Fine structure of electric field waveforms recorded at near and far distances from the lightning channel

We examine the shapes and relative magnitude of slow fronts and fast transitions in electric field waveforms of first return strokes in negative cloud-to-ground lightning recorded simultaneously at near and far distances from the lightning channel. The near and far field-measuring stations are located at Camp Blanding and in Gainesville, Florida, respectively, separated by a distance of about 45 km. A total of five return strokes had been recorded in 2007-2008, four of which were analyzed in detail (one was not suitable for analysis due to saturation of electric field waveform at the far station). The AM and GM zero-to-peak risetimes for four strokes were 7.2 μs and 6.6 μs, respectively, at the near station, and 7.0 μs and 6.5 μs, respectively, at the far station. The AM and GM 10-to-90% risetimes were 4.9 μs and 4.6 μs, respectively, at the near station, and 4.0 μs and 3.6 μs, respectively, at the far station. Three of the four first strokes exhibited the two distinct phases, the slow front and fast transition, in the initial rising portion of their electric field waveforms. For two return strokes the amplitude of the slow front was 49% of the peak at the near station and 44% of the peak at the far station, while for one it was 43% and 18% of the peak at the near and far stations, respectively. The AM and GM 10-to-90% risetimes of the fast transition for the three strokes were both 0.6 μs at the near station versus 0.9 μs at the far station. It is shown, via modeling, that the slow front in electric field waveforms at far distances is primarily due to the radiation field component, while at near distances it is composed of more or less equal contributions from all three components of electric field. For both measured and model-predicted waveforms, the durations of the slow front appear to be similar at near and far distances from the lightning channel.