The fine structure in the field change produced by positive ground strokes

Several features of the field change waveforms originating from positive first return strokes in cloud-to-ground lightning flashes were clarified on the basis of observations of electric and magnetic field changes with bandwidths of 32 Hz to 1.2 MHz and 160 Hz to 1.2 MHz, respectively. In most positive waveforms, the rapidly rising portion (fast transition) was seen to follow the slowly rising portion (slow front), as is usually observed in the waveforms of negative return strokes. The average 10–90% rise times of field waveforms associated with positive first return strokes in Niigata, located in Japan's winter thunderstorm area, in winter and in Tokyo in summer were 8.7 and 6.7 μs, respectively. These values were about twice the 10–90% rise times for the negative first return strokes observed over the corresponding period in each area. Owing to the significantly longer slow front of positive waveforms, the average zero-to-peak rise time for the positive return strokes obtained from the same recordings was 22.3 μs, which is about 3 times larger than that for negative strokes: 7.5 μs. Only 1/3–1/4 of the waveforms of positive first return strokes observed in both summer and winter had discernible pulses preceding return stroke field change. The average time interval between leader pulses within a 100-μs period just prior to the first return strokes observed in winter in Niigata was 17.4 μs, approximately equal to the reported value for negative first return strokes in Florida in summer.