Luminous Crown Residual Vs. Bright Space Segment: Characteristical Structures for the Intermittent Positive and Negative Leaders of Triggered Lightning

Abstract

We identify and investigate the intermittent propagation of upward positive and negative leaders in rocket-triggered lightning based on comprehensive observations of fine time-resolved optical, current, and electromagnetic fields. There is an abrupt luminous crown blooming due to the achievement of a positive leader step, after which the channel weakens and the head degrades. During the positive leader pausing, residual structure is recognized at area of previous luminous crown blooming, in the form of a floating segment. Our observation point to a possible mechanism that it connects with the positive leader head, causing a forward-step and a sharp current pulse. This is to some extent similar to negative leader stepping, in which the space leader emerges and connection between the space leader and the channel head occurs. The generation of clustered space leaders give rise to negative channel branching, but the residual structure in positive leader just leads to individual step.

Polarity symmetry of leaders in moist air

“…The space stem/leader described above seems not the luminous crown residual 25 , but there may be some relations with the space-leader-like segment reported by previous literature 26 . The results reported here powerfully proves the existence of space stem/leader in positive leader steps, ending a longstanding argument 27 .…”

Observations of the Channel Illuminations During Dark Periods in Long Positive Sparks

“…The capacitive current can help support the conductivity of the residual channel, which is good for the ionization propagation when the reillumination phenomenon happens (Bazelyan and Popov, 2020). Therefore, findings in this paper may help to better understand the abrupt elongation and stepping propagation of positive leaders in the laboratory (Gallimberti, 1979; Kostinskiy et al., 2018; Shah et al., 2018) and in natural lightning (R. Jiang et al., 2020; Srivastava et al., 2019). Moreover, as experimental results in this paper provide sufficient evidence that the dark period is not really dark, it seems that the term “dark period” is no longer applicable on these occasions, and the term “dim period” is recommended as an alternative.…”

“…These processes are always not so continuous (Gallimberti, 1979; Kostinskiy et al., 2018; Shah et al., 2018; Y. Yue et al., 2015), during which dark periods are very common. Also, the dark period can be found in a stepped positive leader in natural lightning (R. Jiang et al., 2020; Srivastava et al., 2019). The dark period is defined as a time duration of no current, no channel illumination, and lasting a few microseconds to hundreds of microseconds, during which the existing observations tend to imply the absence of ionization as the resolution may be not high enough.…”

Elongation and branching of stem channels produced by positive streamers in long air gaps

“…The new positive leader propagation mechanism that is here suggested does not only have just implications for long air discharges in the laboratory. As the streamers optically observed at the heads of natural positive leaders 11 – 13 show some similarities with those in the laboratory discharge, the proposed mechanism also introduces a new physical process which needs to be assessed when studying leader-related lightning phenomena such as intermittent propagation of positive leader 39 – 44 , recoil leaders 45 – 47 and needles 12 , 48 , 49 . According to this new mechanism, the leader will propagate stepwise using short steps, as previously suggested by Bazelyan 17 .…”