Measured close lightning leader‐step electric field–derivative waveforms

We characterize the measured electric field–derivative (dE/dt) waveforms of lightning stepped‐leader steps from three negative lightning flashes at distances of tens to hundreds of meters. Electromagnetic signatures of leader steps at such close distances have rarely been documented in previous literature. Individual leader‐step three‐dimensional locations are determined by a dE/dt TOA system. The leader‐step field derivative is typically a bipolar pulse with a sharp initial half‐cycle of the same polarity as that of the return stroke, followed by an opposite polarity overshoot that decays relatively slowly to background level. This overshoot increases in amplitude relative to the initial peak and becomes dominant as range decreases. The initial peak is often preceded by a “slow front,” similar to the slow front that precedes the fast transition to peak in first return stroke dE/dt and E waveforms. The overall step‐field waveform duration is typically less than 1 μ s. The mean initial peak of dE/dt, range‐normalized to 100 km, is 7.4 V m −1 μ s −1 (standard deviation (S.D.), 3.7 V m −1 μ s −1 , N = 103), the mean half‐peak width is 33.5 ns (S.D., 11.9 ns, N = 69), and the mean 10‐to‐90% risetime is 43.6 ns (S.D., 24.2 ns, N = 69). From modeling, we determine the properties of the leader step currents which produced two typical measured field derivatives, and we use one of these currents to calculate predicted leader step E and dE/dt as a function of source range and height, the results being in good agreement with our observations. The two modeled current waveforms had maximum rates of current rise‐to‐peak near 100 kA μ s −1 , peak currents in the 5–7 kA range, current half‐peak widths of about 300 ns, and charge transfers of ∼3 mC. As part of the modeling, those currents were propagated upward at 1.5 × 10 8 m s −1 , with their amplitudes decaying exponentially with a decay height constant of 25 m.