Numerical modelling of VLF propagation in the Earth-Ionosphere Waveguide

This video shows the E field strength in dB for the TM modes as function of distance from the transmitter and height above ground. The simulation show the effect of the sunrise terminator as it moves from right to left, lowering the effective reflection height from 87 km to 75 km. The calculation is not based on solving for modes but uses FFTs to propagate a scalar field forwards in small steps, correcting for the Earth's curvature at each step. The model uses a perfectly conducting ground (i.e. sea water) and neglects the effect of the Earth's magnetic field on the refractive index of the ionosphere.