Leader observations during the initial breakdown stage of a lightning flash

Abstract

This paper describes luminosity and leader propagation during the initial breakdown (IB) stage of a cloud-to-ground flash, beginning at 6.06 km altitude and 31.86 ms before the return stroke (RS). High-speed video (50,000 frames per s) and time-correlated electric field change (E-change) data show multiple branch ends advance concurrently in the first 6 ms of the flash; each branch begins with IB pulses and propagates first via bursts as an initial leader. Burst luminosity (pixel intensity) is directly related to IB pulse amplitude. Some initial leader branches transition to advancing as stepped leaders after a few milliseconds. Each initial leader branch end makes the transition to a stepped leader branch end at a different time, resulting in a complex E-change waveform including relatively narrow step-type pulses during the IB stage and no apparent Intermediate stage prior to the Leader stage. There is no visible evidence of an upward propagating leader end prior to the RS nor of any light above the earliest visible IB luminosity prior to or during the RS. During the RS, the topmost visible portion of the channel that developed as an initial leader (above 5.1 km) behaves differently from the channel below, indicating it is less conductive. Radar and time-of-arrival lightning source data indicate that the IB luminosity visible to the camera comes from about 6 km inside the thundercloud echo. The 1276 m long initial leader transitions to a stepped leader at 4.9-5.0 km, near the altitude of the radar bright band.

Initial breakdown and fast leaders in lightning discharges producing long‐lasting disturbances of the lower ionosphere

“…In the cases where the IB‐RS intervals were exceptionally short (e.g., <2 ms), it is possible that the measured VLF radiation was produced by leader processes not occurring during the IB stage. The measured IB‐RS intervals indicate the maximum duration of the leader process, as leaders can initiate either during or immediately following the IB stage [ Stolzenburg et al , ]. For a leader path length that is less than 7 km, the mean IB‐RS interval of 3.5 ms results in an average leader speed ≥2 × 10 6 m s −1 , and the median IB‐RS interval of 2.4 ms results in an average leader speed ≥3 × 10 6 m s −1 (consistent with speeds of initial leaders or the “ β ‐type” portion of leaders [ Stolzenburg et al , ; Schonland , , ]).…”

“…Wu et al [] observed that measured ratios of IB and return stroke field intensity (IB‐RS ratio) were inversely correlated to measured time delays between IB and the following return stroke (IB‐RS interval). Measured IB‐RS intervals indicate the maximum leader duration, as leaders can initiate either during or immediately following the IB stage [ Stolzenburg et al , ]. Marshall et al [] observed a similar correlation between the range‐normalized intensities of IB pulses and IB‐RS intervals.…”

“…Among all lightning flashes, IB pulses were rarely observed in VLF (with >8 dB SNR) at distances greater than a few hundred kilometers, as will be discussed in the next section. Based on this criterion, we have classified the IB radiation reported in this paper (observed at distances greater than a few hundred kilometers) as “intense.” Recent experimental and modeling studies indicate that peak currents producing IB pulses may be as large as several tens of kiloamps [ Karunarathne et al , ; da Silva and Pasko , ; Kolmašová et al , ], flowing over channels with lengths on the order of a few hundred meters to a kilometer [ Stolzenburg et al , , ; Karunarathne et al , ; da Silva and Pasko , ; Nag and Rakov , ]. Such large currents have been demonstrated to produce initial breakdown observable at a distance of 600 km [ Kolmašová et al , , ].…”

“…LMA sources shown in Figure (first panel) indicate the descent of LMA sources during the IB process (possibly an initial leader) [ Stolzenburg et al , ] and during the stepped leader (SL) process. The LMA sources associated with the IB stage at the start of the flash occur at altitudes between 5 and 6 km, consistent with the inferred altitudes of the bottom of negative charge regions in typical north Florida thunderstorms [ Pilkey et al , ].…”

A Study of Preliminary Breakdown and Return Stroke Processes in High-Intensity Negative Lightning Discharges

“…Rakov and Uman [3] stated that the PB process can be viewed as a sequence of channels extending in seemingly random directions from the cloud charge source with one of them evolving into the stepped leader propagating to ground. More recent studies of preliminary breakdown by Stolzenburg et al [4,5], who used high-speed video and electric field records, indicated that the PB luminosity bursts were correlated with the PB pulses in the corresponding electric field records and suggested that each PB pulse was caused by a substantial current surge traversing a channel segment of the order of hundreds of meters in length. Nag and Rakov [6], via modeling, estimated the PB pulse peak currents to be of the order of tens of kiloamperes, comparable to or even exceeding the corresponding return-stroke peak current.…”