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Effect of Propagation Path Characteristics on Low-Frequency Cloud-to-Ground Lightning Signal Parameters

This work explores the effect of propagation-path characteristics (electrical conductivity of the soil (sigma) and propagation distance (d)) on cloud-to-ground (CG) lightning waveform parameters (rise-time and peak amplitude). The ground wave electromagnetic field propagates long distances (100s of km) over finitely-conductivity soil and can be detected by remote ground-based lightning sensors which respond to the vertical electric field (horizontal magnetic field) component of the resulting waveform. This work has lead to a key insight regarding ground-wave propagation of electromagnetic waves produced by CG lightning. The behavior of waveform parameters (rise-time and amplitude attenuation) as a function of propagation distance (or equivalently, changes in electrical conductivity) is primarily determined by the duration (width) of the initial waveform pulse. This has lead to the inference that the first strokes in CG lightning (which have the widest initial pulse) exhibit the greatest variation in rise-time as a function of distance or conductivity. Conversely, subsequent strokes (which have the narrowest initial pulse) exhibit the greatest attenuation of the field peak as a function of distance or conductivity. Since wider pulses have the greatest dependence of rise-time on the ratio d/sigma, focusing on this sub-population should provide the most sensitive indication of changes in electrical conductivity.