Early phase of lightning currents measured in a short tower associated with direct and nearby lightning strikes

Records of natural lightning currents of negative downward strokes measured at the base of a 60 m high instrumented tower are presented to denote patterns of their early phase. Such patterns in the records of five flashes and two upward unconnected leaders are discussed. Typical profiles of this early phase, comprising the pre-return-stroke pulses of the current flowing upwards the tower in response to the downward stepped leader approaching the ground, the concavity in the initial rising part of the return-stroke waveform, and the abrupt rise around the half-peak of the first-stroke waveform, are remarked. Statistics are developed for the pre-return-stroke pulses of current to characterize their amplitude, duration, interval, and risetime for both the first stroke and upward unconnected leaders, which proved to be quite similar. The ratio of the maximum pulse peak to first stroke peak is typically in the range 0.7% to 2.2% of the analyzed currents, different from the value of around 10% found for the distant electric fields radiated by first-stroke currents.