Physical mechanism of initial breakdown pulses and narrow bipolar events in lightning discharges

Abstract

To date the true nature of initial breakdown pulses (IBPs) and narrow bipolar events (NBEs) in lightning discharges remains a mystery. Recent experimental evidence has correlated IBPs to the initial development of lightning leaders inside the thundercloud. NBE wideband waveforms resemble classic IBPs in both amplitude and duration. Most NBEs are quite peculiar in the sense that very frequently they occur in isolation from other lightning processes. The remaining fraction, 16% of positive polarity NBEs, according to Wu et al. (2014), happens as the first event in an otherwise regular intracloud lightning discharge. These authors point out that the initiator type of NBEs has no difference with other NBEs that did not start lightning, except for the fact that they occur deeper inside the thunderstorm (i.e., at lower altitudes). In this paper, we propose a new physical mechanism to explain the source of both IBPs and NBEs. We propose that IBPs and NBEs are the electromagnetic transients associated with the sudden (i.e., stepwise) elongation of the initial negative leader extremity in the thunderstorm electric field. To demonstrate our hypothesis a novel computational/numerical model of the bidirectional lightning leader tree is developed, consisting of a generalization of electrostatic and transmission line approximations found in the literature. Finally, we show how the IBP and NBE waveform characteristics directly reflect the properties of the bidirectional lightning leader (such as step length, for example) and amplitude of the thunderstorm electric field.

Griffiths and Phelps Lightning Initiation Model, Revisited

“…Figures b and c show the resulting FA waveform (or sferic) that would be observed on the ground at the observation location (5.5‐km plan distance and 6‐km altitude below NBE1's source). The calculations utilize the localized source approximation of equations (7) of da Silva and Pasko (), fully appropriate for the G&P model where the charges and currents occur only at the leading edge of the ionization front. Panel c shows a comparison with the observed sferic.…”

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

“…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 , , ].…”

“…For detailed information regarding the physical nature of IB, we refer the reader to the recent experimental works of Campos and Saba [], Stolzenburg et al [, ], and Wilkes et al []. Recent efforts in modeling currents in initial breakdown processes include the works of Karunarathne et al [], da Silva and Pasko [], and Nag and Rakov [].…”

Electric field change measurements of a terrestrial gamma ray flash

“…[] measured IB pulses using an array of 10 E‐change sensors (described below in section 2.1) and then used a transmission line model to determine six properties of the IB pulses including charge moment change (average value = 0.12 C km) and current propagation speed (average value = 1.3 × 10 8 m/s). A more sophisticated model has been developed assuming that IB pulses involve a negative stepping process [ da Silva and Pasko , ]; in their model the current pulse that produces an IB pulse is caused by the rearrangement of charge on a conducting leader after a negative step. da Silva and Pasko [] modeled one IB pulse from Karunarathne et al [] and obtained good agreement with the findings of Karunarathne et al [].…”

“…A more sophisticated model has been developed assuming that IB pulses involve a negative stepping process [ da Silva and Pasko , ]; in their model the current pulse that produces an IB pulse is caused by the rearrangement of charge on a conducting leader after a negative step. da Silva and Pasko [] modeled one IB pulse from Karunarathne et al [] and obtained good agreement with the findings of Karunarathne et al [].…”