The fine structure of positive lightning return-stroke radiation fields

The electric fields generated by lightning flashes striking the North sea were measured, with a time resolution better than a few tens of nanoseconds, at Fano island in Denmark. The measuring station was located a few tens of meters away from the high water mark at the west coast of the island. This particular location made it possible to capture, with minimal propagation effects, the electromagnetic fields from lightning flashes striking the North Sea. The waveforms were recorded by a measuring system that could provide a time resolution of about 10 ns. The data recorded had the following features. The initial rising part of the positive return-stroke fields contains a slow front followed by a fast transition. The duration of the slow front of the positive return-stroke fields had an average of 8.3 mus and its amplitude, measured as a fraction of the initial peak, had an average of 0.61. The 10%-90% rise time of the fast rising portion of the positive return-stroke fields was about 0.26 mus, on average. The average peak value of the measured radiation fields normalized to 100 km was 15.7 V/m. The mean of the peak value of the time derivative of the radiation fields was 25 V/m/mus. The full width at half maximum of the radiation field derivative had a mean of 170 ns.