On the Transition From Initial Leader to Stepped Leader in Negative Cloud‐to‐Ground Lightning

Abstract

High‐speed video and electric field change data are used to describe the first 5 ms of a negative cloud‐to‐ground flash. These observations reveal an evolution in character of the luminosity and electric field change pulses as two branches of the leader separately transition from initial leader to propagating as a negative stepped leader (SL). For the first time reported, there is evidence of weak luminosity coincident with the initiating event, a weak bipolar pulse 60 μs prior to the first initial breakdown (IB) pulse. During the IB stage, the initial leader advances intermittently at intervals of 100–280 μs, in separate light bursts that are bright for a few 20‐μs frames and are time coincident with IB pulses. In the intervals between IB pulses, the initial leader is dim or invisible during the earliest 1.8 ms. Within 2 ms, the leader propagation begins transitioning to an early SL phase, in which the leader tip advances at more regular intervals of 40–80 μs during relatively dim and brief steps which are coincident with SL pulses having short duration, small amplitude, and typically unipolar waveform. These data indicate that when the entire initial leader length behind the lower end begins to remain illuminated between bursts, the propagation mode changes from IB bursts to SL steps, and the IB stage ends. The results support a hypothesis that the early initial leader development occurs in the absence of a continuously hot channel, thus the initial leader propagation is physically unlike the self‐propagating SL advance.

The Mechanism of the Origin and Development of Lightning From Initiating Event to Initial Breakdown Pulses (v.2)

“…The sequence of events leading to IBP#2 fits with the high-speed video and FA data of Stolzenburg et al (2013Stolzenburg et al ( , 2014Stolzenburg et al ( , 2020, which showed that each new classic IBP extended the previous IBP channel. We note that unlike the generation of the first classic IBP, where the large E was caused by hydrodynamics and statistics, the generation of the second classic IBP is caused by the superposition of electric fields due to (1) E of the polarized plasma of IBP#1 and (2) E caused by hydrodynamics and statistics.…”

“…In seven CG lightning flashes studied using high-speed video data, Stolzenburg et al (2013) observed a burst of light coincident with each classic IB pulse, and successive bursts developed as "linear segments" that extended for 300-1,500 m. Stolzenburg et al (2013) called the extending linear channel an "initial leader." (Using similar video data, Stolzenburg et al, 2014Stolzenburg et al, , 2020 shown that an initial leader can transition to a stepped leader within 5 ms of flash initiation. This transition takes longer in some flashes.)…”

“…c2. The optical radiation of IEs seems to be quite weak (Stolzenburg et al, 2014(Stolzenburg et al, , 2020. Thus, the Mechanism should not contain an initial powerful flash of light like the powerful burst of light that occurs during an IBP.…”

“…These three initial stages of a flash are followed by the well‐understood negative stepped leader stage. The optical radiation of IEs seems to be quite weak (Stolzenburg et al, 2014, 2020). Thus, the Mechanism should not contain an initial powerful flash of light like the powerful burst of light that occurs during an IBP. …”

The initial stage of cloud lightning imaged in high-resolution

“…In particular, it is not known what processes lead to the creation of the primary initial leader (PIL) channel and how that channel propagates. In recent years, the use of lightning mapping arrays (Rison et al, 1999;Edens et al, 2012) and VHF radio interferometers (Rhodes et al, 1994;Yoshida et al, 2010;Stock et al, 2014) augmented with fast antennas and optical measurements (Hill et al, 2011;Montanyà et al, 2015;Qi et al, 2016;Tran & Rakov, 2016) have led to the general picture that lightning initiation begins with an ionization event (Stolzenburg et al, 2020), which could be in the form of a powerful narrow bipolar event (Rison et al, 2016) or much weaker VHF source as seen in (e.g. Marshall et al, 2019;Lyu et al, 2019).…”