Explore the vast range of titles available.

Recoil leaders develop in lightning flash decayed channels. The propagation of a recoil leader depends on the charges stored at its tip and the conductivity of the decayed channel. When the recoil leader propagates over the entire channel, a subsequent return stroke happens. Recoil leaders very often cease propagating before they reach the ground, that is, only part of the decayed channel is reionized. The present work aims to analyze the herein named secondary recoil leader that connect with the primary recoil leaders and cause them to start propagating again. We believe that the secondary recoil leader injects additional charge into the primary recoil leader, allowing the recoil leader reionize the whole decayed channel of the lightning flash. High‐speed videos analysis of upward lightning flashes shows that secondary recoil leaders play an important role on the formation and progression of dart leaders/subsequent return strokes. Plain Language Summary The recoil leader is a phenomenon that occurs in all types of lightning flashes (upward, downward and intracloud flashes). They arise in the remnants of decayed channels of positive leaders, partially or completely rebuilding these channels. The recoil leaders are responsible for some physical processes observed in lightning flashes. Thus, understanding how these physical processes originate is of significant importance. This work presents the role of secondary recoil leaders (recoil leaders that connect to preexisting recoil leaders) in the integral reconstruction of the decayed channels of the analyzed lightning flashes. Key Points Use of high‐speed cameras to study recoil leaders in upward lightning flashes Secondary recoil leaders boost the development of previous recoil leaders Secondary recoil leaders likely influence the development of dart leaders/subsequent return strokes

We have studied the vertical negative leader progression features and the charge structures of 13 typical winter lightning flashes that produced downward terrestrial gamma‐ray flashes (TGFs). All these flashes started at altitudes below 1.5 km with an initial downward negative leader that propagates at a speed ranging from 1.3 to 4.5 × 106 m/s, followed by a strong negative return stroke called “energetic compact stroke” (ECS). After the ECS, usually there exists a radio quiet period lasting more than 10 ms. Interestingly, for more than half of the cases, soon after the resumed activities, an upward negative leader occurred at a position close to the lightning initiation point. Most of TGF lightning occurred under a main negative charge layer at the height of around 2 km. This negative charge layer is usually featured with a thickness of less than 2 km and a horizontal extension of more than 10 km.

The dissonant development of positive and negative lightning leaders is a central question in atmospheric electricity. It is also the likely root cause of other reported asymmetries between positive and negative lightning flashes, including the ones regarding: stroke multiplicity, recoil activity, leader velocities, and emission of energetic radiation. In an effort to contrast lightning leaders of different polarities, we highlight the staggering differences between two rocket‐triggered lightning flashes. The flash beginning with upward positive leaders exhibits an initial continuous current stage followed by multiple sequences of dart leaders and return strokes. On the other, in its opposite‐polarity counterpart, the upward development of negative leaders is by itself the entire flash. As a result, the flash with negative leaders is faster, briefer, transfers less charge to the ground, has lower currents, and smaller spatial extent. We conclude by presenting a discussion on the three fundamental leader propagation modes. Plain Language Summary Lightning flashes that carry positive and negative charges are completely different. In this article, we report on lightning triggered by launching a rocket tethered to the ground toward an electrified cloud. The staggering differences between positive and negative flashes are exposed by a three‐dimensional radio location system and by the current transferred to ground via the trailing wire. Key Points Triggered flashes with positive and negative leaders are contrastingly different with the latter being faster, briefer, and more compact The channel behind triggered positive leaders decays engendering dart leaders and return strokes, which is unparalleled in the negative case Average conductivity is higher in the negative leader channel despite the lack of return strokes and the lower charge transferred to ground

We present time‐correlated ultra‐high‐speed video camera and electromagnetic field measurements of the attachment processes in a natural negative cloud‐to‐ground stroke. The video camera frame exposure time and pixel resolution were 740 ns and 0.91 m/pixel, respectively. The common streamer zone (CSZ) was first observed 2.52 µs preceding the first frame showing the return stroke (RS) in progress, when the upward and downward leader‐tips were 9.8 m apart. In the next frame, the two leaders were observed to have propagated toward each other within the CSZ, with their tips being 0.91 m apart. Our observations show with unprecedented precision/clarity that (a) the slow front in the field waveform is associated with the CSZ, and (b) the “proper” start of the RS is marked by the onset of the fast transition in the field waveform which occurs at the completion of the attachment processes (when the upward and downward leaders have merged). Plain Language Summary Detailed observations of how cloud‐to‐ground lightning attaches to ground remain very difficult to obtain. This is because the associated processes occur on a microsecond‐scale (or faster), making it challenging to perform time‐correlated multi‐instrument measurements. In this study, we present time‐correlated observations of ultra‐high‐speed video and electromagnetic fields, which provide new insights regarding the lightning attachment processes. These processes comprise the so‐called breakthrough phase of lightning attachment when two oppositely charged leaders (one from the thundercloud and one from ground or ground‐based object) merge with each other. Key Points Time‐correlated multi‐sensor observations (with camera exposure time of 740 ns) of the natural lightning attachment processes were made The slow front portion of the field waveform is associated with the time‐evolution of the common streamer zone Fast‐transition onset in the field waveforms occurs when the upward and downward leaders fully merge (“proper” start of return stroke)

Elves are transient luminous events appearing above thunderclouds. They are produced by high peak current lightning events. They occasionally appear as doublets or in general as multiplets. We model elves doublets by setting the temporal and spatial variations of the return stroke current amplitude and we show that our modeling results are consistent with the measurements by Pierre Auger Cosmic‐Ray Observatory. We show the normalized electric fields and the emission rates at the altitudes of elves and we discuss the ability of our model to simulate the time separation of elves rings in doublets for different return stroke parameters. Our model can explain observations which are not compatible with the typical concept of elves doublets being generated by ground reflections of waves generated by intracloud currents.

