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1. Insights on Space‐Leader Characteristics and Evolution in Natural Negative Cloud‐to‐Ground Lightning

   https://doi.org/10.1029/2021GL093614  

   Khounate, Hamza; Nag, Amitabh; Plaisir, Mathieu N.; Imam, Abdullah Y.; Biagi, Christopher J.; Rassoul, Hamid K. (August 2021, Geophysical Research Letters)

   Abstract We present sub‐microsecond‐scale, high‐speed video camera observations of three negative stepped leaders in cloud‐to‐ground flashes with return‐stroke peak currents (estimated by the U.S. National Lightning Detection Network) of −17, −104, and −228 kA. The camera frame exposure times for these observations were 1.8, 1.0, and 0.74 µs, respectively. The 0.74 µs exposure time is the shortest reported to date. We observed the temporal and spatial evolution of space leaders from their inception to their attachment to the pre‐existing leader channel (PELC). For stepped leaders that led to return strokes having higher peak currents, the space leaders appear to have incepted at farther median two‐dimensional distances from their respective PELC‐attachment points. These median distances were 6.1, 16.6, and 17.6 m, respectively, for the three strokes. Our observations indicate that space leader characteristics are likely influenced by stepped‐leader line‐charge‐density, which is expected to be higher in strokes with higher return‐stroke peak currents. 

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2. Inception of subsequent stepped leaders in negative cloud-to-ground lightning

   https://doi.org/10.1007/s00703-019-00702-8  

   Stolzenburg, Maribeth; Marshall, Thomas C.; Karunarathne, Sumedhe (August 2020, Meteorology and Atmospheric Physics)

   null (Ed.)

   Abstract Time-correlated high-speed video and electric field change data for 139 natural, negative cloud-to-ground (CG)-lightning flashes reveal 615 return strokes (RSs) and 29 upward-illumination (UI)-type strokes. Among 121 multi-stroke flashes, 56% visibly connected to more than one ground location for either a RS or UI-type stroke. The number of separate ground-stroke connection locations per CG flash averaged 1.74, with maximum 6. This study examines the 88 subsequent strokes that involved a subsequent stepped leader (SSL), either reaching ground or intercepting a former leader to ground, in 61 flashes. Two basic modes by which these SSLs begin are described and are termed dart - then - stepped leaders herein. One inception mode occurs when a dart leader deflects from the prior main channel and begins propagating as a stepped leader to ground. In these ‘divert’ mode cases, the relevant interstroke time from the prior RS in the channel to the SSL inception from that path is long, ranging from 105 to 204 ms in four visible cases. The alternative mode of SSL inception occurs when a dart leader reaches the end of a prior unsuccessful branch—of an earlier competing dart leader, stepped leader, or initial leader—then begins advancing as a stepped leader toward ground. In this more common ‘branch’ mode (85% of visible cases), there may be no portion of the subsequent RS channel that is shared with a prior RS channel. These two inception modes, and variations among them, can occur in different subsequent strokes of the same flash. 

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3. High‐Speed Video Observations of Needles Evolving Into Negative Leaders in a Positive Cloud‐To‐Ground Lightning Flash

   https://doi.org/10.1029/2023JD039523  

   Wu, Bin; Qi, Qi; Lyu, Weitao; Ma, Ying; Chen, Lyuwen; Lyu, Fanchao; Zhang, Yang; Fan, Yanfeng; Rakov, Vladimir A. (November 2023, Journal of Geophysical Research: Atmospheres)

   Abstract High‐speed video records of a single‐stroke positive cloud‐to‐ground (+CG) flash were used to examine the evolution of eight needles developing more or less radially from the +CG channel. All these eight needles occurred during the later return‐stroke stage and the following continuing current stage. Six needles, after their initial extension from the lateral surface of the parent channel core, elongated via bidirectional recoil events, which are responsible for flickering, and two of them evolved into negative stepped leaders. For the latter two, the mean extension speed decreased from 5.3 × 10⁶to 3.4 × 10⁵and then to 1.3 × 10⁵ m/s during the initial, recoil‐event, and stepping stages, respectively. The initial needle extension ranged from 70 to 320 m (N = 8), extension via recoil events from 50 to 210 m (N = 6), and extension via stepping from 810 to 1,870 m (N = 2). Compared with needles developing from leader channels, the different behavior of needle flickering, the longer length, the faster extension speed, and the higher flickering rate observed in this work may be attributed to a considerably higher current (rate of charge supply) during the return‐stroke and early continuing‐current stages of +CG flashes. 

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4. High‐Speed Video Observations of Recoil Leaders Producing and Not Producing Return Strokes in a Canton‐Tower Upward Flash

   https://doi.org/10.1029/2019GL083862  

   Wu, Bin; Lyu, Weitao; Qi, Qi; Ma, Ying; Chen, Lyuwen; Jiang, Ruijiao; Zhu, Yanan; Rakov, Vladimir_A (July 2019, Geophysical Research Letters)

   Abstract High‐speed video and electric field change data have been used to examine the initiation and propagation of 21 recoil leaders, 7 of which evolved into dart (or dart‐stepped) leaders (DLs) initiating return strokes and 14 were attempted leaders (ALs), in a Canton‐Tower upward flash. Three DLs and two ALs clearly exhibited bidirectional extension. Each DL was preceded by one or more ALs and initiated near the extremity of the positive end of the preceding AL. The positive end of each bidirectional DL generally appeared to be inactive (stationary) or intermittently propagated along the positive part of the preceding AL channel and extended into the virgin air. A sequence of two floating channel segments was formed ahead of the approaching positive end of one DL, causing its abrupt elongation. 

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5. Spectral Hardness of X‐ and Gamma‐Ray Emissions From Lightning Stepped and Dart Leaders

   https://doi.org/10.1029/2023JD040397  

   Contreras‐Vidal, Luis; Sanchez, James T; da_Silva, Caitano L; Sonnenfeld, Richard G; Aulich, Graydon; Edens, Harald E; Eack, Kenneth B; Smith, David M (April 2024, Journal of Geophysical Research: Atmospheres)

   During the 2022 New Mexico monsoon season, we deployed two X‐ray scintillation detectors, coupled with a 180 MHz data acquisition system to detect X‐rays from natural lightning at the Langmuir Lab mountain‐top facility, located at 3.3 km above mean sea level. Data acquisition was triggered by an electric field antenna calibrated to pick up lightning within a few km of the X‐ray detectors. We report the energies of over 240 individual photons, ranging between 13 keV and 3.8 MeV, as registered by the LaBr3(Ce) scintillation detector. These detections were associated with four lightning flashes. Particularly, four‐stepped leaders and seven dart leaders produced energetic radiation. The reported photon energies allowed us to confirm that the X‐ray energy distribution of natural stepped and dart leaders follows a power‐law distribution with an exponent ranging between 1.09 and 1.96, with stepped leaders having a harder spectrum. Characterization of the associated leaders and return strokes was done with four different electric field sensing antennas, which can measure a wide range of time scales, from the static storm field to the fast change associated with dart leaders. 

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