extra computational burden. Based on the original geodesic FDTD formulations, the 2D subgrid technique is developed, and its computational stable relation is derived and analyzed. Then, possible three-dimensional (3D) subgrid structure is proposed. Finally, potential applications for the subgrid technique are suggested.
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