We have developed and deployed a 3D Interferometer-type Lightning Mapping Array (InLMA) for observing winter lightning in Japan. InLMA consists of three broadband interferometers installed at three stations with a distance from 3 to 5 km. At each interferometer station, three discone antennas were installed, forming a right triangle with a separation of 75 m along their two orthogonal baselines. The output of each InLMA antenna is passed through a 400 MHz low-pass filter and then recorded at 1 GS/s with 16-bit accuracy. A new method has been proposed for finding 3D solutions of a lightning mapping system that consists of multiple interferometers. Using the InLMA, we have succeeded in mapping a positive cloud-to-ground (CG) lightning flash in winter, particularly its preliminary breakdown (PB) process. A study on individual PB pulse processes allows us to infer that each PB pulse process contains many small-scale discharges scattering in a height range of about 150 m. These small-scale discharges in a series of PB pulses appear to be continuous in space, though discontinuous in time. We have also examined the positive return stroke in the CG flash and found a 3D average return stroke speed of 7.5 × 107 m/s.

Simultaneous observations of ultraviolet (337 nm) and near‐infrared (777 nm) lightning emissions provide insight into streamer and leader processes. However, coordinated ground‐based measurements of these bands during return strokes remain scarce. We present multiband optical and broadband radio (0.1 Hz–5 MHz) observations of 11 cloud‐to‐ground flashes (0.6–4.2 km range). For first return strokes within the photometer field of view, the 337 nm peak precedes the 777 nm peak by 38 μs on average, and 45 μs for subsequent strokes. The 337 nm emission shows faster rise times (∼5 μs) than 777 nm (∼30–40 μs) and correlates more strongly with peak current (R2 = 0.62 vs. 0.25). Temperature estimates indicate a hotter channel at the 777 nm maximum. Our findings demonstrate that near‐UV emissions provide a sensitive ground‐based diagnostic of the lightning attachment process and may offer improved optical proxies for estimating peak current.

This study analyzes the two‐dimensional speed profiles of 107 stepped leaders and 93 dart leaders recorded by high‐speed cameras in Utah (USA), together with data from lightning location system. The results shows that the final and average speed of the stepped leader has a very strong (R = 0.82) and strong (R = 0.71) correlation with the peak current of the return stroke. It also shows that 91% of the stepped leaders increased their speed near the ground (average increase of 69%). The same analysis for dart leaders shows weak correlation with the peak current of the prospective return stroke (R = 0.39 to average speed and R = 0.28 to final speed). This paper briefly discusses why peak current is better correlated with final speed than with the average speed, and why stepped leaders exhibit a significant correlation, while dart leaders do not. Plain Language Summary This study looks at how fast stepped leaders and dart leaders of lightning flashes propagate from the cloud base to ground, using high‐speed camera videos and data from a lightning location system. The results show that the final and average speed of the leaders are well correlated to the return stroke current, the return stroke current being more closely related to the final speed than the average speed. In contrast, dart leaders showed a weak correlation between their speed and the return stroke current. It also shows that most stepped leaders sped up as they got closer to the ground. Key Points The return stroke peak current is better correlated with final speed than with the average speed of the stepped leader No significant correlation was found between dart leaders speed and stroke peak current The stepped leaders increase their propagation speed near the ground

Negative‐polarity lateral discharges on pre‐ionized negative channels during a positive cloud‐to‐ground lightning flash were captured by very high frequency interferometric observations. Prior to the return stroke (RS), as the positive leader (PL) advanced steadily and the negative leader (NL) weakened, flickering lateral re‐discharges with small scale, resembling needles on PLs, propagated toward the NL tip at approximately 8.0 × 104 m/s. Unlike needles concentrated near PL tips, these re‐discharges occurred along nearly the entire horizontal negative channel. Following the RS, both fast discharges along existing negative channels and new lateral discharges breaking into virgin air were observed, rapidly extending the negative channels and sustaining the continuing current. These re‐discharges appeared to be closely linked to channel potential variations: gradual potential changes before the RS reactivation preceded negative branches, while abrupt potential jumps after the RS initiation triggered intense axial and lateral discharges.

Channel cutoff is the process by which an active lightning channel cools down, goes dark, and loses its ability to effectively conduct electricity. Current cutoff precedes several lightning phenomena, notably dart leaders and return strokes, but the process by which a hot, highly‐conducting leader channel can undergo current cutoff is poorly understood. In this work, we present self‐consistent simulations of positive leader propagation, which include coupled treatment of electrodynamics and plasma physics. The unstable positive leader spontaneously undergoes channel cutoff and creates the conditions for the emission of dart leaders, that is, it accumulates a packet of negative charge at the intersection between cutoff and conducting channel segments. A residual conductivity in the cutoff section facilitates the conversion of the electric field enhancement into a traveling wave, which retraces and reionizes the channel. This is critical for allowing the frail positive leader to keep propagating.