An Analysis of Current and Electric Field Pulses Associated With Upward Negative Lightning Flashes Initiated from the Säntis Tower

Abstract

We present a study on the characteristics of current and electric field pulses associated with upward lightning flashes initiated from the instrumented Säntis Tower in Switzerland. The electric field was measured 15 km from the tower. Upward flashes always begin with the initial stage composed of the upward‐leader phase and the initial‐continuous‐current (ICC) phase. Four types of current pulses are identified and analyzed in the paper: (1) return‐stroke pulses, which occur after the extinction of the ICC and are preceded by essentially no‐current time intervals; (2) mixed‐mode ICC pulses, defined as fast pulses superimposed on the ICC, which have characteristics very similar to those of return strokes and are believed to be associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC‐carrying channel at relatively small junction heights; (3) “classical” M‐component pulses superimposed on the continuing current following some return strokes; and (4) M‐component‐type ICC pulses, presumably associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC‐carrying channel at relatively large junction heights. We consider a data set consisting of 9 return‐stroke pulses, 70 mixed‐mode ICC pulses, 11 classical M‐component pulses, and 19 M‐component‐type ICC pulses (a total of 109 pulses). The salient characteristics of the current and field waveforms are analyzed. A new criterion is proposed to distinguish between mixed‐mode and M‐component‐type pulses, which is based on the current waveform features. The characteristics of M‐component‐type pulses during the initial stage are found to be similar to those of classical M‐component pulses occurring during the continuing current after some return strokes. It is also found that about 41% of mixed‐mode ICC pulses were preceded by microsecond‐scale pulses occurring in electric field records some hundreds of microseconds prior to the onset of the current, very similar to microsecond‐scale electric field pulses observed for M‐component‐type ICC pulses and which can be attributed to the junction of an in‐cloud leader channel to the current‐carrying channel to ground. Classical M‐component pulses and M‐component‐type ICC pulses tend to have larger risetimes ranging from 6.3 to 430 μs. On the other hand, return‐stroke pulses and mixed‐mode ICC pulses have current risetimes ranging from 0.5 to 28 μs. Finally, our data suggest that the 8‐μs criterion for the current risetime proposed by Flache et al. is a reasonable tool to distinguish between return strokes and classical M‐components. However, mixed‐mode ICC pulses superimposed on the ICC can sometimes have considerably longer risetimes, up to about 28 μs, as observed in this study.

Electromagnetic Fields Associated With the M‐Component Mode of Charge Transfer

“…The peak value of all the RS pulses was considered to be 10 kA, while that of MM pulses were assumed to be 5 kA. These values correspond to the arithmetic mean values derived in He, Azadifar, Rachidi, et al (). The considered RS and MM pulses are shown in Figure…”

“…The current associated with the M‐component mode of charge transfer to ground has a more or less symmetrical waveform (Rakov et al, ). He, Azadifar, Rachidi, et al () proposed to use a coefficient called asymmetrical waveform coefficient (AsWC) to distinguish between M‐component‐type ICC pulses and mixed‐mode pulses by quantifying the asymmetry of the observed current pulses. The AsWC is defined as (He, Azadifar, Rachidi, et al, ) where t 50%‐100% is the time interval during which the current rises from 50 to 100% of its peak value and FWHM is the full width at half maximum.…”

“…The analysis presented by He, Azadifar, Rachidi, et al () suggests that the M‐component‐type pulses during the initial stage (ICC) have very similar characteristics to those of classical M‐component pulses occurring during the continuing current (CC) after some return strokes. Their study also confirmed the similarity of mixed‐mode pulses and return strokes, as already suggested in other studies (e.g., Azadifar et al, ; Flache et al, ; Zhou et al, ).…”

“…He, Azadifar, Rachidi, et al () also proposed a new criterion based on the current waveform symmetry to distinguish between mixed‐mode pulses and M‐component‐type pulses, the latter exhibiting a more symmetrical waveform than the former.…”

A New Engineering Model of Lightning M Component That Reproduces Its Electric Field Waveforms at Both Close and Far Distances

“…Rakov et al (), considering simultaneously measured channel‐base currents and close electric fields (30 m) for the M component process, proposed a guided‐wave (two‐wave) mechanism, in which a downward moving current wave would be followed by an upward moving wave of the same polarity. The model proposed by Rakov et al (, ) was implemented in some studies to reproduce the millisecond‐scale electromagnetic field signature of the M component process (He et al, ; Jiang et al, ; Zhang et al, ). Optical evidence in support of the guided‐wave mechanism of M components is reported by Jiang et al ().…”