On the influence of the soil stratification and frequency-dependent parameters on lightning electromagnetic fields

We present an analysis of lightning electromagnetic fields taking into account the soil stratification and frequency dependence of its electrical parameters. Two current waveforms corresponding to typical first and subsequent return strokes are considered for the analysis. Different cases for the soil (homogeneous, 2-layer, frequency-dependent/constant electrical parameters) are considered. The analysis is carried out considering different distance ranges: close (50 m), intermediate (5 km) and distant (100 km). The obtained results confirm that the vertical electric field and the azimuthal magnetic field at close range can be evaluated assuming the ground as a perfectly conducting plane. The impact of the soil stratification and frequency-dependent parameters on the vertical electric field and azimuthal magnetic field appear at intermediate and distant ranges. On the other hand, the horizontal electric field is found to be very sensitive to the ground stratification for all the considered distance ranges. However, at close range, the impact of the soil becomes less significant for observation points that are located at above-ground heights of 10 m or higher. It is shown that the three field components are affected more markedly by the soil stratification than by the frequency dependence of its electrical parameters, especially for intermediate and distant ranges (i.e., 5 km and 100 km). Furthermore, subsequent return stroke fields are more significantly affected by the soil stratification and frequency-dependence compared to first return stroke fields. The impact of the frequency-dependent soil parameters on the considered field components is more noticeable in a poorly conducting soil compared to a good conducting soil. We present also a comparison between simulation results with simultaneous measurements of current and distant vertical electric fields associated with rocket-triggered lightning flashes. It is shown that the computed vertical electric field waveforms for the case of a two-layer soil follow to a much better extent the corresponding experimental waveforms compared a non-stratified ground model. The frequency-dependence of the soil affects slightly the early-time response of the field. However, the late-time response of the field is essentially determined by the soil stratification.