Insights into the ground attachment process of natural lightning gained from an unusual triggered-lightning stroke

[1] We present measured electric and magnetic field and current derivatives from an unusual triggered-lightning stroke that was initiated by a dart-stepped leader and that involved an upward connecting leader. The initial rising portion of the current (integrated dI/dt) waveform is composed of a "slow front'' rising to 20 kA in 2.2 mu s, followed by a fast transition'' from 20 to 27 kA in 0.2 mu s. A similar slow-front/fast-transition sequence has been observed in the currents and in the distant radiation fields of natural-lightning first strokes. Field derivatives measured at 15 and 30 m for the triggered-lightning stroke are similar to those measured for natural-lightning first strokes occurring within a few hundred meters. Two versions of the transmission line model, with the measured current derivative and assumed propagation speeds as input, are able to reasonably reproduce the slow fronts and fast transitions in the field derivatives measured at 15 and 30 m (the electric field slow front being primarily electrostatic) and predict the slow front in the distant radiation fields. These results suggest that the source of the slow fronts observed in the currents and in the distant radiation fields of natural first strokes is likely to be a pair of microsecond-scale current waves, each having a peak of up to some tens of kiloamperes, propagating in opposite directions from the junction of the descending and upward connecting leaders at a speed on the order of 10(8) m s(-1), rather than the upward connecting leader itself, as is often thought.