Features of Upward Positive Leaders Initiated From Towers in Natural Cloud‐to‐Ground Lightning Based on Simultaneous High‐Speed Videos, Measured Currents, and Electric Fields

Abstract

Original simultaneous records of currents, close electric field, and high‐speed videos of natural negative cloud‐to‐ground lightning striking the tower of Morro do Cachimbo Station are used to reveal typical features of upward positive leaders before the attachment, including their initiation and mode of propagation. According to the results, upward positive leaders initiate some hundreds of microseconds prior to the return stroke, while a continuous uprising current of about 4 A and superimposed pulses of a few tens amperes flow along the tower. Upon leader initiation, the electric field measured 50 m away from the tower at ground level is about 60 kV/m. The corresponding average field roughly estimated 0.5 m above the tower top is higher than 0.55 MV/m. As in laboratory experiments, the common propagation mode of upward positive leaders is developing continuously, without steps, from their initiation. Unlike downward negative leaders, upward positive leaders typically do not branch off, though they can bifurcate under the effect of a downward negative leader's secondary branch approaching their lateral surface. The upward positive leader's estimated average two‐dimensional propagation speed, in the range of 0.06 × 106 to 0.16 × 106 m/s, has the same order of magnitude as that of downward negative leaders. Apparently, the speed tends to increase just before attachment.

Abrupt Elongation (Stepping) of Negative and Positive Leaders Culminating in an Intense Corona Streamer Burst: Observations in Long Sparks and Implications for Lightning

“…It looks like a space leader inside the positive streamer zone, with its forward extending positive part being pronounced and branched and the negative part being undetectable. Alternatively, the floating formation was actually connected to the primary channel, in which case it was part of the transition from the corona streamer burst to the regular positive streamer zone. Comparison with the features of positive and negative leaders observed in lightning shows that for either polarity, the stepping in long sparks appears to be qualitatively similar to the stepping in lightning. Our results for positive‐leader stepping, together with those published by Les Renardieres Group for positive sparks and by Wang et al () for UPL in natural upward lightning, argue against the notion (e.g., Saba et al, ; Visacro et al, ) that positive leader cannot (or unlikely to) produce true steps in positive leaders, only fluctuations of current and luminosity induced by a simultaneously developing negative stepped leader.…”

“…5. Our results for positive-leader stepping, together with those published by Les Renardieres Group for positive sparks and by Wang et al (2016) for UPL in natural upward lightning, argue against the notion (e.g., Saba et al, 2015;Visacro et al, 2017) that positive leader cannot (or unlikely to) produce true steps in positive leaders, only fluctuations of current and luminosity induced by a simultaneously developing negative stepped leader.…”

High‐Speed Video and Lightning Mapping Array Observations of In‐Cloud Lightning Leaders and an M Component to Ground

“…LMA sources at the time of the second stroke and the ratio of the leader to return stroke field change indicate that the second stroke occurred in clear air below 1 km (see also, for reports of flashes in clear air; Krider, ; Waldteufel et al, ). Additionally, the final current pulse before the return stroke (bottom‐left panel of Figure ) does not have a coincident d E /d t pulse measured (typically due to stepping of the downward leader), suggesting that that current pulse is associated with stepping of the upward connecting leader (see also Biagi et al, , ; Guimaraes et al, ; Hill et al, ; Visacro et al, , ).…”

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

“…When the rocket was launched to trigger the lightning, the electric field at ground was 4.2 kV/m, as detected by the electric field meter CS110 (Campbell Scientific, Inc., United States). The absolute humidity was 20.6-21.8 g/m (Lalande et al, 1998(Lalande et al, , 2002Lu et al, 2014;Visacro et al, 2017). As the leader gradually enhanced and the background current increased to several amperes, the impulsive feature of the pulses became less pronounced and the waveform transformed to a more or less rippled type, which is clear in Figure 2b.…”