Characteristics of the initial rising portion of near and far lightning return stroke electric field waveforms

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). Field waveform characteristics, including overall zero-to-peak and 10-to-90% risetimes, duration of slow front, fast transition 10-to-90% risetime, and magnitude of slow front relative to the peak, were found to be similar to those reported from other studies, in which the field propagation path was over ground (as opposed to sea water). 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.