Comparing Upward Negative Stepped Leader and Preliminary Breakdown Pulses

The study explores the characteristics of upward negative stepped leader pulses recorded at the Säntis Tower in Switzerland. Analysis of simultaneous channel-base current and 14.7-km vertical electric field data revealed two distinct types of pulses associated with upward negative stepped leaders [2]. Category A pulses were characterized by bipolar electric field signatures with initial positive half-cycles, correlated with negative unipolar current pulses. The E-field pulses had an average duration of 23.7 (± 11.7) μs and exhibited time-dependent characteristics, including increased frequency and slower risetimes. Category B pulses were characterized by unipolar (positive or negative) or bipolar field signatures that lacked correlation with any major current pulses. These had narrower temporal widths compared to Category A pulses. As discussed in Azadifar et al. 2018 [3], notable similarities exist between these two categories and, respectively, “Classical” and “Narrow” Preliminary Breakdown Pulses (PBPs) observed in the initial stages of downward negative leaders [6]. Herein, we present a statistical analysis of 45 Category A pulses from 5 Type-II upward positive flashes, which confirms their similarity to Classical PBPs, particularly in regards to key characteristic timescales reported in the literature, such as the aforementioned pulse duration [1,4,5,6,10], 10-90% risetime (6.1 ± 3.6 μs) [1,9], and zero-crossing time (11.9 ± 6.1 μs) [1,9]. In this dataset, 7 (~16%) of these bipolar pulses were observed to be inverted (with a negative initial half-cycle), and were excluded from this preliminary analysis, though it is of note that a similar phenomenon has been observed in downward stepped leaders as well [8]. The temporal widths of the initial and second half-cycles were observed to be linearly correlated (with correlation coefficient ρ = 0.77), as were their peak amplitudes (ρ = -0.80). Further linear correlations were found to exist between the peak E-field and current amplitudes (R2 = 0.74), as well as their risetimes (R2 = 0.73), with E-field pulses generally rising faster than current pulses. To the best of our knowledge, these specific relationships have not been reported in the literature, though correlations between PBP amplitude and: duration [7], and return stroke peak current [10] have been observed. These findings enhance our understanding of upward lightning phenomena and associated electromagnetic radiation, revealing parallels with the Breakdown, Intermediate, and Leader stages of downward negative flashes. This study contributes to the ongoing debate about the underlying physical mechanisms of lightning initiation and propagation, and highlights the need for further research in this area. Observational studies are specifically recommended to validate these correlations and refine proposed modeling frameworks.