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Abstract We examined three descending positive leaders at distances of 5–11 km and three descending negative leaders at distances of 6–7 km, all simultaneously imaged by high‐speed framing cameras operating in the visible and UV ranges. UV images (290–370 nm) of the positive leaders each exhibited a strong embellishment at the lower channel end, which was not observed in the corresponding visible images (480–800 nm). In contrast, none of the negative leaders exhibited channel embellishment in the UV range and their morphology in UV was similar to that in the visible. Additionally, no embellishment was seen in four negative leaders imaged in UV only. The observed UV embellishment, which is likely to be the streamer zone at the positive‐leader tip, appeared to undergo expansion‐contraction cycles. We attributed the lack of detectable streamer‐zone emission in the UV range in negative leaders to a much lower streamer generation rate compared to positive leaders. 

Abstract This review covers selected results of recent observations of lightning discharges performed across the entire electromagnetic spectrum (radiofrequency, optical, and energetic radiation) at the Lightning Observatory in Gainesville, Florida. The most important results include (a) characterization of the preliminary-breakdown, stepped-leader, and return-stroke processes in high-intensity (⩾50 kA) negative lightning discharges, (b) the first high-speed video images of bidirectional leader that made contact with the ground and produced a return stroke, (c) discovery of negative stepped leader branches colliding with the lateral surface of neighboring branches of the same leader, (d) new data on the occurrence context and properties of compact intracloud discharges, and (e) observation of a terrestrial gamma-ray flash that occurred during a bipolar cloud-to-ground lightning discharge. The results serve to improve our understanding of the physics of lightning with important implications for lightning modeling, lightning protection, and high-energy atmospheric physics studies. 

Abstract Infrared (IR) luminosity of lightning channel in the 3–5 μm range usually persisted throughout the entire interstroke interval, which is in contrast to the simultaneously recorded visible (0.4–0.8 μm) luminosity that always decayed to an undetectable level prior to a subsequent return stroke pulse. A longer visible luminosity period at the end of flash tended to be associated with a longer IR afterglow period following the decay of visible luminosity (and by inference current) to an undetectable level. At the end of flash, the IR luminosity persisted up to about 1 s, and the median IR afterglow duration was a factor of 10 longer than the median visible luminosity duration. The IR luminosity often exhibited a hump when the visible luminosity was monotonically decaying or undetectable, with the corresponding channel temperature being likely around 3400 K. 

Abstract Our basic knowledge of downward positive lightning leaders is incomplete due to their rarity and limited ability of VHF mapping systems to image positive streamers. Here, using high‐speed optical records and wideband electric field and magnetic field derivative signatures, we examine in detail the development of a descending positive leader, which extended intermittently via alternating branching at altitudes of 4.2 to 1.9 km and involved luminosity transients separated by millisecond‐scale quiet intervals. We show that the transients (a) are mostly initiated in previously created but already decayed branches, at a distance of the order of 100 m above the branch lower extremity, (b) extend bidirectionally with negative charge moving up, (c) establish a temporary (1 ms or so) steady‐current connection to the negative part of the overall bidirectional leader tree, and (d) exhibit brightening accompanied by new breakdowns at the positive leader end. One of the transients unexpectedly resulted in a negative cloud‐to‐ground discharge. Both positive and negative ends of the transients extended at speeds of 10⁶–10⁷ m/s, while the overall positive leader extension speed was as low as 10³–10⁴ m/s. Wideband electric field signatures of the transients were similar to K‐changes, with their millisecond‐ and microsecond‐scale features being associated with the steady current and new breakdowns, respectively. For transients with both ends visible in our optical records, charge transfers and average currents were estimated to be typically a few hundreds of millicoulombs and some hundreds of amperes, respectively. 

Abstract Using visible‐range and infrared (3–5 µm) high‐speed video cameras, we observed collisions of adjacent branches in downward negative stepped leaders. Typically, a lagging (chasing) branch (CB) approached a leading branch (LB) from aside at about 90° angle and connected to the lateral surface of the LB within some tens of meters or less of its tip. We infer that collisions can be facilitated by the attracting force of upward moving positive‐charge wave associated with stepping at the leading branch tip. Outcomes of branch collisions differ. The chasing branch may be absorbed by the LB, rebound, or temporarily bridge two branches. It appears that a heavily branched negative stepped leader creates a highly structured and rapidly changing electric field pattern inside the volume it occupies. We observed abrupt changes in the direction of branch extension, suggesting that the direction of local electric field can differ significantly from the ambient.