The thunderstorm-driven diurnal variation of the ELF electromagnetic activity level

We present a numerical model which describes the global distribution and the thunderstorm-driven electromagnetic excitation in the extremely low frequency-(ELF) range. The model, in its present stage, builds on the parameterised world-wide distribution and temporal occurrence of thunderstorms and on electromagnetic wave propagation in the Earth-ionosphere cavity. The ionospheric D layer and the surface of the Earth are treated as very good conductors, and the ELF propagation in the Earth-ionosphere waveguide is considered to be isotropic but damped. The return strokes are regarded as the only transmitters and for the purpose of a qualitative simulation these strokes are approximated by vertical Hertz dipoles with normalized dipole moments.

Our model has been checked against data from two full days of ELF observations at a remote field station (located at 50.4° N, 9.3° E). The comparison, while qualitatively satisfactory, has revealed some quantitative limitations of the model which lead to suggestions for its improvement. These include a more accurate description of the world-wide lightning occurrence, with refined spatial and temporal resolution and possibly an account of systematic variations in the lighting dipole moment with respect to geographic latitude and frequency.