Bursts of Fast Pulses in Positive Lightning Current Waveforms Recorded on the Santis Tower

We present in this paper an analysis of bursts of fast pulses in current waveforms associated with positive lightning flashes of type 1 recorded on the Santis Tower in Switzerland. Type-1 flashes are characterized by the presence of a large unipolar current pulse with characteristics similar to those reported by Berger and co-workers. Several milliseconds to several tens of milliseconds prior to the main pulse, a train of damped oscillatory pulses separated by several milliseconds, exhibiting the highest current steepnesses of the flash have been observed in the majority of these flashes. An analysis reveals that these fast pulses can be categorized into two distinctive types: (A) Damped oscillatory pulses with a frequency of about 2 MHz and a duration of about 5 mu s, and (B) large bipolar pulses similar to a doublet of some 200 ns width, followed by a fast oscillation with a repetition frequency of the order of 20 MHz. The peak current derivatives associated with pulses of both types have a mean value of 3.2 kA/mu s. The time separation between two consecutive pulses is less than 15 ms with a mean value of 4.2 ms.