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1. 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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2. Thunderstorms Producing Sferic‐Geolocated Gamma‐Ray Flashes Detected by TETRA‐II

   https://doi.org/10.1029/2020JD033765  

   Smith, Deirdre ; Trepanier, Jill ; Alnussirat, Samer T. ; Cherry, Michael L. ; Legault, Marc D. ; Pleshinger, Donald J. ( July 2021 , Journal of Geophysical Research: Atmospheres)

   The terrestrial gamma‐ray flash (TGF) and Energetic Thunderstorm Rooftop Array (TETRA‐II) detected 22 X‐ray/gamma‐ray flash events associated with lightning between October 2015 and March 2019 across three ground‐based detector locations in subtropical and tropical climates in Louisiana, Puerto Rico, and Panama. Each detector array consists of a set of bismuth germanate scintillators that record X‐ray and gamma‐ray bursts over the energy range 50 keV–6 MeV (million electron volts). TETRA‐II events have characteristics similar to both X‐ray bursts associated with lightning leaders and TGFs: sub‐millisecond duration, photons up to MeV energies, and association with nearby lightning (typically within 3 km). About 20 of the 22 events are geolocated to individual lightning strokes via spatiotemporally coincident sferics. An examination of radar reflectivity and derived products related to events located within the Next Generation Weather Radar (NEXRAD) monitoring region indicates that events occur within mature cells of severe and non‐severe multicellular and squall line thunderstorms, with core echo tops which are at or nearing peak altitude. Events occur in both high lightning frequency thunderstorm cells and low lightning frequency cells. Events associated with high frequency cells occur within 5 min of significant lightning jumps. Among NEXRAD‐monitored events, hail is present within 8 km and 5 min of all except a single low‐altitude cold weather thunderstorm. An association is seen with maximum thunderstorm development, lightning jumps, and hail cells, indicating that the TETRA‐II X‐ray/gamma‐ray events are associated with the peak storm electrification and development of electric fields necessary for the acceleration of electrons to high energies.

    

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3. 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)

   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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4. Energetic Radiation from Subsequent-Stroke Leaders: The Role of Reduced Air Density in Decayed Lightning Channels

   https://doi.org/10.3390/app12157520  

   Kereszy, Istvan ; Rakov, Vladimir ; Czumbil, Levente ; Muresan, Alexandru ; Ding, Ziqin ; Micu, Dan ; Cooray, Vernon ( August 2022 , Applied Sciences)

   Leaders of subsequent strokes in negative cloud-to-ground lightning are known to produce X-ray/gamma-ray emissions detectable at distances of a few kilometers or less from the lightning channel. These leaders usually develop in decayed but still warm channels of preceding strokes. We computed electric field waveforms at different points along the path of subsequent leader as those points are traversed by the leader tip. For a typical subsequent leader, the electric field peak is a few MV/m, which is sufficient for production of energetic radiation in a warm (reduced air density) channel. We examined the dependence of electric field peak on the leader model input parameters, including the prospective return-stroke peak current (a proxy for the leader tip potential) and leader propagation speed, and compared model predictions with measurements. 

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5. VHF Radio Spectrum of a Positive Leader and Implications for Electric Fields

   https://doi.org/10.1029/2021GL093145  

   Pu, Yunjiao ; Cummer, Steven A. ; Liu, Ningyu ( June 2021 , Geophysical Research Letters)

   Electric fields associated with a developing natural lightning leader are difficult to measure. This work demonstrates a new approach to indirectly probing the electric fields in the streamer zone of a lightning leader. Using a 10–250 MHz broadband lightning interferometer, very high frequency (VHF) radio emissions from the tip of a positive cloud‐to‐ground (CG) leader were measured and localized. We specially use a normalized spectral analysis to avoid the challenge of absolute system calibration to show that the positive leader spectrum exhibits a clear cutoff frequency at 80 MHz. Compared with theoretical predictions, this cutoff frequency corresponds to a streamer growth rate ofand an average electric field of 0.9 times the conventional breakdown fieldin streamer bursts from the positive leader. Implications for the detectability of positive leaders through VHF emissions and for the production of X‐rays by positive leaders are analyzed.

    

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