Test of the transmission line model and the traveling current source model with triggered lightning return strokes at very close range

Abstract

We test the two simplest and most conceptually different return stroke models, the transmission line model (TLM) and the traveling current source model (TCSM), by comparing the first microsecond of model‐predicted electric and magnetic field wave forms and field derivative wave forms at 15 m and 30 m with the corresponding measured wave forms from triggered lightning return strokes. In the TLM the return stroke process is modeled as a current wave injected at the base of the lightning channel and propagating upward along the channel with neither attenuation nor dispersion and at an assumed constant speed. In the TCSM the return stroke process is modeled as a current source traveling upward at an assumed constant speed and injecting a current wave into the channel, which then propagates downward at the speed of light and is absorbed at ground without reflection. The electric and magnetic fields were calculated from Maxwell's equations given the measured current or current derivative at the channel base, an assumed return stroke speed, and the temporal and spatial distribution of the channel current specified by the return stroke model. Electric and magnetic fields and their derivatives were measured 15 m and 30 m from rocket‐triggered lightning during the summer of 2001 at the International Center for Lightning Research and Testing at Camp Blanding, Florida. We present data from a five‐stroke flash, S0105, and compare the measured fields and field derivatives with the model‐predicted ones for three assumed lightning return stroke speeds, v = 1 × 108 m/s, v = 2 × 108 m/s, and v = 2.99 × 108 m/s (essentially the speed of light). The results presented show that the TLM works reasonably well in predicting the measured electric and magnetic fields (field derivatives) at 15 m and 30 m if return stroke speeds during the first microsecond are chosen to be between 1 × 108 m/s and 2 × 108 m/s (near 2 × 108 m/s). In general, the TLM works better in predicting the measured field derivatives than in predicting the measured fields. The TCSM does not adequately predict either the measured electric fields or the measured electric and magnetic field derivatives at 15 and 30 m during the first microsecond or so.