Leader Pulses Research Articles

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.

The electric fields produced by lightning stepped leaders

The electric fields produced by stepped and dart-stepped leaders which immediately precede return strokes in lightning discharges to ground have been recorded in Florida and Arizona. The mean interval time between normal steps is about 16 μs, and the mean interval between dart steps is 6–8 μs. The amplitudes of leader pulses in Florida increase just prior to the return stroke, the largest usually being about 10% of the return stroke peak. In Arizona the leader pulse amplitudes are smaller than those in Florida, in relation to the return stroke, and are not as easy to identify. The shapes of the fields produced by normal steps are similar to dart steps, and the dart steps are very similar to regular sequences of pulses produced by many intracloud discharges. The 10–90% rise times of individual step wave forms are often less than 0.3 μs, and the full width at half maximum of a step pulse is typically 0.4–0.5 μs under conditions where the propagation distortion is minimal. The amplitudes and the shapes of leader step wave forms suggest that the peak step current is at least 2000–8000 A close to the ground and that the maximum rate of change of step current is 6–24 kA/μs or larger. A rough estimate of the minimum charge lowered during the formation of a step is (1–4) × 10−3 C.

Radiation field wave forms produced by lightning stepped leaders

Broadband electric field data are presented which show stepped leader wave forms preceding records of distant lightning return strokes. The majority of leader pulses are characterized by a large initial peak with a small and slow opposite overshoot. Total pulse durations range from 15-40 microsec several milliseconds before the return stroke to 2-10 microsec immediately preceding the return stroke. Close to the ground the stepped leader pulses occur at regular 10- to 20-microsec intervals and are almost unipolar with rise times of about 1 microsec and full widths at half maximum in the range from 1 to 3 microsec. The ratio of the peak of the last leader pulse to the subsequent return stroke peak is typically 0.1, which suggests a peak step current near the ground of about 10% of the return stroke peak current.