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An effective FDTD scheme for the propagation of VLF-LF radiowaves in the Earth-Ionosphere waveguide Summary form only given. Very low frequency (VLF) and low frequency (LF) waves remain used to transmitting toward submarines because they can be received a few meters under the sea surface, a possibility not allowed by other frequency bands. To predict the propagation of such waves, the waveguide and wavehop methods have been developed during years 1960-1980. Both are effective in terms of computational cost, but have some drawbacks. During years 1990-2000, a third method was introduced to compute VLF-LF propagation, the finite-difference time-domain (FDTD) method (Thévenot et al, Annales des Télécommunications, 1999). The price of the full wave method is a far larger computational time, but this is a drawback increasingly small as the computers become increasingly powerful.After developing a FDTD computer code for VLF-LF, several papers appeared on FDTD schemes in the Ionosphere. Although they address other applications, an interesting idea in these papers is the use of an implicit scheme for solving the auxiliary equation that governs the current density in the medium. Introduced in non magnetized plasmas (S. A. Cummer, IEEE Trans. Ant. Propag., 1997) and later used in the magnetized Ionosphere (S. A. Cummer, IEEE Trans. Ant. Propag., 2000), the implicit scheme has some drawbacks, as larger memory requirements, but its stability condition is the same as in a vacuum, conversely to that of the explicit scheme which is more severe. For this reason, using the ideas in Cummer’s papers, a new implicit scheme has been developed for VLF-LF propagation. The scheme is more compact and effective than the one in (W.H. Hu and S.A. Cummer, IEEE Trans. Ant. Propag., 2006). It permits significant reductions of the computational time, and it renders the FDTD time step independent of the Physics of the Ionosphere, which is a highly desirable feature. The paper presents the new implicit scheme and discusses its advantages and drawbacks in comparison with- the explicit scheme. Numerical experiments are provided to validate the new scheme and illustrate its effectiveness in VLF-LF calculations. With the computer code based on the new scheme, the CPU time for a 8000 km radiopath at frequency 20 kHz is typically smaller than 1 min on a 3GHz PC. It is about 10 min for a 5000 km radiopath at 60 kHz. These CPU times can be reduced further to about 10 s and 1-2 min by taking advantage of the multithread possibilities of PC’s.