Istvan Kereszy

Using a high‐speed imaging system operating in UV (290–370 nm) range at the Lightning Observatory in Gainesville (LOG), Florida, we observed, for the first time, what appeared to be lightning channel fragmentation starting <1 ms after the return‐stroke onset. The phenomenon was observed in 14 out of 15 strokes in 2 negative cloud‐to‐ground flashes at distances of a few hundred meters from LOG. Channel luminosity profiles exhibited pulsations, superimposed on the slowly‐varying UV luminosity waveform, with a typical period of some hundreds of microseconds over a typical time interval of some milliseconds. For higher‐order strokes, the pulsation time interval was longer, up to tens of milliseconds. The pulsation period was often influenced by the occurrence of M‐component. The presence of slowly‐varying UV luminosity and occurrence of M‐components during the pulsation time interval, along with the continuous visible‐range luminosity, are all indicative of the fragmented channel carrying a steady background current.

Development of hot lightning channels is facilitated by essentially cold streamers emanating from their extremities. Thus, streamers play a crucial role in lightning, yet they are poorly understood, because they emit primarily ultraviolet (UV) light, which usually goes undetected. Here, we present unique UV images of lightning, including interaction between positive and negative streamers, leading to new inferences on lightning propagation and attachment. We observed an abrupt change in the lightning propagation path to ground, which resulted in an aborted attachment process and a different ground connection point, 200 m away from the expected one. This change was likely due to the self‐choking effect of large negative space charge that was suddenly introduced by the intense streamer burst ahead of the lower end of the descending negatively‐charged channel. Further, the unique UV images show that positive and negative streamer zones are an order of magnitude longer than previously thought.

Lightning often strikes tall (≥200 m) objects and poses significant threat to the infrastructure. Using the characteristic electric field signatures of the tower-terminated lightning, we developed a machine learning approach to identify strikes to towers in large lightning datasets. The ground-truth data were acquired at the Lightning Observatory in Gainesville (LOG), Florida. The classification model used in this study is based on supervised multi-layer perceptron model, aiming to capture complex pulse patterns with the neural network architecture. The results show that tower-terminated lightning and non-tower-terminated lightning can be accurately identified. We found that the model performance depends on the configuration of the training dataset. Using the Local Interpretable Model-agnostic Explanations (LIME), we found that our machine learning model is capable of capturing the key features of electric field signatures produced by lightning strikes to tall towers.

In a three‐stroke negative cloud‐to‐ground flash, the leader of Stroke 2, while forming a new, heavily branched path to ground, briefly collided with the defunct (nonluminous) channel/branches of Stroke 1 at heights of about 2.0, 1.6, again 2.0 km, and 90 m above ground level. Each of the first three (higher‐altitude) collisions was associated with luminosity waves originating from the collision point, including an upward reflection‐type wave along the colliding leader channel and one or more transmitted waves along the residual branches of the preceding stroke. Based on the estimated extension speeds and RF field signatures, we attributed the transmitted luminosity waves to dart‐stepped leaders developing in shorter segments of the decayed channel. As a result of the last collision, at a height of 90 m above ground level, a branch of the Stroke 2 leader entered the lower part of the residual (nonluminous) Stroke 1 channel and connected to the ground, producing a return stroke. Additionally, the heavily branched Stroke 2 leader created a ground termination about 950 m away from its ground termination common with Stroke 1. We observed four X‐ray pulses of unknown origin with peaks ranging from 130 to 750 keV during a 3.5‐ms or so interval around the time of the higher‐altitude collisions during the leader stage of Stroke 2, but no detectable X‐ray pulses during the leader stages of Strokes 1 and 3. The observed occurrence of X‐ray pulses is unlikely to be random and appears to be linked to the Stroke 2 leader.

In a three-stroke negative cloud-to-ground flash, the leader of Stroke 2, while forming a new, heavily branched path to ground, briefly collided with the defunct (non-luminous) channel/branches of Stroke 1 at heights of about 2.0 km, 1.6 km, again 2.0 km, and 90 m above ground level. Each of the first three (higher-altitude) collisions was associated with luminosity waves originating from the collision point, including an upward reflection-type wave along the colliding leader channel and one or more transmitted waves along the residual branches of the preceding stroke. Based on the estimated extension speeds and RF field signatures, we attributed the transmitted luminosity waves to dart-stepped leaders developing in relatively short segments of the decayed channel. As a result of the last collision, at a height of 90 m above ground level, a branch of the Stroke 2 leader entered the lower part of the residual (non-luminous) Stroke 1 channel and connected to the ground, producing a return stroke. Additionally, the heavily-branched Stroke 2 leader created a ground termination about 950 m away from its ground termination common with Stroke 1. We observed four X-ray pulses with peaks ranging from 130 to 750 keV during a 3.5-ms or so interval around the time of the higher-altitude collisions during the leader stage of Stroke 2, but no detectable X-ray pulses during the leader stages of Strokes 1 and 3. The probability of random occurrence of 4 X-ray pulses within a 3.5-ms time interval at our site is 0.00099.

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.

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.

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.